Compression pressure
Compression pressure is a term used in reciprocating piston engine technology and describes the pressure inside a cylinder, for example in internal combustion engines or compressors , that is generated by the piston stroke.
Measure compression pressure
In internal combustion engines, the spark plugs or injection valves are replaced by the measuring device to measure the compression pressure . The manufacturers of compression pressure recorders provide the appropriate adapters for this purpose. The suitable connection piece for the motor can be determined via an adapter database. The engine is brought up to starting speed with the starter and the peak pressures are measured cylinder by cylinder one after the other . In gasoline engines, measurements are taken with the throttle valve fully open in order to avoid intake throttling.
As a rule, compression measuring devices are to be found separately according to gasoline and diesel measuring ranges. The measuring device can be a writing device that generates an arc line on a cardboard card with a writing pointer from the spring-loaded pressure membrane , the end value of which shows the compression pressure. With these devices, the card for the next cylinder is pulled out a little so that a parallel curved line is created with further measurement. Measuring devices without a written record contain a one-way valve that maintains the maximum pressure in the display and allows the pressure to be released for the following measurement by pressing a button. Another variant is a pressure gauge with a drag pointer, which is set to zero by hand for the next measurement.
The usual compression values of modern vehicle engines are in the range of 11 to 15 bar for new, run-in Otto engines and approx. 30 to 55 bar for diesel engines; Pre-chamber and swirl chamber diesel engines have higher compression values than direct-injection diesels. Values below 8 bar for gasoline engines or below 20 bar for diesel engines are unusual in most modern engines and indicate wear or a defect. Older engine designs (for example VW boxer engines) have correspondingly lower default and limit values. There are also special designs with different compression data (two-stroke engines, engines with positively controlled valves , "medium-pressure engines " from Audi from the 1960s). In common workshop manuals, the regularly expected measured pressure values are specified broken down according to engine type.
In the event of a suspected leak in one of the cylinders, the measurement is repeated in order to narrow down the type of leak more precisely. For this purpose, a small amount of engine oil is added to the combustion chamber before the second measurement, which improves the seal between the piston and the cylinder through the piston rings. If the measured value is then significantly higher, this indicates advanced wear in the area of the cylinder, piston and piston rings. Consistently poor compression values, however, suggest leaks in the area of the cylinder head, head gasket and valves.
To assess wear, the delivery rate at operating pressure is usually measured for compressors, and for pumps for liquids, and not the compression pressure.
Alternative measurement methods
The pressure loss measurement is also used instead of the compression pressure measurement. When the machine is at a standstill, a defined pressure is built up in the cylinder. Then the pressure supply is turned off and the time for the pressure drop between two reference pressures is measured. The pressure loss measurement can be carried out with gases (e.g. air) as well as with liquids (e.g. oil).
In the past, "indirect compression measurements" were sometimes carried out on vehicles: in the first Volkswagen with a diagnostic socket in the 1970s, attempts were made to draw conclusions about the compression of the individual cylinders from the periodic course of the voltage drop during the starting process - a rather imprecise and possible error only roughly indexing method. If error indications were discovered in this way, it was usually advisable to carry out a "regular" compression test after the first diagnosis.
Since EURO6: Modern glow plugs in diesel engines can also be equipped with integrated pressure sensors. Today's Euro6 engines use z. B. one of these pressure sensor glow plugs in one cylinder and conventional glow plugs in the remaining cylinders. As soon as, as in individual diesel engines, all cylinders were equipped with such glow plugs with pressure sensors, the compression pressure measurement would also be possible, which would take place via the engine control unit.
Conclusions from the measurement result
In addition to the level of compression pressure, the uniformity between the cylinders is also important. Therefore, in addition to the target value for the compression pressure and the wear limit, the maximum permissible pressure difference between the cylinders is specified for internal combustion engines. Consistently poor compression levels usually occur with a high mileage engine approaching its wear limit. Very different compression values between the cylinders are a characteristic of a local defect and can also occur in new engines. Smaller differences can (in rare cases, after running-in procedures) sometimes improve or approach one another.
literature
- Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology. 27th edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 2001, ISBN 3-8085-2067-1
- Peter A. Wellers, Hermann Strobel, Erich Auch-Schwelk: Vehicle technology expertise. 5th edition, Holland + Josenhans Verlag, Stuttgart, 1997, ISBN 3-7782-3520-6