ENGINE COMPRESSION ISSUES

Compression Definition -

The compression ratio is a single number that can be used to predict the performance of any engine (such as an internal-combustion engine or a Stirling Engine). It is the ratio between the volume of the cylinder, when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke. A high compression ratio allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. High ratios place increased oxygen and fuel molecules into a reduced space; thus, they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree. Higher compression ratios, however, also make engine knocking, also known as detonation or pre-ignition, more likely and this can reduce an engine's efficiency and even physically damage it. This is especially true of Turbo engines due to the added combustion chamber density.

Checking Compression and compression issues

The only way to check compression is by removing your engine spark plugs and using a pressure testor in each cylinder to see what the compression is. The purpose of a compression test is to be certain that there is not a headgasket issue. If the engine compression is within 20 points of each other you can assume that the headgasket is likely ok. What is meant is here is that each cylinder should be as close to the same as possible ie..... Cylinder 1 compression is 160, Cylinder 2 is 160, Cylinder 3 160, Cylinder 4 is 160. The actual number is not as important as the numbers being close. When a head gasket fails you will often find that compression in one or two (usually side by side) cylinders are down 40 points or more depending on the severity of the headgasket leak.

NOTE: One thing to keep in mind when checking engine compression is this. If the engine cylinder you are checking is dry then it WILL alter the compression readings. The best thing to do is to squirt an ounce or two of engine oil in each cylinder before testing that way you know that all cylinders are damp before testing.

Saab Variable Compression Engine Prototype

Because cylinder bore diameter, piston stroke length and combustion chamber volume are almost always constant, the compression ratio for a given engine is almost always constant, until engine wear takes its toll.

One exception is the experimental Saab Variable Compression engine (SVC). This engine, designed by Saab Automobile, uses a technique that dynamically alters the volume of the combustion chamber (Vc), which, via the above equation, changes the compression ratio (CR).

To alter Vc, the SVC 'lowers' the cylinder head closer to the crankshaft. It does this by replacing the typical one-part engine block with a two-part unit, with the crankshaft in the lower block and the cylinders in the upper portion. The two blocks are hinged together at one side (imagine a book, lying flat on a table, with the front cover held an inch or so above the title page). By pivoting the upper block around the hinge point, the Vc (imagine the air between the front cover of the book and the title page) can be modified. In practice, the SVC adjusts the upper block through a small range of motion, using a hydraulic actuator.

The SVC project was shelved by General Motors, when it took over Saab Automobile, due to cost.