1.2.1 The Four-Stroke Gasoline Engine

A gasoline internal combustion engine converts the chemical energy of fuel to mechanical energy through a process called the four-stroke cycle. This process is also known as the Otto cycle in honor of its inventor, Nikolaus Otto. Each four-stroke cycle makes two engine revolutions. Cylinders, pistons, valves, and spark plugs are the primary components of the engine during the combustion process. As illustrated in Figure 1.3, the four-stroke process includes the intake stroke, compression stroke, power stroke, and exhaust stroke as described below:

1. Intake stroke: This is the first stroke of the cycle. Initially, the intake valve opens and the piston is at the top of the cylinder. The piston subsequently slides down the cylinder to create a low-pressure environment. While the outlet (exhaust) valve remains closed, the low-pressure environment draws a measured volume of fuel and air mixture inside the cylinder. When the piston reaches the bottom of the stroke (to the maximum volume position), the intake valve closes and this stroke ends.
2. Compression stroke: While both the intake and outlet valves are closed, the piston starts moving upwards and squeezes the air–fuel mixture from the top of the cylinder. The stroke finishes when the piston reaches the minimum volume position. The air–fuel compression results in an increase in pressure, temperature, and fuel mixture density.
3. Power stroke: Once the piston reaches its maximum compression ratio (the top of the cylinder), the compressed air–fuel mixture ignites with a spark plug. This stage generates lots of heat, and the resulting high temperature and pressure gases push the piston downward. The intake and outlet valves remain closed during this stroke. Gasoline engines, occasionally referred to as SI engines, use the spark ignition (SI) system. The SI system is an electrical system that includes a lead-acid battery and an induction coil. The system provides a high-voltage electric spark to ignite the air–fuel mixture inside the cylinder. The battery is recharged on-board using an alternator driven by the engine.
4. Exhaust stroke: This is the fourth and final stroke of the cycle. The leftover combustion by-products and gases in the power stroke, called the exhaust, are vacated during the exhaust stroke. Once the piston reaches the bottom of the cylinder at the end of the power stroke, the outlet (exhaust) valve opens and the exhaust stroke begins. The crankshaft pushes the piston upwards to expel the exhaust from the cylinder through the outlet valve. At the end of this stroke, the exhaust valve closes. As the exhaust valve closes, the intake valve opens and the sequence cycle repeats.

1.2.2 The Four-Stroke Diesel Engine

Like the gasoline engine, the diesel engine is also a four-stroke engine, though it operates slightly differently. Rather than a spark ignition system, diesel engines use a compression heating ignition system in which the ignition of the air–fuel mixture occurs because of compression instead of a spark plug. In other words, ignition occurs when the diesel fuel is sprayed into the cylinder filled with compressed air.

When compared to gasoline engines, additional advantages of the diesel engine include its superior fuel economy, higher compression ratio, and enhanced durability. The desirable fuel economy is a result of its properties, high compression ratio, and lower fuel price. The higher compression ratio produces better thermal efficiency, consequently resulting in highly effective mechanical work. Diesel fuel is also cheaper than gasoline when measured in fuel volume. Moreover, despite both fuel types having similar amounts of energy in specified weights, the amount of energy in a specific volume of diesel fuel is still 10% greater than that of gasoline. However, the high compression ratio of a diesel engine results in very high pressure, which consequently produces a high level of stress on engine materials. Furthermore, diesel engine components such as cylinders, pistons, rods, and valves are much heavier and thicker, which causes the diesel engine to be sluggish. Another issue with the diesel engine is its complicated fuel injector system. Since diesel fuel does not evaporate easily, high-pressure injection nozzles are required to produce the appropriate air–fuel mixture. As illustrated in Figure 1.4, the four-strokes of the diesel cycle are intake stroke, compression stroke, power stroke, and exhaust stroke as described below:

1. Intake stroke: The intake valve opens and draws air in the cylinder while the piston slides downward. The intake valve closes when the piston reaches the maximum volume.
2. Compression stroke: The piston pushes upwards and compresses the air to about 1/16th of its initial volume. Air temperature can reach up to 1000 °C in this stage.
3. Power stroke: Diesel fuel enters the cylinder by the fuel injector just before the peak of compression. The air–fuel mixture starts burning due to the high pressure and temperature within the combustion chamber, and the resulting thermodynamic energy pushes the piston down. Throughout the course of this stage, both intake and exhaust valves remain closed.
4. Exhaust stroke: The exhaust valve opens and the piston slides upwards while the combustion gases emits through the exhaust valve. Once the piston reaches the head of the cylinder, the exhaust valve closes and the intake valve opens, restarting the cycle.

1.2.3 ICE Performance Characteristics

An established performance baseline evaluates and compares several parameters and variables of IC engines (ICEs). Assessing and comparing efficiencies is a method of evaluating the performance of an engine. This includes aspects such as fuel efficiency, thermal efficiency, work efficiency, and engine emissions, among others. The following subsection discusses the most significant parameters, including power-to-weight ratio, air-to-fuel ratio, power-to-volume ratio, and volumetric efficiency.

1.2.3.2 Air–Fuel Ratio

Both gasoline and diesel fuels are a composition of hydrocarbons (made from hydrogen, oxygen and carbon). They react with the oxygen available in the air to kindle burning. In an internal combustion engine, the air (oxygen) available in the cylinder can burn a portion of fuel. If the amount of fuel is greater than the air available (rich fuel mixture), some unburned fuel will remain after combustion occurs. The vehicle expels the unburned fuel into the environment through the exhaust valves and tailpipe, consequently polluting the air and environment. On the other hand, if the amount of air is more than the amount of fuel (lean fuel mixture), there will be more oxygen available in the exhaust gases produced. Note, in a gasoline engine, lean fuel mixtures may cause engine knocking, an abnormal combustion in which the remaining air–fuel mixture in the chamber detonates due to high-pressure and heat. Engine knocking produces aggravating noises, reduces engine efficiency, and may damage engine materials. Additionally, a lean air–fuel mixture may also generate more nitrogen–oxide pollutants. Ultimately, the engine performance is a function of the direct relationship between an engine's air flow and its fuel requirements.

In engine design, the definition of air–fuel ratio is a measure of the air–fuel mixture quality. This ratio measures the weight ratio between air and fuel in the air–fuel mixture within a combustion chamber at any given moment. A desirable air–fuel ratio is one that allows the engine to achieve maximum power and the best fuel economy while producing the least emissions. However, these objectives are conflicting since the fuel requirements of an engine change, based on variations in temperature, load, and speed conditions.

The stoichiometric air–fuel ratio is an air–fuel mixture with a certain weight of air that oxidizes available fuel without leaving behind excess oxygen or unburned fuel. This ratio is 14.7 for gasoline engines and 14.5 for diesel engines. A mixture that is less than a stoichiometric ratio is a rich mixture, while a mixture greater than a stoichiometric ratio is a lean mixture. Gasoline engines need lean mixtures to attain optimal fuel economy through minimum fuel consumption while they need rich mixtures to suppress combustion knock and to attain maximum power (Figure 1.5). Idle heavy loads and high-speed conditions require a rich mixture and normal cruising and light load conditions require a lean mixture. The air–fuel ratio needed to start an engine is approximately 9, and the best possible fuel economy is attainable with an air–fuel ratio of 16. Acceleration, idling, and full power requirements need an air–fuel ratio of approximately 12.

