Piston Profile:
The piston is the “heart” of an automobile engine, which bears alternating mechanical and thermal loads, and is one of the key components in the engine with the harshest working conditions. The function of the piston is to withstand the gas pressure and drive the crankshaft to rotate through the piston pin to the connecting rod, and the piston top is also an integral part of the combustion chamber.
The types of pistons are generally divided into: diesel vehicle pistons, gasoline vehicle pistons, and general-purpose pistons.
The function of the piston is to withstand the gas pressure and drive the crankshaft to rotate through the piston pin to the connecting rod, and the piston top is also an integral part of the combustion chamber. Working conditions: The piston works under the conditions of high temperature, high pressure, high speed, and poor lubrication. The piston is in direct contact with the high-temperature gas, and the instantaneous temperature can reach more than 2500K, therefore, the heating is serious, and the heat dissipation conditions are very poor, so the temperature of the piston is very high, the top is as high as 600~700K, and the temperature distribution is very uneven; The top of the piston bears a lot of gas pressure, especially the maximum power stroke pressure, the gasoline engine is as high as 3~5MPa, and the diesel engine is as high as 6~9MPa, which makes the piston have an impact and bear the effect of side pressure; The piston reciprocates at a very high speed (8~12m/s) in the cylinder, and the speed is constantly changing, which produces a great inertia force and makes the piston subject to a large additional load. The piston works under such harsh conditions, which can deform and accelerate wear, as well as additional loads and thermal stresses, as well as the chemical effects of gas.
Working conditions of the piston:
The piston works under conditions of high temperature, high pressure, high speed, and poor lubrication. The piston is in direct contact with the high-temperature gas, and the instantaneous temperature can reach more than 2500K, therefore, the heating is serious, and the heat dissipation conditions are very poor, so the temperature of the piston is very high, the top is as high as 600~700K, and the temperature distribution is very uneven; The top of the piston bears a lot of gas pressure, especially the maximum power stroke pressure, the gasoline engine is as high as 3~5MPa, and the diesel engine is as high as 6~9MPa, which makes the piston have an impact and bear the effect of side pressure; The piston reciprocates at a very high speed (8~12m/s) in the cylinder, and the speed is constantly changing, which produces a great inertia force and makes the piston subject to a large additional load. The piston works under such harsh conditions, which can deform and accelerate wear, as well as additional loads and thermal stresses, as well as the chemical effects of gas.
Structure of the piston:
Generally, the piston is cylindrical, according to the working conditions and requirements of different engines, the structure of the piston itself is various, and the piston is generally divided into three parts: the head, the skirt and the piston pin seat.
The head refers to the top of the piston and the part of the ring groove. The top of the piston depends entirely on the requirements of the combustion chamber, and the flat or near-flat top design of the top helps the piston to reduce the contact area with the hot gas, so that the stress is evenly distributed. Most gasoline engines use flat-topped pistons, and some engines (such as direct-injection diesel engines and new in-cylinder injection gasoline engines) need to have a more complex shape at the top of the piston with a certain depth of pits as part of the combustion chamber in order to improve the combustion efficiency and minimize the deflagration in order to improve the combustion efficiency and minimize the deflagration for the needs of gas mixture formation. The grooves of the piston are called ring grooves and are used to install the piston rings. The function of the piston ring is to seal, prevent air leakage and prevent oil from entering the combustion chamber.
The piston skirt refers to the lower part of the piston, which is used to maintain the vertical posture of the piston in reciprocating motion as much as possible, that is, the guiding part of the piston.
The shape of the piston skirt is very particular, especially for light passenger cars such as cars, the designer starts from the structure and performance of the engine, and often uses his brain on the piston skirt to make the engine structure compact and smooth as much as possible.
The piston pin seat is the supporting part of the piston that is connected to the connecting rod by the piston pin, which is located above the piston skirt. The special feature of the piston pin seat of high-speed engine is that the pin seat hole is not necessarily on the same centerline plane as the piston, but can be offset a little to one side, that is, to the side where the piston touches the cylinder wall during the power stroke, so that when the piston changes direction to the top dead center, the degree of piston hitting the cylinder wall will be reduced, thereby reducing the engine noise.
