Compression in the car. What compression should the engine have? Engine compression measurement

In the broadest sense of the word, compression is the amount of pressure that is created in the cylinder at the end of the compression stroke. This parameter depends within very wide limits on the conditions under which it is measured, and to a large extent on the technical condition of the motor. That is why compression is “adopted” as a diagnostic factor that allows you to objectively assess the health of the engine.
As with any evaluative indicator, measurement modes and methods should always be the same. For compression, they mean that the engine is warmed up to operating temperature, the throttle valve is fully open, the spark plugs in all cylinders are turned out, the starter is working, the battery is fully charged.
A few words about compression meters - instruments that are necessary for this purpose. There are many of them, but in essence they differ from each other only in their design. Typically, a compression gauge consists of a tip inserted into the spark plug hole, a check valve at the inlet, and a pressure gauge. The pressure gauge and tip can be connected by a hose or metal tube. The valve in the tip of the compression gauge is necessary so that the pressure gauge needle, when measuring, is fixed at the level of the highest pressure that arises in the cylinder. Service stations often use more complex instruments, where instead of a dial pressure gauge, a recorder is installed that records the value of the measured pressure on a special form or digital display.
How to evaluate the results when measuring compression? In the instructional materials, domestic manufacturers provide the minimum compression values ​​permissible when operating a vehicle. But this is not the only condition. It is very important that the difference between the cylinders in terms of compression is minimal. Identical or similar values ​​indicate an equivalent condition of the parts and the degree of wear of the cylinders. Let's take this example: in one of the engine cylinders the compression meter registered 10.1 kgf/cm2, and in the rest - 11.6-11.8 kgf/cm2. Despite the fact that the absolute values ​​of compression in each cylinder are within acceptable limits, their difference is a signal of some kind of malfunction. Therefore, technical standards allow a difference between the highest and lowest compression readings in the cylinders of no more than 1 kgf/cm 2.
Let us dwell on the case when compression does not meet factory requirements in one or more cylinders. How and where to look for the reason?
The simplest and well-known method: pour about a tablespoon of engine oil into the “suspected” cylinder through the spark plug hole and measure the compression again. If the pressure has increased noticeably, the tightness of the piston rings is insufficient; if not, the defect is caused by other reasons (leakage of valves or gaskets under the cylinder head, a crack or burnout in the walls of the combustion chamber, in the piston bottom).
If it is possible to eliminate the malfunction on your own, it is advisable to conduct a more detailed examination, which will help determine the specific cause of the low compression. To do this you need to make a simple device. You will need a failed spark plug, from which the insulator is knocked out, and a steel valve from an unusable car or motorcycle camera is welded to the metal body (can be firmly soldered). Set the piston of the cylinder under test to the ignition moment position, screw the manufactured adapter into the spark plug hole and attach the tire pump hose to it. Then, having first removed the radiator and oil filler plugs and asked an assistant to pump air into the cylinder, they can determine by ear which way it leaks from the combustion chamber. If air enters the exhaust pipe, the exhaust valve is leaking; if air enters the suction manifold, the intake valve is leaking. If the cylinder head gasket is leaking, air can enter the cooling system jacket, which will appear as bubbles in the upper radiator tank. Insufficient sealing of the piston rings can often be detected by the hissing of air heard through the oil filler neck. Such a check helps to more accurately understand the nature of the defect, the scope of work to be done, and avoid unnecessary disassembly of the engine.
So far we have been talking about compression in connection with the identification and elimination of defects. But it is equally important to know what compression depends on in a working engine and what factors influence its value.
It is clear that the most obvious factor is the compression ratio in the engine. The higher it is, the higher the pressure in the cylinder after the compression stroke. The simplicity of this relationship often leads to the erroneous conclusion that the compression amount should be numerically equal to the compression ratio. In fact, it is noticeably larger. Where does the additional pressure come from?
It is appropriate to recall that the compression ratio is a purely design parameter, showing the relationship between geometric parameters - the total volume of the cylinder above the piston at bottom dead center (BDC) and the volume of the combustion chamber above the piston at top dead center (TDC). As for the compression of real gas when the piston moves from BDC to TDC, then the laws of thermodynamics come into play. Compressing a gas (working mixture or air) requires energy, which causes its temperature to increase (think of the heating of a tire pump hose), which in turn leads to an increase in pressure in the combustion chamber at the end of the compression stroke. Additionally, this is facilitated by the heating of the gas from the walls of the cylinder and combustion chamber, which are heated to approximately 90 ° C, while the intake air is at an external temperature. In fact, thermal processes are, of course, much more complex than what is said here, but the point is that the actual value of pressure during compression depends on these phenomena, occurring in a strictly defined time period. From here it is clear how important it is to maintain the specified thermal conditions of the engine and the crankshaft speed when measuring compression.
There is another factor that has a practical impact on the result of compression measurements. In a simplified form, we can assume that before the compression stroke the cylinder is filled with gas, which always has the same pressure, practically atmospheric. What if it is lower? Then, naturally, the final pressure value will be lower. The cylinder is filled during the suction stroke, when a vacuum is created in it and a portion of air passes through the entire intake tract - the air filter, the intake manifold. The lower the resistance to gas flow, the greater the filling of the cylinder before compression. The conclusion is simple - an air filter clogged with dust can significantly distort the measurement result, even if the air and throttle valves are open as they should be.
There is also a design factor due to which engines of different models can differ from each other in compression, despite the fact that their compression ratio is the same. It is believed that compression in the cylinder begins as soon as the piston begins to move upward from BDC. This is only possible if the intake valve, through which the cylinder was filled during the previous stroke, is already completely closed. However, in modern engines the intake valve closes much later, when the piston has already passed part of the way to TDC. The magnitude of this “lag”, expressed in crankshaft rotation angles, is 40°-70°.
This design solution ensures high power at high crankshaft speeds, since at the beginning of the compression stroke, when the pressure in the cylinder is still low, the flow of the fuel mixture continues to flow into the cylinder by inertia and thereby recharges it. The picture is different at low speeds, when the speed and inertia of the mixture flow are small. At a low crankshaft rotation speed, which is achieved by the starter, by the time the intake valve closes, the piston has time to push back into the intake tract part of the gas that filled the cylinder, and in fact compression begins only after the valve closes.
Thus, the engine of each model has individual characteristics that also depend on the valve timing. In real life, each motor may have noticeable deviations from the phase parameters intended by the designers. The reason for this is wear of drive parts and violation of their adjustments. Therefore, this factor must be taken into account in practice.
Let us once again emphasize the role of compression: it provides a good opportunity to objectively assess the “health state” of the engine, without requiring complex equipment. Regular compression checks - at a service station or on your own - should become the rule, as this will keep the engine in good condition and, ultimately, save fuel, oil and money for routine repairs.

