The Miller cycle is an operating cycle for forced induction engines which is similar to the conventional (Otto) cycle, but has changes to the inlet valve timing which increase thermal efficiency.

In normal piston engines the compression ratio and expansion ratio are the same. Raising the expansion ratio enables more complete combustion and improves efficiency, but normally means increasing the compression ratio at the same time – which can cause problems in a forced induction engine

A Miller cycle engine is designed with a high geometric compression ratio, so that the expansion ratio will also be high, but then the effective compression ratio is lowered by reducing the amount of intake charge in the cylinder, in one of two ways:

  • the intake valves are closed early, while the piston is still descending on the induction stroke, so there is less time for intake charge to reach the cylinder
  • the intake valves are closed late, after the piston has started rising on the compression stroke, so some intake charge is forced back into the induction system – the so-called 'fifth cycle'

The result is an engine where the expansion ratio is greater than the compression ratio. This leads to high thermal efficiency, but causes a drop in maximum power compared to an Otto cycle engine.

The Miller cycle was named after engineer Ralph Miller, who developed it in the 1940s. It has recently been adopted for some high-efficiency road car engines from Mazda and Nissan.

Mileage has two meanings. In the UK, mileage means the total distance travelled by a car during its lifetime – an important factor in establishing the value of a vehicle.

In the US mileage usually refers to gas mileage, a measure of fuel consumption.

A micro hybrid has a conventional internal combustion engine fitted with start/stop and recuperation systems which capture waste energy during braking for re-use.

This is often achieved by running electric systems from the vehicle battery with the alternator disengaged. The alternator is only engaged when the car is braking, so that it converts some of the vehicle's kinetic energy into electricity.

Mercedes-Benz BlueHybrid is a mild hybrid system

A mild hybrid has a petrol or diesel engine and an electric motor. The electric motor supplements the internal combustion engine during heavy power demand (eg when accelerating hard) but a mild hybrid cannot run on electric power alone.

Because the engine is not used to charge the batteries or to provide power for the electric motor, a mild hybrid can be classed as a parallel hybrid.

Mild hybrid systems do not provide the same advantages as full hybrids in city emissions performance and overall fuel efficiency. However, they can use smaller electric motors and traction batteries, so they are lighter and cheaper to implement than full hybrid systems.

Common mild hybrid vehicles include:

  • Mercedes-Benz S400 BlueHybrid
  • BMW ActiveHybrid 7
  • Chevrolet Silverado PHT

Jaguar development mule

A mule is a test vehicle which uses the bodywork from an existing production car with new mechanical components. Mules are most commonly used for development of the powertrain and suspension.

Often the bodywork is modified to suit the new mechanical components and to alter the mule's centre of gravity, overall weight and weight distribution. These changes are often made quite crudely, so mules often have a shabby appearance.

In some cases mules are built with some production-spec bodywork. For instance, the nose of the new model might be grafted onto a body largely composed of old-generation panels. This might be essential for developing engine cooling, braking and HVAC systems.

Mules are often painted black and/or white, and may carry disguise panels so that it is more difficult to discern the make of car – in fact some manufacturers have been known to deliberately incorporate styling cues from rival vehicles in their disquise, such as BMW 'twin kidney' grilles.