10324 Maximum Gradability

The maximum gradability condition must be checked once the sizing of the IC engine and the electric motor is done from the previous three requirements. The maximum gradability of a vehicle is given by

If the condition is not met, then the size of the engine or the motor or both must be increased or the gear ratio must be changed to meet the gradability requirements.

Although the design philosophy outlined above states that the IC engine sizing primarily comes from the rated cruising velocity, and the electric motor sizing comes from the initial acceleration, the practical design involves extensive computer simulation using various drive cycles, parameters of the vehicle, and characteristics of chosen battery, motor, generator, and IC engine. As in all systems cases, the sizing and design of the components of the EV and the HEV is an iterative process that ends when all the design requirements are met. However, the discussions presented on design provide the theoretical basis for initial estimates, avoiding unnecessary oversizing of components.

The specifications of the components of an example HEV (Toyota Prius) are given in the following. The example is not necessarily for the best or optimum HEV, but merely a representative one from today's production HEVs. The Toyota Prius uses a series-parallel combination hybrid, as shown in Figure 10.3. During starting and moving under light load, the electric motor supplies the power, while the IC engine is turned off. The IC engine also turns off during deceleration or regenerative braking, saving energy and reducing pollution. During normal operation, the IC engine provides the power with assistance from the electric motor. Part of the power generated by the IC engine is used to generate electricity through the series generator, which supplies power to the motor (refer to Figure 10.3 to check the power flow direction). The generated power is also used to keep the batteries charged. During full throttle operation, extra energy is drawn from the battery to overcome demanding situations. The Toyota Prius performs like a regular ICEV, but the addition of the electric motor power unit helps reduce the CO2, HC, CO, and NOx emissions. The battery capacity of the vehicle is rather low to give a good range by drawing energy only from the batteries. Specifications of Toyota Prius

Performance:

0-400 m in 19.4 s Top speed—160 km/h Dimensions:

Overall height, width, and length—57.6 in*66.7 in*169.6 in Aerodynamic drag coefficient—CD=0.29 Curb weight—258.5kg IC engine specifications:

1500 cc four-stroke engine using high expansion Atkinson cycle DOHC 16 valve, variable valve timing, electronic fuel inj ection Compression ratio—13: 1 Peak power—42.5 kW at 4000 rev/m Peak torque—102 Nm at 4000 rev/m Electric motor specifications: PM synchronous motor Maximum power—3 0 kW Rated speed—940 rev/m Constant power range—940-2000 rev/m Peak torque—305 Nm from standstill to 940 rev/m System voltage—288 V Energy storage:

Battery pack—40 NiMH batteries

Battery output voltage—273.6 V (228, 1.2 V cells)

Battery energy capacity—1.9 kWh

Rated battery capacity—6.5 Ah

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