Compression Peak Pressure:
During the compression stroke, the intake and exhaust valves are closed, and the piston moves upward. The air (or air/fuel mixture) is compressed as a result. Once the piston reaches TDC (top dead center), the maximum compression pressure is achieved. This is called the compression peak pressure. As soon as fuel is added to the air, the spark plug will ignite the mixture. The combustion pushes the piston down and rotates the crankshaft.
The compression peak pressure is influenced by factors such as the compression ratio.
Measuring Compression:
With a compression peak pressure that is too low, the maximum energy cannot be extracted from the fuel, causing power loss. If only one cylinder’s compression peak pressure is too low, the engine will shake and vibrate, often resulting in a fault code related to cylinder misfire.
By measuring compression, one can assess the compression peak pressure of the engine. A self-recording compression gauge registers the pressure built up in the cylinder. The technician determines if the compression peak pressure is acceptable based on this reading.
Procedure to measure compression:
1. Ensure the engine is at operating temperature. The engine components expand due to heat, making the readings realistic.
2. Remove the spark plugs.
3. If possible, disable the fuel supply by disconnecting the injector connectors. The injectors won’t be activated during cranking, preventing unburned fuel from entering the engine.
4. Insert the compression gauge into the spark plug hole. The rubber end of the compression gauge provides a seal between the gauge and the cylinder head.
5. Have someone else start the engine while keeping the accelerator pedal fully pressed. This maximizes the throttle opening, preventing the intake air from being restricted.
6. During engine starting, press the compression gauge firmly against the cylinder head. Hissing noises indicate that air is escaping past the compression gauge, resulting in lower readings.
7. Once the needle of the compression gauge no longer moves to the right, stop cranking. Usually, cranking for 3 to 5 seconds is sufficient for an accurate measurement.
Repeat steps 4 through 7 for each cylinder. When measuring another cylinder, ensure the measurement is performed on a different chart section. Click the button on the compression gauge for this. The chart will slide up. Below are some scenarios observed in practice:
The compression peak pressure of all four cylinders is sufficiently high, with no cylinder deviating. The measurement indicates that the engine’s compression peak pressure is good.
Cylinder 3 has lower pressure compared to the others. Insufficient pressure is built in cylinder 3, indicating a problem. This could be due to poor sealing of one or more valves or an issue with the piston rings.
A deviation in two adjacent cylinders indicates a possible crack in the head gasket or cylinder head between the two cylinders. During cylinder 2’s compression stroke, air leaks into cylinder 3, and vice versa.
If the compression peak pressure in all cylinders is too low, several factors could be responsible, like worn or stuck piston rings.
A compression test can determine if the compression peak pressure is inadequate. However, it cannot ascertain the cause. Diagnostic options include:
- Pouring a little engine oil into the spark plug hole (not too much!). In the case of worn compression rings, the oil temporarily improves sealing, leading to a better or acceptable second measurement.
- Disassembling engine components for a visual inspection.
- Performing a cylinder leak test.
Cylinder Leak Test:
A cylinder leak test can identify the cause of compression loss. Using compressor air, pressure is applied to the cylinder space. A gauge on the leak tester indicates the amount of leakage as a percentage. A reading above 0% signifies leakage. If the pressure in the combustion chamber remains constant, the gauge will read 0%. When performing this test, ensure the valves are closed; conduct the test with the piston at TDC during the compression stroke. If the piston is near the top but in the exhaust or intake stroke, the valve might be slightly open due to valve overlap, but this does not indicate a defect. Optionally, remove the valve cover to check if the camshaft lobes are pointing up.
Procedure for cylinder leak test:
- Ensure the engine is at operating temperature. The engine components expand due to heat, making the readings realistic.
- Position the piston of the cylinder to be tested at TDC. Ensure the engine is in the compression stroke, so the valves are closed.
- Engage the parking brake and place the car in gear. This prevents the air pressure from pushing the piston down. The vehicle should not be on the lift.
- Apply compressor air to the cylinder.
- Read the gauge.
If the gauge reads 0%, no leakage is present. Connect the leak tester to the next cylinder. If the gauge shows a value, leakage is present. With compressor air applied to the cylinder, air leaks somewhere. Possible scenarios include:
- Hissing noise in the air intake pipe of the air filter: intake valve is leaking.
- Hissing noise in the exhaust: exhaust valve is leaking.
- Hissing noise after removing the oil filler cap: air leakage to the oil pan, possibly due to a defective head gasket or worn piston rings.
- Hissing noise at cylinder 3 while air pressure is on cylinder 2; the head gasket between cylinders 2 and 3 is cracked.
- Air bubbles in the coolant system: head gasket or cylinder head cracked.
Relative Compression Test with Oscilloscope:
The compression test can also be graphically represented with an oscilloscope. In this method, no engine components (e.g., spark plugs) need to be removed. Compression measurement is based on the starter motor’s current draw. The test is performed during engine cranking, ensuring the engine does not start by disconnecting the injector connectors. This way, no fuel is injected, and the engine won’t start. Note that you should not just disable the ignition! If only the ignition coils of a gasoline engine are disconnected, the engine will still inject fuel, which will exit through the exhaust unburned.
This oscilloscope image is from a relative compression test performed on a three-cylinder engine.
The current is displayed against time. The test was conducted by connecting a current clamp between the vehicle’s chassis ground and the battery, measuring the current during cranking. Each compression stroke causes the starter motor to “struggle” more to turn. During compression, the starter motor requires more current to rotate. This is visible in the peaks of the oscilloscope image. If no variations are observed in the peaks, the relative compression test result is satisfactory.
The displayed oscilloscope image is of a three-cylinder engine where one cylinder has no compression. Compared to the previous oscilloscope image, one peak is missing, indicating compression loss. There is a high peak (cylinder 3), a low peak (cylinder 2), and an intermediate peak (cylinder 1). The lowest peak (cylinder 2) indicates insufficient compression peak pressure for that cylinder. The starter motor requires less effort to rotate the crankshaft during this compression stroke. The intermediate peak is also affected by the cylinder with compression loss, although this does not mean that there is compression loss for this cylinder. This is explained with the help of the illustration below:
Because the firing order of cylinders 2 and 1 follow each other (firing order 1-3-2), the compression loss in cylinder 2 affects the oscilloscope image of cylinder 1. As the piston of cylinder 2 moves up (and the starter motor struggles less due to compression loss), the piston of cylinder 1 also begins to move up.
Without compression loss, the crankshaft rotation speed would decrease with each compression stroke. However, because the rotation speed at cylinder 2 has not decreased much, it also affects the crankshaft rotation speed of cylinder 1.
The above oscilloscope images can determine whether the engine is in good condition. If all peaks are equally high, the relative compression test is satisfactory. In case of a discrepancy, these oscilloscope images cannot specify which cylinder is causing the issue. To identify this, channel B can be used for an ignition measurement. This channel can be displayed in the same screen. The ignition oscilloscope image will appear above the line of the compression measurement, allowing the specific cylinder to be identified.
This page will soon be expanded with instructions for the WPS-500x, measuring cylinder pressure during engine operation.