The hybrid Toyota is a well known and claimed fuel efficient car. We all know its somehow got a battery and an engine. But what is the idea? Why does this make it more efficient?
Essentially the Toyota is more efficient (well a bit) than many similar cars, because the engine operates on what is known as an Atkinson* cycle, rather than the normal Otto cycle for petrol engines.
In case you are interested we explain how this differs from a normal car engine later. But the point is that by fiddling around with the engine valve timing ( the valves regulate when in the rotation of the engine, the air is admitted to the engine, and exhaust gas is let out ), you can make an ordinary petrol engine (Otto cycle) quite a bit more efficient.
The problem is, this seriously lowers the power output of a given size (and therefore weight) of engine. This means the acceleration is poor – there is less power to accelerate more weight.
The Toyota Prius gets round this by having a battery and a motor / generator on board. When the driver wants to accelerate hard, the feeble engine is assisted by the energy stored in the battery driving the motor.
The battery is recharged by using the motor as a generator to slow the car (- regenerative breaking) or using the engine to top it up when it is not working hard at accelerating but cruising.
So the regenerative breaking, which saves some of the energy otherwise dissipated as heat in the breaks can be re-used which improves the mileage in towns.
But contrary to what you might suppose, the engine is more efficient than the equivalent Otto engine even when cruising, so mileage will be improved even for driving outside town.
The downside is that whilst the whole thing is on paper more efficient, you have to carry a heavy battery and motor around, and this eats up some of the savings.
The result is a car that performs as well as an ordinary Otto cycle, but a bit more efficiently, but is more expensive.
If you want real fuel economy, you are probably better of with a diesel.
The car certainly isn’t a partly electric car even if it can be charged from the mains – without any petrol the car will only travel about 1 mile – which shows how irrelevant the battery is in terms of using mains renewable power to substitute for petrol.
Fred Starr / Dave Andrews
*Atkinson cycle – this is an ordinary Otto cycle engine which operates on the well known four stroke cycle (suck, squeeze, bang, blow) but with the inlet valve closing considerably delayed compared to the Otto. This has the effect of decreasing the amount of air compressed by the engine considerably and less energy is thus used to compress it, and more energy can be extracted during the expansion phase, since this arrangement allows the exhaust gases to more fully expand than in the Otto cycle.
In a turbocharged engine, the Atkinson cycle is for some reason known as a Miller cycle – a number of manufacturers e.g. Wartsila and Cummins make such a Miller cycle gas engine because it has higher efficiency but lower specific output.
See Wikipedia for more info.