ENGINE DIAGNOSTICS WITH PRESSURE TRANSDUCER
The position of the characteristic points and sections of the cylinder pressure waveform of a gasoline engine, makes it possible to determine the proper synchronizationbetween the crankshaft and the gas-distributor shafts. Measurement and comparison of the absolute cylinder pressure values at certain characteristic points, allows determining the condition of the cylinder seals. The pressure at the Top Dead Center (TDC) on a straight four-cylinder engine, fluctuates between 4.5 and 6 bars. Smaller values indicate that there may be serious mechanical defects in the tested cylinder, and the larger ones – an extra air is entering the cylinder or there is high engine load.
During the fuel suction of the air mixture, the dilution in the cylinder reaches 0.65...0.75 bars and is greater in absolute value than the average dilution in the intake manifold. In the exhaust flow rate, the pressure in the cylinder practically does not exceed the atmospheric pressure. Increasing of this pressure can be caused by clogging of the drainage ducts, a small stroke of opening the exhaust valve, clogged catalyst or exhaust.
1. Distinctive features when observing the measured waveform
Cylinder pressure waveform allows you to determine:
- The real ignition advance angle by the ratio of TDC to the high voltage pulse. For this purpose, besides the pressure sensor in the cylinder of the second channel of the oscilloscope, an inductive probe has to be connected to the high voltage cable of the test cylinder.
It should be noted that the angle thus obtained is real and may not match the angle read by the code reader. If there is a large discrepancy between the two values, it is good to check the crankshaft disk.
- The condition of the engine mechanics by observing the pressure difference before and after the compression (approx.).
Fig.1. Normal engine operation
When engine is operating properly, the pressure at point A should be approximately equal to the pressure at point B. If the cylinder has serious mechanical defects (burnt valve, broken segments, failure in the gas distribution mechanism), the pressure at point A will be noticeably larger compared to the pressure at point B due to significant leakage of compressed gases in the cylinder. An example is shown below:
Fig.2. Cylinder mechanical defect
- The correct position of the exhaust camshaft by observing the opening angle of the exhaust valve.
It is necessary to use the 720 degree measuring line on the automotive oscilloscope that is used to perform this test. Measure the angle between the TDC until the exhaust valve is opened. On most engines this angle is between 140 and 145 degrees, and only on some Opel engines, this angle is 160 degrees.
- The correct position of the intake camshaft by observing the valve overlap position and the opening of the intake valve. The moment of closing the intake valve should be 580 degrees on the cylinder pressure waveform, for example. The correct alignment of the intake camshaft can be determined both by the valve overlap position, and the moment of closing (580ᴼ) of the intake valve.
-The condition of the exhaust valve guide sleeve by the shape of the waveform.
The section of the waveform during the period of time during which the exhaust valve opens completely, should be as smooth as possible. The presence of pulsations indicates that there is considerable wear on the exhaust valve guide sleeve. This is shown on the waveform below:
Fig.3. Significant wear of theexhaust valve guide sleeve
- The permeability of the exhaust system by the pressure value at the exhaust phase in the area of the waveform between 180ᴼ and 360ᴼ. During this period, the pressure in the cylinder is equal to the pressure in the exhaust manifold. In other words, the cylinder pressure fluctuates slightly above the atmospheric pressure. This pressure is assumed to be perfectly normal if it is within the range of 0.1-0.15 bar (relative). If it is considerably larger, up to 1-1.5 bar, this indicates that there is clogging of the exhaust.
- The presence and the vacuum value in the intake manifold.
The average value of the vacuum in the intake manifold for the upright engine, must be about 0.6 bar.
- The presence of a gap in the timing belt by the difference in the angles of overlapping of the valves from one frame of the waveform to the next frame.
When considering the waveform when the engine is idling, if it is indicated that the position (360ᴼ) of the intake and exhaust valves changes from frame to frame, this is indicative of timing belt loosening.
2. Specific points and sections in the cylinder pressure waveform
For greater convenience, the specific points and sections of the measured cylinder waveforms, are numbered.
In order to perform a diagnostics of the engine mechanical condition by the pressure graph in the cylinder, it is necessary to:
- put the pressure sensor in place of the spark plug on the cylinder which you want to diagnose. For this purpose, an ignition extension lead will most likely have to be used;
- an ignition discharger must be connected to the high voltage cable of this cylinder in order not to damage the ignition system;
- power the pressure sensor by plugging the red and black cables to the corresponding battery terminals;
- connect the pressure sensor signal cable to the oscilloscope input;
- the engine must be preheated to operating temperature and idling;
- Measurement time should not exceed 2-3 minutes to keep the pressure sensor temperature below 70 degrees.
Fig.4. Cylinder pressure waveform
Point 1 (or TDC 0°)
At point 1, the cylinder pressure reaches its max. At this point, the plunger is at least a distance from the block head. This position of the piston is called the Top Dead Center (TDC). At the moment of TDC, intake and exhaust valves are closed. Cylinder pressure in point 1 can vary considerably depending on compression rate, cylinder seal state, crankshaft rotation speed, and the amount of compression mixture in the cylinder. The more fuel mixture in the cylinder, the higher the value will reach the cylinder pressure at point 1.