The main and important indicator of engine performance is the tightness of the combustion chamber. Compression in the engine cylinders determines the degree of efficient fuel combustion, and, accordingly, directly affects its reliable starting, regardless of the ambient temperature, as well as stable operation both at idle and while driving.

Basic Concepts

Compression is a parameter for the amount of pressure created in the cylinder at the end of the compression stroke. The value depends on a fairly large number of factors. It is important to follow the measurement rules in order to reduce the percentage of errors when assessing the technical condition of the engine.

General rules of measurement

For a preliminary general assessment of the compression ratio in the cylinders of a gasoline engine, the following conditions must be met:

• warmed up to operating temperature;
• spark plugs are removed from all cylinders;
• the fuel supply is turned off (on injection engines it is enough to turn off the crankshaft position sensor, on carburetor engines - turn off the fuel supply from the fuel pump);
• the battery is fully charged or additionally connected to the starter;
• the starter is working;
• the clutch pedal is fully depressed;
• the throttle valve is fully open;
• The measuring modes for all cylinders are the same in terms of the starter cranking time (or the number of strokes).

Standards and norms

There is an opinion among owners of modern cars that the compression of a hot engine can be from 8 to 10 atm.

The compression rate in the cylinders of any engine is from 12 atm, with rare exceptions.

Cars use engines of various configurations, determined by the number of valves and camshafts, the geometry of the intake manifold, and the installed connecting rod and piston group. In accordance with this, its specific compression ratio is calculated - this is the ratio of the total volume of the cylinder to the volume of the combustion chamber.

The higher the compression ratio, the higher the compression value. For gasoline engines, the compression ratio is in the range of 8 - 12 units, which is indicated in the technical documentation of a particular car. Theoretically, it is not difficult to determine what the compression in the cylinders should be for a particular engine. It is enough to multiply the compression ratio by a factor of 1.3.

