What is the working principle of a cylinder pressure gauge?

[LeWeston]

The working principle of a cylinder pressure gauge is: when gas pressure enters the curved tube, the tube straightens, thereby driving the pointer on the dial to swing through the lever and gear mechanism, indicating the magnitude of the gas pressure. A cylinder pressure gauge is a specialized gas pressure gauge composed of a pressure gauge head, tubing, a one-way valve, and a connector. The pressure gauge head is mostly of the Bourdon tube type, with its driving element being a flat tube bent into a circular shape, with one end fixed and the other end movable. Reasons for excessively high cylinder compression pressure: 1. Excessive oil entering due to carbon buildup; 2. Overly rich air-fuel mixture causing excessively low engine temperature; 3. Incomplete combustion over a prolonged period and poor exhaust flow.

[MacTheo]

I think a cylinder compression tester is like a blood pressure monitor for an engine. It screws into the spark plug hole via a special adapter. When you crank the engine, the piston moves upward to compress the air-fuel mixture, and the resulting pressure pushes the needle on the gauge. Inside the gauge, there's a precision spring mechanism—the greater the pressure, the more the spring compresses, and the further the needle deflects. Last time I tested the compression on my old car, I found the third cylinder only had 7 bar, significantly lower than the other cylinders' 10 bar. This indicates possible leakage in the piston rings or valves, no wonder the engine was shaking so badly. A compression difference exceeding 1.5 bar between cylinders is a red flag, just like a large blood pressure difference between a person's left and right arms would signal trouble. Remember to fully depress the throttle during the test, as insufficient throttle opening can affect reading accuracy.

[Terrence]

Measuring cylinder pressure with a compression gauge is a fundamental skill in auto repair. First, remove the spark plug and tightly screw the correct adapter into the spark plug hole—ensuring no air leakage. Then, disconnect the fuel injector circuit and have an assistant fully depress the accelerator while cranking the engine for 2-3 seconds. The gauge needle will surge to its peak before settling—this maximum reading represents the actual compression pressure. The principle is straightforward: the crankshaft drives the piston to compress air in the cylinder, transmitting pressure through tubing to the Bourdon tube spring inside the gauge. The spring's deformation rotates gears that move the needle, displaying the value on the dial. Ensure the battery voltage is sufficient during testing, as low starter motor RPM from weak voltage will yield falsely low readings.

[SophiaFitz]

The core component of a cylinder pressure gauge is a curved Bourdon tube made of copper. When high-pressure gas from the engine cylinder enters the Bourdon tube through the hose, the tube attempts to straighten. This movement is amplified by gears to drive the dial pointer. Modern digital gauges also incorporate sensors to convert mechanical motion into electrical signals for display. The cylinder's sealing directly affects the readings during testing: worn piston rings result in pressures below 8 bar, leaking valves cause the pointer to rise slowly, and a damaged head gasket leads to low pressure in two adjacent cylinders. Remember, cold engine tests yield readings about 10% lower than hot engine tests, which is crucial.