As a result, the lowering in power recovering efficiency in a condition where an actual expansion ratio is smaller than a designed expansion ratio is suppressed.

Thereby, a lowering of power recovery efficiency is suppressed even under conditions where the actual expansion ratio is smaller than the designed expansion ratio.

This means the compression ratio is smaller than the expansion ratio.

In an engine low-load operation, the mechanical compression ratio is so maximized that the expansion ratio is 20 or more.

When the catalyst temperature is predicted to drop to an active temperature or lower, the real expansion ratio is lowered while keeping the real compression ratio identical or while increasing the real compression ratio.

In an Air Expander and Firing (AEF) mode of the engine, the engine has a residual expansion ratio at XovrE valve closing of 15.7 to 1 or greater, and more preferably in the range of 15.7 to 1 and 40.8 to 1.

In addition, the relaxed rate of 80 percent of accelerated depreciation was reduced by half.

To maximise the benefits of the Miller cycle, the effective expansion ratio, the ratio of the expansion ratio to the compression ratio and the compression ratio itself must all be maximized together.

The effective compression ratio is reduced (for a time the air is escaping the cylinder freely rather than being compressed), but the expansion ratio is unchanged.

The combustion rate is controlled by exhaust gas recirculation or spray of water into a cylinder or an intake passage, and high thermal efficiency is ensured by a high expansion ratio.

In another form of embodiment, the regulation is carried out according to an expansion ratio in the turbine corresponding to a ratio between the pressures P3m and P4m.

At an engine low-load running time, the mechanical compression ratio is made so higher than that at an engine high-load running time that the expansion ratio may be 20 or higher.

A control device applied to a system having an engine adapted so that the expansion ratio of the engine can be changed.

In the internal combustion engine, an expansion ratio is set higher during a low-load engine operation, as compared with that during a high-load engine operation.