The throttle and fuel injector regulate the air–fuel ratio in a gasoline engine vehicle. The throttle is a valve that varies the engine airflow, while the electronic fuel injector sprays a measured amount of fuel into the cylinder. The function of the throttle is to adjust the amount of air entering the intake manifold. The vehicle driver controls this mechanism through the throttle (accelerator) pedal.

Older vehicles (pre-1990) used a carburetor to mix air and fuel. A carburetor is a mechanical device that regulates the amount of fuel drawn into the airstream based on the speed and pressure of airflow entering the engine. Fuel injection systems replaced carburetors and they are no longer used in production vehicles due to their lower fuel efficiency and higher emission rates.

In modern vehicles, the Engine Control Unit (ECU) determines the amount of fuel that the injector sprays into the intake manifold. The ECU calculates this amount based on information obtained from sensors installed within the vehicle. Oxygen, mass airflow, throttle position, and coolant temperature sensors send the most significant data. The exhaust pipe houses the oxygen sensor, which detects rich and lean mixtures by monitoring the amount of available oxygen. The throttle position sensor monitors the throttle pedal position. The mass airflow sensor measures the air mass entering the intake manifold. The coolant temperature sensor measures the engine temperature to determine if it has reached the appropriate operating conditions. When the driver steps on the throttle pedal to demand more power from the engine, the mass airflow sensor sends the ECU information about the amount of air mass entering the engine. The ECU subsequently injects more fuel into the intake manifold based on this information. Likewise, the ECU also acquires temperature data from the coolant temperature sensor. If the engine has not reached proper working temperature, the ECU accordingly injects more fuel to help heat up the engine. In cruise and light load conditions, the ECU keeps the air–fuel ratio close to stoichiometric proportions based on the amount of oxygen leaving the exhaust pipe, as measured by the oxygen sensor. On the other hand, high speed/high load conditions require a proper rich air–fuel mixture. In these circumstances, the injector feeds a measured amount of fuel regardless of the information given by the sensors. This variation is necessary to ensure a higher margin against detonation.

With some exceptions, diesel engines generally draw an uncontrolled amount of air into the cylinders because they do not have a throttle valve or throttled system. Regulating the amount of fuel injected at the end of a compression stroke controls the speed and power. Unlike gasoline engines, which can run on rich, lean, and stoichiometric fuel mixtures, diesel engines mostly run on a lean mixture. The combustion temperature of lean fuel mixtures is lower than that of stoichiometric mixtures because it burns less fuel. Lower temperature environments lessen the amount of heat lost in the engine, allowing more heat energy to convert into mechanical work by pistons during combustion.

1.2.4 ICE Vehicle Emissions

Vehicle emissions are one of the key contributors to environmental air pollution and global climate change. Tank-to-wheel and well-to-wheel emissions are the most common ways to discuss motor vehicle emissions. Tank-to-wheel refers to emissions produced during vehicle operation and during fuel combustion. On the other hand, well-to-wheel emission refers to discharges during the production and distribution of fuel, as well as throughout vehicle operation. The subsequent section discusses tank-to-wheel emissions and their effects on the global environment.

As mentioned earlier in this chapter, IC engines generate propulsion power by combusting air and a fuel, usually gasoline or diesel, within a combustion chamber. Gasoline and diesel fuels are a mixture of hydrocarbons (made of hydrogen, oxygen and carbon atoms), while air is mainly composed of nitrogen (N₂) and oxygen (O₂). In a perfectly operating engine with ideal combustion conditions, hydrocarbons would react with oxygen to produce water vapor (H₂O) and carbon dioxide (CO₂), while nitrogen would pass through the engine unaffected. However, depending on the operating conditions and the fuel-air ratio, an incomplete combustion procedure may occur, causing the vehicle to emit pollutants as well.

The amount of pollution that a vehicle emits depends on many factors. The most important factors include fuel rate consumption, driving conditions (e.g., the speed, acceleration, and load on the vehicle), the type of fuel used (e.g., gasoline or diesel), and the technology used to control emissions (e.g., catalysts), among others.

The main emissions from vehicles are carbon dioxide (CO₂), carbon monoxide (CO), hydrocarbons (HC), particulate matters (PM), nitrogen oxides (NO_x), nitrous oxide (N₂O), and methane (CH₄). The reaction of oxygen with existing sulfur and carbon fuel impurities can also result in sulfur monoxides (SO) and sulfur dioxide (SO₂) emissions, which contribute to acid rain formation. Vehicle emissions belong to two categories: greenhouse gas emissions and air pollution emissions. Greenhouse gas emissions contribute to climate change, particularly global warming, whereas the term air pollution emission refers to harmful smog-forming pollutants released by vehicles. Carbon dioxide, nitrous oxide, and methane are greenhouse gas emissions, whereas hydrocarbons, particulate matters, nitrogen oxides, sulfur monoxides, and sulfur dioxide are air pollution emissions [8–15].

1.2.4.4 Gasoline Engine Emissions vs. Diesel Engine Emissions

The majority of emissions from diesel-fueled vehicles are particulate matter, nitrogen oxides, and hydrocarbons, whereas gasoline-fueled vehicles release mostly carbon monoxide, hydrocarbons, and nitrogen oxides. The amount of emissions released by a vehicle depends on a number of factors, including operation conditions, environment, and engine temperatures, fuel quality, and most importantly, air–fuel ratio. Figures 1.6 and 1.7 illustrate the effect of air–fuel ratio variations on gasoline and diesel vehicle emissions. In these figures, λ represents a normalized ratio based on the proportion of air–fuel ratio and stoichiometric air–fuel ratio. As illustrated in Figure 1.6, the amount of emissions produced under lean mixtures is less than the amount produced by rich mixtures in a gasoline engine. However, lean mixtures can cause engine knocking and reduce engine power. Furthermore, lean mixtures generate high amounts of nitrogen oxide emissions when they have an air–fuel ratio near stoichiometric proportions. This is because a stoichiometric ratio produces the high temperature gases necessary to form nitrogen oxide. As such, the designs of gasoline engines need to meet the conflicting requirements necessary for efficiency, power, and emissions. As Figure 1.7 illustrates, diesel engines have greater amounts of hydrocarbons and particulate matter at high air–fuel ratios (lean mixtures). This occurs because after combustion, the heat generated by lean mixtures is not high enough to burn out residual hydrocarbons. The burning of low air–fuel ratios (rich mixtures) in a diesel engine increases particulate matter emissions because it lacks the oxygen necessary for oxidation. A common problem with gasoline and diesel engines is that reducing nitrogen oxide emissions results in the increase of hydrocarbon emissions.

1.3.1 Advanced Engine Design

Emissions from an ICE can be reduced by improving the engine design and controlling the combustion process. Engine design should be optimized for efficiency and performance under a variety of driving conditions, while minimizing its emissions. Properly controlling the engine and combustion process variables such as ignition timing, air–fuel ratio, the volumetric efficiency, and the compression ratio can also significantly reduce the level of engine emissions. Advanced control technologies such as fuel injection systems, electronic engine units, controlled air induction systems, variable valve timing, and turbocharging systems have been shown to be effective in reducing emissions of both diesel and gasoline vehicles. Generally, the most important of these are variable valve timing and turbocharging systems. The subsequent section discusses these two technologies in detail.

