Ditex Oscilloscope Relative Compression part 3 Bodgit And Leggit Garage

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1. Introduction and principle of operation
Correct functioning of the engine depends on many factors like correct sensor readings and proper functioning actuators. The Engine itself also needs to be in good condition and with the relative compression test you can easily determine whether all cylinders have about the same compression. The relative compression analysis does not measure the actual pressure, but just compares one cylinder to the next. Hence the term "relative".
The principle of the relative compression test is based on tracking the battery current/voltage changes during cranking to determine the compression values of all cylinders. Analysis of the current/voltage changes gives the comparative compression values of all cylinders. An engine with poor compression in one or more cylinders can be quickly identified using the relative compression test.
To perform the relative compression test, the fuel system must be disabled to prevent the engine from starting.
There are two methods to perform the relative compression test – Current and Voltage:
The current method is based on tracking the starter motor current changes during engine cranking. The oscilloscope then analyses the compressions by monitoring the current in each cylinder compression stroke. The better the compression, the higher the current, and vice versa. It is important that you have an equal current draw on all cylinders. The current required to crank the engine depends on several factors, fuel type - gasoline or diesel, including the engine volume, number of cylinders, viscosity of the oil, condition of the battery, the starter motor, condition of the starter motor's wiring circuit, the compression in each of the cylinders and the mechanical condition of the cylinder. The typical current of a four-cylinder gasoline engine is about 100A to 200A. The initial peak current up to 500A is the current required to overcome the continuous friction and inertia to rotate the engine. Once the engine starts rotating, this current lower.Voltage method is based on tracking the battery voltage changes during engine cranking to determine the compression values of all cylinders. The oscilloscope then analyses and compares the compressions by monitoring the battery voltage drops in each cylinder compression stroke. The better the compression, the lower the voltage, and vice versa. It is important that you have an equal voltage drops on all cylinders.
2. Advantages of the Relative Compression Test
quick test to confirm if there are significant engine mechanical issues or not;
all cylinders can be tested at once;
no pressure sensors are needed to check each cylinder;
no need to remove spark plugs, glow plugs, injectors or disconnect spark plug leads;
suitable for any vehicle, regardless of manufacturer - cars, trucks, buses and etc.
suitable for gasoline and diesel engines;
only an oscilloscope with a specific mode for this test and a current clamp are needed;
irrespective of the number of cylinders (up to 12)
3. Disadvantages of the Relative Compression Test:
only shows relative difference in cylinder’s compression expressed as percentage;
if all of the cylinders have about the same loss of compression due to wear of cylinder parts, the relative compression test would not detect this condition;
typical error is about 10% but in some cases, if specific engine units are not functioning correctly, it can rise up to 20%;
specific oscilloscope knowledge is needed to perform this test;
cannot be used on rotary Wankel engines;
4. Performing a Relative Compression Test with an automotive lab scope

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