Point 2 (30°)
After TDC 0 °, the piston changes its direction of motion, starting to move away from the cylinder block head. As a result, the volume between the piston and the cylinder head begins to increase and the cylinder pressure decreases. When the crankshaft rotates 30 ° after TDC 0 °, the cylinder pressure will be half the maximum at point 1 and the minimum in the cylinder (point 4). This is indicated on the waveform as point 2.
Point 3 (90°)
When the piston passes point 3, it continues to move away from the cylinder head at an increasing speed until the crankshaft does not rotate 90 ° after TDC 0 °. The piston has passed half of its stroke and its maximum speed is here. After this point, 90 ° after TDC 0 °, the piston stroke speed begins to decrease. This point is indicated as point 3. At this point the pressure in the cylinder will be close to the atmospheric with a deviation of ± 0,5 bar. As the piston movement continues, the volume between the piston and cylinder head continues to increase and a dilution occurs in the cylinder after point 3.
Before the piston reaches the Bottom Dead Center (BDC), the exhaust valve opens. The moment of opening of the exhaust valve is indicated as point 4. The piston still moves away from the cylinder head and the volume between the piston and the cylinder block continues to increase. But, starting from point 4, the absolute pressure in the cylinder increases. This is because the exhaust gases enter the exhaust manifold through the opening exhaust valve.
Section 5 (BDC 180°)
The entry of gases from the exhaust manifold into the cylinder occurs because the absolute pressure in the exhaust manifold is close to the atmospheric, which is greater than the absolute pressure in the cylinder. On the cylinder pressure waveform, the area where the exhaust gas entering the exhaust manifold enters the cylinder is marked as section 5. The center of section 5 must be at the bottom dead center (BDC 180 °).
Fig.5. Section 5
If the center of section 5 lies within the range of 170 ° - 195 ° after TDC 0 ° (-10 ° ... + 15 ° from BDC 180 °), then the start of the opening of the exhaust valve is assumed to be correct.
The pressure in the cylinder increases until it equalizes with the pressure in the exhaust manifold. The point of the waveform, where the cylinder pressure equals the pressure in the exhaust manifold, is marked with as point 6.
Once the piston has reached BDC 180 °, it starts to move towards the cylinder head, resulting in a reduction in the volume between the plunger and the cylinder head. This gradual reduction in volume forces the gases in the cylinder to flow into the exhaust manifold through the opened exhaust valve. The piston speed continues to rise until the crankshaft rotates to 270 ° (90 ° after BDC 180 °). After passing 270 degrees, the piston speed decreases. The area through which the gases in the cylinder flow into the exhaust manifold is indicated by as section 7. During this section, the pressure in the cylinder must be nearly equal to the atmospheric pressure. Increasing the pressure in the cylinder by more than 0.5 bar relative to the atmospheric pressure in the middle of the section 7 is an indicator of a difficult gas leakage from the cylinder. This may be due to inadequate opening of the exhaust valve or clogged muffler.
Point 8 (330° ÷ 360°)
Approximately between 30 ° before TDC (360 °), the intake valve opens. The moment of the opening of the valve is indicated as point 8. When the intake valve is opened, the cylinder is connected to the intake manifold where the absolute pressure is significantly less than the cylinder pressure. But the pressure in the cylinder continues to equalize with the pressure in the exhaust manifold through the still opened exhaust valve. Because of this, it is not possible to precisely locate point 8 of the cylinder pressure waveform.
When the piston reaches TDC 360 °, the exhaust valve is closed. The intake valve continues to open. As a consequence, the pressure in the cylinder begins to equalize with the pressure in the intake manifold. Since the absolute cylinder pressure is close to atmospheric, the gases from the cylinder start to flow into the intake manifold, where the pressure is significantly less than the atmospheric pressure. This area of the plot is indicated as section 9. The center of section 9 should be 380 ° after TDC 0 ° (20 ° after TDC 360 °).
Fig.6. Section 9
If the center of section 9 lies within the range of 370 ° to 390 ° after TDC 0 °, the moment of opening of the intake valve is assumed to be set correctly. For VVTi engines, the center of section 9 should be within the 380 ° to 400 ° range after TDC 0 °.
At point 10, the pressure in the cylinder is equal with the pressure in the intake manifold as the intake valve is substantially opened.
Section 11 to the second BDC (540°)
Although the piston moves downwards and the distance between the cylinder head and the cylinder head increases, the cylinder pressure drop is not observed as air from the intake air manifold flows through the intake valve.
Section from the second BDC (540°) to Point 12
After BDC 540 °, the piston begins to approach the cylinder head again. In other words, the compression phase begins, but the intake valve still remains open for some time. This serves to improve the filling of the cylinder with a fuel air mixture.
Point 12 (580°)
This is the end of the intake valve closing. From this moment on, the absolute cylinder pressure begins to increase relatively intensively. If the moment of complete close of the intake manifold is within the range of 560 ° to 600 ° after TDC 0 °, the moment for the end of the intake valve closing is assumed to be set correctly.
Point 13 (630°)
At this point, the pressure in the cylinder will be close to the atmospheric ± 0.5 bar. But due to the movement of the piston at closed intake and exhaust valves, the compression continues to increase.
Point 14 (690°)
This point is 30 ° before TDC 720 °, where the cylinder pressure will be approximately half of the minimum in the cylinder (point 12) and the maximum cylinder pressure (point 1). Once the piston has passed this position, the compression continues to increase until it reaches TDC 720 °.