1.3.1.2 Turbocharging Systems

The primary function of a turbocharger is to increase the amount of oxygen inhaled into the combustion chamber by compressing the air intake. This compression results in enhanced volumetric efficiency, reduced particulate matter and hydrocarbon emissions, and improvements in fuel efficiency. However, due to the increase in combustion pressure, turbochargers can also increase the rate of nitrogen–oxide emissions. A turbocharger is composed of a turbine, a compressor, and a center housing/hub rotating assembly (see Figure 1.8). Hot exhaust gases drive the turbine and they leave the engine through the exhaust ports. The captured kinetic energy of the exhaust gases drives the compressor through a shaft in the turbo housing. The blades of the compressor draw ambient air inside and accelerate it back into the engine. Before the intake air enters the intake manifold, the compressor increases its mass, compresses it, and increases its pressure.

Turbochargers are capable of providing volumetric efficiency greater than 100% because in these engine types, the intake manifold pressure exceeds atmospheric pressure. However, the engine efficiency comes at the expense of reduced power from the cylinders. This reduction occurs because the turbine in the exhaust causes the exhaust stroke of the engine to push harder against higher backpressure. The two major problems of turbochargers are turbo lag and high intake air temperature. Turbo lag is a sluggish (delayed) response from the engine during the initial push on the throttle pedal. It occurs due to the initial inertia of the turbocharger. Essentially, at low engine speeds, the exhaust gas flow is not strong enough to push the turbine quickly. The compression of the intake air results in increased air pressure, which in turn causes the temperature to rise. Excessive intake air temperature and pressure may cause detonation (engine knocking) or pre-ignition phenomena, which reduces the output power. Pre-ignition is an abnormal combustion in which high pressure and temperature cause the air–fuel mixture to ignite earlier than it would by spark plug fire. To cool down the intake air, an intercooler located in the middle of the piping between the turbocharger's compressor and the engine's air intake valve is integrated. The waste-gate usually regulates and limits air intake pressure. The waste-gate controls air pressure by bypassing some of the exhaust gas flow before entering the turbine as it reaches the intake pressure threshold.

An advanced type of turbocharger technology is the variable turbine geometry turbocharger, also known as a variable geometry turbocharger or a variable nozzle turbine. This type of turbocharger contains a set of adjustable vanes in the turbine housing. These vanes are modified in angle throughout the engine speed to maximize and control boost pressure over a wide range of engine operations. The vanes guide the exhaust flow towards the turbine while an actuator adjusts their angles. The purpose of a variable geometry turbocharger is to partially close the vanes at low-speed conditions. Partially closing these vanes directs exhaust gas towards the turbine and pushes the turbine blades to the right angle. This will cause the turbine to spin faster before the engine reaches proper speed. At higher speeds where the exhaust flow is sufficiently strong, the vanes open completely to capture the high kinetic energy of the exhaust gas. This reduces the exhaust pressure in the turbocharger by removing the waste-gate from the circuit.

A variable geometry turbocharger reduces turbo lag at low engine speeds, provides cleaner exhaust gas, and improves fuel economy while maintaining power and performance. Furthermore, these turbochargers also effectively reduce particulate matter emissions from diesel engines by providing lean combustion in the engine. Diesel engine vehicles mainly use variable geometry turbochargers; they have achieved some success with gasoline engine vehicles as well. The high temperature of exhaust gases is the main obstruction preventing complete integration of variable geometry turbochargers within gasoline engines.

1.3.2 Catalytic Converters

Positioned in the exhaust pipe, catalytic converters are emission control devices that convert unsafe exhaust emissions into harmless compounds through a combination of catalysts. A catalyst is a chemical material that increases the rate of a chemical reaction without changing itself in the process. Platinum, rhodium, and palladium are the most commonly used catalysts.

The catalytic converter plays a significant role in vehicle emission reduction, which is why it is the main emission control technology used in vehicles. However, leaded fuels adversely affect the efficiency of catalytic convertors. These fuels form deposits that coat the catalyst and prevent proper contact between exhaust gasses and catalysts. The catalytic converters used in gasoline vehicles include oxidation (two-way) catalysts and oxidation–reduction (three-way) catalysts, while diesel vehicles use diesel oxidation catalysts, selective catalytic reduction, and nitrogen–oxide adsorber catalysts. Important factors that affect the performance of a catalytic converter are catalyst temperatures, mixture air–fuel ratio, and hydrocarbon mix.

1.3.3 The Diesel Particulate Filter (DPF)

A diesel particulate filter (DPF) collects and removes diesel particulate matters or soot from the exhaust of the engine by passing exhaust gases through the walls between numerous channels (see Figure 1.9). A DPF accumulates soot and particulate matters over time; however, since filters have limited capacity, they need to be removed or cleaned in order to avoid backpressure blockage on the engine. If left dirty, this blockage may result in engine damage or destruction.

Some filters are designed for single use and can easily be discarded or replaced. For other filters, it is necessary to remove the accumulated particulate matters. A practical approach to dispose of the trapped materials is to burn or oxidize the particulate matters within the filter once the exhaust gases reach a certain temperature. The process of cleaning by burning the trapped materials is called filter regeneration.

A passively regenerated filter is a filter regenerated by available exhaust heat or by using a catalyst. On the other hand, an actively regenerated filter is one that uses active means during the filter regeneration process. For example, actively regenerated filters can either use the injection of diesel fuel into an upstream of DOC, or utilize a fuel burner that heats the filter to particulate matters combustion temperatures. Passive regeneration cannot be used if the exhaust temperature is lower than the threshold, whereas in active regeneration techniques, it is possible to integrate various engine controls to provide filter regeneration conditions on demand. Integrating a combination of passive and active strategies can ensure the completion of filter regeneration under all possible vehicle operating conditions. Additionally, reduction in particulate matters and nitrogen oxide emissions can be simultaneously achieved if a diesel particulate filter is used in conjunction with exhaust gas recirculation, nitrogen–oxide adsorber catalysts, or selective catalytic reduction.

1.3.4 Exhaust Gas Recirculation (EGR)

Figure 1.10 depicts a schematic of an exhaust gas recirculation (EGR) for a gasoline engine. This technology reduces nitrogen oxide emissions in both gasoline and diesel engines during operating periods in which high combustion temperatures occur. The principal function of an EGR is to recirculate a controlled portion of exhaust gases into the engine combustion chamber through a valve. This technology recirculates some exhaust gases to the intake manifold where the temperature of recirculated exhaust gases usually decreases by passing gases through an intercooler. The recirculated exhaust gases subsequently mix with the incoming intake manifold air, resulting in a diluted air–fuel mixture. This dilution causes a lower heat release and a lower peak cylinder temperature, which subsequently reduces nitrogen oxide formation. The recirculated exhaust gases must mix homogeneously with incoming air to have a consistent EGR distribution per cylinder. Improper EGR distribution may cause one cylinder to receive higher amounts of EGR, resulting in a higher rate of particulate matters emissions. Conversely, cylinders receiving low amounts of EGR produce higher rates of nitrogen oxide emissions. The EGR system prolongs the engine life because it reduces the peak of combustion temperature.

In a gasoline engine, an EGR system provides different types of exhaust gas flow. For example, EGR systems use a high flow of exhaust gases during operating conditions with high combustion temperatures, such as cruising and mid-range acceleration. Likewise, it provides a low flow of gases during low speed or light load conditions. The EGR system stops recirculating gases at full power demands as the recirculation process reduces engine operating efficiency, and also during idling to avoid higher level of emissions.

It is important to precisely regulate the amount of recirculated exhaust gases. This amount is a trade-off between a reduction in nitrogen oxide emissions and engine efficiency. Excessive recirculation of exhaust gases degrades the engine performance and efficiency, while minimal recirculation results in engine knocking and a decrease in EGR efficiency. Modern vehicles utilize ECU, sensors, and servo-driven EGR valves to balance engine efficiency and vehicle drivability by regulating the recirculation flow rate.

