12.4 Expand the Concepts and Develop the Concept Fragments
Expanding the Propulsion Function
In Chapter 7, we noted that concepts can often be broken down into constituent ideas, which we call concept fragments. Step 2 in creating the concept is to expand the concept and develop fragments. For the Hybrid Car, the specific process we identified is driving—a concept rich in meaning! In order to expand this concept, we are going to decompose the driving process to identify concept fragments. We begin with the concept fragment of propulsion as one of the constituents of driving, and then we will examine seven additional concept fragments.
Specifically, we will categorize hybrids first by how their propulsion systems are combined. Three types of overarching vehicle concept classifications exist based on the dependence of the car’s propulsion system on external energy sources: monovalent, bivalent, and multivalent architectures.
Monovalent Architectures: Cars that exhibit a propulsion system dependent on one external energy source. Most cars today are monovalent cars that use an internal combustion engine with one liquid fuel such as gasoline or diesel. Hybrid cars that exhibit a secondary internal fuel source in the form of a high-voltage battery are also considered monovalent, because such cars remain dependent on one external fuel source.
Bivalent Architectures: Cars that exhibit a propulsion system with two external energy sources. An example of a bivalent car, or fuel-flexible car, is a plug-in hybrid electric car wherein two external energy sources are transferred and stored within the vehicle: electricity and fuel.
Multivalent Architectures: Vehicle architectures that exhibit a propulsion system with more than two external energy sources. These cars are designed to obtain and store three or more sources of energy. An example of such a system is the Fiat Siena Tetrafuel, which is designed to run on gasoline, E20 to E25 blends, on pure ethanol (E100), or as a bi-fuel with natural gas (CNG). [4]
This decomposition highlights that our original concept was anchored to the bivalent architectures, but there is a much broader space in multivalent architectures that would still solve the System Problem Statement. Notice, however, that the choice among these types of hybrid propulsion is not a concept fragment in that it does not map function to form.
Decomposing the propulsion function further into energy carrying/storage and vehicle moving (typically referred to as the powertrain) may help ideate additional concepts. Four general energy storage concept pathways are known to date. These include steam, internal combustion engine, battery, and fuel-cell-based concepts, along with multiple combinations thereof. Four powertrain concepts are summarized, together with their input energy sources, in Figure 12.5. Note how the diagram illustrates two primary internal functions: energy carrying and vehicle moving.
Figure 12.5 Four primary vehicle-moving concepts (ICE, HEV, PHEV, and BEV), together with the primary energy sources they rely on.
Steam-based concepts, which dominated the early automotive market from approximately 1790 to 1906, have not seen successful commercialization at a large scale since. Attempts have been made to combine the early steam concepts with other concepts, such as a steam–hybrid electric car or the use of internal combustion engine exhaust gases to generate steam. [5] The BMW turbo-steamer project developed a proof of concept that could use the combustion engine exhaust gases to generate steam and use the excess energy to boost the car’s torque by 10%, but at a weight increase of 220 pounds. Arguably, the overall vehicle efficiency of steam, steam-electric, and ICE-steam combinations lies below that of traditional cars today.
Gasoline-powered spark ignition (SI) engines and diesel-powered compression ignition (CI) internal combustion engine (ICE) types are well-known alternatives. Other combustion engine alternatives for cars, such as turbine engines, have also been studied but have failed to achieve fuel consumption equivalent to that of SI and CI engines. [6]
Battery electric powertrains are now gaining favor as environmental demands for the reduction of exhaust gases in transportation have become a leading issue. Hybrid electric vehicles (HEVs) represent the first commercially available alternative to the conventional ICE. Initial hybrids feature small electric systems that assist the internal combustion engine in delivering power to the wheels. These first successful hybrid models are expected to lead the way for larger battery electric concepts that feature external battery charging, as in the case of plug-in hybrid electric vehicles (PHEVs). Battery electric vehicles (BEVs) will gain importance for city driving and short commuting customer use cases. [7] The key advantages of the battery-based concepts are the ability to reduce tailpipe emissions and increased flexibility in selecting less CO2-emission-intensive production of electricity from primary energy sources. The greatest limitation to the battery-based concept is the battery itself. Improvement in battery life, energy density limitations, and costs will be crucial in making the battery-based pathway a success. [8]
Finally, the fuel cell concept is considered to be several decades of development away from market-readiness. [9] The first commercial fuel cell vehicles are expected to combine a large battery electric system and a fuel cell range extender with the ability to chemically convert fuel into electricity to be used in powering electric motors. Fuel cell powertrains may feature hydrogen as a fuel or a variety of liquid fuel carriers, such as methanol.
Concept Fragments for Seven Additional Internal Functions of the Hybrid Vehicle
Just the propulsion system of a car is clearly a rich multifunctional concept (Figure 12.5). In the interest of expanding to view the concept fragments, we will create a more complete list of the internal functions. Seven basic hybrid vehicle internal functions encompass the value added of hybrid systems for the customer: motor start-stop, regenerative braking, power boost, load level increase (battery charging), electric driving, external battery charging, and gliding. These functions are briefly discussed below.
Concept Fragment 1: Motor Start-Stop
The motor start-stop function is a basic function found in all hybrid vehicle concepts. As soon as the hybrid control system senses that the vehicle will come to a complete stop (for example, at a traffic light), the engine will shut off and be prevented from idling. The engine is restarted by means of an electrical motor or starter-generator as soon as there is a power requirement that merits its starting again. In micro hybrid systems that do not offer electric driving, the automatic start-stop feature is able to start the engine and have it available for acceleration in less than a second. The driver’s signal to start the engine is normally depressing the clutch (for manual transmission cars) or releasing the brake (for automatic transmission cars).
