Compression force refers to the pressure generated by the cylinder when the engine is working. There may be many reasons for insufficient cylinder pressure. The most common are the following three aspects:

1. Large intake resistance
The increase of the air intake resistance reduces the air intake. For example, the air filter is blocked, the valve opening is reduced, and the valve air phase is not equal, which may cause the increase of the air intake resistance.

2. The compression ratio becomes smaller
The compression ratio of the cylinder becomes smaller, that is, the volume of the combustion chamber increases. After the volume of the combustion chamber increases, the cylinder pressure will drop; the reason for the increase of the volume of the combustion chamber is improper or untimely repair, such as the thick cylinder gasket. The surface of the cylinder head was reamed due to an unreasonable valve reamer. When grinding the crankshaft, the radius of gyration was reduced. When repairing the connecting rod, the center distance between the large and small heads was reduced.

3. Compression system leaks
Due to wear damage, looseness and misalignment, an unnecessary gap appears between the parts constituting the compression system, which does not have a sealing effect, and causes the air in the cylinder to leak during the compression process.
(1) Air leakage of cylinder head gasket
The edge of the cylinder head gasket leaks air, causing gas to escape during compression and work strokes. The reasons for cylinder head gasket leakage are: insufficient pre-tightening force of the cylinder head fixing bolts, or failure to tighten evenly in the required tightening sequence; warpage of the joint plane of the cylinder head and cylinder block; insufficient protrusion height of the cylinder liner, or similar The protruding height difference between the adjacent two cylinders is too large, the working temperature of the diesel engine is too high and the cylinder head gasket is burned; the compression ratio is too high, which causes the explosion pressure to be too high.
(2) Piston ring leaks
You can inject some clean engine oil into the cylinder. If the cylinder pressure is significantly increased after inspection, it means that the piston ring is not tightly sealed. Otherwise, it means that the cylinder pressure has nothing to do with the piston ring. In addition, you can also observe the discharge from the oil filling port when the diesel engine is working. Judge the amount of exhaust gas. Reasons for piston ring air leakage: serious wear of the piston, piston ring, and cylinder, excessive matching clearance, insufficient piston ring elasticity, broken or stuck in the ring groove by carbon deposits, and can't move, the end clearance and side clearance of the piston ring are too large .
(3) Valve leakage
Including the air leakage between the valve and the valve seat and the valve seat and the cylinder head.
The reasons for valve leakage are: too much carbon deposits or bending of the valve stem, which makes the valve movement inflexible, resulting in untimely or lax closing, and the carbon deposits fall into the inclined surface of the contact ring between the valve and the valve seat, causing the valve to close Lax; the contact ring belt of the valve and the valve seat is worn out, ablated or the contact ring is too wide, causing the valve to not close tightly, the valve gap disappears, the valve spring is too small or broken, so that the valve is not tightly closed, and the valve seat ring When the valve seat ring is loosened or not sealed, it will cause air leakage.

The method of judging air leakage: Run the diesel engine for a certain period of time, and when the operating temperature of the diesel engine rises above 50 degrees, stop and crank the crankshaft. At this time, if the valve leaks, the compression force of each cylinder will be felt. A long hissing sound can be heard at the air intake pipe. If there is a serious air leak, the "chichi" sound can be clearly heard when the diesel engine is working.