Driving Cycle ð Vehicle Resistance ð Transmission ð Combustion Engine ð Fuel Consumption Figure 3.2 Calculation Logic: Mechanical Drivetrain
The base vehicle with an internal combustion engine coupled to a mechanical transmission is related to the specified driving cycle as shown in Figure 3.2. The calculation starts with the chosen driving cycle, specified as an array of vehicle velocity versus time (at intervals of one second). From these two inputs, the vehicle acceleration is calculated. This information is used to calculate the instantaneous power needed to operate the vehicle, by adding
aerodynamic drag, tire rolling resistance, and inertial force (vehicle mass times acceleration). The required total power is converted to the torque needed to drive the tires, which through an automatic, manual, or continuously variable transmission is converted to the torque needed at the engine output shaft.
In addition to the power required as engine output, all the engine losses (due to engine cycle inefficiencies, engine friction, changes in rotational kinetic energy, and auxiliary component power requirements) are summed together to obtain the total rate at which fuel chemical
energy is consumed. Using the lower heating value2 (the stored useable chemical energy of a
fuel), this "fuel power” is converted to the amount of fuel needed, thus generating the desired result—energy consumption per unit distance traveled. This logic diagram applies to the
current, evolutionary gasoline, and the advanced3 gasoline and diesel vehicles presented in
this study.
2
Two fuel heating values are defined, a lower and higher, depending on whether the water in the combustion products is vapor or liquid. We follow the usual engine convention here. The energy, fuel consumption and CO2 predictions are unaffected since the heating value cancels out.
3
Here, "advanced" is used to denote components where plausibly practical new technologies which improve performance have been incorporated.
3-9 Driving Cycle ð Vehicle Resistance ð Electric Motor ð Battery Status Figure 3.3 Calculation Logic: Battery Electric Drivetrain
The electric vehicle with batteries driving an electric motor is modeled in a similar manner, as shown in Figure 3.3. In many ways, this electric vehicle is simpler, having a single gear transmission, and easier to predict motor and battery characteristics. Again, the model begins with the chosen driving cycle and takes into account vehicle resistances. Then, the total required energy at the tires is converted to the torque needed at the output of the electric motor. With the motor efficiency and the discharging efficiency of the batteries, the desired energy consumption per unit distance traveled can be calculated. With an electric drivetrain, regenerative braking— the conversion of vehicle kinetic energy to stored energy in the batteries during vehicle braking, with losses due to generator (motor) and recharging inefficiencies—is included here, also.
This logic diagram applies only to the pure battery electric vehicle, a case presented in this study primarily to illustrate the required battery performance characteristics for EVs to be competitive. Limits in battery technology (too low energy storage per unit weight, short life, and high cost) currently prevent such vehicles from being commercially viable. Also note that the energy consumption for the EV will be lower than that of an ICE vehicle, because the efficiency of the motor and battery combined is substantially higher than that of any
"engine". However, this tank-to-wheels estimate does not take into account the efficiency of electricity generation from the primary energy source and transmission over the grid, or electricity generation at a local recharging station. The losses during the battery recharging process from the grid are accounted for separately.
ð Transmission ð Combustion Engine ð Fuel Consumption ò Driving Cycle ð Vehicle Resistance ð Logic Control ïð Electric Motor ïð Battery
Figure 3.4 Calculation Logic: ICE − Battery Electric Parallel Drivetrain
The parallel hybrid simulation combines the logic of these two models and uses both the combustion engine and the electric motor, as shown in Figure 3.4. The additional logic control block determines the power flow required from the engine and the battery,
respectively, based on the amount of power required and the state of charge of the batteries. The objective here is to operate the engine at higher loads where it is more efficient, switch the engine off during idling and low power requirements, and use the battery and engine together at peak power levels so both components can be kept as small and light as possible.
3-10
ð Fuel Cell ð Fuel Consumption ò Driving Cycle ð Vehicle Resistance ð Electric Motor ð Logic Control ïð Battery Figure 3.5 Calculation Logic: Fuel Cell − Battery Electric Drivetrain
In fuel-cell powered vehicles, the fuel cell system is combined with a battery, as a hybrid, for similar reasons: to maintain fuel cell operation in its high efficiency (part load) region as much as possible, and benefit from regenerative braking energy recovery. Its logic is shown in Figure 3.5. During idling and low-power operation, the batteries supply the necessary power. Over a certain threshold, the fuel cell turns on; extra power is used to recharge the batteries if they are below a set state of charge. When the power required exceeds the maximum fuel cell stack capabilities, the batteries again supplements peak loading. Since the fuel cell directly converts chemical energy to electrical energy, a mechanical transmission is not required. Also, the fuel cell requires energy, even during vehicle operations when it is not supplying power directly; hence it creates an addition drain on the battery system. Finally, if a liquid fuel (methanol or gasoline) is stored on the vehicle, then a fuel reformer system, which converts the liquid fuel to hydrogen on board, is included.