For example, the compression ratio in the car’s characteristics is indicated as 9.5 units, multiplying 9.5 by a factor of 1.3 to obtain a calculated value of 12.35 atm.

Compression testers

A compression gauge is a device consisting of a pressure gauge with a scale, a pressure relief valve and tips. The most convenient is a flexible hose with a threaded end, in which a nipple is installed to prevent air from escaping back out of the device. Checking compression in engine cylinders that include spark plug wells in their design is carried out by screwing a flexible hose into the threaded hole of the spark plug, which eliminates the loss of compressed air from loose fitting of rigid tips without threads.

Measure the compression in the cylinders with your own hands

First of all, it is necessary to establish from the technical documentation what the normal compression of a gasoline engine is specifically for this car and determine the difficulty of accessing the spark plugs. If access requires disassembling other engine components (throttle assembly, intake receiver), preventing them from being unscrewed, and there is confidence in their successful disassembly and reassembly, then you can begin measuring.

How to measure compression

There is an opinion that in order to correctly measure compression in an engine, you need to turn the crankshaft two or three turns with the starter. This method will lead to incorrect identification of the malfunction and, as a result, to unjustified costs for engine repairs.

The test is performed in different modes in order to broadly analyze the condition of the motor:

• with closed throttle;
• with the damper fully open;
• with the addition of oil to the cylinder;
• on a “cold” engine.

Checking with the throttle valve closed

This measurement method is needed to determine small engine defects that are sensitive when there is a small amount of air entering the cylinders. This could be a crack on the valve disc, a slight burnout of the edge, or a lack of tightness in the seat-valve pair. When measuring compression, the flow of air through a closed throttle valve is limited, and its value will be low (from 10 to 11 atm). Due to the small amount of air entering the cylinders, sensitivity to leaks increases, as a result of which the results of pressure parameters are underestimated.

Full throttle test

To determine severe wear of the internal combustion engine, compression is checked with the throttle valve fully open, ensuring maximum air flow into the cylinders. An increased amount of air contributes to an increase in pressure, but leaks also increase, but in comparison with the mass of incoming air they are so small that the magnitude of the compression drop is insignificant and reaches 12 - 13 atm. If there are “gross” defects in the engine, then reduce to 8 - 9 atm. Possible reasons:

• breakage or sticking of rings in the piston grooves;
• piston crown failure or burnout;
• severe burnout of the valve disc or deformation of its axis;
• as a result of the ingress of foreign materials, scoring of the cylinder wall occurred;
• After repairing an engine with an end of life, associated with the replacement of piston rings, the ovality of the cylinders was not taken into account and they were not bored to a different size of the piston and rings.

Poor compression in the engine

The car's behavior has changed. The engine does not start from the first crank with the starter or requires prolonged rotation, and at sub-zero ambient temperatures it may not start at all. If you unscrew the spark plugs and inspect them visually, the reason for the failure to start becomes clear - the spark plugs are “wet” due to an excess of fuel entering the cylinders. These are symptoms of poor compression in the engine.

Signs

• the engine starts, after warming up it idles smoothly, but when driving there is a loss of power;
• failures during acceleration and increased fuel consumption;
• works with misfires - “troit”.

What to do if the above symptoms are identified

In order to avoid further and rapid wear, it is necessary to establish the causes as soon as possible and identify components that need to be repaired.

Based on the results of compression measurements, the following indicators are possible:

• high - more than 15;
• low - less than 12;
• no - 0.

An increase in compression in the engine is visible in the exhaust gases - the color becomes bluish-white and oil consumption increases. When accelerating, the piston pins ring. High compression causes an increase in carbon deposits on the valve plates and guides, and a layer of coke forms on the walls of the combustion chamber, reducing the volume of the chamber and accordingly changing the compression ratio of the cylinders.

High compression gradually causes engine detonation knocks, which destroy the connecting rod and piston group.

Reason for high compression

Analysis of the engine revision shows that there is more than one reason. The table shows the reasons for increased compression in the engine:

Reason for low compression

Weak compression is detected when it is started, especially in cold weather, and signs are also found in loss of dynamics and increased fuel consumption.

Low compression increases the rate of engine wear, and if the defect is in one cylinder, then the idle speed is unstable, the car often stalls or lowers the idle speed.

