What is compression in an engine and how to measure it

What is engine compression and how to measure it.

Engine compression, more correctly engine cylinder compression, is a measure of an engine’s viability. Scientifically, the definition of engine compression would look like this – “The maximum rate of air pressure in the combustion chamber achieved at the moment the piston reaches the top dead center during the compression stroke.” – which of course is correct.

As mentioned earlier, engine compression is one of the main indicators of engine health. Many engine processes – fuel burnability, oil consumption, ease of starting an engine (with low compression, the engine starts badly), engine sneezing and farting can also be caused by low or no compression in one or more cylinders of the engine, the engine can thrash due to weak compression in the cylinders. Low compression can also be the reason for the engine power drop, more precisely with the drop in engine power is to check the compression.

No compression, no compression, let’s look at the causes of lost compression. Signs of the disappearance of compression are described above. Let’s return to the reasons for the loss of compression. There can be quite a few of them.

– Engine overheating – may well be the main cause of low compression. Severe overheating often causes burrs on pistons and cylinders, pistons can melt and burn out, there were cases when a piston burned out inside, i.e. a hole was formed in the middle of the piston. VAZ engines also often suffer, when overheating or from service life the piston ring baffles tend to fall apart, as a result low compression, the engine farts, doesn’t go, needs repair.

– Faults in the timing system can also cause loss of compression. Burnt-out valve – compression is 2 or 3 – you need to open your head and check valves, replace them, lapping.

– Timing belt or chain broke – you are lucky if valves are not bent, new belt, go further – low compression – valves probably bent – head should be removed, valves should be replaced, lapped. A few teeth missed on the pinion, the marks were wrong when installing the camshaft – at a minimum wrong valve timing, the engine farts, unstable throttle, does not take or does not reduce speed – maximum bent valves.

– Valve adjustment – no gap, valve doesn’t close, doesn’t hold (one of the reasons for burnt valves), resulting in low or no compression. Too much clearance – valve doesn’t open enough due to misalignment or wear of parts, less air is getting into cylinders, signs – knocking under valve cover, not hard to recognize, not as bad as a jammed valve.

– A blown cylinder-head gasket – the cause is obvious, hard not to notice when the engine is running. Or the gasket is burnt out and the gases go either into the cooling system or into the oil line.

– Wear of piston rings, pistons and wear and tear and ellipse on the cylinder walls, also not infrequently accompanied by increased oil and fuel consumption.

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– Cracks in the cylinder head.

– A clogged air filter makes it harder for air to flow into the cylinders, and thus also affects compression.

How to check the compression in the engine cylinders.

To check, to measure the compression we need a special device – a compressometer. In fact it is a pressure gauge with extensions to it, so that it was possible to get to the most inaccessible hole, as well as adapters for different sizes of plugs. Compressometers also come separately for diesel and gasoline engines, as the compression in the cylinders of a diesel engine is much higher than that of a gasoline engine. At one time two sets of compression meters cost me 50 bucks, the diesel one was more expensive for some reason.

So, how to check the compression of the engine – in principle, if you have a compressometer, everything else will find itself. Before you begin all preparations for compression measurement, be sure that you are dealing with a well-charged battery and a live starter.

– In the beginning we need to unscrew all plugs, but before that clean out all garbage near plugs (blow out with compressor, if there is any), so it would not get into combustion chambers.

– Then we prepare the compressometer itself, choose the necessary adapter with an extension cord, and connect it all.

– Then accurately screw the adapter into a hole for a spark plug (diesels have to unscrew nozzles) and twist the starter.

– Check each cylinder.

– Watch the pressure gauge, analyze the difference in readings between all cylinders. Find out what compression should be in your engine models at this stage of mileage and based on the data we make conclusions about the state of the piston group and the timing.

Compression check

Checking engine compression is done to find faults in the internal combustion engine. Compression is the compression of the mixture in the cylinder under the influence of external forces. It is measured as the compression ratio multiplied by 1.3. By measuring the compression you can find a cylinder that is malfunctioning .

If the machine has various kinds of problems, like loss of power, loss of oil, throbbing in the engine – then check the plugs, sensors, inspect the engine for damage and leaks. When such checks are fruitless, then the compression measurement is resorted to. This video shows how to determine it by the example of VAZ classic.

It is possible to check the compression independently with a compressometer . At service stations, such checks are made with a compressograph or motor tester.

Causes of decreased compression in the cylinders

There are many causes for decreased engine compression:

  • piston and piston group parts wear and tear;
  • Incorrect adjustment of the timing; and pistons.

To specifically determine the cause of the malfunction and is carried out measuring engine compression while the engine is hot and cold. Let’s find out how to carry out such a procedure both with and without a compressometer.

