HSD replaces the gear box, alternator and starter motor with a pair of electrical mo tor-generators, a computerized shunt system to control them, a mechanical power splitter that acts as a second differential, and a battery pack that serves as an energy reservoir. A motor-generator is a transducer that converts electricity to motion or vice-versa. The mechanical connections of the system allow the computer to convert mechanical power from the engine between three forms: extra torque and rotation at the wheels (under constant torque and rotation speed), and electricity. This achieves the benefits of a continuously variable transmission, except that the torque/speed conversion uses electricity rather than direct mechanical connection. In practice, HSD cars can be driven a mile or two without gasoline, as an emergency measure to reach a gas station.
Process:
One of the motor-generators (MG2) is mounted on the driveshaft, and thus couples torque into or out of the driveshaft: feeding electricity into MG2 adds torque at the wheels. The engine end of the driveshaft has a second differential; one leg of this differential is attached to the gasoline engine and the other leg is attached to a second motor generator (MG1). The differential relates the rotation speed of the wheels to the rotation speeds of the engine and MG1, with MG1 used to absorb the difference between wheel and engine speed. The differential a "power split device"; that and the two motor-generators are all contained in a single housing that is bolted to the engine. Special couplings and sensors monitor rotation speed of each shaft and the total torque on the driveshaft for feedback to the control computer.
The drive works by shunting electrical power between the two motor generators and the battery pack to even out the load on the gasoline engine. Since a power boost is available for periods of acceleration, the gasoline engine can be sized to match only the average load on the car, rather than its peak load: this saves fuel because smaller engines are more power efficient. Furthermore, during normal operation the gasoline engine can be operated at its ideal speed and torque level for power, economy, or emissions, with the battery pack absorbing or supplying power as appropriate to balance the demand placed by the driver.
Phases of Operation:
The HSD operates in distinct phases depending on speed and demanded torque. Here are a few of them:
- Engine start : To start the engine, MG1 is fed negative voltage, so that it acts as a starter motor. The engine is forced into forward motion. Because both motor generators are sized to drive the entire car, turning the engine does not stress the motors and the conventional starter motor sound is not heard: engine start is silent. Engine start can occur when the car is stopped or moving.
- Low gear (equivalent): When accelerating at low speeds in normal operation, the engine turns much more rapidly than the wheels, but does not develop as much torque as is needed. MG1 is forced rapidly backwards, and the computer pulls electricity from MG1. The electricity is shunted to MG2, adding torque at the driveshaft, so that the drive train develops power at low speed and high torque.
- High gear (equivalent) : When cruising at high speed, the engine turns more slowly than the wheels, but develops more torque than is needed. The computer pulls electricity from MG2, reducing the torque available at the wheels. The electricity is shunted to MG1, which boosts the speed of the driveshaft. Because the engine supplies mechanical energy to the whole system, conserva tion of energy is not violated: the power that is shunted from MG2 to MG1 is less than the total power developed by the engine, and so power is delivered to the wheels.
- Reverse gear : There is no reverse gear as in a conventional gearbox: the computer feeds negative voltage to MG2, applying negative torque to the wheels. If the battery is low, the system can simultaneously run the engine and draw power from MG1, although this will reduce available reverse torque at the wheels.
- Silent operation : At slow speeds and moderate torques the HSD can drive without running the gasoline engine at all: electricity is supplied only to MG2, allowing MG1 to rotate freely (and thus decoupling the engine from the wheels). This is popularly known as "Stealth Mode." Provided that there is enough battery power, the car can be driven in this silent mode for some miles even without gasoline.
- Neutral gear : Most jurisdictions require automotive transmissions to have a neutral gear that decouples the engine and transmission. The HSD "neutral gear" is achieved by turning the engine off. Under this condition, the planetary gear is stationary (if the vehicle wheels are not turning); if the vehicle wheels are turning, the ring gear will rotate, causing the sun gear to rotate as well (the engine inertia will keep the carrier gear stationary unless the speed is large), while MG1 freewheels so no power is dissipated.
- Regenerative braking : by drawing power from MG2 and depositing it into the battery pack, the HSD can simulate normal compression braking while saving the power for future boost. The HSD system has a special transmission setting labelled 'B' (for Brake), that takes the place of a conventional automatic transmission's 'L' setting for engine braking on hills. If the battery is full, the system switches to conventional compression braking, drawing power from MG2 and shunting it to MG1, speeding the engine with throttle closed and so slowing the vehicle. The regenerative brakes in an HSD system absorb a significant amount of the normal braking load, so the conventional brakes on HSD vehicles are undersized compared to brakes on a conventional car of similar mass.
- Electric boost : The battery pack provides a reservoir of energy that allows the computer to match the demand on the engine to a predetermined optimal load curve, rather than operating at the torque and speed demanded by the driver and road. The computer manages the energy level stored in the battery, so as to have capacity to absorb extra energy where needed or supply extra energy to boost engine power.
Battery charging : The HSD can charge its battery without moving the car, by running the engine and extracting electrical power from MG1. The power gets shunted into the battery, and no torque is supplied to the wheels. |