The table shows the main reasons:

If there is no compression in one or more cylinders, then the engine starts very poorly, and once started, it immediately begins to work unbalanced, with strong vibrations and shaking.

If compression is lost in all cylinders at the same time, the engine will not start. One of the reasons may be a broken timing belt while the engine is running and a collision of the piston with the valves. After an impact the valves bend. The gas distribution system loses its tightness, and the compressed air-fuel mixture is directed at high speed into the intake and exhaust manifolds.

Compression also disappears in one of the cylinders due to severe burning of the valve disc. There are examples when from 40 to 50% of the valve disc area is melted and compression is zero. This is easy to determine without a compression gauge. It is enough to wrap a spark plug in this cylinder and when cranked by the starter, the crankshaft will rotate easily and uniformly, without highlighting the compression strokes.

When measuring compression, it is advisable to observe the needle of the pressure meter (pressure gauge) and monitor the dynamics of its increase. By the rate of increase in compression, you can indirectly determine in which group of parts there is severe wear. If on the first stroke the pressure gauge shows low compression (3-5 atm), and during subsequent strokes the rate of pressure increase increases, then with a high probability it can be established that the piston rings are worn out. To double-check the readings, you can use the method of artificially creating an oil film on the cylinder wall by adding 5-10 ml of oil through the spark plug hole. If, with added oil, the compression increases sharply on the first stroke and settles at nominal, then we can specifically state that the compression rings are worn out. If you measure in two adjacent cylinders and obtain a low result without oil and with oil, you can conclude that there is a defect in the cylinder head gasket.

Another option is also possible, when 6-9 atm is reached on the first stroke and on subsequent strokes the pressure gauge needle hangs in the same position. In this situation, it is assumed that the valve or cylinder head gasket is not tight. You can more accurately determine the cause with other diagnostic devices, such as a pneumatic tester or an electronic oscilloscope.

Why is there no compression in the engine?

Situations also arise when the engine has been repaired and a new piston group has been installed, the cylinders have been bored, and the valves have been ground to the seats. In other words, the repair was carried out strictly according to the technological map. A compression test was carried out and there was no pressure in the cylinders.

After repairs have been carried out, hydraulic compensators can put pressure on the valve stems, as a result of which they can open. After a certain time, the hydraulic compensator will fill with oil and begin to function.

Permissible compression in a carburetor engine

The permissible pressure in a carburetor engine when using AI-76 gasoline (low octane gasoline) is about 8-10 atm, and when using high-octane gasoline 11-12 atm.

Evaluation of measurement results

Obtaining correct data is the key to accurately identifying the fault.

The same values ​​obtained during the measurement indicate a uniform condition of the parts of the gas distribution mechanism and the same degree of wear of the connecting rod and piston group.

If the pressure in one of the cylinders drops by 1 atm, in comparison with other cylinders, it is necessary to apply other diagnostic methods in order to determine the exact cause.

Compression recovery

It can be restored if the gas distribution mechanism on the cylinder head is not damaged. Piston ring sticking may occur. In this case, you can increase the pressure without disassembling the engine. To do this, you need to buy a liquid for dissolving coke and carry out the “decoking” procedure on a hot engine. The cost of the procedure is minimal compared to disassembling the engine. To evaluate the recovery of decoking fluid from the piston rings, measure them before and after “decoking.” In most cases, this method temporarily restores the engine.

There is also a method of pouring oil into the cylinders before starting the engine. Allows you to increase the compression solely for starting the engine, especially in cold weather. The essence of the method is to forcibly create an oil film on the cylinder walls, which briefly blocks gas leaks into the engine crankcase.

Checking on a removed internal combustion engine

In garage conditions, it is possible to measure compression by connecting the starter terminals to the battery. But it is necessary to take into account that the temperature will be equal to the ambient temperature.

Stuck rings or cracked valve - much more commonreasons for decreased compression than engine wear.

Compression is a vulgarism. That's right - the pressure at the end of the compression stroke. This is the pressure that is created in the cylinder when the ignition is turned off (or without fuel supply - for a diesel engine) with the piston position at top dead center. So, many diagnosticians, based on the measured compression (sorry, science, for the jargon!) give a conclusion: “the patient is alive” or “to the morgue,” that is, for major repairs. According to many advanced motorists, compression is almost everything for an engine! But is it?

