Dead rings or a crack in the valve are significantly more common causes of reduced compression than engine wear.
Compression is vulgarism. Correct - the pressure of the end of the compression stroke. This is the pressure that is created in the cylinder when the ignition is off (or without fuel supply for a diesel engine) when the piston is at top dead center. So, many diagnosticians in terms of measured compression (sorry, science, for jargon!) Give a conclusion: “the patient is alive” or “to the morgue”, that is, for major repairs. According to many advanced motorists, compression for the motor is almost everything! But is it?
Compression and compression ratio are one and the same thing: the first 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 amount of space above the piston when it is in TDC (it is also called the compression end volume - it’s the same). It’s incorrect to call it a combustion chamber, since fuel combustion occurs in the entire cylinder volume.) Compression depends on the compression ratio, but the compression ratio does not! Compression also depends on a bunch of parameters: the pressure of the beginning of compression, the adjustment of the valve timing, the temperature at which measurement is carried out, and leaks from the combustion chamber. And leaks are determined by the wear of the rings and cylinders. “Compression” is the maximum pressure that we measure in the cylinder with the ignition off.
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Raised the compression - increased power: the second tale
Not certainly in that way. Compression can be raised in two ways - to increase the degree of compression or to reduce leaks from the combustion chamber. Let's see what will happen in each case: we have a stand at our disposal. First, we’ll reduce the volume of the compression chamber. The easiest way to do this is to grind the lower plane of the cylinder head. The base engine of the Eleventh VAZ has a cylinder displacement of just over 370 cubes. With a standard compression ratio of 9.8, the volume of the compression chamber will be 42.6 cm³. It can be calculated 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 motor. And now, just out of interest, we remove another 2 mm. The compression ratio is already increasing to 12.6. In the textbook we find the necessary 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%. Wow! And now we put these heads on the bench motor and remove the moment 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 small loads. The increase in power is even less: from a force of 2-3%, moreover, in the zone of small and medium revolutions. But at high - no effect … Everything is clear: with an increase in the compression ratio, the pressure in the cylinder increases sharply, this growth 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 temperature at the outlet is growing, and so the risk of burning valves and pistons with such an engine is much higher. The second way is to reduce leaks. Let's go from the opposite: compare what will become of the moment characteristic if we replace the rings so that the gaps in them become more, say, two times. Made. For the new engine - everything is fine, for all cylinders the compression is 13.2 … 13.4 bar. For spoiled rings with large gaps - 10.8 … 11.1. And what did the power measurements show? In the low-speed zone, the power of the damaged motor fell slightly, but when 2500 rpm went over, the torque curves almost merged. This is because leaks from the combustion chamber to the crankcase, which should reduce power, are noticeable only at low revs, and at high their mass decreases sharply in one cycle, because with a decrease in cycle time with increasing crankshaft speed, the leakage time also decreases. Compression increased sharply, but power did not. Together with compression, detonation woke up, and the ignition timing had to be shifted back. And it affects power more.
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No compression - immediately to kapitalku: the third tale
Usually, a mechanic who finds a low compression immediately declares: "The engine is worn out, capital is required." Is it all so clear? Of course not! For the argument, we can name twenty possible reasons for the decrease in compression. There are problems with the gas distribution mechanism, and mechanical or thermal damage to engine parts, and the 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 the methods of dealing with them. But this is the topic of a separate article.
The higher the compression, the better: the fourth tale
Often from apologists for various additives, one has to hear how the compression jumped after the next processing of the motor. Growth up to 15 bar, up to 17 bar! But we must keep in mind that in a normal state, even having restored the gaps to the state of a new engine, compression cannot be obtained above the standard one. Where did the numbers come from? Usually on a disassembled engine it is seen that the combustion chamber after processing is overgrown with incomprehensible what and, as a result, the volume of the compression chamber has decreased. But these deposits disrupt the heat removal from the combustion chamber. Hence the detonation, ignition and so on. So an unprecedented increase in compression is not necessary to rejoice, but rather. Change in specific fuel consumption at fixed speeds (2500 rpm) in two engine versions - basic and with rings in which the gaps are increased. Compression has fallen, but in terms of consumption it is noticeable only at low loads.