The compression ratio 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".
Cylinder with the piston at the bottom of its stroke containing 1000 cc of air. 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. High ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression - causing better mixing and evaporation of the fuel droplets. Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree.
Higher compression ratios will however make gasoline engines subject to engine knocking, also known as detonation and this can reduce an engine's efficiency or even physically damage it.
Diesel engines on the other hand 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.
Typical compression ratios
Petrol/gasoline engine
Due to detonation, the CR in a gasoline/petrol powered engine will usually not be much higher than 10:1, although some production automotive engines built for high-performance from 1955-1972 had compression ratios as high as 12.5:1, which could run safely on the high-octane leaded gasoline then available.
A technique used to prevent the onset of knock is the high "swirl" engine that forces the intake charge to adopt a very fast circular rotation in the cylinder during compression that provides quicker and more complete combustion. Recently, with the addition of variable valve timing and knock sensors to delay ignition timing, it is possible to manufacture gasoline engines with compression ratios of over 11:1 that can use 87 MON (octane rating) fuel.
Petrol/gasoline engine for racing
Motorcycle racing engines can use compression ratios as high as 14:1, and it is not uncommon to find motorcycles with compression ratios above 12.0:1 designed for 86 or 87 octane fuel.
Racing engines burning methanol and ethanol often exceed a CR of 15:1. Consumers may note that "gasohol", or 90% gasoline with 10% ethanol gives a higher octane rating (knock suppression).
Gas-fuelled engine
In engines running exclusively on LPG or CNG, the CR may be higher, due to the higher octane rating of these fuels.
Diesel engine
In an auto-ignition diesel engine, (no electrical sparking plug--the hot air of compression lights the injected fuel) the CR will customarily exceed 14:1. Ratios over 22:1 are common. The appropriate compression ratio depends on the design of the cylinder head. The figure is usually between 14:1 and 16:1 for indirect injection engines and between 18:1 and 20:1 for direct injection engines.
Measuring the compression pressure of an engine, with a pressure gauge connected to the spark plug opening, gives an indication of the engine's state and quality. There is, however, no formula to calculate compression ratio based on cylinder pressure.
If the nominal compression ratio of an engine is given, the pre-ignition cylinder pressure can be estimated using the following relationship:
p = po X Cr h
Where po is the cylinder pressure at bottom dead center (BDC) which is usually at 1 atm, Cr is the compression ratio, and h is the specific heat ratio for the working fluid, which is about 1.4 for air, and 1.3 for methane-air mixture.
For example, if an engine running on gasoline has a compression ratio is 10:1, the cylinder pressure at top dead center (TDC) is
pTDC = (1bar) X 10 1.4 = 25.1 bar
Variable Compression Ratio (VCR) Engines
the first VCR engine being built and tested by Harry Ricardo in the 1920s. This work led to him devising the octane rating system that is still in use today. SAAB has recently been involved in working with the 'Office of Advanced Automotive Technologies', to produce a modern petrol VCR engine that showed an efficiency comparable with that of a Diesel. Many companies have been carrying out their own research in to VCR Engines, including Nissan, Volvo, PSA/Peugeot-Citroën and Renault but so far with no publicly demonstrated results.
The Atkinson cycle engine was one of the first attempts at variable compression. Since the compression ratio is the ratio between dynamic and static volumes of the combustion chamber the Atkinson cycle's method of increasing the length of the power stroke compared to the intake stroke ultimately altered the compression ratio at different stages of the cycle.
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".
Cylinder with the piston at the bottom of its stroke containing 1000 cc of air. 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. High ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression - causing better mixing and evaporation of the fuel droplets. Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree.
Higher compression ratios will however make gasoline engines subject to engine knocking, also known as detonation and this can reduce an engine's efficiency or even physically damage it.
Diesel engines on the other hand 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.
Typical compression ratios
Petrol/gasoline engine
Due to detonation, the CR in a gasoline/petrol powered engine will usually not be much higher than 10:1, although some production automotive engines built for high-performance from 1955-1972 had compression ratios as high as 12.5:1, which could run safely on the high-octane leaded gasoline then available.
A technique used to prevent the onset of knock is the high "swirl" engine that forces the intake charge to adopt a very fast circular rotation in the cylinder during compression that provides quicker and more complete combustion. Recently, with the addition of variable valve timing and knock sensors to delay ignition timing, it is possible to manufacture gasoline engines with compression ratios of over 11:1 that can use 87 MON (octane rating) fuel.
Petrol/gasoline engine for racing
Motorcycle racing engines can use compression ratios as high as 14:1, and it is not uncommon to find motorcycles with compression ratios above 12.0:1 designed for 86 or 87 octane fuel.
Racing engines burning methanol and ethanol often exceed a CR of 15:1. Consumers may note that "gasohol", or 90% gasoline with 10% ethanol gives a higher octane rating (knock suppression).
Gas-fuelled engine
In engines running exclusively on LPG or CNG, the CR may be higher, due to the higher octane rating of these fuels.
Diesel engine
In an auto-ignition diesel engine, (no electrical sparking plug--the hot air of compression lights the injected fuel) the CR will customarily exceed 14:1. Ratios over 22:1 are common. The appropriate compression ratio depends on the design of the cylinder head. The figure is usually between 14:1 and 16:1 for indirect injection engines and between 18:1 and 20:1 for direct injection engines.
Measuring the compression pressure of an engine, with a pressure gauge connected to the spark plug opening, gives an indication of the engine's state and quality. There is, however, no formula to calculate compression ratio based on cylinder pressure.
If the nominal compression ratio of an engine is given, the pre-ignition cylinder pressure can be estimated using the following relationship:
p = po X Cr h
Where po is the cylinder pressure at bottom dead center (BDC) which is usually at 1 atm, Cr is the compression ratio, and h is the specific heat ratio for the working fluid, which is about 1.4 for air, and 1.3 for methane-air mixture.
For example, if an engine running on gasoline has a compression ratio is 10:1, the cylinder pressure at top dead center (TDC) is
pTDC = (1bar) X 10 1.4 = 25.1 bar
Variable Compression Ratio (VCR) Engines
the first VCR engine being built and tested by Harry Ricardo in the 1920s. This work led to him devising the octane rating system that is still in use today. SAAB has recently been involved in working with the 'Office of Advanced Automotive Technologies', to produce a modern petrol VCR engine that showed an efficiency comparable with that of a Diesel. Many companies have been carrying out their own research in to VCR Engines, including Nissan, Volvo, PSA/Peugeot-Citroën and Renault but so far with no publicly demonstrated results.
The Atkinson cycle engine was one of the first attempts at variable compression. Since the compression ratio is the ratio between dynamic and static volumes of the combustion chamber the Atkinson cycle's method of increasing the length of the power stroke compared to the intake stroke ultimately altered the compression ratio at different stages of the cycle.
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