Compression and compression ratio are the same thing: the first fairy tale

No not like this! Compression is the pressure in the cylinder, the compression ratio is a dimensionless parameter that describes the geometric parameters of the cylinder: this is the ratio of the total volume of the cylinder to the volume of the compression chamber (the compression chamber is the volume of space above the piston at its position at TDC (it is also called the volume of the end of compression - this is the same thing). Calling it a combustion chamber is incorrect, since fuel combustion occurs throughout the entire volume of the cylinder.) Compression depends on the compression ratio, but the compression ratio does not! Compression also depends on a bunch of parameters: compression start pressure, valve timing adjustment, temperature at which the measurement is carried out, leaks from the combustion chamber. And leaks are determined by wear of the rings and cylinders. “Compression” is the maximum pressure that we measure in the cylinder when the ignition is turned off.

Raised the compression - increased the power: the second fairy tale

Not certainly in that way. Compression can be increased in two ways - increasing the compression ratio or reducing leaks from the combustion chamber. Let's see what happens in each case: we have a stand at our disposal. First, let's reduce the volume of the compression chamber. The easiest way to do this is to grind the bottom plane of the cylinder head. The base engine of the “eleventh” VAZ has a cylinder displacement of slightly more than 370 cubic meters. With a standard compression ratio of 9.8, the volume of the compression chamber will be 42.6 cm³. You can calculate that by removing 2 mm from the seating surface of the cylinder head, we reduce the volume of the compression chamber by 5.1 cm³. The new compression ratio will be 11 units, that is, 1.2 higher than that of the base engine. Now, just out of interest, let's remove another 2 mm. The compression ratio increases to 12.6. In the textbook we find the required formula and get: the thermal efficiency of the piston engine cycle should theoretically increase in the first case by at least 4%, in the second - by 9%. Great! Now we put these heads on the bench motor and take the torque characteristics. The reduction in fuel consumption is significantly less than the theory promised - by 2.5% in the first case and by 4.5% in the second. Moreover, the effect is more pronounced in the zone of low loads. The increase in power is even less: 2–3% at most, and in the low and medium speed zone. But at high levels there is no effect... Everything is clear: with an increase in the compression ratio, the pressure in the cylinder increases sharply, this increase provokes detonation, the corresponding sensor catches it and shifts the ignition timing back. Consequently, the power drops. And therefore the theoretical effect is significantly reduced. But the exhaust temperatures increase, which means that the risk of burning valves and pistons with such an engine is much higher. The second method is to reduce leaks. Let's start from the opposite: let's compare what will happen to the torque characteristic if we replace the rings so that the gaps in them become, say, twice as large. Done. For a new engine, everything is fine, for all cylinders the compression is 13.2...13.4 bar. For one damaged by rings with large gaps - 10.8...11.1. What did the power measurements show? In the low speed zone, the power of the damaged motor dropped slightly, but when we reached 2500 rpm, the torque curves almost merged. This is because leaks from the combustion chamber into the crankcase, which should reduce power, are noticeable only at low speeds, and at high speeds their mass per cycle drops sharply, because with a decrease in cycle time and an increase in crankshaft rotation speed, the time for leakage also decreases . Compression increased sharply, but power did not. Along with compression, detonation arose, and the ignition timing had to be shifted back. And it affects the power more.

No compression - straight to capital: the third tale

Typically, a mechanic who discovers low compression immediately declares: “The engine is worn out and needs some capital.” Is everything so clear? Of course not! For argument's sake, we can name twenty possible reasons for decreased compression. There are problems with the gas distribution mechanism, mechanical or thermal damage to engine parts, and coking of the piston rings. And only one of them will be associated with catastrophic wear of the engine. It is important to be able to distinguish between these causes, understand the degree of their danger and know methods to combat them. But this is a topic for a separate article.

The higher the compression, the better: the fourth tale

Often, from apologists for various additives, you hear how the compression jumped after the next treatment of the engine. Growth up to 15 bar, up to 17 bar! But we must keep in mind that in normal condition, even after restoring the gaps to the state of a new engine, you will not get compression higher than standard. Where do the numbers come from? Usually, on a disassembled engine, it is clear that after processing the combustion chamber is overgrown with something unknown and, as a result, the volume of the compression chamber has decreased. But these deposits interfere with heat removal from the combustion chamber. Hence detonation, glow ignition, etc. So we shouldn’t rejoice at the unprecedented increase in compression, but vice versa. Change in specific fuel consumption at fixed speeds (2500 rpm) in two engine variants - the base one and with rings in which the gaps are increased. Compression has dropped, but in terms of consumption this is noticeable only at low loads.