In diesel engines, reducing nitrogen–oxide emission occurs at the cost of an increase in particulate matters' emissions. The two main types of EGR systems used by diesel engines include high pressure loop EGR and low-pressure loop EGR (Figure 1.11). A high-pressure loop EGR diverts the exhaust gases before they reach the turbocharger turbine section. Conversely, the low-pressure loop EGR captures the exhaust gases after passing through the turbocharger and the diesel particulate filter. Incorporating a diesel particulate filter or an oxidation catalyst in an EGR loop helps reduce the amount of particulate matters re-routed to the combustion process.

Integrating a combination of high- and low-pressure loop systems with a variable geometry turbocharger can enhance the EGR system efficiency over a wide range of operating conditions. The low-pressure EGR recirculates exhaust gases at low engine speeds and loads, while high-pressure EGR functions under higher engine speeds and loads. An optimized combination of the technologies will provide a more effective and efficient reduction of nitrogen oxide.

1.3.5 Crankcase Emission Control System

Blow-by gases are gases that leak into the crankcase during power or compression strokes while an engine is running. They are composed of mostly unburnt or partially burnt hydrocarbons and combustion by-products, which leak through the clearances between the piston rings and the cylinder walls.

The ventilation of blow-by gases helps prevent pressure build-up and increases engine longevity. In uncontrolled crankcase emission, a road draft tube releases blow-by gases and other vapors directly into the atmosphere. Hydrocarbon and particulate matters emissions are the classifications of these gases, and, if they enter the cabin, can be a significant source of emission exposure for drivers and passengers.

The crankcase emission control system, shown in Figure 1.12, is designed to re-burn the blow-by gases by diverting them into the intake system of a gasoline or diesel engine. When a gasoline engine vehicle is moving, fresh air flows to the crankcase through an air inlet path called the breather. The incoming air transfers and recirculates the blow-by gases from the crankcase into the intake system, allowing it to mix with an incoming stream of air.

A check valve adjusts the gas flow between the crankcase and the intake manifold; the positive crankcase ventilation (PCV) valve controls the intake system pressure of the check valve. Then the diverted mix of air and blow-gases is injected into the combustion chamber where it is re-burned. Valve malfunctions may result in the blockage of blow-by gases and since the gases are under pressure, they may find other ways to escape from the crankcase to other parts of the engine compartment. These malfunctions can cause damage to engine parts, and result in oil leaks and sludge formation within the engine.

In a diesel engine, a multi-stage filter collects and returns the emitted lube oil to the engine's sump. Then, filtered gases reroute with a balanced differential pressure to the intake system. The crankcase emission control system of diesel vehicles often includes filter housing, a pressure regulator, a pressure relief valve, and an oil check valve.

1.4.1 Natural Gas Vehicles (NGVs)

As a clean-burning alternative to diesel/gasoline fuel, natural gas is a combustible mixture of hydrocarbon gases. It primarily consists of methane (approximately 80%), but also includes lesser amounts of propane, ethane, and butane. It is possible to use it in vehicles in the form of Compressed Natural Gas (CNG) or, less commonly, Liquefied Natural Gas (LNG). Light and medium-duty vehicles generally use CNG, whereas heavy-duty vehicles use LNG.

Dedicated, retrofitted, bi-fuel, and dual-fuel engines are all possible designs for a natural gas vehicle (NGV). Both dedicated and retrofitted engines operate merely on natural gas fuel. A bi-fuel engine runs on either natural gas or gasoline while a dual-fuel engine functions on a combination of natural gas and diesel fuel. A retrofitted engine is an engine modified from an engine designed for gasoline vehicles, while dedicated engines are those engines specifically designed and optimized to run on natural gas. Bi-fuel vehicles use two tanks to store fuels separately, allowing the engine to run on one fuel at a time. Conversely, dual-fuel vehicles burn a blend of two fuels stored in a single tank. In comparison with dedicated and dual-fuel vehicles, bi-fuel vehicles offer fuel flexibility at the expense of passenger/cargo space. Dedicated and bi-fuel engines are spark-ignited while dual-fuel engines need a diesel igniter to operate. The air-gas mixture in the cylinder of dual-fuel engines ignites with the injection of a small amount of diesel fuel, which self-ignites. Natural gas vehicles produce significantly lower nitrogen oxides, particulate matters, and carbon dioxide emissions than conventional gasoline/diesel vehicles because of their low carbon content and high compression ratio.

Since the air mixture in the combustion chamber of a NGV is completely gaseous, the emissions associated with cold start ignitions do not exist. However, because the main constituent of natural gas is methane, these vehicles emit considerably more methane than gasoline and diesel vehicles. The stoichiometric ratio of CNG is 17.2, higher than that of diesel and gasoline fuels. Gasoline or diesel engines retrofitted to operate on CNG may not provide the amount of emission benefits as a dedicated engine. Burning LPG in an engine optimized for gasoline/diesel fuel stoichiometric conditions will result in a rich-fuel condition.

Economically, CNG is usually cheaper than gasoline due to the more stable market and abundance of resources. The Energy Protection Agency (EPA) in the United States selected the Honda Civic GX – designed as a dedicated CNG vehicle – for eight years till 2012 as the cleanest-burning combustion engine vehicle. Although most major automotive industries offered vehicle models powered by CNG, the Honda Civic GX is the most commercialized CNG vehicle currently available in the global market.

Despite its advantages, the market for NGVs is very narrow because of its limitations in design technology, fuel infrastructure, and driving range. The primary issue is onboard fuel storage. Currently, CNG is stored in a high-pressure cylinder, and LNG is stored in an insulated tank. Both storage systems are considerably heavier, more expensive, and bulkier than storage systems for equivalent amounts of gasoline or diesel, thereby reducing the vehicle's power-to-weight ratio. Furthermore, NGVs require frequent refueling in comparison to their gasoline and diesel counterparts for similar travel distances due to lower volume energy density of natural gas in comparison with gasoline and diesel. The performance of these vehicles is comparable to gasoline and diesel vehicles when the CNG tank is under full pressure. However, when the pressure falls below a certain threshold, NGVs display limited performance at high speeds and uphill settings. Another important obstacle preventing the widespread commercialization of NGVs is the lack of refueling infrastructure for natural gas. Moreover, the use of gas storage cylinders that have limited shape flexibility is inconvenient for vehicle designers. Despite clean performance and abundant resources, NGVs still run on unsustainable fuel resources.

Due to economic factors, most NGVs use retrofitted engines. Unfortunately, the performance of retrofitted engines is not optimal compared to dedicated engines that are designed with the express aim of operating with natural gas fuels. In addition, as a result of extremely limited access to CNG refueling stations, most retrofitted NGVs are bi-fuel. Currently, fleet vehicles such as taxi cabs, transit vehicles, and school buses are the main applications of natural gas-fueled technologies.

1.4.2 Liquefied Petroleum Gas Vehicles (LPGVs)

As a by-product of natural gas and refined crude oil, liquefied petroleum gas (LPG) is composed of propane and butane. This fuel is also known as propane and autogas, and is currently the most prevalent alternative fuel. LPG is a fuel for conventional vehicles while producing lower levels of toxic and smog-forming air pollutants compared to fossil fuels.

