18 Electric and Hybrid Vehicles 1.8.1 e fficiency c omparison In order to evaluate the efficiencies of different types of vehicles on a level ground, the complete process in both systems starting from crude oil to power available at the wheels must be considered, i.e., the analysis must be carried out based on WTW efficiencies. The EV process starts not at the vehicles but at the source of raw power whose conversion efficiency must be considered to calculate the overall efficiency of EVs. The power input P IN to any vehicle ultimately comes from a primary energy source even before it is stored in a vehicle tank.
The power extracted from a piece of coal by burning it is an example of primary power obtained from a primary energy source. The power that is available in a vehicle from an energy storage tank or device is applied power obtained from a sec- ondary source of energy. The applied or secondary power is obtained indirectly from raw materials. The electricity generated from crude oil and delivered to an electric car for battery charging is an example of secondary power. The raw or primary power is labeled as P IN RAW , while the secondary power is designated as P IN PROCESS . An example efficiency comparison is presented here based on the complete WTW processes involved in an EV and an ICEV.
The complete EV process can be broken down into its constituent stages involving power generation, transmission and usage as shown in Figure 1.8. The primary power from the source is fed to the system only at the first stage, although the secondary power can be added in each stage. Each stage has its efficiency based on total input to that stage and output delivered to the following stage. The efficiency of each stage must be calculated from input–output power considerations, although the efficiency may vary widely depending on the technology being used. Finally, the overall efficiency can be calculated by multiplying the efficiencies of all the indi- vidual stages.
The overall efficiency of the EV system shown in Figure 1.8 is as follows: P P P P P P P P P η EV = = 0 0 = = 0 6 5 4 3 2 1 η η η η η η η P IN P P 0 + ∑ 7 1 2 3 4 5 6 7 P 6 P 5 P 4 P 3 P 2 P 1 P IN LOSSi i = 1 The ICEV process details are illustrated in Figure 1.9. The process starts from the conversion of crude oil to fuel oil in the refinery, and then includes the transmission of fuel oil from refinery to gas stations, power conversion in the IC engine of the vehicle and power transfer from the engine to the wheels through the transmission.
The efficiency of the ICEV process is the product of the efficien- cies of the individual stages indicated in Figure 1.10 and is given by η ICEV = η η 1 2 3 4 η η P IN PROCESS1 P IN PROCESS2 P IN RAW Refinery: Raw to Fuel oil η 1 Electricity Generation Transmission & Distribution Battery Charger Battery Charge & Discharge P LOSS5 η η η 2 3 4 P LOSS1 P LOSS2 P LOSS3 P LOSS4 Transmission Axle Power Converter & Electric Motor P O η η 7 P LOSS6 P LOSS7 6 FIGURE 1.8 The complete EV process from crude oil to power at wheels.