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For compression ratio in data compression, see data compression ratio.
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.
Picture a 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 autoignition will cause late ignition which will also lead to engine knock.

Contents

1 Formula
2 Typical compression ratios 

2.1 Petrol/gasoline engine
2.2 Petrol/gasoline engine with pressure-charging
2.3 Petrol/gasoline engine for racing
2.4 Gas-fuelled engine
2.5 Diesel engine
3 Fault finding and diagnosis
4 Saab Variable Compression engine
5 Variable Compression Ratio (VCR) Engines
6 Dynamic Compression Ratio
7 Compression ratio versus overall pressure ratio
8 See also
9 External links 

//

 Formula
The ratio is calculated by the following formula:

, where
b = cylinder bore (diameter)
s = piston stroke length
Vc = volume of the combustion chamber (including head gasket). This is the minimum volume of the space into which the fuel and air is compressed, prior to ignition. Because of the complex shape of this space, it is usually measured directly rather than calculated. 

 Typical compression ratios

 Petrol/gasoline engine
Due to pinging (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.
In engines with a 'ping' or 'knock' sensor and an electronic control unit, the CR can be as high as 13:1 (2005 BMW K1200S). 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.

 Petrol/gasoline engine with pressure-charging
In a turbocharged or supercharged gasoline engine, the CR is customarily built at 9:1 or lower.

 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.

 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 p0 is the cylinder pressure at bottom dead center (BDC) which is usually at 1 atm, Cr is the compression ratio, and ? is the ratio of specific heats of 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

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 enough oil to facilitate reasonable piston ring seal artificially give a misleadingly high figure, on engines with compromised ring seal.
This 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 effect 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 (> 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 cracked.
If a problem is suspected then a more comprehensive test using a leak-down tester can locate the leak.

 Saab Variable Compression engine
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).
To alter Vc, the SVC 'lowers' the cylinder head closer to the crankshaft. It does this by replacing the typical one-part engine block with a two-part unit, with the crankshaft in the lower block and the cylinders in the upper portion. The two blocks are hinged together at one side (imagine a book, lying flat on a table, with the front cover held an inch or so above the title page). By pivoting the upper block around the hinge point, the Vc (imagine the air between the front cover of the book and the title page) can be modified. In practice, the SVC adjusts the upper block through a small range of motion, using a hydraulic actuator.

 Variable Compression Ratio (VCR) Engines
The SAAB SVC is an advanced and workable addition to the world of VCR engines, the first 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-Citro?n 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 powerstroke 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 (bottom dead centre - BDC), and that the volume compressed is the actual volume.
However: intake valve closure (sealing the cylinder) always takes place after BDC, which causes 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. This "corrected" compression ratio is commonly called the "dynamic compression ratio".
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 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 (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 and cylinder head designs determine how effectively an engine scavenges. 

 Compression ratio versus overall pressure ratio
Compression ratio and overall pressure ratio are interrelated as follows:

Compression ratio
1:1
3:1
5:1
10:1
15:1
20:1
25:1
35:1

Pressure ratio
1:1
2:1
10:1
22:1
40:1
56:1
75:1
110:1
The reason for this difference is that compression ratio is defined via the volume reduction,

,
Pressure ratio is defined as the pressure increase

.
From the combined gas law we get:

Since T2 is much higher than T1 (compressing gases puts work into them, i.e. heats them up), CR is much lower than PR.

 See also

Overall pressure ratio - a closely related ratio for jet engines 

 External links

SVC
Cam Timing vs. Compression Ratio Analysis 

Categories: Piston engines | Engine technology | Saab engines | Engineering ratios

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The Positive Crankcase Ventilation valve, or PCV valve, is a one-way valve that ensures continual evacuation of gases from inside a gasoline internal combustion engine's crankcase.

Contents

1 Explanation
2 History
3 PCV system
4 PCV valve
5 Operation 

//

 Explanation
As an engine runs, high-pressure gases are contained within the combustion chamber and prevented from passing into the crankcase (containing the crankshaft and other parts) between the side of the piston and the cylinder bore by piston rings which seal against the cylinder. However, some amount of gas always leaks past the piston rings into the crankcase. This amount is very small in a new or properly rebuilt engine, provided that the piston rings and cylinder walls are correctly "broken in", and increases as the engine wears. Scratches on the cylinder walls or piston rings, such as those caused by foreign objects entering the engine, can cause large amounts of leakage. This leaked gas is known as blow-by because the pressure within the cylinders blows it by the piston rings. If this blow-by gas could not escape then pressure would build up within the crankcase.
Before the invention of Crankcase Ventilation in 1928 the engine oil seals were designed to withstand this pressure, oil leaking to the ground was accepted and the dipstick was screwed in. The hydrocarbon rich gas would then diffuse through the oil in the seals into the atmosphere. It is therefore an emissions requirement as well as a functional necessity that the crankcase has a ventilation system. This must maintain the crankcase at slightly less than atmospheric pressure and recycle the blow-by gas back into the engine intake. However, due to the constant circulation of the oil within the engine, along with the high speed movement of the crankshaft, an oil mist is also passed through the PCV system and into the intake. The oil is then either burnt during combustion or settles along the intake tract, causing a gradual build-up of residue inside the inlet path. For this reason many engine tuners choose to replace the PCV system with an oil catch can and breather filter which vents the blow-by gases directly to atmosphere and retains the oil in a small tank (or returns it to the sump), although this technically fails to meet most engine emission legislation.

 History
Prior to the early 1960s, automobile gasoline engines vented combustion gases directly to the atmosphere through a simple vent tube. Frequently this consisted of a pipe (the "road draft tube") that extended out from the crankcase down to the bottom of the engine compartment. The bottom of the pipe was open to the atmosphere, and was placed such that when the car was in motion a slight vacuum would be hopefully obtained, helping to extract combustion gases as they collected in the crankcase. The oil mist would also be discharged, resulting in an oily film being deposited in the middle of each travel lane on heavily-used roads. The system was not positive though, as gases could travel both ways, or not move at all, dependent on conditions. Most modern diesel engines still use this type of system to dispose of crankcase fumes. During World War II however, a different type of crankcase ventilation had to be invented to allow tank engines to operate during deep fording operations, where the normal draft tube ventilator would have allowed water to enter the crankcase and destroy the engine. The PCV system and its control valve were invented to meet this need but the need for it on automobiles was not recognized.
In 1952, Professor A. J. Haagen-Smit, of the California Institute of Technology at Pasadena, postulated that unburned hydrocarbons were a primary constituent of smog, and that gasoline powered automobiles were a major source of those hydrocarbons. After some investigation by the GM Research Laboratory (Dr. LLoyd L. Withrow) it was discovered in 1958 that the road draft tube was a major source, about half, of the hydrocarbons coming from the automobile. GM's Cadillac Division, which had built many tanks during WWII, recognized that the simple PCV valve could be used to become the first major reduction in automotive hydrocarbon emissions. After confirming the PCV valves' effectiveness at hydrocarbon reduction, GM offered the PCV solution to the entire U.S. automobile industry, royalty free, through its trade association, the Automobile Manufacturers Association (AMA). In the absence of any legislated requirement, the AMA members agreed to put it on all California cars voluntarily in the early 1960s, with national application following one year later.
Following its introduction into production, several years later the PCV became the subject of a Federal grand jury investigation in 1967, when it was alleged by some industry critics that the AMA was conspiring to keep several such smog reduction devices like the PCV on the shelf to delay smog control. After eighteen months of investigation by U.S. Attorney Samuel Flatow, the grand jury returned a "no-bill" decision, clearing the AMA, but resulting in a "Consent Decree" that all U.S. automobile companies agreed not to work jointly on smog control activities for a period of ten years.

 PCV system
The PCV valve is only one part of the PCV system, which is essentially a variable and calibrated air leak, whereby the engine returns its crankcase combustion gases. Instead of the gases being vented to the atmosphere, gases are fed back into the intake manifold, to re-enter the combustion chamber as part of a fresh charge of air and fuel. The PCV system is not a classical "vacuum leak." All the air collected by the air cleaner (and metered by the mass air flow sensor, on a fuel injected engine) goes through the intake manifold. The PCV system just diverts a small percentage of this air via the breather to the crankcase before allowing it to be drawn back in to the intake tract again. It is an "open system" in that fresh exterior air is continuously used to flush contaminants from the crankcase and into the combustion chamber.
The system relies on the fact that, while the engine is running, the intake manifold's air pressure is always less than crankcase air pressure. The lower pressure of the intake manifold draws air towards it, pulling air from the breather through the crankcase (where it dilutes and mixes with combustion gases), through the PCV valve, and into the intake manifold.
The PCV system consists of the breather tube and the PCV valve. The breather tube connects the crankcase to a clean source of fresh air, such as the air cleaner body. Usually, clean air from the air cleaner flows in to this tube and in to the engine after passing through a screen, baffle, or other simple system to arrest a flame front, to prevent a potentially explosive atmosphere within the engine crank case from being ignited from a back-fire in to the intake manifold. The baffle, filter, or screen also traps oil mist, and keeps it inside the engine.
Once inside the engine, the air circulates around the interior of the engine, picking up and clearing away combustion byproduct gases, including a large amount of water vapor, then exits through a simple baffle, screen or mesh to trap oil droplets before being drawn out through the PCV valve, and into the intake manifold.

 PCV valve

PCV valve on Ford Taunus V4 engine in a Saab 96, between left valve cover and intermediate flange on intake manifold
The PCV valve connects the crankcase to the intake manifold from a location more-or-less opposite the breather connection. Typical locations include the opposite valve cover that the breather tube connects to on a V engine. A typical location is the valve cover(s), although some engines place the valve in locations far from the valve cover. The valve is simple, but actually performs a complicated control function. An internal restrictor (generally a cone or ball) is held in "normal" (engine off, zero vacuum) position with a light spring, exposing the full size of the PCV opening to the intake manifold. With the engine running, the tapered end of the cone is drawn towards the opening in the PCV valve, restricting the opening proportionate to the level of engine vacuum vs. spring tension. At idle, the intake manifold vacuum is near maximum. It is at this time the least amount of blow by is actually occurring, so the PCV valve provides the largest amount of (but not complete) restriction. As engine load increases, vacuum on the valve decreases proportionally and blow by increases proportionally. Sensing a lower level of vacuum, the spring returns the cone to the "open" position to allow more air flow. At full throttle, there is nearly zero vacuum. At this point the PCV valve is nearly useless, and most combustion gases escape via the "breather tube" where they are then drawn in to the engine's intake manifold anyway.

 Operation
Should the intake manifold's pressure be higher than that of the crankcase (which can happen in a turbo charged engine or under certain conditions, such as an intake backfire), the PCV valve closes to prevent reversal of the exhausted air back into the crankcase again. Positive is not a synonym for 'one way', but for 'real', 'definite', 'incontestable' i.e. one of its other meanings. It simply means there is a constant and definite flow of air through the system, as compared to the hit-and-miss road draught system used previously, in which air may flow in either direction or not at all. In many cases PCV valves were only used for a few years, the function being taken over by a port on constant depression carburettors such as the SU. This has no moving parts or diaphragm to jam, block or rip like many PCV valves. It also doesn't have a 'one-way' function but the lack of it was never a problem in intake backfire.
It is critical that the parts of the PCV system be kept clean and open, otherwise air flow will be insufficient. A plugged or malfunctioning PCV system will eventually damage an engine. PCV problems are primarily due to neglect or poor maintenance, typically engine oil change intervals that are inadequate for the engine's driving conditions. A poorly-maintained engine's PCV system will eventually become contaminated with sludge, causing serious problems. If the engine's lubricating oil is changed with adequate frequency, the PCV system will remain clear practically for the life of the engine. However, since the valve is operating continuously as one operates the vehicle, it will fail over time. Typical maintenance schedules for gasoline engines include PCV valve replacement whenever the air filter or spark plugs are replaced. The long life of the valve despite the harsh operating environment is due to the trace amount of oil droplets suspended in the air that flows through the valve that keep it lubricated.
Not all gasoline engines have PCV valves. Engines not subject to emission controls, such as certain off-road engines, retain road draft tubes. Dragsters use a scavenger system and venturi tube in the exhaust to draw out combustion gases and maintain a small amount of vacuum in the crankcase to prevent oil leaks on to the race track. Small gasoline two stroke engines use the crank case to compress incoming air. All blow by in these engines is burned in the regular flow of air and fuel through the engine. Many small four-cycle engines such as lawn mower engines and small gasoline generators, simply use a draft tube connected to the intake, between the air filter and carburetor, to route all blow by back into the intake mixture. The higher operating temperature of these small engines has a side effect of preventing large amounts of water vapor and light hydrocarbons from condensing in the engine oil.
 [1] - PCV images

Categories: Auto parts | Engine valves | Engine technology | Pollution control technologies

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