05-14-2009 06:42 PM
I have four engines of the type MAK 9 M20 C in the engine room on my ship. On the engine is put a sign telling med that "decompression valves" must not be used for indicating. The cylinder heads are equipped with indicator valves and I want to make measurements off compression and max pressure using a Kiestler peak pressure indicator.
I am in doubt about if I can do this measurement due to the sign on the engine, as I find hard to believe. I have been a marine engineer for 30 years now, and this is the first time I hear about that the indicator valves can not be used for what they are supposed to be used for. Can anybody give a comment or explanation for this. Is it due to the design of the cylinder head or the quality of the valve who is to poor to do this important task as a pressure measurement is. I will appreciate a comment on this matter. Thanks in advance.
05-20-2009 02:59 AM
That is right, with electronic indicators one can figure out a lot of things from the (combustion) pressure pattern measured on the indicator valve, e.g. determination of power output or ignition point and compression pressures, etc., etc.
But all this only makes sense if the pressure at the indicator connection equals combustion pressure.
However, due to the indicator bore in the cylinder head that causes pipe resonances this is not the case!
That is why the sign is provided on the engine.
Hope that this helps.
05-20-2009 03:43 AM
Thanks for the explanation on the meaning of the sign on the MAK M20 engine.
I am well aware of that measuring the combustion process on medium speed engines may give a wrong impression due to resonans and pressure fluctrations in the bore in the cylinder head. Due to this fact I agree on that the electronic indicator is not suitable for power measurement on this type of engine.
However I see that most engine manufacturers accept the use of electronic indicators like Kiztler or Premet to get a monitoring of compression and peak pressures. This only for monitoring purpose and to give the chief engineer and owner some information on that the engine is in good and proper running condition.
From your answer I understand that there is no risk for any damage to the engine by measuring compression and combustion pressures, but that the results from the reading should be considered as not an exactly description of the combustion process in the cylinder and therefore not be used for any adjustment on the engines settings.
06-15-2009 08:10 AM
Sorry for not answering earlier, but maybe the following response gives you more information:
The valves provided in the cylinder head of our M 20 engines are decompression valves and should not be used for indicating.
Due to factory pre-setting and the tight tolerances met in the manufacture an analysis of firing pressures is not required any more on our M 20 engines.
The decompression hole in the cylinder head is designed such that a mechanical indicator does not record the pressure surges and the superimposed
vibrations correctly any more and would produce a wrong impression of the firing pressure.
On the MaK test bed the settings of the engines are made with an electronic indicator and they must not be readjusted or changed in the course of further operation.
The ground spacers under the injection pumps are intended for adjusting the commencement of fuel delivery/firing point. This is carried out before factory acceptance test and the spacers are then pinned and it is not permissible to change or replace the spacers. The output of the engine and with that also the firing pressure of the cylinders is controlled via the fuel rack position of the injection pumps.
Of course, the customer may indicate, but the measured values should only be used to compare the cylynders to each other.
06-29-2009 06:20 AM
"Due to factory pre-setting and the tight tolerances met in the manufacture an analysis of firing pressures is not required any more on our M 20 engines."
Can i apply this to my engine? I'm actually using a electronic Premet XL indicator on my 12 VM43 A.
01-11-2010 10:06 AM
The combustion pressure of all MaK engine types (legacy engines, M25, M32, M43) can be measued. If there is any need to measure the combustion pressure on M20 engine due to serve engine problems it is no problem. Be aware that the absolute values are not correct. But to get a hint about the origin of the trouble the values are sufficient for comparison.