The difference between the piston of gasoline engine and diesel engine:
There is no difference in the main structure, the biggest difference between the gasoline engine and the diesel engine is the ignition mode and air intake: the gasoline engine enters the cylinder with combustible gas mixed with oil and gas, and the diesel engine is pure air; The gasoline engine is ignited by a spark plug, and the diesel fuel is injected into the diesel from the fuel injection hole after the compressed air reaches the spontaneous combustion temperature of the diesel, and the diesel fuel spontaneously combusts due to the high temperature in the cylinder at this time.
If you want to talk about the difference, the most likely is in the skirt of the piston, because most diesel engines have a longer piston stroke than gasoline engines, and there may be a special shape in the skirt to match the movement of the flywheel, but not all diesel engines are like this.
Let’s compare the piston of a 2-valve gasoline engine with a diesel engine: the difference is at the top of the piston. The gasoline engine piston is flat, and the diesel engine piston has a combustion chamber on the top. That’s the biggest difference. Then compare the multi-valve gasoline engine piston and the diesel engine piston: both have combustion chambers on the top. But the gasoline engine is simpler. The distance between the piston ring grooves is larger than that of a diesel engine than that of a gasoline engine. The groove of the oil ring groove is perforated for the diesel engine. Gasoline engines do not. Overall, there is no difference.
Judgment and treatment of piston knocking abnormal sound
When the engine is running in the vehicle, there is a piston knocking abnormal sound. The abnormal sound of the piston knocking cylinder occurs in the upper part of the cylinder, which is a rhythmic “click” sound similar to hitting the concrete floor with a small hammer. When the engine is idling, the sound is clear and clear. Especially when the engine is running at low temperature, the sound is obvious, and when the temperature rises, the sound decreases or even disappears.
Judgment method of piston knocking cylinder:
(1) Cut off the oil cylinder by cylinder. Take the method of cutting off the oil cylinder one by one to determine the position of the cylinder, if the oil is cut off to a certain cylinder, the sound is significantly reduced or disappears, and when the oil supply is resumed, the obvious “click” sound can be heard, indicating that the cylinder piston knocks the cylinder.
(2) In order to further confirm that the cylinder piston knocks the cylinder, the injector of the cylinder can be removed, a small amount of CD-grade supercharged engine oil (playing a sealing role) is added to the cylinder, and then the injector is installed, the engine is started, the knocking sound disappears or weakens, and the knocking sound appears again after running, which is the piston knocking on the cylinder undoubtedly.
The main reasons for the knocking of the piston cylinder in production are as follows:
(1) The gap between the piston and the cylinder wall is too large. The standard clearance between the piston skirt and cylinder of WD615 series engine is 0.143-0.182, and the maximum wear limit is 0.35-0.40.
(2) After the engine runs for a period of time, the cylinder piston is worn, coupled with poor lubrication, the matching gap between the piston and the cylinder increases due to wear, and a more serious step appears at the lower part of the first air ring, so that the piston strikes the cylinder and makes an abnormal sound.
(3) After running for a period of time, the piston skirt and cylinder are seriously worn, resulting in serious outof-rounding and knocking of the cylinder.
(4) Individual connecting rods are deformed due to various reasons, resulting in piston eccentricity, and the gap becomes larger and the cylinder is knocked. Piston knocking on the cylinder will lead to high engine fuel consumption, engine oil channeling, high fuel consumption, and poor economy. When the piston knocks the cylinder seriously, it will also crush the piston and damage the cylinder, so that the connecting rod will break and the cylinder block will be damaged.
When encountering piston knocking, the following methods can be used:
(1) After the engine starts, there is a knocking sound when running at low temperature, and after the temperature is normal, the sound disappears, and it can be temporarily disposed of and continue to run.
(2) When the engine temperature is normal and there is an obvious knocking sound, high-speed operation should be avoided as much as possible and disassembled and repaired as soon as possible.
(3) After removing the cylinder head and pulling out the piston, it is found that the cylinder is seriously out of round, strained, or the gap between the piston and the cylinder is too large, it should be replaced, and the cylinder liner, piston and piston ring should be replaced.
(4) If the connecting rod is found to be deformed, it should be replaced as much as possible.
Post time: Jan-04-2024