Miller Cycle | Sequential Valve Timing (S-VT) | Continuously Variable Transmission (CVT)

The key to improving fuel efficiency lies in raising an engine’s thermal efficiency. This can be done by increasing the expansion ratio. The expansion ratio is the amount of work the engine does each time the air-fuel mixture in the cylinders detonates. However, in conventional engines the expansion ratio is the same as the compression ratio, so increasing the expansion ratio will also raise the compression ratio. This is a problem because a high compression ratio causes abnormal combustion, or knocking.

01-compression ratio-expansion ratio-miller cycle engine

The answer is the Miller-cycle engine. By delaying the closure of the intake valves, compression actually begins part way through the compression stroke, which results in a reduced compression ratio. At the same time, changing the shape of the piston crown decreases the combustion chamber minimum volume, resulting in a larger expansion ratio. In this way we can decrease the compression ratio and while increasing the expansion ratio. In other words, the Miller-cycle engine has a higher expansion ratio than compression ratio.

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02-MZR engine-Miller Cycle Engine-Higher expansion ratio

Mazda’s naturally-aspirated MZR 1.3L Miller-cycle engine delays the closure of the intake valves to improve the thermal efficiency (high expansion ratio). Sequential-valve timing (S-VT) is also employed to optimize intake valve timing and ensure sufficient torque for cruising and accelerating. Furthermore, the engine is mated to a continuously variable transmission (CVT) for a perfect blend of responsive acceleration, smooth gear shifts and top-class fuel economy.

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  • isn’t the miller timing that the intake valve is closed earlier (before bottom dead center) so there is some expansion of the intake air first before the compression. this lowers the temperature before compression and therefor also after compression. this leads to lower combustion temperatures and therefor to better thermal efficiency and lower NOx emissions.

  • danny,

    Your statement is true of Miller cycle engines as implemented with turbo diesels in the marine industry. I believe the low-end torque loss using this method would make it unsuitable for a street-driven, mass-produced automobile unless the shear displacement of the engine could compensate as with the diesels. Might be fun to try though.

    • that explains a lot. i geuss the biggest problem with a auto-mobile would be to get sufficient fresh air into the cilinder. for marine and industrial diesel they raise the scavenge air presure for this, so you would need a other turbo charger..

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