The third step in our concept development framework is evolving and refining the integrated concepts. One of the benefits of concept fragments is that they lend themselves to recombination and modularity. The intent of this third step is to search the space of concepts systematically, by enumerating integrated concepts from combinations of concept fragments. One way to analytically organize the search of the combinatorial space among the concept fragments is to structure the search according to the different possible mappings between functions and forms. Recall how in Chapter 7 we produced a variety of transport concepts using the morphological matrix. For example, having a propeller perform each of lifting, propelling, and guiding, we produced a helicopter. In Part 4 we showcase computational methods for reasoning exhaustively, but in this section we treat this topic analytically.

From the energy storage and powertrain concept fragments, we can begin by analyzing the spectrum of architectures within the “electric pathway” at various levels of electrification, including hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs).

Specifically, in the hybrid context, the relevant form-to-function mapping is whether the energy storage function is an input to the vehicle-moving function, implying that the form is shared between these two functions, or whether energy storage and vehicle-moving decompose to separate forms.

This mapping is referred to as parallel hybrids vs. series hybrids and is illustrated in Figure 12.8. Parallel hybrid powertrain systems are additive systems that combine both drives; by contrast, the series hybrid powertrain system works in a sequential manner, where the fuel-based powertrain provides power to the electric powertrain. The energy load consumption is the energy needed to propel the vehicle. Vehicles that can mix features of both parallel and series hybrid operating modes are referred to as combined hybrid systems. Combined hybrid vehicles may split the energy from the fuel converter into the series and parallel hybrid energy paths simultaneously, as in the case of power split hybrids. Or they may have a distinct switch that allows for only parallel operation, or only series operation, at any one time.

For brevity, we have excerpted the search process among the potential combinations of parallel and series hybrids, skipping directly to the integrated concepts. The conceptual typology of hybrid vehicle architectures can be described in a two-dimensional solution space of electrical driving range and degree of electrification, as depicted in Figure 12.9. The first dimension entails the electric distance the car can achieve with the electrical propulsion system alone, whereas the degree of electrification is determined by the ratio of cumulative peak electric motor power to maximum combined electric and engine power. A degree of electrification of zero describes a conventional internal combustion engine car with no electric system, and a degree of electrification of one describes a battery electric vehicle with no internal combustion engine installed.

There are seven integrated concepts that populate the car architecture solution space: conventional ICE, micro hybrid, mild hybrid, full hybrid, plug-in hybrid, battery electric vehicle, and fuel cell electric vehicle. These are obtained by combining concept fragments for energy storage and vehicle moving with the seven additional concept fragments describing internal functionality.

• Conventional Internal Combustion Engine (ICE). The conventional ICE car concept is the current dominant architecture. The most popular conventional cars feature a diesel or Otto cycle engine and are defined by a degree of electrification and electric driving range of 0 (zero), meaning no electrical propulsion system is installed.

• Micro Hybrid Electric Vehicle (Micro HEV). Micro hybrids achieve propulsion exclusively by an ICE but offer some functionality of hybrid vehicles, most importantly the motor start-stop function. The start-stop function of a micro hybrid can be achieved by a 12-V to 16-V electric battery system and requires a more robust starter-generator system. Some micro hybrids also exhibit limited regenerative braking. Micro HEVs are sometimes referred to as advanced ICE cars because they are essentially conventional cars with no electric driving capability and minimal electrification.

• Mild Hybrid Electric Vehicle. Mild hybrids differentiate themselves from micro hybrids in that they offer limited functionality in electric driving (some mild hybrids do not offer electric-only driving). Mild hybrids thus include the three key components of the electric drive system: a high-voltage battery, an electric starter-motor for propulsion, and a control system that determines when the electric and the combustion engine systems will work together. Mild hybrids are solely parallel systems that can offer additional functionality of motor assist and expanded regenerative braking.

• Full Hybrid Electric Vehicle. Full hybrids display larger degrees of electrification (10 to 30%) than mild hybrids and are characterized by short electric driving distances (for example, from 500 m to 3 km). The primary propulsion system still remains with the internal combustion engine, but the electric system can assist in providing power to the wheels. Most full hybrids exhibit parallel or combined configurations. Full ­hybrids exhibit all functions of mild hybrids and have a larger capacity for ­regenerative braking.

• Plug-In Hybrid Electric Vehicle. Plug-in HEVs are differentiated from other HEV types by the ability to charge the high-voltage battery externally through a battery charger and to plug into an external energy source. PHEVs come in a wide range of architectures, including parallel, series, and combined configurations, and they offer extended electric driving ranges (from 5 km to about 160 km). PHEVs offer a degree of electrification typically above 35%. The range extender (ICE plus generator) of ­series plug-in hybrids (sPHEV) can vary from large ICE power systems to small “limp home” emergency IC engines.

• Battery Electric Vehicle (BEV). The battery electric vehicle falls outside of the HEV solution space with a degree of electrification equal to 1. BEVs have no internal combustion engine installed and are plug-in vehicles by definition: Electricity is their only energy source. The electric range of BEVs depends on the level of electrification installed.

• Fuel Cell Electric Vehicle (FCEV). Fuel cell cars exhibit an electrochemical cell that converts a source of fuel into an electrical current. FCEVs are primarily configured in a series architecture where the fuel cell power system provides energy to the electrical power system for propulsion.

In summary, these seven integrated concepts represent the decision facing us. These were generated by analytically combining functions and forms for the two primary functions and then combinatorially pairing the seven concept fragments. In this manner, we were able to generate coverage of the space of integrated concepts. Our focus here was on presenting the results; it is difficult to represent the creative process, particularly infeasible or conflicting options. In Part 4, we will examine methods for searching the space computationally, which provides the side benefit of tracking conflicting concept fragments and infeasible combinations.