For concepts that have electric driving capability, the transition from rest to starting the engine can be delayed by using the electric driving mode as a first means of propulsion before starting the engine for additional power.
Figure 12.6 shows a 5 to 7% saving in fuel consumption from a reference conventional car facilitated by the elimination of idling through the motor start-stop function during city driving conditions. [10] Under optimal control strategy, it is estimated that there is a further 5 to 9% saving available by combining the functions of load level increase, boosting, electric driving, and gliding (to be discussed below). Of our seven concept fragments, note that only external battery charging is not present in Figure 12.6; it adds energy to the system outside the nominal operations of driving. The values shown in Figure 12.6 are representative of full hybrid powertrains with limited electric driving and are not representative of plug-in hybrid systems that can essentially replace fuel consumption in greater proportions.
Figure 12.6 Fuel consumption savings potential for full hybrid systems. [11] (Source: M. Ehsani, A. Emadi, and Y. Gao, Modern Electric, Hybrid Electric, and Fuel Cell Vehicles: Fundamentals, Theory, and Design, CRC Press, 2009)
Concept Fragment 2: Regenerative Braking
The term “regenerative braking” refers to the capturing of braking energy that would normally be lost to friction and heat in conventional car systems. Brake energy recuperation is achieved by setting the electric traction motor in a generative mode that serves as a counterforce to the vehicle’s direction of movement. The energy obtained through regenerative braking can be directly stored in the high-voltage battery and later used for boosting or powering the electrical system components. Figure 12.6 indicates an additional 5 to 9% saving from regenerative breaking.
The use of electric motors as brakes could be sufficient for most braking situations. However, redundant friction braking systems are still required for safety purposes. Hybrids with high-voltage battery systems (greater than 42 V) display a regenerative braking capability that can prolong the life of traditional friction brakes as an added benefit to the customer. Regenerative braking is limited by the battery system’s ability to allow for impulse power storage in short time scales. Super-capacitors have been proved to be well suited for regenerative braking in the case of micro and mild hybrid systems, where two or three seconds of high power inputs and outputs are used in charging and discharging from the capacitor device. Sustained electric driving is not currently possible with super-capacitors.
Concept Fragment 3: Power Boost
When the driver’s situation requires excess acceleration power beyond what the combustion engine can deliver, the electric motors provide additional torque to the wheels known as boosting. Power-boosting situations also include driving on inclines and towing. In this mode, the battery charge is depleted, and power is delivered through the electric motors as an additional source of power.
Boosting is particularly effective in improving a car’s 0–100 km/h (0–60 mph) acceleration specifications. Figure 12.7 shows that the electric motor delivers the highest moment starting from rest and low RPM values (0 to 900 RPM), whereas the typical Otto-cycle internal combustion engine achieves maximum power at higher RPM values (2000 to 2500 RPM). In a typical Hybrid Car, the resulting system performance is enhanced when accelerating from rest by initially using the torque that the electric motor supplies to the drive train.
Figure 12.7 Moment versus speed (in RPM) for an electric motor and a combustion engine. Boosting in hybrid systems allows for additional torque for acceleration, especially when starting from rest.
Concept Fragment 4: Load Level Increase
The load level increase or generative mode allows the engine to deliver some of its excess power to generate electricity, together with an electric motor in generative mode. The extra load level can be used to increase the engine torque and RPM to a more efficient operating point when excess power is available. The generated electricity can be used to charge the battery or power other electrical system loads.
Concept Fragment 5: Electric Driving
Electric driving is achieved by using electric energy stored in the high-voltage battery to power the traction motor to power the wheels. During the electric driving mode, the combustion engine is decoupled from the powertrain. It is either shut off or used to generate electric power. Electric driving is limited by the energy availability of the electrical storage system.
Concept Fragment 6: External Battery Charging
External battery charging differentiates plug-in hybrid concepts from all other hybrid vehicle concepts. In addition to the typical hybrid components, a battery charging unit can be added to the car, offering the possibility of plugging into an external electrical grid. Otherwise, an electrical charging station is required. Both charging strategies impose a limitation on the PHEV market, because customers are forced to have access to plug in their cars at home or at a charging station.
The option of connecting hybrid and electric cars to the electrical grid opens up possibilities for nighttime charging when electricity is cheapest and the electric load capacities of local power stations are at their lowest. Vehicle-to-grid studies within electric mobility research are complementary areas of study that have garnered recent attention. [12]
Concept Fragment 7: Gliding
The last hybrid function of “gliding” is somewhat trivial but nevertheless useful in optimizing a hybrid control strategy. Gliding consists of decoupling both the engine and the electrical system from the wheels and using the force of gravity to propel the vehicle without friction losses of powertrain loads. Conventional vehicles can glide when placed in neutral during downhill operation. Hybrids, however, must have the ability to rapidly connect the appropriate powertrain that best suits the driving situation while moving in and out of a gliding operating environment.
In summary, these seven concept fragments represent potential features of a hybrid vehicle. Some (such as power boost) are directly related to the driving experience, whereas others contribute to the overall performance without direct traceability to the operating experience of the driver. Having developed these concept fragments, we return to the question of the integrated concept. We will compose our integrated concepts from our two primary internal functions, energy storage and vehicle moving, and the seven concept fragments. How do these concepts interact? Do they conflict? Are they synergistic?