Compression Ratio: What effect does it have on the engine?

Engine pistons
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The compression ratio is one of the basic characteristics of a piston combustion engine, which indicates the cylinder volume ratio between the piston in the bottom and the piston in the top dead center.

The compression ratio is thus the ratio of the entire working volume of the cylinder to the volume of the compression space. In other words, the ratio between the volume of the mixture sucked into the cylinder and the volume of the mixture compressed in the cylinder.


Compression Ratio Calculation

The compression ratio calculation differs for a reciprocating piston engine and a rotary piston engine.

Calculation of the compression ratio for an engine with rectilinear reciprocating piston movement:

εk = (VK + VZ) : VK

Explanation of variables:

  • εk – compression ratio

  • VK – compression space/volume

  • VZ – stroke volume

The piston in the cylinder performs the reciprocating movement. While the farthest position of its movement from the crankshaft is called the top dead center, and the least far position of its movement from the crankshaft is called the bottom dead center.

The space between the bottom and top dead center is called the stroke volume. The stroke volume depends on the cylinder's bore diameter and the piston's stroke. The space in the cylinder when the piston is at the top dead center is called the compression space.

Calculation of the compression ratio for a rotary piston engine:

In these engines, the compression ratio is defined as the ratio of the largest and smallest volume of the working space during the revolution of the piston.

εk = V1 : V2

Explanation of variables:

  • εk – compression ratio

  • V1 - the largest volume of the workspace

  • V2 - the smallest volume of the workspace

How Does the Compression Ratio Affect the Engine?


In a classic piston combustion engine, the compression ratio is constant and is always a compromise between different driving modes. However, some engines can smoothly change the compression ratio according to the load.

Such an engine can thus operate with a large compression ratio during low load and, conversely, with a low compression ratio during high load.

At high loads, it is advisable that the compression ratio be low and thus prevent detonations. At low loads, it should be higher for the best possible engine efficiency. The greater the compression ratio, the greater the compression of the mixture before ignition.

The compression ratio fundamentally affects:

  • The achievable efficiency of the internal combustion engine and thus also its power and torque

  • Engine emissions

  • Fuel consumption

Disadvantages of an increased compression ratio:

  • Premature self-ignition of the fuel (detonation combustion) can occur, especially within the gasoline engines

  • Engine parts wear more over time than with a lower compression ratio, so such an engine must be equipped with more durable parts, which are much more expensive (ceramic and titanium parts)

Compression Pressure Measurement

3D model of a working V8 engine. Pistons and other mechanical parts are in motion.

Measuring cylinder compression pressures is a method that provides accurate information about the engine's condition. Compression pressure is measured using a compression gauge. Before measuring the compression pressure, the engine is warmed to operating temperature to define the clearances between the piston and the cylinder.

The compression gauge is screwed into the cylinder head instead of the spark plug. Subsequently, the engine is revved using the starter while the throttle is fully open (accelerator pedal fully pressed). The compression pressure is shown on the needle of the compression meter, which records the highest pressure reached.

Compression pressure is the maximum achievable pressure at the end of the engine's compression stroke when the mixture is not yet burning. The size of the compression pressure depends on the compression ratio, the engine speed, the degree of filling of the cylinders, and the tightness of the combustion chamber. All these parameters, except for the tightness of the combustion chamber, are unchanged and are determined by the engine design.

If during the measurement, it is found that one of the cylinders does not reach the value specified by the manufacturer, this indicates a leak in the combustion chamber. It is also crucial that the compression pressure of all cylinders is the same.

What Causes Lower Compression Pressures?

  • worn or damaged piston ring

  • worn engine cylinder

  • damaged or cracked cylinder head

  • damaged gasket under the cylinder head

  • damaged valve

  • damaged valve spring

  • worn valve seat

If the combustion chambers are in order, the maximum difference in compression pressures on the individual cylinders is up to 1 bar (0.1 MPa). Compression pressures range from 1.0 to 1.2 MPa for petrol engines and 3.0 to 3.5 MPa for diesel engines.

What are the Normal Values for an Engine's Compression Ratio?

To prevent detonation (premature self-ignition of the fuel), the compression ratio is not greater than 10:1 for gasoline engines. However, engines with a detonation combustion sensor, an electronic control unit and other devices can achieve a compression ratio of up to 14:1.

In turbocharged gasoline engines, the compression ratio is around 8.5:1 because part of the compression of the working substance is done in the turbocharger. Diesel engines have a compression ratio of 20:1 or even higher because they work on the principle that the injected fuel ignites from the heat of compression.

This is why the compression ratio of diesel engines must be higher than gasoline engines. The load from the pressures in the engine cylinder limits the compression ratio of the diesel engine.

Video of Jason from Engineering Explained explaining compression ratio: