The internal combustion engine has been around for over 100 years! An engine is subject to enormous loads and thermal events that will eventually cause wear to engine components and the engine itself. When this occurs it can cause a rough running engine, engine noise and general drive-ability engine performance issues.
Since the inception of the Otto engine mechanics of past and present are still tasked with diagnosing the cause of a rough running engine. It is imperative that the technician be able to validate the breathing, sealing, compressing actions of the engine. The technician must be able to verify that the mechanical functions of the engine are operating as designed.
A very essential test is the static compression test. This test utilizes a compression gauge that will measure the peak compression that each cylinder can produce. Like a doctor, the technician must not only do one test to validate the diagnosis. If the compression is lower than normal, the technician then should perform a cylinder leakage test. This test will utilize a pressure differential gauge that will insert a pressure into a cylinder when it is at TDC. What we are looking for is the ability for the cylinder to seal properly.
It's not uncommon for a technician to find that the compression pressure and leak-down pressure results indicate proper engine integrity and still have a mechanical problem with the engine! How is this possible you might ask if the peak pressure is good and the cylinders can seal properly.
From my experience I have found that the most effective way to validate cylinder integrity is to test the cylinders 4 strokes while the engine is running! Yes...........this test is done while the engine is running! Why you might ask! We must remember that 99.9% of your engine problems will occur when the engine is running. So why not test it when its running! The traditional compression test happens at cranking rpms of 200-350 rpms give or take. A running engine at idle can have rpms of 500+. Your compression test at engine running speeds will be able to test peak pressures under real engine dynamics and thermal loads.
What new tools are needed to do a running compression test. First off lets get all the names of this test out of the way. The general terms used for this test are Dynamic compression test, running compression test, cylinder dynamic test, in cylinder running compression and the list goes on! I will not go into the specifics about how this test is physically done but will describe the benefits of this test.
The Dynamic compression test is testing the true peak pressure at running speeds. This pressure will actually be much less than static cranking compression for a couple of reasons. The technician must understand this so he does create a mis-diagnosis. The pressure is less due to the fact that the 4 strokes are simply occurring faster and the actual breathing and compression is affected by this fact. An engine is actually a very energy wasting machine. Thermal losses, frictional losses of energy rob the engine of power. Another loss is what is called throttling losses, the fact that the air intake has a throttle plate restricting the air intake into a cylinder will lower that compression pressure. The technician must be aware of all of this to be able to execute an effective and accurate diagnosis from the test.
Now I will delve into the cons of the running compression test! Yes of course every test has its draw backs. The draw back is the simple fact that this running compression test is again only measuring peak pressure. I was once asked during an interview with the leading Hybrid Mfr in the world how an engine works! It was a sly question that came out of left field. My answer was simple, 4 strokes.....but lets visual this, think of a sine wave going from positive to negative peaks over and over. Intake would being the downward slope of the sine, compression the upward part of the sine, power a downward sine and exhaust an upward sine!
Since the basic compression gauge is only measuring peak pressure, how can I truly see all the stroke events and its effect on vacuum/intake, compression, power and exhaust strokes? How can I see this sine wave effect of the 4 strokes? My tool of choice involves advanced engineering tools that involve a High quality Digital storage Oscilloscope coupled with a pressure transducer. This tool will allow the technician to........be continued!
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