The engine technologies behind LPG-fueled vehicles are similar to those of NGVs. The design of an LPG engine can be as a dedicated or bi-fuel engine, though it is possible to convert conventional combustion engines to use LPG as well. However, the use of LPG in dual-fuel diesel engines is uncommon due to its poor knock resistance.

Converting a gasoline or diesel engine to operate with LPG is cheaper than converting it to operate with natural gas because of lower fuel tank costs. Another advantage to CNG is the convenience of carrying LPG onboard the vehicle. The principal function of a LPG engine is similar to that of a spark-ignition engine. Like natural gas, LPG can function with a higher compression ratio, which results in increased power output and reduced exhaust emissions. LPG engines generate lower amounts of emissions under cold start conditions and perform better in low-speed and light-throttle conditions as well. This is because LPG injects into the combustion chamber as a vapor and the air mixture within the chamber is completely gaseous. The power output and torque of LPG engines are similar to gasoline engines. As such, LPG-driven vehicles can perform comparably to gasoline vehicles in terms of climbing slopes and traveling in mountainous areas.

Unlike gasoline engine vehicles, LPGVs produce near-zero particulate emissions, very little carbon monoxide, and only moderate amounts of hydrocarbons. Additionally, the carbon monoxide emissions of LPG vehicles are typically lower as well, while nitrogen oxide emissions are similar to the amount released from gasoline vehicles. LPGVs produce much higher levels of carbon monoxide and hydrocarbon emissions compared to NGVs. Like CNG, LPG has a stoichiometric ratio (A/F = 15.7) higher than that of gasoline/diesel fuel. As such, a retrofitted LPG engine may not be as efficient as a dedicated engine in terms of emission benefits.

Toyota made the first LPG vehicles in the 1970s. Despite the advantages of LPG-vehicles, they have a limited share of vehicles on the road. Like NGVs, the main obstacles preventing widespread commercialization of LPG-equipped vehicles are driving range, vehicle fuel availability, and infrastructure. The current LPG vehicles suffer from limiting vehicle space due to the additional tank required for the LPG fuel. With a similar volume of fuel, LPG vehicles travel shorter distances compared to diesel and gasoline vehicles due to the lower volumetric energy density of LPG, making it necessary for the vehicle to refuel frequently. Moreover, LPG is also a by-product of substances with limited resources. Additionally, despite producing lower level of emissions compared to gasoline/diesel-fueled vehicles, LPG vehicles still produce pollutants that affect air quality. Using a three-way catalyst or an oxidation catalyst in conjunction with an LPG engine further reduces the emission level.

1.4.3 Biodiesel

Biodiesel is a natural alternative fuel for compression-ignition (diesel) engines that can be derived from a variety of renewable biological energy sources, including vegetable oil, animal fat, and cooking oil. Biodiesel is the result of a chemical process known as trans-esterification. This process involves extraction of the basic compounds of biodiesel such as esters from natural oil resources through a catalytic chemical reaction with methanol. Currently, the primary feed stocks used in the production of biodiesel fuels are soybeans, canola oil, and yellow grease. Biodiesel fuel or their blends in most diesel engines work with little to no modification to the engine. Usually, the concentration of biodiesel in fuel blends is denoted by a system known as the “B” factor. For example, the denotation of 100% (pure) biodiesel is B100 while a diesel fuel with 20% biodiesel and 80% petro-diesel is known as B20.

The expected use of biodiesel in pure or blended form reduces carbon dioxide, particulate matters, and hydrocarbon emissions of diesel engines, in addition to slightly increasing nitrogen–oxide emissions. These actions occur because of the oxygen available in biodiesel, which allows the fuel to burn more completely. Moreover, nitrogen–oxide emission control technologies can eliminate or significantly reduce the nitrogen oxide emissions that the combustion of these fuels emits. In addition to reducing emissions, biodiesel fuel offers other important advantages compared to fossil fuels. Some of these benefits include: renewability (significantly beneficial for the environment), limited sulfur–dioxide production (can directly reduce the formation of acid rain), safer disposal of extraneous fuel (biodiesel fuel is non-toxic and degrades faster than petroleum diesel fuel), and better lubricant properties (biodiesel fuel is a very effective lubricant and acts very much like a solvent, capable of cleaning the engine).

Despite these benefits, biodiesel fuel still has a number of disadvantages. The main problem is a conflict with the engine warranty policies of major auto manufacturers, who have not yet accepted the use of blends more concentrated than B5. This lack of legal policy restricts the widespread use of biodiesel fuel. Additionally, biodiesel is more expensive to purchase on a recurring basis. They have lower energy content and fuel economy than fossil fuels – approximately 10% lower for B100 and 2% lower for B20 blends. The B100 blend raises further concerns due to its potentially adverse impact on engine durability over prolonged periods of usage, as well as its suitability for lower temperatures.

1.4.4 Hydrogen

Hydrogen is an alternative fuel for vehicle propulsion when burnt in an internal combustion engine using the combustion of a mixture of hydrogen and air. Hydrogen fuel is a decarbonized energy carrier that does not naturally occur in pure form. Hydrogen can be obtained mainly either by converting combustible molecules into hydrogen or by consuming electric energy. The common methods used to produce hydrogen include water electrolysis, natural gas reformation, or partial oxidation and steam reformation of other fossil fuels. Hydrogen production is available domestically from renewable sources such as wind, solar, and nuclear. Currently, natural gas most often functions to generate hydrogen. While there are environmental consequences of producing hydrogen from reformed fossil fuels, studies comparing the well-to-wheel emissions of hydrogen and petroleum productions have shown overall reductions in ozone and GHG while using hydrogen. However, an even more distinct advantage would be to use renewable energy to generate hydrogen. Hydrogen is appealing because it can reduce or eliminate carbon dioxide emissions from vehicular propulsion systems by deprecating the use of fossil fuels. Regardless of the amount of emissions released from the formation of hydrogen, hydrogen-fueled ICE vehicles can potentially have near-zero tailpipe emissions. The only harmful emission released from these vehicles would be nitrogen–oxide emissions, which occur because of the high temperature from hydrogen oxidation during the combustion process. Integrating nitrogen–oxide emission control technologies is the most convenient way to reduce this type of emission in hydrogen vehicles.

Since a hydrogen-fueled engine burns fuel similarly to conventional gasoline engines, a slight modification to a gasoline-based internal combustion engine can create a hydrogen-fueled engine. François Isaac de Rivaz designed the first hydrogen engine in 1807. However, it was not until 1970 that Paul Dieges added a patented modification to Rivaz's design. This modification allowed the engine to run using hydrogen, and eliminated much of the negative environmental effects. Modified internal combustion engines (e.g., Mazda's Wankel engine) are a low-cost and viable alternative technology. A number of automakers, including BMW, Mazda, and Ford, have separately launched production-bound hydrogen ICE vehicles. Some critics assert that hydrogen is not an ideal fuel solution for ICE engines because it increases the propensity for pre-ignition. However, certain experiments by auto-makers have completely dismissed these concerns. Currently, there are even suggestions for a gradual withdrawal from conventional fossil fuels to make way for complete reliance on hydrogen fuels.

A major concern regarding the use of hydrogen as a fuel is its storage. The most common ways to carry hydrogen on a vehicle includes storing it as a compressed gas, liquid, or in the form of metal hybrids by using chemical storage. Reforming liquid fuel sources such as methanol, ethanol, natural gas, and liquid petroleum gas can produce hydrogen on-board a vehicle. However, doing so requires a process to convert the existing liquid fuel sources into hydrogen. These requirements increase the overall cost while also adding maintenance costs to the system. Moreover, reforming fuel sources also releases carbon dioxide. Nevertheless, the amount of emissions released by engines operating on reformed fuel is much less than that of gasoline-powered engines. However, the volumetric energy density of compressed hydrogen gas is also much smaller than that of gasoline. As a result, hydrogen-powered vehicles have shorter traveling distances and less torque output.

