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. For example, a cylinder and its combustion
chamber with the piston at the bottom of its stroke may contain 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. 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 autoignition
will cause late ignition, which will also lead to engine knock.
Formula The ratio is calculated by the following formula:
, where = cylinder bore
= piston stroke length = clearance volume. It is the volume of the
combustion chamber. This is the minimum volume of the space at the end of the compression
stroke, i.e. when the piston reaches top dead center. Because of the complex shape of this
space, it is usually measured directly rather than calculated.
Typical compression ratios Gasoline engine
The compression ratio in a gasoline or petrol-powered engine will usually not be much higher than
10:1 due to potential engine knocking and not lower than 6:1. Some production automotive
engines built for high performance from 1955–1972, used high-octane leaded gasoline or '5 star'
to allow compression ratios as high as 13.0:1. 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.
It is possible to manufacture gasoline engines with compression ratios of over 11:1 that
can use 87/2 fuel with the addition of variable valve timing and knock sensors to delay ignition
timing. Such engines may not produce their full rated power using 87 octane gasoline
under all circumstances, due to the delayed ignition timing. Direct fuel injection, which
can inject fuel only at the time of fuel ignition, is another recent development which also allows
for higher compression ratios on gasoline engines.
The CR can be as high as 13.5:1 in engines with a 'ping' or 'knock' sensor and an electronic
control unit. In 1981, Jaguar released a cylinder head that allowed up to 14:1 compression;
but settled for 12.5:1 in production cars. The cylinder head design was known as the
"May Fireball" head; it was developed by a Swiss engineer Michael May.
In 2012, Mazda released new petrol engines under the brand name SkyActiv with a 14:1
compression ratio, to be used in all Mazda vehicles by 2015.
Petrol/gasoline engine with pressure-charging In a turbocharged or supercharged gasoline
engine, the CR is customarily built at 10.5:1 or lower. This is due to the turbocharger/supercharger
already having compressed the fuel/air mixture considerably before it enters the cylinders.
Petrol/gasoline engine for racing Motorcycle racing engines can use compression
ratios as high as 14:1, 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 18000 RPM.
Ethanol and methanol engines Ethanol and methanol can take significantly
higher compression ratios than gasoline. Racing engines burning methanol and ethanol fuel
often incorporate a CR of 14.5-16:1. Gas-fueled engine
The CR may be higher in engines running exclusively on LPG or CNG, due to the higher octane rating
of these fuels. Diesel engine
There is no electrical sparking plug in an auto-ignition diesel engine; the heat of compression
raises the temperature of the mixture to its auto-ignition point. The CR will customarily
exceed 14:1 and 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 direct
injection engines, and between 18:1 and 23:1 for indirect injection
Kerosene engine A compression ratio of 6.5 or lower is desired
for operation on Kerosene. 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. Fault finding and diagnosis
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:
where is the cylinder pressure at bottom dead center which is usually at 1 atm, is the compression
ratio, and 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 of 10:1, the cylinder pressure at top dead center is
This figure, however, will also depend on cam timing. Generally, cylinder pressure for
common automotive designs should at least equal 10 bar, or, roughly estimated in pounds
per square inch as between 15 and 20 times the compression ratio, or in this case between
150 psi and 200 psi, depending on cam timing. Purpose-built racing engines, stationary engines
etc. will return figures outside this range. Factors including late intake valve closure
can produce a misleadingly low figure from this test. Excessive connecting rod clearance,
combined with extremely high oil pump output can sling enough oil to coat the cylinder
walls with sufficient oil to facilitate reasonable piston ring sealing. In engines with compromised
ring seals, this can artificially give a misleadingly high compression figure.