And it's not a fairy tale at all...

So what does compression do? For a lot! The main thing is the starting properties of the motor, especially at low temperatures. This primarily applies to diesel engines, where the pressure and temperature at the end of compression determine whether the fuel in the cylinder will ignite or not. But gasoline engines in a cold state are also sensitive to changes in compression: it affects the volatility of the fuel, which, during a cold start, should only theoretically evaporate on the way to the cylinder. But in reality, it gets there in the form of non-flammable liquid drops. Reduced compression increases crankcase gas pressure. In this case, a larger volume of oil vapor flies through the ventilation system to the engine inlet. This is bad: toxicity increases, and the rate of combustion chamber contamination increases sharply. Uneven compression across the cylinders causes engine vibrations, especially noticeable at idle and at low speeds. And this, in turn, harms both the transmission and the engine suspension. And to the driver himself. In short, the role of compression as a diagnostic sign, largely characterizing the condition of the engine, is very great. And our “fairy tales” in no way call for giving up on her - on the contrary! But the desire for an unbridled increase in it in search of additional “horses” is generally futile.

When it comes to the condition of an internal combustion engine, we often talk about compression. If it is low, then the situation is bad - the engine requires repair. What is this parameter?

Compression is the pressure in the cylinder that is created when the piston reaches top dead center. This is official, but now let’s look at this phenomenon in a simple way.

During engine operation, the piston of each cylinder makes cyclic movements up and down. In this case, in the upper part of the cylinder, the corresponding valves open and close at a certain period. As the piston rises, there is a period of time when both the intake and exhaust valves are closed, thus the gas in the cylinder begins to be compressed. When the piston reaches the highest position (Top Dead Center, TDC), the pressure in the cylinder will be at its strongest. This value is called compression.

What does compression depend on?

The overall “efficiency” of the engine depends on compression. At standard pressure in the cylinders, the engine will operate at maximum efficiency (provided that other parameters are normal). When compression is low, combustion of the air/fuel mixture in the cylinder will be inefficient because the pressure that should move the piston down will be partially bleed out of the cylinder. Such an engine will not be able to accelerate the car normally. In the worst case, it won't start at all.

Compression meter - a device for measuring compression in engine cylinders

Engine compression becomes extremely important when starting in cold weather. In order for the fuel mixture to ignite, it must be almost vapor. Gasoline evaporates well, but only when it’s warm outside. When it is thirty degrees below zero, gasoline vapors do not form and it is very difficult to set the mixture on fire. In a healthy engine, compression heats up the fuel mixture and increases the chances of it igniting. If the compression is too weak, this heating does not occur and the engine will not start.

Why does compression drop?

In order for normal pressure to be created in the cylinder, it is necessary that this cylinder be sealed at the right time. As mentioned above, when the piston goes up, both valves in the cylinder head are closed and there is nowhere for the gas volume to escape. In addition to closed valves, compression rings placed on the piston are also responsible for the tightness of the cylinder. They prevent pressure from escaping into the oil sump.

Compression rings are installed on the piston and ensure the tightness of the cylinder

Over time, any engine parts wear out: compression rings wear off, valves burn out or begin to close loosely. The pressure is released through the resulting cracks.

In fact, there are many more reasons for low compression. Here is their extensive list.

Reasons for low compression in the engine:

— Worn compression rings

— Burnt-out valve plates

— Valves that do not close tightly (bent valve stem, destruction of guide bushing, spring wedge, etc.)

— Incorrect valve adjustment

— Incorrect operation of the gas distribution mechanism

— Crack in the cylinder head

— Failure of the cylinder head gasket

— Excessive wear of the cylinder walls

Compression in engine cylinders is measured with a special device - a compression meter.

– an excellent diagnostic method. With its help, you can quite accurately determine many faults without resorting to disassembling the engine. When compression is restored after repair, any driver feels it himself. A serviceable engine with standard compression pleases the owner with throttle response, low fuel consumption and maximum power.

Compression is one of the main indicators of engine viability. Directly affects the longevity of the power unit without major repairs. If the compression is too low, the engine will soon need help. If the parameter is within normal limits, then the power plant will work for tens of thousands of kilometers without problems. In this article we will talk about the role of compression in the life cycle of an internal combustion engine, the features of its testing and possible results.