Compressed hydrogen requires a heavier chassis and greater amount of space to hold equivalent amounts of gasoline. This increases the weight of the vehicle, resulting in a lower power-to-weight ratio. However, with the same fuel mass, this engine can run at higher efficiency levels than a gasoline combustion engine. This increased efficiency is a result of the higher compression and leaner air–fuel ratios of hydrogen fuel. In general, the issues associated with hydrogen engines are production, distribution, storage, and refueling. While sustained research and development efforts are bringing hydrogen-powered vehicles closer to widespread commercialization, the development of these vehicles is still in its early stages. The increased usage of hydrogen vehicles can substantially reduce fossil fuel dependency.

1.5.1 Rear-Wheel Drive Powertrains

Figure 1.13 illustrates a rear-wheel drive powertrain in which the engine power transfers to the rear wheels through the transmission, drive (propeller) shaft, differential, and axle. The front of the vehicle contains the engine, while the transmission is behind the engine. Usually, the engine and transmission are in alignment with longitudinal installation. This type of system provides a good catalytic converter configuration because of its long exhaust system. It also provides more space for the engine, making it more suitable for powerful vehicles. The proper weight distribution that occurs in a rear-wheel drive system results in greater vehicle handling and performance ability. Additionally, the load transfer to the back of the vehicle allows this system to provide better traction on the driven wheels during heavy acceleration. The driveshaft and other components needed to transfer the power to rear wheels result in reduced fuel economy and engine power. Furthermore, the additional weight of these components also increases the weight-to-power ratio of the vehicle. Luxury and racing vehicles usually have rear-wheel drive integration.

1.5.2 Front-Wheel Drive (FWD) Powertrains

As Figure 1.14 shows, in a front-wheel drive (FWD) powertrain, the engine and transmission system are at the front of the vehicle. As such, the engine power transfers directly to the front wheels through the transaxle. The transaxle consolidates the transmission, differential, and the associated components of the driven axle in one unit.

The front-wheel drive arrangement provides better fuel economy and less engine power loss than rear-wheel drive because it contains less moving components. During heavy acceleration, traction loss may occur due to load transfer towards the back of the vehicle. However, FWD vehicles as they deliver the load on steered and driven wheels, have improved road-holding and traction. Moreover, this arrangement provides more space for the cargo and passenger cabin at the expense of reduced space for the engine and other front-located components (e.g., steering mechanism, engine compartments).

There are two types of FWD powertrain systems, transverse or longitudinal, depending on how the vehicle's engine and transmission are mounted. In transverse powertrains, the engine and transmission are mounted parallel to the front axle. Conversely, in the longitudinal powertrain, the engine and transmission are parallel to the longitudinal vehicle axis. The integration of each system also depends on the engine and transmission size, as well as the available space. The transverse front-wheel drive powertrain is found in the majority of modern small and mid-sized vehicles.

1.5.3 Multi-Wheel Drive Powertrains

A multi-wheel drive powertrain delivers the engine torque to all four wheels of the vehicle. In some configurations, the torque transmits to both axles when all the wheels are continuously driven. In other configurations, the engine power transmits to one of the two axles, while another axle can engage manually or automatically when needed. In a multi wheel-drive, the engine torque transfers to the axles through a power-split differential system such as a center differential, transfer case, or another type of gear set. In this power-split system, the torque of each axle routes to the wheels on each side through each axle differential.

Multi-wheel drive systems are further available as four-wheel drive and all-wheel drive (AWD) systems. In four-wheel drive systems, the power-split differential may contain one or more sets of low-range gears with the ability to select two-wheel drive or four-wheel drive operation. The low-range gears reduce the vehicle speed in order to increase the torque routed to the axles. This helps when driving in extreme off-road conditions or on rough terrain. Conversely, the power-split differential of all-wheel drive systems does not include the low-range gears, and it does not offer a manual switch from four-wheel drive to two-wheel drive. Generally, multi-wheel drive systems provide better traction on all road conditions, better acceleration in low gear, and better handling performance. However, these advantages come at the expense of a very complex powertrain system and reduced fuel economy. Figure 1.15 illustrates a schematic of a multi-wheel drive powertrain configuration.

There are also other less common powertrain configurations that vehicles can use, such as a rear-mounted engine rear-wheel drive, or a rear-mounted engine four-wheel drive. These configurations are rare in modern vehicles because they make the vehicle prone to instability, particularly during cornering.

1.6.1 Manual Transmission/Transaxle Systems

The manual transmission system is the first type of transmission that was integrated into a vehicle while the manual transaxle system developed many years later with the introduction of front-wheel drive vehicles. Manual transmission systems and manual transaxle systems adjust the engine power and speed through a set of gears that can be shifted manually by a driver through a clutch pedal and shift knob. Typically, manual transmissions are used in powertrains in which the engine and drive wheels are located on two ends of the vehicle (e.g., front-mounted engine rear-wheel drive or four-wheel drive systems). Likewise, manual transaxle systems are used in powertrains in which the engine and drive-wheels are located on the same vehicle end (e.g., front-mounted engine front-wheel drive). These systems are similar in many ways – they both function to efficiently transfer engine torque to the drive wheels. The primary difference between the two is that a transaxle system transfers engine torque while also providing the differential action to the drive wheels, whereas the differential for a manual transmission system is separate and is connected to the transmission through the driveshaft. The mistakenly interchangeable use of the terms transmission and transaxle is common. The main components of a manual transmission/transaxle include the clutch, the gears, the shafts, and the synchronized collars:

• The clutch: The clutch is a coupling device with the primary function of engaging or disengaging the engine from the transmission system, as necessary. It regulates torque transfer from the engine to the gears, and makes gear shifting easier and smoother. The driver typically operates the clutch by using a foot pedal.
• The gears: Gears are a major component of the manual transmission/transaxle system. They are mounted on two separate shafts, and mesh with one another by sliding in and out of engagement. Gears with different sizes and different numbers of teeth mesh together in order to change the engine torque and speed by transmitting the rotating motion from one shaft to another. The gears provide functionalities such as magnifying the engine torque while reducing engine speed or vice versa, transferring the torque while keeping the engine speed, or changing the torque direction. Driving gears and driven gears are the two types of gears on a manual transmission/transaxle system. The driving gears receive their rotation from input shaft. In manual transmissions, the driven gears transfer the rotation to the output shaft, whereas in manual transaxles, this rotation transfers to the differential. The gear ratio reflects the ratio of the input and output torques of a transmission system. The gear ratio is a function of the number of teeth meshing on the driving and driven gears, as well as their rotational velocities. Most modern transmission/transaxle systems have at least four forward gear ratios. Higher gear ratios belonging to the first and second gears provide higher torque and lower speed to the drive wheel, while lower gear ratios belonging to the third and fourth gears transfer the engine torque with lower or no magnifications. Usually, the fourth gear provides a direct drive at which the input and output shafts rotate at the same speed when the gear ratio is one. If the rotational speed of the driving gear is less than that of the driven gear, the gear ratio is less than one. This is the overdrive gear ratio, supplied by the fifth and upper gears. In addition to less engine wear, overdrive ratios lead to enhanced fuel economy. This fuel efficiency occurs because the overdrive ratio causes the drive wheels to turn faster than the engine at high speeds in which the torque requirement to keep vehicle moving is at its minimum.
• The shafts: The shafts that construct the structure of a manual transmission system are the input shaft, output shaft, and counter shaft. The splines of the input shaft engage with those of the clutch; the engine power is transferred to the transmission system through this shaft. The output shaft transfers the regulated torque to the differential. The countershaft – also known as the layshaft – is an intermediate shaft that transmits the motion from the input shaft to the output shaft. The driving gears affix and attach to the countershaft; the driven gears mount onto the output shaft and can rotate freely, independent of the output shaft rotation. The input and output shafts are two different shafts that are linked together through the countershaft. In most designs, the input and output shafts lie along the same shaft and have the option of combining in a single shaft called the main shaft. However, these shafts rotate separately at different speeds while still linked together through the countershaft.

