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Mazda's Miller cycle engine drops the supercharger

Mazda has developed an unusual fuel-efficient 1.3-litre Miller-cycle engine. Dispensing with a supercharger, it instead combines with the OEM’s first continuously variable transmission (CVT).

The unit, based on Mazda’s 1.3-litre DOHC aluminium engine, which normally achieves 19.2km/litre, gives fuel economy of 23km/litre.

The engine’s increased efficiency comes from a reduction in pumping losses. It delays the closing of the intake valves, effectively dividing the compression stroke into two discrete cycles – the defining feature of a Miller-cycle engine.

As the piston moves upwards, the charge is partially expelled through the still-open intake valve. The valves close at about a quarter of the way into the compression stroke.

As a result, the engine achieves the same compression ratio as it would in a standard Otto engine, but expends less energy in the process. But the loss of air does result in a loss of torque at low engine speeds.

A conventional Miller-cycle engine would compensate for this with a positive-displacement supercharger. If it can compress the charge using less energy than the piston, the result is a fuel saving. But Mazda’s version of the Miller does not use a supercharger – instead a CVT boosts low-end torque.

Mazda controls the valve events using its sequential valve timing technology. It varies intake valve timing by using hydraulic pressure to rotate the cam-phaser.

Despite the efficiency gains, a Miller-Mazda2 will only sell in Japan.

“We have no plans to introduce the engine in Europe,” said Mazda. “It has to be linked with a CVT and demand is limited.”
There is increasing interest in Japan in engines that vary the stroke length in order to improve efficiency.

Mazda’s take on the Miller-cycle is similar to the Prius powertrain, which Toyota calls an Atkinson-engine. The term generically describes an engine in which the power stroke is longer than the compression stroke.

The original Atkinson concept featured extra linkage to allow the intake, compression, power, and exhaust strokes to occur in a single turn of the crankshaft.