Miller Cycle Engines

The Miller cycle is closely related to the Atkinson cycle but takes a different approach. In a Miller cycle engine, the intake valve closes early — before the piston reaches the bottom of the intake stroke. The piston continues down against a partial vacuum, which effectively lowers the compression ratio. But the expansion ratio stays the same because the power stroke is a full stroke. Like the Atkinson cycle, the expansion ratio ends up larger than the compression ratio, which extracts more work from each combustion event.

Because the intake valve closes early, a Miller cycle engine does not fill the cylinder completely on its own. It would make very little power naturally aspirated. That is why Miller cycle engines are always paired with a supercharger or turbocharger. The forced induction crams air in before the valve closes early, making up for the lost volume. The result is an engine that makes good power from the forced induction but has the thermal efficiency benefits of over-expansion.

Mazda took the Miller cycle concept further with their Skyactiv-X engine, which uses Spark Controlled Compression Ignition — SPCCI. The engine runs an extremely lean mixture and uses compression ignition (like a diesel) for most of the combustion, with a small spark-ignited fireball in the center to control the timing of the compression ignition event. It is the first gasoline production engine to use compression ignition. The result is diesel-like efficiency from a gasoline engine. This engine is available in some markets but not widely sold in the United States.

Miller cycle engines with forced induction share the same service considerations as any turbocharged engine — intercooler inspection, boost leak testing, and turbo or supercharger maintenance. On the Skyactiv-X specifically, the engine uses a supercharger for cylinder charge control and has a specialized high-pressure fuel system. The compression ignition strategy means the engine operates at very high cylinder pressures, so proper oil specification and change intervals are critical.