Variable compression ratio engines

The thermodynamic efficiency of an internal combustion engine is an increasing function of the volumetric compression ratio, whether for compression-ignition "diesel engines" or spark-ignition "gasoline engines".

Unfortunately, it is not possible to increase this indefinitely due to other constraints:

• excessively high end-of-combustion pressures incompatible with permissible engine loads ;

• risk of a drop in overall motor efficiency, particularly through a fall in mechanical efficiency;

• risk of auto-ignition of the mixture in the case of gasoline engines, which should generally be avoided.

This thermodynamic efficiency is defined in relation to the engine's theoretical thermodynamic cycle. In low-load areas, engine operation deviates from this theoretical cycle. End-of-compression pressure is no longer directly linked to the volumetric compression ratio, and is highly dependent on load and engine speed. The benefits of a Variable Compression Ratio (VCR) device will be predominant in these areas. Increasing the volumetric compression ratio will restore the end-of-compression pressure to the value required for optimized engine operation; this value may be guided by consumption or pollution imperatives.

In the case of automobiles, a very low-load situation is encountered in urban traffic. It is particularly detrimental to the spark-ignition engine, since load regulation is achieved mainly by limiting the quantity of air admitted.

The potential of a variable compression ratio for diesel engines is not as great, but the application of this technology to this type of engine is not without interest.

The advantages and benefits of a variable compression ratio have been known for a very long time. As early as 1932, the first patent was filed, and in recent years there has been an explosion of variable compression ratio engine solutions. Today, no vehicle is equipped as standard with a variable compression ratio engine, due to the complexity of its construction and the additional costs involved. However, in view of future constraints in terms of reducing CO ₂ emissions from motor vehicles, EUCAR (European Council for Automotive R&D) is considering the application of this technology in the years 2015.

After a study detailing the benefits of variable compression and the means to achieve it, the various constructive solutions proposed will be presented.