What is engine compression?

Compressio (“compression”) is translated from Latin as “compression.” This term refers to the maximum level of air pressure in the combustion chamber created during cranking by the starter at the moment the piston reaches its highest point. Compression is the pressure measured at the end of the compression stroke. The technical condition of the motor and the following processes depend on its level:

• oil consumption;
• combustion of gasoline or diesel fuel;
• ease of launch.

The higher the compression parameter, the better the working mixture is compressed. As a result, foreign gases do not enter the engine crankcase. This increases the efficiency of the power unit. Compression corresponding to the indicator established by the automaker is a guarantee of optimal operation of the internal combustion engine.

In what units of measurement is it indicated?

• pascals (Pa);
• kilograms per square centimeter (kg/cm2);
• atmospheres (atm).

About the differences between compression ratio and compression

It is worth distinguishing between the concepts of “compression” and “compression ratio”. The first is measured in certain units (pascals, atmospheres, etc.), but the second is not. The compression ratio shows how many times the size of the combustion chamber is smaller than the total volume of the cylinder. This is a constant value that is indicated by the car manufacturer in the technical documentation.

It turns out that the compression ratio directly affects the compression. But not the other way around! Compression changes due to a huge number of operating parameters (adjustment of valve timing, etc.). It is also affected by the presence or absence of leaks in the combustion chamber, which appear due to problems with the cylinders and rings.

The influence of compression on the performance of the power unit

Starting the engine depends on this pressure indicator, especially during the winter period of vehicle operation.

Compression should definitely be of interest to owners of cars with a diesel power plant. In engines running on heavy fuel, the ignition process of the fuel is directly affected by both temperature and pressure at the end of compression.

Gasoline units are better adapted to compression changes. However, it is still necessary to monitor it. The main reason is that compression affects the volatility of the fuel. This is very important when starting the engine “cold”.

About reduced and uneven compression across the cylinders

Different compression values ​​in fuel cylinders contribute to the appearance of vibrations and unpleasant jerking at idle and low speeds. This mode of operation of the internal combustion engine negatively affects the technical condition of the gearbox and engine suspension. Moreover, the driver and passengers become less comfortable during the trip.

A reduced compression rate causes increased exhaust gas pressure. As a result, the toxicity of the exhaust increases, and the combustion chamber becomes dirty faster.

What compression is considered optimal for a diesel and gasoline engine?

To determine the optimal pressure at the end of the compression stroke, you must use the following formula.

Compression = special coefficient for internal combustion engines x compression ratio

The coefficient is indicated for each class of power plants. For 4-stroke gasoline engines with an injection power system, this parameter is 1.2-1.3. Thus, for power plants with a compression ratio of 8 to 9, the compression ranges from 10.4 to 11.7 atm.

The situation is different with diesel engines. Compression in a diesel engine is significantly higher compared to gasoline engines. In order for the fuel to heat up to the temperature required for ignition, a pressure of 25 to 33 kg/cm2 will be required. The final value depends on the outside temperature and the technical condition of the engine.

What factors influence compression?

The fuel cylinder is a working chamber of volumetric displacement. It is a closed-type system consisting of a large number of parts:

• valves;
• piston and rings;
• cylinder walls.

Each element affects the performance of the system. If one of them is damaged or severely worn, the pressure decreases. A cracked piston will lead to exhaust gas leakage. This will have a bad effect on power characteristics.

The pressure level often decreases due to damaged or dirty rings. Incorrectly set valve timing, low-quality composition of the working mixture, and an incorrect injection timing angle also cause the formation of carbon deposits in the valves. This ultimately leads to loss of pressure and further problems.

How to check compression correctly and with what?

The main diagnostic equipment used to measure compression is called a compression meter. It looks like a device with a check valve, a connecting hose and a special fitting. Measurements are carried out as follows: when the crankshaft rotates, air is pumped into the hose until the pressure in it and the cylinder is equalized.

When measuring, adhere to the following rules:

1. It is prohibited to take measurements on a newly started engine. The power unit must run for some time. Then the fuel supply should be stopped. This can be done by turning off the fuel pump.
2. The spark plugs must be unscrewed. Please note that it is necessary to dismantle each device to ignite the working mixture.
3. Measurements should be carried out with a working starter and a charged battery.

