Compression Ratio

The compression ratio of an engine  is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity.

In a piston engine, it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.

For example, a cylinder and its combustion chamber with the piston at the bottom of its stroke may contain 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio.

A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. This occurs because internal combustion engines are heat engines, and higher efficiency is created because higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature. It may be more helpful to think of it as an "expansion ratio", since more expansion reduces the temperature of the exhaust gases, and therefore the energy wasted to the atmosphere. Diesel engines actually have a higher peak combustion temperature than petrol engines, but the greater expansion means they reject less heat in their cooler exhaust.

Higher compression ratios will however make gasoline engines subject to engine knocking if lower octane-rated fuel is used, also known as detonation. This can reduce efficiency or damage the engine if knock sensors are not present to retard the timing. However, knock sensors have been a requirement of the OBD-II specification used in 1996 model year vehicles and newer.

On the other hand, Diesel engines operate on the principle of compression ignition, so that a fuel which resists auto-ignition will cause late ignition, which will also lead to engine knock.

The ratio is calculated by the following formula;

where

b = cylinder bore (diameter)
s = piston stroke length
V_c = clearance volume. It is the volume of the combustion chamber (including head gasket). This is the minimum volume of the space at the end of the compression stroke, i.e. when the piston reaches top dead center (TDC). Because of the complex shape of this space, it is usually measured directly rather than calculated.

Relationship between compression ratio and fuel efficiency

Automotive engineers can improve fuel efficiency and fuel economy by designing engines with high compression ratios.

The higher the ratio, the more compressed the air in the cylinder is. When the air is compressed, you get a more powerful explosion from the air-fuel mixture, and more of the fuel gets used. Think about it this way: If you had to be near an explosion, you'd probably choose to be near one somewhere outside, because the force of the explosion would dissipate, and it wouldn't seem as powerful. In a small room, however, the force would be contained, making it feel much more powerful. It's the same thing with compression ratios. By keeping the explosion in a smaller space, more of its power can be harnessed.

By increasing the compression ration from 8:1 to 9:1, for example, you can improve fuel economy by about 5 to 6 percent.

Then, why not just keep on increasing the compression ratio?

One of the limiting factors in compression ratio is called detonation (this manifests as engine knocking or pinging) where instead of burning in a controlled fashion, the air/fuel mixture explodes, potentially damaging the engine. Also, a higher compression engine tends to have less clearance between the piston at top dead center and the valves fully opened, and running at a high rpm can lead to valve float which can lead to contact between the valves and piston which is bad news.

The trouble with petrol is that if it is compressed too much it starts igniting of its own accord, rather than when you want it to. This is known as detonation or 'knocking', and it's this that prevents engines running very high compression ratios.

Compressing a gas raises its temperature, so as the piston rises and the fuel/air mix is compressed, it heats up. In an ideal world the compressed fuel/air mix ignites from a central point in the combustion chamber (where the spark plug is), expanding out evenly in all directions and pushing the piston down (because that's the only bit that can move at this point). Raise the compression ratio too far and a spontaneous auto-ignition occurs towards the outer edges of the combustion chamber, instigated by the sudden rise in cylinder pressure when the spark plug ignites, instead of burning outwards from a central point.

This uneven, spontaneous burn produces amplified pressure waves in the combustion chamber. A metallic knocking sound can even be heard from the engine as these high pressure waves hit the piston crown and the sides of the combustion chamber, hence the term 'knocking'. While these pressure waves are very short-lived, they are higher than normal combustion pressure - and temperature - and can cause all sorts of problems, from blown head gaskets to damaged pistons and overstressed rods and bearings. Overheating can also be a problem.

A fuel's octane rating is a measure of its resistance to detonation - the higher the rating the more it can be compressed without detonation occurring. Normal unleaded pump fuel with a 95-Ron octane rating is fine for most standard engines' compression ratios, but even mildly-tuned engines might need 97/98-Ron super unleaded fuel to run safely.

Sources: How Stuff Works, Wikipedia, Tim Dickson

TDC = Top Dead Centre

BDC = Bottom Dead Centre