Mazda is a company that isn’t afraid to think outside the box, especially when it comes to designing engines. The company is well known for its use of the Wankel rotary engine, especially in the RX-7 and RX-8. More recently, the Mazda announced its revolutionary SkyActiv-X family of engines, which could potentially rival electric vehicles in terms of ‘well-to-wheel’ emissions.

However, there is one particularly bizarre Mazda engine you may not have heard of: the supercharged KJ-ZEM V6. Used only in the Mazda Millenia (sold as the Xedos 9 in Europe), the KJ-ZEM is definitely not your typical V6 engine. It has a displacement of just 2.25 litres, less than many mainstream four-cylinder engines. But what made the KJ-ZEM particularly unusual was its unconventional combustion cycle. In fact, it was the first mass-produced passenger car engine to run on the Miller cycle.

Most four-stroke petrol engines run on what’s known as the Otto cycle, as demonstrated by the above animation. The Miller cycle has the same four strokes (intake, compression, power, exhaust) with one key difference.

During the compression stroke in an Otto cycle engine, the intake valve is closed as the piston compresses the fuel-air mixture against the cylinder wall. In a Miller cycle engine, the compression stroke occurs in two stages. During the first stage, the intake valve actually remains open, allowing for some of the fuel-air mixture to re-enter the intake manifold. The second stage occurs when the piston reaches about 25 per cent of its stroke length. At this point, the intake valve closes, and the piston compresses the remaining fuel-air mixture until it is ignited by the spark plug. This is actually similar to the Atkinson cycle engine, which is commonly used today in hybrid-electric vehicles. Because the piston doesn’t have to compress as much fuel-air mixture, the engine’s overall efficiency is greater than the typical Otto cycle engine.

The main drawback to the Atkinson cycle engine is that the lower compression ratio makes it less powerful than an Otto cycle engine. The Miller cycle engine solves this problem by using forced induction (typically a Roots-style supercharger) to maintain compression while increasing the expansion ratio, or the rate at which the fuel-air mixture expands after ignition. The end result is an engine that is more powerful, refined and efficient than a conventional unit.