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How to measure compression in the engine

To begin with you need to prepare the engine for the test. To do this we need to warm the engine to a high temperature of 70-90 degrees. After that you need to disconnect the gasoline pump, so that no fuel is supplied and turn out the spark plugs.

Be sure to check the starter and battery charge. The last preparation step is to open the throttle and air valve.

After all this we proceed to checking the compression:

  1. Insert the tip of the compression gauge into the plug connector and crank the engine with the starter until the pressure growth stops.
  2. The crankshaft should rotate at about 200 rpm.
  3. If the engine is good, the compression should increase in a matter of seconds. If it happens for a long time, the piston rings are burning out. If the pressure does not increase at all, most likely the block gasket should be replaced. The minimum pressure in a petrol engine must be 10 kg/cm² or more (in a diesel engine over 20 kg/cm²).
  4. After taking the reading, release the pressure by unscrewing the cap on the gauge.
  5. Similarly check all other cylinders.

Illustration of the stages of measuring compression in a cylinder

There is another way of checking, which differs from the above in that engine oil is poured into the cylinder being checked. An increase in pressure indicates worn piston rings, if the pressure does not increase, the cause is in the cylinder head gasket , or in general there is a leak in the valves.

If the engine is normal, the compression in it should be 9,5 to 10 atmospheres (gasoline engine), while in the cylinders it should differ no more than by one atmosphere.

You can also diagnose poor compression by malfunctions in the carburetor. If air is leaking you need to check the tightness of the bypass valve. If the air leaks through the top of the radiator, then a faulty cylinder head is to blame.

What affects engine compression

  1. Throttle position . Throttle opening or closing can reduce compression.
  2. Clogged air filter .
  3. Wrong valve timing, when the valve is not in the correct timing. This can occur if the timing belt or chain is not installed correctly.
  4. Valve closing not in time because of valve drive clearances.
  5. Motor temperature . The higher its temperature, the higher the temperature of the mixture. Consequently, the pressure is lower.
  6. Air suction . Air leaks, reduce compression. They are caused by damage or natural wear of the combustion chamber gaskets.
  7. Oil ingress into the combustion chamber increases compression.
  8. If the fuel enters in the form of droplets, the compression is reduced – the oil that plays the role of the seal is washed away.
  9. Lack of tightness in the compressor or in the check valve.
  10. Speed of rotation of the crankshaft . The higher it is, the higher the compression, there will be no leaks due to depressurization.

Above described how to measure the compression in the internal combustion engine, running on gasoline. In the case of a diesel engine, the measurements are made differently.

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Measuring the compression in a diesel engine

  1. In order to disconnect the diesel engine intake, it is necessary to disconnect the fuel valve from the power supply. This can also be done by clamping the cutoff lever on the high pressure pump.
  2. Measurements on the diesel engine are made with a special compressometer, which has its own features.
  3. When checking, you do not need to press the accelerator pedal, as there is no throttle in such engines. If there is one, it must be cleaned before checking.
  4. Each type of engine is provided with a special instruction on how to measure compression on it.

Compression measurement on a diesel engine.

Compression measurement on the injector car

It is worth remembering that compression measurements can be inaccurate. When measuring for the most part, you need to consider the pressure difference in the cylinders, not the average compression value.

It is necessary to take into account such parameters as oil temperature, engine temperature, air temperature, engine speed, etc. Only taking into account all the parameters, you can draw a conclusion about the degree of wear of pistons and other parts that affect compression. And as a result of all these malfunctions to give a conclusion about the need for an engine overhaul.

How to check the compression without a compression meter

It will not be possible to measure the compression without a device. Since the word “measurement” itself, implies the use of a measuring device. So it is impossible to measure the compression in the engine without a compressometer . But if you need to check, to determine whether it is there at all (for example after the break of timing belt or long idle time of the car, etc.), there are, some of the easiest ways how to check the compression without a compression meter. A sign of poor compression is atypical car behavior, when, for example, at low revs it works sluggishly and unstably, and at high “wakes up”, with blueish smoke from the exhaust, and if you look at the plugs, they appear to be covered with oil. With decreasing compression, the crankcase pressure increases, the ventilation system gets dirty faster, and as a result the CO toxicity, combustion chamber fouling increases.

Checking compression without instruments

The most basic check of engine compression without instruments is by ear. Since usually if there is compression in the engine cylinders, turning the starter you can hear the engine working out every stroke of compression with a characteristic sound. In most cases, the engine may wobble slightly. When there is no compression, you will not hear any distinct strokes, nor will there be any wobbling. Such behavior is often indicative of a timing belt failure.

Video how to check engine compression without instruments

Having plugged the plug of a suitable diameter (rubber, cork plastic or dense rag) the candle well, having turned out beforehand the plug of some of the cylinders, you can check if there is any compression. If it is there, the plug will fly out with a characteristic pop. If there is no compression, it will remain where it was.

