How Hybrids Work

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Hybrid electric vehicles (HEVs) combine the benefits of gasoline engines and electric motors. They can be designed to meet different goals, such as better fuel economy or more power.

Most hybrids use several advanced technologies:

Regenerative Braking. Regenerative braking recaptures energy normally lost during coasting or braking. It uses the forward motion of the wheels to turn the motor. This generates electricity and helps slow the vehicle.

Electric Motor Drive/Assist. The electric motor provides power to assist the engine in accelerating, passing, or hill climbing. This allows a smaller, more-efficient engine to be used. In some hybrids, the electric motor alone propels the vehicle at low speeds, where gasoline engines are least efficient.

Automatic Start/Stop. Automatically shuts off the engine when the vehicle comes to a stop and restarts it when the accelerator is pressed. This reduces wasted energy from idling.

HYBRID ENGINES

The most common hybrid cars are petrol-electric, like the Prius. Petrol-electric hybrid cars use a normal petrol engine, just like you’d find in any other car, but in addition, there are one or two high-torque electric motor-generators. The motor-generator(s) draw power from a bunch of car batteries stored either in the floorpan of the car (for a low center of gravity) or in the rear (for convenience). With power supplied to the motor-generator, it behaves like an electric motor. When no power is supplied but the shaft is turning, it becomes a generator to create power. In this mode, you get regenerative braking, where the energy required to slow the vehicle down is all taken up in the motor-generator to re-charge the battery packs. Both the petrol engine and the motor-generator(s) are connected to an onboard computer system which has been programmed by men in white coats to work as efficiently as possible. There are three mainstream technologies in the hybrid market at the time of writing, each championed by a different company or group of companies. Note: the diagrams below all show rear wheel drive for ease of explanation, but hybrid drives can be any of the standard drivetrains from front wheel only to 4-wheel drive.

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IMA – INTEGRATED MOTOR ASSIST (HONDA)

The motor-generator (electric motor and regenerative generator) is in-line with the petrol engine, typically built into the bell-housing in front of the gearbox. The motor-generator is used to assist the petrol engine, thus reducing the load on it and allowing it to be smaller than it would otherwise be for a vehicle of the same weight. For example, the Civic hybrid uses a 1.3l engine where the non-hybrid uses a 1.8l engine. The motor-generator cannot turn without turning the petrol engine too. First-generation systems didn’t have enough power to be able to run the car on electric alone. Current generation ones do through higher powered motors and the ability to shut off the petrol engine when coasting. Because the motor-generator is in-line, the regenerative braking works very simply – as you start to brake, the motor becomes the generator. Conversely, it is also used as the primary starter motor for spinning the petrol engine up quickly after it has been turned off, for example at traffic lights. There is also a backup ‘regular’ starter motor for cold-starts and emergencies. Of the three mainstream hybrid technologies, IMA is by far the simplest to implement, maintain and repair. In the following images, red is the battery pack, green is motor-generator 1 and blue or purple is motor-generator 2.

HYBRID SYNERGY DRIVE (TOYOTA)

Toyota’s take on the hybrid drive has a pair of motor-generators, one in-line like the Honda IMA design, one not. The key to its success is the compound planetary gearset in the transmission. In the Toyota system, the petrol engine and one motor-generator are connected to one of the inputs, the second motor-generator to the second input and the wheels to the third. Through a clever use of electronics, the planetary gearbox can be locked and unlocked in various configurations depending on what is required. For example under modest acceleration, the petrol engine drives the planetary gearbox as well as the first motor-generator. The output from that is fed to the second motor-generator along with the output from the gearbox to drive the wheels. In pure electric mode, the first motor-generator freewheels, the petrol engine is turned off and all the electric power is fed to the second motor-generator. Under regenerative braking, the second motor-generator becomes the generator as it does in the IMA system above. The difference is that if the battery pack is full, the energy derived from the second motor-generator is redirected to the first motor-generator which in turn uses it to induce drag in the petrol engine to slow the vehicle down. As a result, the actual brakes in a Toyota Hybrid car do not wear very quickly at all because most of the braking is provided by the motor-generators. Only in severe cases do the brake pads actually engage the brake rotors. This is all made possible by the central engine computer and throttle-by-wire / brake-by-wire system.

DUAL-MODE OR 2-MODE HYBRID (GM)

The third hybrid system comes from GM and has two operating modes as opposed to the single mode of IMA or HSD. It again uses two motor-generators. In first and second gears, the first motor-generator sends power to the second motor-generator, and that coupled with the petrol engine provide the power to the wheels. In higher gears or under heavier loads, the petrol engine always runs (as opposed to the IMA and HSD systems where it can be turned off or have cylinders deactivated). The difference is how the motor-generators work in cooperation with it. As speed increases, the first motor-generator gets to the point where it’s providing no useable input to the drivetrain. At this point, it begins to freewheel and the second motor-generator begins to act as a generator. As speed increases further, the first motor-generator begins to act as a generator again and at this point, its power is once again fed to the second motor-generator which now becomes a motor. Coupled with variable intake timing, direct common-rail injection and a host of other technologies, these all come together to give GM’s take on hybrid technology.

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Most hybrids have an energy display screen mounted either in the instrument cluster or in the centre console. This is a small LCD which gives you, the driver, information about what mode you’re driving in, and where the power is going. Again, the most recognisable and famous of these displays to date is that of the Toyota Prius (see right). The only real problem with these displays is the fascination they provide to the novice hybrid driver. Watching the animations spin around and the energy arrows scroll here and there as you drive is certainly informative but not really conducive to safe driving. One benefit, however, is the constantly-updated gas-mileage chart. Many Prius owners report that this spurs them to attempt to get videogame-like high scores in their cars, driving them in such a fashion as to get the highest recorded mpg from their cars. If nothing else, the energy display affects most drivers in terms of educating them as to how their driving style directly impacts their gas mileage.