The compression ratio is the ratio of the total cylinder volume of the diesel engine to the volume of the combustion chamber. Its change not only affects the dynamic performance and economy of the diesel engine, but also affects its starting performance. Each diesel engine is designed with an optimum compression ratio, but in use, the compression ratio tends to decrease due to various related factors. The main reasons for the reduction of the compression ratio and some problems that should be noted in the maintenance process are briefly described as follows:
First, the position of the piston at the end of compression is low
(1) The relevant parts have a large clearance. When the crankshaft main bearing and the main journal, the connecting rod bearing and the connecting rod journal, the connecting rod bushing and the piston pin or the matching gap between the piston pin and the pin seat hole are too large, the piston top dead center is often caused during the compression process. The actual position is shifted down so that the compression ratio decreases. Therefore, these fit clearances should be controlled within the allowable range of repairs.
(2) Deformation or main dimensions of the relevant parts are extremely poor. For example, when grinding the connecting rod journal of the crankshaft, the eccentricity is not adjusted so that the radius of rotation of the crankshaft after grinding becomes smaller; the connecting rod is bent so that the center distance between the large and small end holes of the connecting rod is shortened; The distance between the center line of the piston pin seat hole and the piston top plane is shortened. These factors will cause the piston to move down in compression at top dead center and the compression ratio will drop. Therefore, during the repair, you should follow the operating rules and ensure the quality of repairs. At the same time, do not forget to check when changing parts. Do not mistakenly replace or install defective parts. The contents to be inspected when changing parts include: radius of crankshaft gyration, center distance between large and small end holes of the connecting rod, distance between the centerline of the piston pin seat hole and the top plane of the piston, and between the upper plane of the machine body and the centerline of the main bearing seat hole distance.
Second, the combustion chamber is too large
(1) The valve and valve seat are heavily worn and the valve sinking is too large (even exceeding the limit). The valve and door seat should be replaced at this time.
(2) The thickness of the cylinder head gasket exceeds the design requirement or artificially increases the thickness of the cylinder head gasket. At this point should replace the cylinder pad that meets the requirements.
(3) The pit on the top of the piston (combustion chamber component) ablates the defect or replaces the wrong part so that the volume of the pit is too large (check with water injection contrast method). In this case, use a qualified piston.
(4) The vortex chamber on the cylinder head is burned, or the quality is unqualified and the volume is too large (check with water injection contrast method). The qualified cylinder head should be replaced at this time.
It is worth mentioning that among the many factors that lead to the reduction of the compression ratio, the influence of a single factor is not significant, but the cumulative effect of multiple factors on the compression ratio is large. As mentioned above, in order to control the compression ratio of the diesel engine within the specified range, the key is to improve the repair quality, make repairs, measurements, and matching of parts, and try to use smaller assemblies within the scope of technical requirements. gap. When the diesel engine has difficulties in starting, power drops, oil consumption rises, cylinder sealing, gas distribution, and oil supply are normal, and it seems that the cause of the failure cannot be found, do not ignore the inspection of the compression ratio.
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