34.11.
Combined Power Source Vehicles (Hybrid Drives)
Internal combustion engines produce polluting emissions as well as deliver poor efficiency at part load. Electric drives on the other hand produce “no” emissions but have a limited range.
The concept of hybrid drive is to combine the best aspects of each and minimise the worst. The principle of the hybrid drive system is shown in Fig. 34.14.
One way of combining these two drives is to use the electric drive in slow traffic and towns, and to use the IC engine on the open road. This is most effective way of reducing pollution in towns. Sophisticated control systems can permit even better usage, like under certain conditions both the electric motor and the engine can be used.
Fig. 34.14. Principle of the hybrid drive system.
34.11.1.
Types of Hybrid Drives
The hybrid drive concept can be applied in a number of ways as shown in Fig. 34.15. Also it is possible to adapt different types of engine, like petrol, diesel or gas turbine engines. Both series and parallel arrangements of the drives can be used. In series layout engine drives a generator that operates a motor, which in turn drives the wheels. In parallel layout both engine and motor drive the wheels. This system permits the engine to recharge the battery while powering the vehicle. The parallel arrangement provides greater flexibility. The series arrangement however permits the fossil fuel engine to run at a constant speed while driving the generator. This makes the combustion engine to run more efficiently, however the double energy conversion process (mechanical to electrical to mechanical) is less efficient than direct transmission of the vehicle. Additional advantage provided by the series connection is that it avoids the use of transmission (gear box).
Nelco Hybrid System.
Nelco has developed a hybrid EV drive system, which is a drop in drive package with parallel
layout (Fig. 34.16). The system is compatible with existing internal combustion engine cars. The performance of the system is equivalent to a conventional front wheel drive car, with two thirds of the fuel consumption and just one third of noxious emissions. The vehicle exhibited a range of 800 km and a top speed of 160 km/h. The main components of the layout include a deep discharge tolerant lead acid battery, a permanent magnet brushless DC motor and a Norton rotary engine.
Fig. 34.15. Principle of hybrid drives.
Fig. 34.16. Parallel layout used for the Nelco system.
The battery incorporates lead in foil plate construction, and a flat array of cells was placed under the passenger compartment of the vehicle. The pack weighs 170 kg and can supply 7.5 kWh. The battery can withstand 1100 discharge cycles to 80% depth of discharge (DOD) and 11000 cycles to 20% DOD. The thermal management system keeps the lead acid cells at a constant 303 to 313 K, the most efficient operating temperature. Hence battery is expected to have a long life.
Norton rotary engine used in the system has a fast warm up and provides a starting torque of 8 Nm. Two electrically preheated catalytic converters are used. The fuel injection system operates the engine on a lean burn setting at high load. The engine supplies a constant output, and the electric motor supplements power for transient loads.
The sectional view of the permanent magnet brushless DC motor is shown in Fig. 34.17. The motor weighs 45 kg and is oil cooled to prevent freezing. A sophisticated inverter along with control circuit controls the motor. The battery voltage
Fig. 34.17. Permanent magnet brushless DC motor.
is converted from the 216 V DC of the batteries to a 300 V DC stabilised rail, which is supplied to the motor. The motor is supplied with three phase power as either trapezoidal or square waves, the phase of which can be altered to control braking or acceleration. The accelerator position gives an input to the control module and a Hall effect rotor position sensor provides a feedback signal to ensure that the three phases of the motor are energized in the correct order.
The whole power unit weighs about 100kg, compared with 200 kg for a conventional system. The batteries, however, add a further 130 kg above the normal, but provide a 480 km range without running the engine.
Toyota Hybrid System.
Toyota hybrid technology (Fig. 34.18) works on both series and parallel layouts. The system divides the engines power through a split drive using planetary gear to power both drive shaft and the generator. The generator drives the motor and a part of its inverted to DC to charge nickel metal hybrids batteries.
Fig. 34.18. Layout of Toyota hybrid system.
The system incorporated 1.5 L gasoline engine that works on Atkinson cycle and a high compression ratio. Atkinson cycle engines offer much highs thermal efficiency than conventional engines, but require super-charging for producing sufficient power. The engine develops a maximum speed of 4000 rpm. The engine is quite small and lighter, and thus incorporates a thinner crankshaft, lower tensile strength piston rings and reduced valve spring loads. As a result there is considerable reduction in friction loss. The engine uses smaller combustion chamber of tilted quish type.
The engine is boosted with a motor whenever necessary. This allows the engine to operate at maximum thermal efficiency than maximum output, resulting in much better fuel economy. The engine automatically shuts-off when the vehicle is stopped or decelerated at a low speed. Depending on conditions, the system controls the division of power between engine and motor so that engine always operates in its maximum torque range. The engine also automatically operates within a constant rpm range to maximize fuel economy.
When the vehicle decelerates, the motor serves as a generator, converting the vehicle’s kinetic energy into electricity and sending it through an inverter to charge the battery. The Toyota hybrid system doubles the fuel economy of conventional vehicle due to the use of generated electricity (improvement by 80%) and energy recovery during deceleration (improvement by another 20%).
During starting, driving at extremely low speeds, going down a moderate slope and/or operating in other conditions in which engine does not work at maximum efficiency, the engine shuts down and motor drives the vehicle powered by battery. During full throttle acceleration, the battery also supplements power to the motor’s output.
