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.