1. The shaft design of a manual transaxle system is different. Typically, this system includes two shafts known as the primary and secondary shafts. The primary shaft receives the torque from the engine while the secondary shaft transmits the regulated torque directly to the differential inside the transaxle by using an additional gear. Unlike the fixed gears of the secondary shaft, the gears on the primary shaft can rotate independent of the shaft rotation.

• Collars: The function of a collar varies based on its intended system type. In a manual transmission, it connects or disconnects the gears to the output shaft; in a transaxle system, it connects or disconnects the gears to the primary shaft. A collar slides between the space of two gears and can be engaged in one gear at one time. As such, if there are more than two gears in the transmission/transaxle system, the required number of collars increases. The process of connecting or disconnecting the collar and gears may cause noises and grinding because of the different shaft speeds and rotating gears. In modern transmission/transaxle systems, a synchronizer is used to synchronize the rotational speed of the gear and shaft while shifting in order to prevent gears from grinding or clashing while they are engaged.

1. Usually, cars and SUVs are equipped with a transmission/transaxle system with four to six forward gear ratios, as well as a reverse gear ratio. However, the transmission/transaxle systems of some cars can provide up to eight gear ratios. For heavy-duty vehicles such as trucks, the transmission/transaxle design provides at least nine gear ratios since these vehicles need to have a wider range of power band from the engine. Most manual transmission/transaxle systems have the flexibility of choosing any forward gear ratio while driving; however, certain vehicles (e.g., racing cars) also allow sequential gearshifts. This is a sequential transmission system. The main advantages of manual/transaxle systems include its simple structure and high power transmission efficiency. However, their function also requires effort and skill from the driver.

1.6.2 Automatic Transmission/Transaxle Systems

General Motors introduced the first automatic transmission system in the early 1930s. Currently, this system is prevalent in most modern vehicles, especially passenger cars. Unlike a manual transmission/transaxle system in which gear selection and shifting are manual functions by the driver, an automatic transmission/transaxle system provides automatic gear shifting through a control system. This control system is electrical or mechanical and operates through a fluid coupling device called the torque converter. As the vehicle moves, gear selection and shift decisions are a function of: engine load prediction, vehicle speed, throttle position, and other specific factors. The fundamental difference between an automatic and manual transmission/transaxle system is the gear sets and engine connection. Automatic systems utilize a torque converter and planetary gear sets rather than a clutch, and gear sets operating by sliding in and out of engagement. The automatic transmission/transaxle system offers more comfortable driving than a manual transmission/transaxle at the cost of a more complicated and expensive structure. The early designs of automatic systems were less fuel-efficient than their manual counterparts. However, as a result of continuous development, modern automatic transmission/transaxle systems can now provide fuel economy similar to that of manual systems. The primary components of an automatic transmission/transaxle system are the torque converter, planetary gear sets, clutches, and the hydraulic control system unit.

1.6.2.1 The Torque Converter

A torque converter is a hydraulic mechanical device that replaces the function of a traditional manual transmission clutch. The main functions of the converter include transferring engine output power to the gear unit, making the gear shifts smoother, providing torque multiplication at low speed conditions, and avoiding engine stalls in the process of stopping or while the vehicle is idling.

The torque converter contains special oil and has a donut-shaped case that contains three sets of vanes (see Figure 1.16). These vanes are the impeller, the turbine, and the stator. Bolts secure the impeller to the flywheel of the engine, while the turbine is attached to the main shaft of the gear unit. A hydraulic pump forces the oil fluid into the rotating impeller vanes. The centrifugal force created by the impeller pumps the fluid towards the turbine vanes. The fluid striking on the turbine vanes causes the turbine to rotate. This rotation directly affects the gearset because it connects to the turbine. At low speed conditions, the fluid returns towards the impeller after striking the turbine vanes. However, the rotational direction of the returned fluid is opposite to that of the impeller, which results in engine power loss. This loss occurs because the impeller needs to redirect the returned fluid. A stator is located between the impeller and the turbine, and increases the efficiency of the torque converter under such conditions. It is mounted on a fixed shaft through a one-way clutch, allowing it to rotate with the direction of the impeller. When the stator is locked by a one-way clutch, it redirects fluid flow returning from the turbine to the rotational direction of the impeller before hitting the impeller vanes again. The angles and direction of the stator vanes cause the pressure of the returned fluid to increase before reaching the impeller. The impeller once again pumps out fluid towards the turbine, but with even higher pressure. This pressure results in torque multiplication and makes the automatic transmission/transaxle powerful enough to provide a high torque at low speeds. The fluid speed and pressure escalate as the impeller speed rises, resulting in an increase in turbine speed. The high turbine speed consequently prevents fluid redirection towards the impeller and allows more engine torque to be directed toward gear unit. When the turbine speed increases, the one-way clutch starts unlocking the stator and allows it to rotate. The operating point at which the impeller, stator, and turbine are rotating at the same speed is the coupling phase, where torque multiplication does not occur.

Fluid slippage in a torque converter makes it impossible to achieve a perfect coupling phase. Specifically, it causes engine power loss, which negatively affects the high-speed efficiency of automatic transmission/transaxle systems. Modern torque converters are equipped with an internal locking clutch, known as a converter lock-up clutch. These locking mechanisms eliminate fluid slippage during vehicle cruising by locking the torque converter components. They also improve the fuel efficiency of automatic transmission/transaxle systems and reduce the amount of heat that the system generates. The lock-up clutch disengages at low speeds to prevent engine stalls. An electrical solenoid controls the function of the lock-up clutch. The fluid from the impeller still strikes the turbine vanes while the vehicle is idling; however, it does not carry enough kinetic energy to rotate the turbine. This causes the gear set to disconnect from the engine in an attempt to prevent the engine stalling.

1.6.3 Automated Manual Transmissions (AMTs)

As previously mentioned, a simpler structure, better fuel economy, and superior efficiency are the most desirable aspects of a manual transmission while being more comfortable to drive and easier to use are the best features of an automatic transmission system. An intermediate solution bridging these two systems is the automated manual transmissions, also known as semi-automatic transmission systems. The design of the system takes advantage of the best features of both transmissions.

In this transmission, the driver has the option of using a fully automated mode or shift manually by using switches or levers. Two main variations of the automated transmission system include sequential manual transmission and dual-clutch transmission. The automated transmission system provides better fuel efficiency, lower emissions, lower energy loss, and a more comfortable ride than either manual or automatic transmission systems.

