MAZDA:Miller-cycle engine | Environmental Technology

Two important characteristics of an engine are its thermal efficiency and fuel efficiency. In order to improve these, it is necessary to increase the expansion ratio. This means making the air-fuel mixture in the cylinders expand more when the fuel detonates.
In most engines, the expansion ratio is more-or-less equal to the compression ratio, so increasing the expansion ratio will also raise the compression ratio. The problem is that engines with a large compression ratio suffer from abnormal combustion (knocking). In other words, if you can find a way of increasing the expansion ratio while keeping the compression ratio small, it is possible to achieve high thermal efficiencies.
*In this article, the terms "compression ratio'' and "expansion ratio'' mean the same as the scientific terms, "effective compression ratio'' and "effective expansion ratio.''

The answer to this problem is the Miller-cycle engine
By delaying the closure of the intake valves, compression begins part way through the compression stroke. This reduces the actual compression ratio and makes it possible to have an expansion ratio larger than the compression ratio.

Increased expansion ratio and delayed closure of the intake valves
By changing the shape of the piston head, the expansion ratio can be increased to 11:1, while the compression ratio remains at 10:1, the same value as Mazda's standard MZR 1.3L (Otto cycle) engine. Then, by adjusting the profile of the intake side camshaft to delay closure of the intake valves (from 59° after bottom dead center to 80°), the thermal efficiency improves without risk of knocking, and pumping losses are also reduced. Fuel efficiency is also enhanced during cruising and acceleration.

MZR 1.3L Miller-cycle engine performance curve

Expanded use of sequential-valve timing (S-VT)
The S-VT system was improved to run at higher revolutions and over a greater range of phase angles. This enabled us to minimize the torque reduction that arose from to the lower compression ratio.
Using CVT to enhance engine performance
By linking the engine and CVT control units, the CVT shift pattern is used to maintain the best possible engine speed at all times. This produces an optimum balance of performance and fuel economy.