Cylinder Head

The cylinder head is installed on the cylinder block to seal the cylinder from above and form the combustion chamber. It is often in contact with high temperature and high pressure gas, so it bears a great thermal load and mechanical load. A cooling water jacket is made inside the cylinder head of a water-cooled engine, and the cooling water hole on the lower end of the cylinder head communicates with the cooling water hole of the cylinder block. Use circulating water to cool high-temperature parts such as the combustion chamber.

The cylinder head is also equipped with intake and exhaust valve seats, valve guide holes, which are used to install intake and exhaust valves, as well as intake and exhaust channels. The cylinder head of a gasoline engine is machined with holes for installing spark plugs, and the cylinder head of a diesel engine is machined with holes for installing fuel injectors. The cylinder head of the overhead camshaft engine is also machined with a camshaft bearing hole for installing the camshaft.

Cylinder heads are generally made of gray cast iron or alloy cast iron. Aluminum alloy has good thermal conductivity, which is beneficial to increase the compression ratio. The cylinder head is an integral part of the combustion chamber. The shape of the combustion chamber has a great influence on the operation of the engine. Due to the different combustion methods of gasoline and diesel engines, the parts of the cylinder head that make up the combustion chamber are quite different. The combustion chamber of a gasoline engine is mainly in the cylinder head, while the combustion chamber of a diesel engine is mainly in the pit on the top of the piston.

The engine cylinder head is one of the most critical parts of the engine. Its precision requirements are high, the processing technology is complex, and the processing quality directly affects the overall performance and quality of the engine. Therefore, the processing of the engine cylinder head is particularly important. The key parts of the valve seat ring and The processing of the conduit hole is the most important thing.

Engine intake and exhaust valve seats are important working parts that control gas intake and exhaust gas discharge. During operation, they will be subjected to air erosion and valve impact friction at high temperatures. The working conditions are very harsh: during normal operation, The valve seat ring has been exposed to a high temperature of (600~800) ℃ for a long time. Factors such as high temperature gas corrosion and part deformation will cause the valve guide and seat ring cone surface to be more damaged, resulting in a loose valve seal. It is wasted, thus greatly reducing the power of the engine. Therefore, the valve seat ring and duct hole should have good high temperature wear resistance, corrosion resistance, heat transfer and high temperature strength, high temperature creep resistance, and thermal expansion coefficient matching the cylinder head. At the same time, if the center of the valve and the center of the valve seat ring are too large when the engine is working, the engine power will decrease and the fuel consumption will increase, and the wear of the valve and the duct hole will also be accelerated. Therefore, the machining accuracy of the valve seat ring and the duct hole, especially the runout of the valve seat ring working cone face the duct hole, stipulates strict tolerance limits.

The general requirements for the machining accuracy of the engine cylinder head valve seat ring and the duct hole are: the coaxiality tolerance requirement of the gasoline engine is (0.015~0.025) mm; while the diesel engine is only (0.01~0.015) mm, and the cone runout is ( 0.03~0.05) mm, and the hole accuracy requirement is generally H7.

1. The cylinder head bolts should be tightened evenly and the oil supply time should be adjusted correctly.

2. Soft water should be added to the water tank, and the water should be changed as little as possible.

3. The engine should avoid long-term overloading.

4. When the engine is working and the water tank is occasionally short of water, do not turn off the engine immediately, but slowly add water at low speed. Do not add cold water after the engine is hot. After stopping, wait until the water temperature is below 40°C before draining the water. It is not possible to add boiled water immediately during the cold winter season, but the water should be heated before adding boiled water.

5. When the engine is working and the water tank occasionally runs short of water, the flame should not be turned off immediately, and the water should be added slowly by running at low speed. Do not add cold water after the engine is hot, and after stopping, wait for the water temperature to fall below before draining the water, cold winter tenderloin

6. When assembling, check whether the cooling water holes are unblocked. Clean the cooling system with alkaline solution regularly to remove scale and oil stains in time.

Cylinder head suffers from low-cycle thermal fatigue damage, high-cycle thermal fatigue damage and creep damage during operation, and its life and reliability are important indicators of the engine. During the start-stop process of the engine (start-up cycle), the cylinder head is heated and cooled rapidly, resulting in a large cyclic thermal stress and suffers from low-cycle thermal fatigue damage. In each working cycle after the engine is started (inhalation-compression-work-exhaust cycle process), the cylinder head undergoes a relatively small temperature change and suffers high-cycle thermal fatigue damage. Partial materials of the cylinder head work for a long time in an environment higher than the creep temperature, and suffer creep damage.

1) The low-cycle thermal fatigue damage, high-cycle thermal fatigue damage and creep damage of the cylinder head are theoretically analyzed. The main cause of cylinder head failure is low-cycle thermal fatigue damage, and the number of starts is its main life indicator;

2) Creep has less direct damage to the cylinder head, but it can affect the average stress of low-cycle thermal fatigue. Therefore, the creep of the engine-low-cycle thermal fatigue can be equivalent to heat with a constant strain amplitude and a certain average stress- Mechanical fatigue, using thermomechanical fatigue test instead of creep-thermal fatigue test can greatly reduce the test time.