1.6.4 Continuous Variable Transmissions (CVTs)

Unlike the limited number of gear ratios that automatic and manual transmission/transaxle systems offer, a continuous variable transmission (CVT) system provides an unlimited number of gear ratios within the upper and lower limits of their operating range. CVTs do this by properly adjusting the geometrical configuration of the system without using a set of gears. A specific type of CVT is the infinitely variable transmission (IVT) system, in which the gear ratio ranges from zero to the highest ratio. The zero ratio decouples the engine load from the driveshaft, thus making idling possible.

Generally, CVT transmission systems allow the engine to function at an optimal and efficient range of operation under varying driving conditions. This efficiency results in enhanced fuel economy, reduced emissions, and improved vehicle performance. Since there are no gears in CVTs, the gear ratio refers to the speed ratio of the input and output shafts of the transmission system. Additionally, the continuous and smooth gear change of CVTs offers more comfort, superior acceleration performance, and a lightweight transmission system with a compact structure. Conversely, the negative aspects of CVTs include its noisier performance and lower torque capacity. Due to technological advances and the afore-mentioned advantages, these transmission systems have found their way into the automotive industry. Currently, several car manufacturers have integrated CVT technology into their products.

There are various types of CVTs currently available, including the spherical, hydrostatic, magnetic, ratcheting, cone, radial roller, and toroidal CVTs. A general way to classify CVTs is by determining whether they are mechanical or non-mechanical systems. Mechanical systems are further available as friction and ratcheting systems, while non-mechanical systems can be sub-divided between hydraulic and electrical. Currently, the most commonly used CVT systems are variable-diameter pulley or pulley-based CVTs, toroidal or roller-based CVTs, and hydrostatic CVTs.

1.6.4.2 Toroidal CVTs

Like pulley-based CVTs, toroidal CVTs are also friction-based. The primary advantage of toroidal CVTs is their parallel power flow, which allows the system to have a high torque capacity and compact design. The main operation of a toroidal CVT is similar to that of a pulley-based CVT. However, unlike the belts and pulleys used by pulley-based CVTs, toroidal CVTs use discs and power rollers or wheels (Figure 1.17). In toroidal CVTs, one disc connects to the engine crankshaft while the other one connects to the driveshaft. Power transfers from the driving disc to the driven disc through the rollers or wheels located between them, which spin around the horizontal axis. The rollers located between the discs adjust the gear ratio based on how they are angled relative to the discs. As such, even a slight tilt in the roller/wheel angle around the vertical axis can incrementally adjust the gear ratio. For example, lower gear ratios can be achieved when the rollers/wheels are in contact with the driving disk near the center while they are in contact with the driven disk near the rim.

Two variations of toroidal CVTs are the full toroidal and the half-toroidal CVT. The difference between these two subtypes is the position of the tilting rotation center. In a half-toroidal CVT (Figure 1.18a), the power roller tilting rotation center is outside the center of the cavity between the discs. As shown in Figure 1.18b, in a full toroidal CVT, however, the titling rotation center of the power roller coincides with the center of cavity between discs. Ultimately, half-toroidal CVT designs are more efficient than full-toroidal drives at the expense of a lower torque capacity. The disadvantages of toroidal CVTs are similar to those of pulley-based CVTs.

1.7.1 Open Differentials

Figure 1.19 is a schematic of an open differential system. Open differentials, also known as standard differentials, are the most common differentials in modern vehicles. They include two side gears, a set of spider gears (known as planetary gears), the ring gear, and the differential case. The driveshaft rotates the ring gear bolted to the differential case. The spider gears then rotate with the differential case and can also rotate along their own shaft axes. Each side gear is connected to an axle shaft and is driven by the spider gears. These spider gears allow the outside and inside wheels to move at different velocities during cornering.

On paved roads, the open differential system works well and provides adequate handling performance. However, if the wheels encounter unequal traction conditions, power routes to the wheel with the least amount of traction. This causes unexpected handling performance for the driver and can hinder the vehicle's motion. For example, if one wheel spins on a slippery surface while another is on a dry surface, all the power is directed towards the wheel on the slippery road, leaving the other wheel to receive minimal or no power. The design of limited slip differentials and locking differentials addresses the limitations of open differentials on uneven traction conditions.

1.7.2 Limited Slip Differentials

The design of limited slip differentials resolves the limitations of open differentials in conditions where traction is lost. However, resolving this issue also comes at the expense of a more complex system. “Limited slip” refers to the function of the system in limiting wheel spin. In this design, the differential operation is limited when the traction loss of any wheel exceeds a certain threshold. It then redirects power to the wheel with more traction by up to 80%, thereby maintaining rotational power in both wheels. Limited slip differentials are used mainly in off-road vehicles since it is possible for one of the wheels to lose traction in off-road conditions. In addition, high-speed performance vehicles utilize these differentials to improve vehicle handling during high-speed cornering, when the different velocities of the left and right wheels may lead to higher amounts of differential slip.

Limited slip differentials have various designs; the most important ones include mechanical, viscous coupling, and electronic limited slip differentials. Mechanical limited slip differentials are based on the interaction between two (or more) mechanical parts. Clutch, cone, and helical gear limited slip differentials belong in this category. In the viscous coupling variation, the differential slip is controlled by the hydrodynamic friction from fluids with high viscosity. These differentials are less efficient than the mechanical ones because power loss occurs during activation. However, in modern vehicles, electronic limited slip differentials are used to activate the mechanical components of the system. An electronic limited slip differential utilizes different sensors and microcomputers in order to monitor the wheel slip and vehicle motion.

1.7.3 Locking Differentials

The design of locking differentials, also known as lockers, enables the wheels on an axle to lock relative to each other, avoiding the difference in speed between two wheels on the axle, regardless of the amount of traction on each wheel. Automatic and selectable lockers are two variations of locking differential systems. The automatic differentials lock both wheels on the axle under normal conditions, while they unlock the differential for certain conditions such as cornering. Automatic lockers automatically lock and unlock the differential system, thereby causing fast-changing and strange handling characteristics on the street. The function of the differential is noisy and increases tire wear. Off-road four-wheel drive vehicles are the main applications of the automatic lockers and typically, they are integrated on the rear axles only, since front axle installations can cause difficulties in steering. In contrast, a selectable locker functions as an open differential in normal driving conditions; however, the driver has the option of locking both wheels on the axle. When locked, both axles will then turn at the same speed irrespective of the road surface. The locking process can occur through compressed air, mechanical cable, electric actuator or hydraulic fluid. It is possible to integrate selectable lockers in both front and rear axles without sacrificing vehicle handling performance and tire wear. Off-road vehicles are the main applications of such differentials. The integration of the selectable differentials on on-road vehicles is limited due to the high cost and complexity of installation.

1.7.4 Transfer Case Differentials

Typically, four-wheel and all-wheel drive vehicles have three differentials, two of which are mounted on the front and rear axles, whereas the third one, called the transfer case, is located between the front and rear output shafts to the final drive units. Attached to the rear of the transmission and also to the front and rear axles through two separate driveshafts, the transfer case and center differentials are mechanical devices which split the power between the front and rear axles while varying the distribution of power between the front and rear axles smoothly and over a certain range. In addition, they adjust the speed differences in front and rear axles. The transfer case receives the power directly from the transmission and directs it to either the rear wheels or both the front and rear wheels. The transfer action may occur through a set of gears or through a chain-driven device. Through the transfer case, the driver has the option of selecting two-wheel drive or four-wheel drive mode by a shifter. However, some vehicles accomplish this electronically through a switch. Some transfer cases are not selectable such as all-drive vehicles.