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The compression can be checked by turning the crankshaft. This method of checking the compression has no accuracy at all, but, nevertheless, people sometimes use it. It is needed to take out all plugs, except for the first cylinder, and turn by hand, by the crankshaft pulley bolt, until the compression stroke is over (determined by the timing marks). Then repeat the same procedure with all the other cylinders, approximately remembering the effort applied. As the measurements are quite conventional, so it is preferable to use a compressometer. Every car owner should have such device, because its price is so high not to buy it, and its help may be needed at any moment. You can find out the required value of compression for your car from the service manual or at least know the compression ratio of your car engine, then the compression can be calculated by the formula: compression ratio * K (where K=1.3 for gasoline engines and 1.3-1.7 for diesel engines).

According to the condition of the exhaust or the condition of the spark plugs, only an experienced mechanic can determine the compression without the device, and that is also self, – relatively.

This method is relevant for cars with a worn out engine, when the refill becomes more frequent, and a blue-white smoke with a specific odor appears from the muffler. This will tell you that the oil in the combustion chambers began to come in several ways. A competent motor mechanic will determine the exact cause of such smoke and oil consumption, judging by the exhaust and the condition of the spark plugs, as well as having analyzed acoustic noises (to listen to the noises you will need a device, which is a medical stethoscope with a mechanical sensor).

There are two main culprits of oil leakage – oil valve caps or cylinder piston group (rings, pistons, cylinders), which indicates compression deviations.

When the oil seals are worn, often there are oil rings around the plugs and exhaust, and then measuring the compression and can not do. But if after warming up the engine the characteristic smokiness continues or its intensity increases, you can conclude that the engine is worn. And in order to determine what caused the loss of compression, you need to perform several simple tests.

Tests for checking the lost compression

To get an accurate answer, it is required to apply all of the above methods with the comparison of the results obtained.

To determine the wear of the rings, it is enough to spray, with a syringe, into the cylinder literally 10 grams of oil, and repeat the test. If the compression has increased, the rings, or other parts of the cylinder-piston group are tired. In case of invariability of indicators – there is an air leakage through the gasket or valves, and in rare cases because of a crack in cylinder-head cylinder head. And if the pressure has changed literally by 1-2 bars, it is time to sound the alarm – it is a symptom of piston burnout.

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Uniform decrease of compression on cylinders indicates the usual engine wear and is not an indicator for the urgent overhaul.

Compression measurement results

The compression measurement results show the state of an engine, particularly of pistons, piston rings, valves, camshafts and allow making decisions concerning the necessity to repair or just replace the cylinder head gasket or valve caps.

On gasoline engines, the normal compression is within 12-15 bar. If you explore in detail, the tendency will be as follows:

  • Front-drive domestic cars and old foreign cars – 13,5-14 bar;
  • rear-drive carburetor cars – up to 11-12;
  • new foreign cars – 13,7-16 bar, and turbocharged cars and with a large volume up to 18 bar.
  • In cylinders of diesel cars, compression should be at least 25-40 atm.

The table below shows more precise values of compression pressure for different engines:

Engine type Value, bar Wear limit, bar
1.6, 2.0 л 10,0 – 13,0 7,0
1.8 л 9,0 – 14,0 7,5
3.0, 4.2 л 10,0 – 14,0 9,0
1.9 l TDI 25,0 – 31,0 19,0
2.5 l TDI 24,0 – 33,0 24,0

The results of the rising dynamics

When the pressure value is 2-3 kgf/cm² and then rises sharply during cranking, the compression rings are likely to be worn. In the same case, compression rises sharply in the first stroke, if you drop oil in the cylinder.

When pressure immediately reaches 6-9 kgf/cm² and then practically does not change, it is most likely that the valves are not tightly adjacent (lapping will correct the situation) or the cylinder head gasket is worn.

In the case when the compression is reduced (about 20%) in one of the cylinders, and the engine is unstable at idle, there is a high probability of cam wear.

If the results of compression measurement showed that in one of the cylinders (or two adjacent ones), the pressure rises noticeably slower and is 3-5 atm below the norm, then probably the gasket between the block and the head has burned through (pay attention to the oil in the coolant).

By the way, don’t be happy if your engine is old, and the compression is higher than the new one – the growth of compression can be explained by the fact that as a result of a long time combustion chamber has oil deposits, which not only reduce the heat sink, but also reduce its volume, and as a result detonation appears, glow ignition and similar problems.

Uneven compression in the cylinders causes engine vibration (especially noticeable at idle and low rpm), which in turn also damages the transmission and engine suspension. So, having measured the compression pressure, it is necessary to draw conclusions and to eliminate the defect.

Car mechanic with 20 years of experience in repairing and servicing cars of different brands. Main area: diagnostics and mechanics.

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