Compression is measured in 2 ways - with the throttle valve open or closed. Each of them allows you to find faults. Test results will vary. When the damper is closed, a low level of compression is observed (about 0.7 MPa). Air leaks are comparable in volume to the air entering the fuel cylinder. Therefore, testing with the throttle valve closed gives more accurate results. Compression can decrease even with small leaks.

A test with the damper open gives a different result. It will be 0.8-0.9 MPa. As a result, there is a smaller volume of leaks with an increased air supply.

If you compare the two methods, checking the compression with the valve closed allows you to find minor engine defects. To identify more serious damage (burnout of valves, coke formation on rings, etc.), it is better to use the second method.

It is important to monitor the dynamics of pressure increase. If the car owner is attentive and focused, then the nature of the breakdown will become easier to determine. For example, in the first stroke there is a low compression (about 0.4 MPa), and in subsequent strokes its significant increase is noticeable. These results indicate ring wear.

About testers

The tester is another tool for taking measurements. It has a different operating principle. Unlike a compression gauge, it records changes in the ripple current required by the starter during cranking. Thanks to this device, the task is simplified for owners of cars with internal combustion engines that have many cylinders. You can find out the results in a couple of crankshaft revolutions. In this case, there is no need to dismantle the glow plugs.

The main flaw in the work of testers is the display of results in relative units. For example, an electronic device indicates the difference in the performance of one cylinder to another as a percentage. Only the most functional and expensive testers measure pressure and show the result for each cylinder.

What information do measurements provide?

1. The difference in the cylinders is more than 0.5 atm. This is evidence of leaking rings. The problem with increased compression is often solved by replacing these consumables.
2. Reduction in each cylinder by 0.5 atm. If a decrease in the compression level is observed along with an increase in fuel consumption, a decrease in thrust of the internal combustion engine and the appearance of bluish-colored exhaust gases, then the problem is in the rings and valve stem seals.
3. The difference between the cylinders is more than 3 atm. Changes in pressure along with blue or white smoke from the exhaust system indicate problems with the cylinder head gasket.
4. A drop of 5-6 atm in one, two or all cylinders. A burnt-out valve is often to blame for a large reduction in compression.

If the measurement results do not reveal a decrease or increase in compression, but there is an increase in fuel consumption, a drop in power and a change in the color of the exhaust gases to dark gray, then the problem may be as follows:

• fuel injectors;
• glow plugs;
• improper operation of the carburetor;
• wear or damage to oil scraper rings and caps (if there is a decrease in oil level).

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What to do if the compression is low?

With low compression, difficulties arise with cold starting of the power unit. Excessive consumption of fuel and oil and deterioration in acceleration dynamics become noticeable. In power plants running on gasoline, the spark plugs are also filled.

A low level of compression is evidence of leaks in the combustion chamber or the appearance of large gaps between individual parts of the cylinder-piston group. The problem may also be a large amount of carbon deposits on the valves, destruction of the piston, wear of the oil scraper rings, defects in the cylinder head gaskets, etc.

Very often, in case of serious compression problems, it is necessary to disassemble the power unit. For example, for boring or lining a BC, replacing pistons and rings, and performing other work.

If the engine has a high mileage, then major repairs are indispensable. However, low compression is not always a death sentence for an internal combustion engine. Auto mechanics who, after checking, assure the vehicle owner that it is 100% necessary to overhaul the engine, are either mistaken or disingenuous.

The problem is that low compression is associated with less serious defects, the elimination of which does not require large amounts of money. First of all, this applies to:

• Coked rings;
• The need to regulate the gas distribution mechanism.

Therefore, the ability to determine its true cause comes to the fore in solving a problem with low compression. Owners who have little understanding of the technical part of the vehicle are convinced that very high compression is always good. This is partly the merit of auto chemical sellers who talk about the “magical” properties of additives. These drugs really work. However, the increase in compression occurs due to changes in the compression chamber, which simply decreases in volume.

If you disassemble the engine after prolonged use of special additives and additives, you can find a coating of unknown origin. Thus, the purchase and use of auto chemicals is only a temporary solution to the problem. The additive does not correct the problem. Additional formations in the compression chamber impair heat dissipation, which leads to detonation and the appearance of a “glow ignition” effect.

Video: Engine compression