The static compression ratio, (symbol ), of an internal combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity. It is a fundamental specification for many common combustion engines.
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 (also known as detonation) if lower octane-rated fuel is used. This can reduce efficiency or damage the engine if knock sensors are not present to modify the ignition timing.
Static compression ratio ( ) is calculated by the formula
- = displacement volume. This is the volume inside the cylinder displaced by the piston from the beginning of the compression stroke to the end of the stroke.
- = clearance volume. This is the volume of the space in the cylinder left at the end of the compression stroke.
can be estimated by the cylinder volume formula
Because of the complex shape of it is usually measured directly. This is often done by filling the cylinder with liquid and then measuring the volume of the used liquid.
Typical compression ratiosEdit
Gasoline (petrol) enginesEdit
In production gasoline (petrol) engines from the past 20 years, compression ratios are typically between 8:1 and 12:1. Several production engines have used higher compression ratios, including:
- Cars built from 1955–1972 which were designed for high-octane leaded gasoline, which allowed compression ratios up to 13:1.
- Some Mazda SkyActiv engines released since 2012 have compression ratios up to 14.0:1. The SkyActiv engine achieves this compression ratio with ordinary unleaded gasoline (95 RON in the United Kingdom) through improved scavenging of exhaust gases (which ensures cylinder temperature is as low as possible before the intake stroke), in addition to direct injection.
- The 2014 Ferrari 458 Speciale also has a compression ratio of 14.0:1.
When forced induction (e.g. a turbocharger or supercharger) is used, the compression ratio is often lower than naturally aspirated engines. This is due to the turbocharger/supercharger already having compressed the air before it enters the cylinders. Engines using port fuel-injection typically run lower boost pressures and/or compression ratios than direct injected engines because port fuel injection causes the air/fuel mixture to be heated together, leading to detonation. Conversely, directly injected engines can run higher boost because heated air will not detonate without a fuel being present.
Diesel engines use higher compression ratios than petrol engines, because the lack of a spark plug means that the compression ratio must increase the temperature of the air in the cylinder sufficiently to ignite the diesel. Therefore compression ratios are often between 14:1 and 23:1 for direct injection diesel engines, and between 18:1 and 23:1 for indirect injection diesel engines.
Kerosene engines typically use a compression ratio of 6.5 or lower. The petrol-paraffin engine version of the Ferguson TE20 tractor had a compression ratio of 4.5:1 for operation on tractor vaporising oil with an octane rating between 55 and 70.
Motorsport engines often run on high octane petrol and can therefore use higher compression ratios. For example, motorcycle racing engines can use compression ratios as high as 14.7:1, and it is common to find motorcycles with compression ratios above 12.0:1 designed for 86 or 87 octane fuel. F1 engines come closer to 17:1, which is critical for maximizing volumetric/fuel efficiency at around 18,000 RPM.
Variable compression ratio enginesEdit
Variable compression ratio is a technology to adjust the compression ratio of an internal combustion engine while the engine is in operation. This is done to increase fuel efficiency while under varying loads. Variable compression engines allow the volume above the piston at top dead centre to be changed.
Higher loads require lower ratios to increase power, while lower loads need higher ratios to increase efficiency, i.e. to lower fuel consumption. For automotive use this needs to be done as the engine is running in response to the load and driving demands.
The 2019 Infiniti QX50 is the first commercially available car that uses a variable compression ratio engine.
Relationship with the pressure ratioEdit
Based on the assumptions that adiabatic compression is carried out (i.e. that no heat energy is supplied to the gas being compressed, and that any temperature rise is solely due to the compression) and that air is a perfect gas, the relationship between the compression ratio and overall pressure ratio is as follows:
This relationship is derived from the following equation:
- where is the ratio of specific heats (air: approximately 1.4)
However, in most real-life internal combustion engines, the ratio of specific heats changes with temperature and that significant deviations from adiabatic behavior will occur.
Dynamic compression ratioEdit
The "static compression ratio" discussed above— calculated solely based on the cylinder and combustion chamber volumes— do not take into account any gasses entering or exiting the cylinder during the compression phase. In most automotive engines, the intake valve closure (which seals the cylinder) takes place during the compression phase (i.e. after bottom dead centre), which can cause some of the gasses to be pushed back out through the intake valve. On the other hand, intake port tuning and scavenging can cause a greater amount of gas to be trapped in the cylinder than the static volume would suggest. The "dynamic compresion ratio" accounts for these factors.
The dynamic compression ratio is higher with more conservative intake camshaft timing (i.e. soon after BDC), and lower with more radical intake camshaft timing (i.e. later after BDC). Regardless, the dynamic compression ratio is always lower than the static compression ratio.
The absolute cylinder pressure is used to calculate the dynamic compression ratio, using the following formula:
- where is a polytropic value for the ratio of specific heats for the combustion gasses at the temperatures present (this compensates for the temperature rise caused by compression, as well as heat lost to the cylinder)
Under ideal (adiabatic) conditions, the ratio of specific heats would be 1.4, but a lower value, generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines based on design, size and materials used. For example, if the static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be (7.5)^1.3 × atmospheric pressure, or 13.7 bar (relative to atmospheric pressure).
The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength. An engine with high static compression ratio and late intake valve closure will have a dynamic compression ratio similar to an engine with lower compression but earlier intake valve closure.
- Lutjen, D; Müller, M (1984). Kfz-Rechnen. B.G. Teubner Stuttgart. p. 12. ISBN 9783519067214.
- Encyclopædia Britannica, Compression ratio, retrieved 2009-07-21
- "High Compression!". Popular Science. Bonnier Corporation. 154: 166–172. January 1949. ISSN 0161-7370. Retrieved 14 July 2019.
- "Calculated Compression Ratios". www.s-86.com. Archived from the original on 7 September 2009.
- "2012 Mazda 3 gets 40-mpg SkyActiv engine option; diesel expected in 2014". Autoweek. 2011-04-22. Archived from the original on 2012-02-29. Retrieved 2012-05-29.
-  Archived March 12, 2012, at the Wayback Machine
- VANDERWERP, DAVE (August 2010). "Mazda Engine News: Mazda Sky Gas and Diesel Details". Car and Driver. Retrieved 2012-05-29.
- "Tractor Vaporising Oil". Web.archive.org. 2005-04-18. Archived from the original on October 12, 2007. Retrieved 2014-08-10.
- "Variable Compression Engine". www.fs.isy.liu.se. Archived from the original on 11 March 2005.
- "Cam Timing vs. Compression Analysis". www.victorylibrary.com. Retrieved 14 July 2019.