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.
Miller Cycle for Sequential Valve Timing (S-VT)
The answer is the Miller-cycle engine. By delaying the closure of the intake valves, compression actually begins partway 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 while increasing the expansion ratio. In other words, the Miller-cycle engine has a higher expansion ratio than the compression ratio.
Mazda’s naturally-aspirated MZR 1.3L Miller-cycle engine delays the closure of the intake valves to improve 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.