# Compression ratio

The compression ratio (especially in connection with internal combustion engines ) is the ratio of the entire cylinder space before compression ( total volume ) to the space remaining after compression (residual volume): ${\ displaystyle \ varepsilon}$

${\ displaystyle \ varepsilon = {\ frac {V_ {h} + V_ {k}} {V_ {k}}} = 1 + {\ frac {V_ {h}} {V_ {k}}}> 1}$

With

• ${\ displaystyle V_ {h}}$= Displacement (displacement = piston stroke x piston cross-sectional area)
• ${\ displaystyle V_ {k}}$ = Compression space (remaining volume)
• ${\ displaystyle V_ {h} + V_ {k} = V_ {h}}$ Combustion chamber (total volume).

The compression ratio must not be confused with the pressure ratio ( ambient pressure to compression pressure ). Since the temperature rises with the compression , a compression ratio of z. B. 10: 1 that the pressure of the air introduced increases to more than ten times.

For engines with the Atkinson or Miller cycle , the (maximum) cylinder space cannot be used as a basis for calculation.

## history

The compression ratio was continuously increased in the course of the development of internal combustion engines. At the beginning of the Mercedes-Knight (1910–1916) it was only 4: 1. In 1912 it was increased to 4.6: 1 and in 1916 to 5: 1. In 1928 the 27/170/225 PS model was 6.2: 1. The SSKL type (1929–1932) achieved 7: 1.

For the M116 / M117 engines, which also come from Mercedes-Benz, it was 9: 1 until autumn 1987. Then an electronic knock control was introduced and the result was 10: 1. With current engines such as the 4.2-l V8-FSI from Audi, the compression is 12.5: 1.

For a long time, the gradual increase in compression was primarily due to the fuel formulation. Octane numbers up to 102 RON are available at petrol stations today . Older engines tend to have lower fuel quality requirements because the fuel quality was not as high as when they were developed. The shape of the combustion chamber and the intake system also contribute to the octane rating. Harry Ricardo researched the combustion process in engines in the 1920s and developed the Ricardo head, in which the piston almost touches part of the cylinder head at top dead center in order to reduce the surface area of ​​the combustion chamber.

Gasoline engines with an unusually high compression for their production period are the "medium pressure engine" of the Audi F 103 from 1965 with a compression ratio of 11.2: 1 and the 1.3-l engine with 55 kW of the VW Polo from 1982 with a Compression ratio of 11: 1, which requires 98 RON gasoline. What these engines have in common are compact combustion chambers with pinch edges, in the Audi in the piston (Heron combustion chamber) and intake ducts, which give the flow a swirl at the expense of a lower filling .

Turbocharged engines are usually less compressed. An example is the R-2600 multiple radial engine from Curtiss-Wright , which is only 6.9: 1 compressed and requires 100 octane jet fuel . It is charged with a radial compressor . The turbo-charged engines of the BMW 2002 turbo and the Porsche 930 also have low compression ratios of 6.9: 1 and 6.5: 1.

Another factor for today's high compression ratios is direct injection and the use of knock sensors. The point in time and the amount of fuel added is precisely controlled, which means that premature ignition - and thus knocking - can always be prevented.

## technology

The compression ratio of an uncharged Otto engine in motor vehicles is now 10: 1 to 14: 1. Aircraft engines optimized for reliability and low load that are operated with AVGAS often run at only 7: 1. Sports engines with high specific power use compressions of up to 15: 1, in series motorcycles compressions of up to 13: 1 are used. The maximum compression pressure is 25 to 30 bar, depending on the fuel.

For diesel engines without supercharging, the compression is around 19: 1 to 23: 1. Large turbocharged diesel engines have a lower compression, around 14: 1 to 16: 1. Due to the fuel, the maximum compression pressure is 50 to 60 bar.

With a higher compression ratio (more precisely: expansion ratio) the thermal efficiency is also higher, but the increase is lower with higher compression and with very high compression it is compensated by the then poorer mechanical efficiency. In the ideal Otto cycle , the efficiency with compression and the adiabatic exponent (can be assumed to be 1.4 for air when compressed and 1.3 for fuel gas when expanding) is:${\ displaystyle \ eta}$${\ displaystyle \ varepsilon}$${\ displaystyle \ varkappa}$
${\ displaystyle \ eta _ {\, th _ {\, \ mathrm {Otto}}} = 1 - {\ frac {1} {\ varepsilon ^ {\ varkappa -1}}} = 1 - {\ frac {1} {10 ^ {\, 0.3}}}}$

In the case of gasoline engines, however, the tendency to knock (premature combustion) also increases. During operation, knocking can be prevented by gasoline with a higher octane number and by setting the ignition late. Modern engines have knock sensors that detect the vibrations when knocking occurs and signal it to the control computer, which then shifts the ignition point slightly towards retarded.

In terms of design, the tendency to knock can be influenced by the shape of the combustion chamber . Double ignition and direct injection, which lower the compression end temperature of the fresh gas and, through subsequent injection and combustion, lower the maximum temperature, also have knock-reducing effects.

## Increase in compression

The thickness of the cylinder head gasket can influence the compression ratio to a small extent. Furthermore, during the plane grinding of the cylinder head that is necessary as part of an overhaul, more material is sometimes removed in order to slightly increase the compression and thus the performance and the efficiency.

In the case of increased compression, the ignition point may need to be corrected, as the mixture in the smaller compression chamber can only be ignited later to avoid knocking .