This simple technique has changed the way the internal combustion engine is diagnosed around the world. If you are not aware of the power of this simple truth please read the Motor Age articles “Mastering In-Cylinder Pressure Testing” I, II, and III. These articles cover the basics of in-cylinder compression waveforms at crank, idle and snap throttle.
This technique is not only good when used to find failures, but is great in eliminating possible problems as well. I can recall a time when checking a drivability problem would include checking the ignition timing, not necessarily to find an ignition timing problem but to rule one out. The in-cylinder compression waveform can be used in the same way. Many times in my career I have found myself diagnosing a problem with an engine I thought was of a mechanical nature, but with no way to confirm my suspicions. Now, with this game-changing technique, you can test the mechanical condition of the engine quickly and accurately.
The spark plug is removed from cylinder No. 8 and a 300 PSI transducer installed. The engine is then started and allowed to idle. The in-cylinder running pressure waveform taken from cylinder No. 8 is shown in Figure 1. This waveform clearly shows a problem, which can be seen by comparing it to an in-cylinder compression waveform taken from good cylinder No. 6, as seen in Figure 2. Let’s look at problem cylinder No. 8 in Figure 1. Point A is where the piston came as close to the cylinder head as mechanically possible; this is a true Top Dead Center (TDC) position. Point B is where the exhaust valve opened far enough to establish flow. This can be seen by the pressure changing direction at the 40-degree marker located before Bottom Dead Center (BDC). The pressure then starts to rise and crosses the BDC marker with -6 PSI, this would indicate a problem is present with cylinder No. 8. This pressure at the BDC marker should have risen to 0 PSI. Let’s look at Figure 2. It can clearly be seen at point B that the exhaust valve opening occurred and the pressure is changing direction at the 40-degree marker located before BDC. Once the exhaust valve has opened, the pressure changes direction, quickly rising to 0 PSI at point C located just before the BDC marker. The pressure rise to 0 PSI happens in less than 40 degrees of crankshaft rotation. Now, let’s look at Figure 2. At point C where the pressure achieves 0 PSI, there is some 110 degrees of crankshaft rotation after the exhaust valve opened. As the piston continues to rise toward the cylinder head, the pressure continues to rise to its peak pressure of 13 PSI, located at point D just before the TDC marker. When compared to good cylinder No. 6 in Figure 2, the pressure located at D is very close to 0 PSI.
Now let’s look at Figure 3. This is a snap throttle waveform of cylinder No. 8. Point A is the peak pressure within the cylinder located at TDC. Point B is where the exhaust scavenge cycle is started. As the piston moves toward the cylinder head, the volume becomes less, forcing the exhaust out of the cylinder and into the exhaust system. At point C the pressure has increased to 64 PSI and then drops just before TDC. This drop occurs due to the intake valve opening, allowing the high pressure within the cylinder to move to the low pressure within the intake system. In Figure 4, good cylinder No. 6 is shown on a snap throttle event and it can be clearly seen that no pressure increase occurs as the piston comes up to the TDC marker located at point C.