This phenomenon can actually be used to some slight advantage. If a compression test does
give a low figure, and it has been determined it is not due to intake valve closure/camshaft
characteristics, then one can differentiate between the cause being valve/seat seal issues
and ring seal by squirting engine oil into the spark plug orifice, in a quantity sufficient
to disperse across the piston crown and the circumference of the top ring land, and thereby
affect the mentioned seal. If a second compression test is performed shortly thereafter, and
the new reading is much higher, it would be the ring seal that is problematic, whereas
if the compression test pressure observed remains low, it is a valve sealing issue.
If there is a significant difference between cylinders, that may be an indication that
valves or cylinder head gaskets are leaking, piston rings are worn, or that the block is
cracked. If a problem is suspected, then a more comprehensive
test using a leak-down tester can locate the leak.
Variable Compression Ratio engines Because cylinder-bore diameter, piston-stroke
length and combustion-chamber volume are almost always constant, the compression ratio for
a given engine is almost always constant, until engine wear takes its toll.
One exception is the experimental Saab Variable Compression engine. This engine, designed
by Saab Automobile, uses a technique that dynamically alters the volume of the combustion
chamber, which, via the above equation, changes the compression ratio.
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.
Dynamic compression ratio The calculated compression ratio, as given
above, presumes that the cylinder is sealed at the bottom of the stroke, and that the
volume compressed is the actual volume. However: intake valve closure always takes
place after BDC, which may cause some of the intake charge to be compressed backwards out
of the cylinder by the rising piston at very low speeds; only the percentage of the stroke
after intake valve closure is compressed. Intake port tuning and scavenging may allow
a greater mass of charge to be trapped in the cylinder than the static volume would
suggest ( This "corrected" compression ratio is commonly called the "dynamic compression
ratio". This ratio is higher with more conservative
intake cam timing, and lower with more radical intake cam timing, but always lower than the
static or "nominal" compression ratio. The actual position of the piston can be determined
by trigonometry, using the stroke length and the connecting rod length. The absolute cylinder
pressure is the result of an exponent of the dynamic compression ratio. This exponent is
a polytropic value for the ratio of variable heats for air and similar gases at the temperatures
present. This compensates for the temperature rise caused by compression, as well as heat
lost to the cylinder. Under ideal conditions, the exponent 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, but provides useful results for purposes of
comparison. 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^1.3 × atmospheric pressure,
or 13.7 bar. 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
DCR similar to an engine with lower compression but earlier intake valve closure.
Additionally, the cylinder pressure developed when an engine is running will be higher than
that shown in a compression test for several reasons.
The much higher velocity of a piston when an engine is running versus cranking allows
less time for pressure to bleed past the piston rings into the crankcase.
a running engine is coating the cylinder walls with much more oil than an engine that is
being cranked at low RPM, which helps the seal.
the higher temperature of the cylinder will create higher pressures when running vs. a
static test, even a test performed with the engine near operating temperature.
A running engine does not stop taking air & fuel into the cylinder when the piston reaches
BDC; The mixture that is rushing into the cylinder during the downstroke develops momentum
and continues briefly after the vacuum ceases. This is called scavenging. Intake tuning,
cylinder head design, valve timing and exhaust tuning determine how effectively an engine
scavenges. Compression ratio versus overall pressure
ratio Compression ratio and overall pressure ratio
are interrelated as follows: The reason for this difference is that compression
ratio is defined via the volume reduction: ,
while pressure ratio is defined as the pressure increase:
. In calculating the pressure ratio, we assume
that an adiabatic compression is carried out. We also assume that air is a perfect gas.
With these two assumptions, we can define the relationship between change of volume
and change of pressure as follows:
where is the ratio of specific heats for air. The values in the table above are derived
using this formula. Note that in reality the ratio of specific heats changes with temperature
and that significant deviations from adiabatic behavior will occur.
See also Mean effective pressure
Overall pressure ratio - a closely related ratio for jet engines
Notes
External links "Here Comes High Compression Engines " 1949
highly detailed article in Popular Science with photos and cutaway drawings
Variable compression engine Cam Timing vs. Compression Ratio Analysis
Calculating Compression Ratio changes with engine modifications