The static 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
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.
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.
2 Typical compression ratios
Gasoline (petrol) engine
2.2 Petrol/gasoline engine with pressure-charging
2.3 Petrol/gasoline engine for racing
2.4 Ethanol and methanol engines
2.5 Gas-fueled engine
2.6 Diesel engine
3 Fault finding and diagnosis
4 Variable Compression
Ratio (VCR) engines
5 Dynamic compression ratio
Compression ratio versus overall pressure ratio
7 See also
9 External links
The static compression ratio is calculated by the following formula
for 4-cycle OVERHEAD VALVE DESIGNS:
COMPRESSION RATIO (CR) = SUM OF THE SWEPT VOLUME BY THE PISTON IN THE
CYLINDER/HEAD COMBUSTION CHAMBER PLUS THE CLEARANCE VOLUME DIVIDED BY
THE CLEARANCE VOLUME
displaystyle mbox CR = frac tfrac pi 4 b^ 2 s+V_ c V_ c
= cylinder bore (diameter)
= piston stroke length
displaystyle V_ c ;
= clearance volume. This is the minimum volume of the space in the
cylinder head at the end of the compression stroke, i.e. when the
piston reaches its maximum upward position or top dead center (TDC).
Notes: The piston/cylinder head design illustrated above(A
four-stroke engine...) is typical of a modern interference engine.
Because of the complex shape of
displaystyle V_ c ;
this is usually measured directly* rather than calculated.
*by filling with liquid for example
Typical compression ratios
Gasoline (petrol) engine
The compression ratio in a gasoline (petrol)-powered engine will
usually not be much higher than 10:1 due to potential engine knocking
(detonation) 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
A technique used to prevent the onset of knock is the high "swirl"
engine that forces the intake charge to adopt a 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 (MON + RON)/2
(octane rating) 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
(similar to a diesel engine), is another recent development which also
allows for higher compression ratios on gasoline engines.
The compression ratio can be as high as 14:1 (2014 Ferrari 458
Speciale) 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.
Mazda released new petrol engines under the brand name
SkyActiv with a 14:1 compression ratio (U.S. models have a 13:1
compression ratio to allow for 87 AKI octane), 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 air before it
enters the cylinders. Port fuel injected engines typically run lower
boost than direct fuel injected engines because port fuel injection
allows the air/fuel mixture to be heated together which leads to
detonation. Conversely, directly injected engines can run higher boost
because heated air will not detonate without a fuel being present. In
this instance fuel is injected as late as 60 degrees before top dead
center to avoid heating the mixture to the point of compression
Petrol/gasoline engine for racing
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.
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.
The CR may be higher in engines running exclusively on LPG or CNG, due
to the higher octane rating of these fuels.
There is no spark plug in an auto-ignition diesel engine; the heat of
compression raises the temperature of the air in the cylinder
sufficiently to ignite the diesel when this is injected into the
cylinder; after the compression stroke. 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 23:1 for direct injection engines, and
between 18:1 and 23:1 for indirect injection.And also in crdi
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
displaystyle p=p_ 0 times text CR ^ gamma
displaystyle p_ 0 ;
is the cylinder pressure at bottom dead center which is usually at 1
displaystyle text CR
is the compression ratio, and
displaystyle gamma ;
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
displaystyle p_ text TDC =1 text bar times 10^ 1.4 =25.1 text
This figure, however, will also depend on cam (i.e. valve) timing.
Generally, cylinder pressure for common automotive designs should at
least equal 10 bar, or, roughly estimated in pounds per square inch
(psi) 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 (relatively speaking for
camshaft profiles outside of typical production-car range, but not
necessarily into the realm of competition engines) can produce a
misleadingly low figure from this test. Excessive connecting rod
clearance, combined with extremely high oil pump output (rare but not
impossible) 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 (or more rarely head gasket, or
breakthrough piston or, rarer still, cylinder-wall damage) issue.
If there is a significant (greater than 10%) 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
If a problem is suspected, then a more comprehensive test using a
leak-down tester can locate the leak.
Ratio (VCR) 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
(SVC). This engine, designed by Saab Automobile, uses a technique that
dynamically alters the volume of the combustion chamber (Vc), which,
via the above equation, changes the compression ratio (CR).
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.
On August 15, 2016 Nissan Motor Company announced a new variable
compression engine that can choose an optimal compression ratio
variably between 8:1 and 14:1. That lets the engine adjust moment by
moment to torque demands, always maintaining top efficiency. Nissan
says that the turbo-charged, 2-liter, four-cylinder VC-T engine
averages 27 percent better fuel economy than the 3.5-liter V6 engine
it replaces, with comparable power and torque.
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 (sealing the cylinder) 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 (at a higher than atmospheric pressure) to be
trapped in the cylinder than the static volume would suggest ( This
"corrected" compression ratio is commonly called the "dynamic
This ratio is higher with more conservative (i.e., earlier, soon after
BDC) intake cam timing, and lower with more radical (i.e., later, long
after BDC) 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 (measured
between centers). 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 (adiabatic) 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 (7.5)^1.3 × atmospheric pressure, or 13.7 bar. (×
14.7 psi at sea level = 201.8 psi. The pressure shown on a gauge would
be the absolute pressure less atmospheric pressure, or 187.1 psi.)
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
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 (in the same respect that rapidly opening a
door will create a draft that continues after movement of the door
ceases). This is called scavenging.
Intake tuning, cylinder head
design, valve timing and exhaust tuning determine how effectively an
Compression ratio versus overall pressure ratio
Compression ratio versus pressure ratio
Compression ratio and overall pressure ratio are interrelated as
The reason for this difference is that compression ratio is defined
via the volume reduction:
displaystyle text CR = frac V_ 1 V_ 2
while pressure ratio is defined as the pressure increase:
displaystyle text PR = frac P_ 2 P_ 1
In calculating the pressure ratio, we assume that an 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). 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:
displaystyle P_ 1 V_ 1 ^ gamma =P_ 2 V_ 2 ^ gamma Rightarrow
frac P_ 2 P_ 1 =left( frac V_ 1 V_ 2 right)^ gamma
is the ratio of specific heats (air: approximately 1.4). 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.
Mean effective pressure
Overall pressure ratio
Overall pressure ratio - a closely related ratio for jet engines
^ Encyclopædia Britannica, Compression ratio, retrieved
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Mazda 3 gets 40-mpg
SkyActiv engine option; diesel
expected in 2014". Autoweek. Retrieved 2012-05-29. CS1 maint:
Multiple names: authors list (link)
^  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.
^ Wan, Mark. "AutoZine Technical School". www.autozine.org. Retrieved
"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 changes with engine modifications
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