Cylinder pressure gauge methods and steps for measuring cylinder pressure, how to read the cylinder pressure gauge reading instructions

Methods and steps for measuring cylinder compression pressure

1. Structure

Cylinder pressure gauges generally consist of pressure gauge heads, conduits, one-way valves and connectors. Most of the pressure gauge heads are of the Bourdon-tube type. The driving element is a flat tube bent into a circle, with one end being a fixed end and the other end being a movable end. The movable end is connected to the watch pointer through a lever and a gear mechanism. When gas pressure enters the elbow, the elbow straightens. Therefore, the lever and gear mechanism drive the meter pointer to swing, indicating the gas pressure on the dial.

2. Testing conditions

The engine should be run to normal operating temperature. Use the starter to drive the engine with all spark plugs or injectors removed, and its speed should comply with the original manufacturer's regulations.


3. Detection method
① Start the engine and stop when the engine water temperature rises to about 80°C.
② Use compressed air to blow away the dust on the outside of the spark plug, remove the spark plug of each cylinder, and open the throttle valve and choke valve.
③ Remove the exhaust gas from the cylinder, place the rubber head of the pressure gauge on the spark plug hole of the first cylinder, and press it firmly.
④ Use the starter or hand crank to quickly turn the crankshaft at about 180r/min. Write down the pressure gauge reading. Test and record 2-3 times, and then measure the remaining cylinders in sequence.
⑤ When the cylinder pressure is lower than the standard, you can add 20--30g of new lubricating oil to the top of the piston and try the pressure again. If it rises significantly, it means that the piston ring is worn. If the pressure remains unchanged, it means that the valve or cylinder gasket is leaking.
4. Diagnostic parameter standards
For automobile engines in use, in accordance with the national standard GBl8565-2001 "Comprehensive Performance Requirements and Inspection Methods for Commercial Vehicles", the pressure of each cylinder of the engine should not be less than 85% of the original design specified value;the difference between the pressure of each cylinder and the average pressure of each cylinder : Gasoline engines should be no more than 8%, diesel engines should be no more than 10%. For engines that have been overhauled, in accordance with the provisions of Appendix B of the national standard GB/T 15746.2-1995 "Automotive Repair Quality Inspection and Assessment Standards Engine Overhaul": the cylinder pressure of the engine that has been overhauled should comply with the original design regulations;the pressure of each cylinder and the average pressure of each cylinder The difference: no more than 8% for gasoline engines and no more than 10% for diesel engines.







Using starting current to indirectly measure cylinder pressure

Principle: The starting torque M is proportional to the starting current, and the current is proportional to the pressure in the cylinder. Starting torque overcomes mechanical friction resistance, inertial force, and compression pressure. M is proportional to I and compression pressure.




M=K˙m˙ф˙Is

The electromagnetic torque of the starter is the driving torque, which should be balanced with the starting resistance torque of the engine during stable operation. The starting resistance torque of the engine is composed of mechanical resistance torque, inertial resistance torque and reaction torque of the cylinder compressed air. Under normal circumstances, the first two resistance torques do not change much and can be regarded as constants;while the compressed air reaction torque obviously fluctuates periodically, with a peak value when the piston of each cylinder reaches the top dead center of the compression stroke. If the resistance torque increases. The electromagnetic torque is temporarily smaller than the resistance torque, and the starter speed n decreases;as n decreases, the counter electromotive force will decrease and the armature current will increase. Then the electromagnetic torque increases until it reaches a new balance with the resistance torque. If the resistance torque decreases, the starter will accelerate and rotate, the speed n will increase, and the counter electromotive force will increase accordingly, so the armature current and torque will decrease until they are balanced. It can be seen that when the engine is started, the fluctuation of compression pressure causes The fluctuation of the starter's starting working current is eliminated, and the peak value of the current fluctuation is proportional to the cylinder compression pressure. If the cylinder corresponding to a certain current peak value can be determined, such as the first cylinder, the starting current peak value corresponding to each cylinder can be determined according to the ignition order, and its size can represent the cylinder compression pressure value of the cylinder.




How to read the cylinder pressure gauge, how to read the cylinder pressure gauge

1. First look at the pressure unit on the dial. Some use "Mpa";some use "Bar";some use "Kpa" and so on.
2. Look at the value between the two large scales marked with numbers.
3. Look at how many small divisions there are in each large scale. Divide the value by the number of small divisions to know the value of each small division.

It should be noted that when reading, you should look at the front position of the pointer, because the pointer has a certain width.









Precautions for installing cylinder pressure gauge

1. The pressure gauge must be placed vertically. Use a wrench to tighten it when installing. Do not directly tighten its outer shell by hand, because