Due to the different characteristics of gasoline and diesel, there are differences in working principles and structures between gasoline and diesel engines.
The working principle of gasoline engine (gasoline engine)
A four-stroke gasoline engine mixes air and gasoline in a certain ratio into a good mixture. It is sucked into the cylinder during the intake stroke. The mixture is compressed and ignited to generate heat. The high-temperature and high-pressure gas acts on the top of the piston to push the piston back and forth. Linear movement, external mechanical energy output through connecting rod, crankshaft flywheel mechanism. The four-stroke gasoline engine completes a working cycle in the intake stroke, compression stroke, power stroke and exhaust stroke.
⑴ Intake stroke
The piston moves from the top dead center to the bottom dead center under the drive of the crankshaft. At this time, the intake valve opens, the exhaust valve closes, and the crankshaft rotates 180°. During the movement of the piston, the cylinder volume gradually increases, the gas pressure in the cylinder gradually decreases from pr to pa, a certain degree of vacuum is formed in the cylinder, and the mixture of air and gasoline is sucked into the cylinder through the intake valve and further in the cylinder. Mix to form a combustible mixture. Due to the resistance of the intake system, at the end of the intake, the gas pressure in the cylinder is less than the atmospheric pressure p0, that is, pa= (0.80～0.90)p0. The temperature of the combustible mixture entering the cylinder rises to 340-400K due to the heating of high-temperature parts such as the intake pipe, cylinder wall, piston crown, valve and combustion chamber wall, and mixing with residual exhaust gas.
⑵ Compression stroke
During the compression stroke, the intake and exhaust valves are closed at the same time. The piston moves from bottom dead center to top dead center, and the crankshaft rotates 180°. When the piston moves up, the working volume gradually decreases, and the pressure and temperature of the mixed gas in the cylinder continue to rise after compression. When it reaches the end of compression, its pressure pc can reach 800-2000kPa, and the temperature can reach 600-750K.
⑶ Work stroke
When the piston is close to the top dead center, the spark plug ignites the combustible mixture, and the mixture burns to release a large amount of heat energy, which makes the pressure and temperature of the gas in the cylinder increase rapidly. The highest combustion pressure pZ reaches 3 000～6 000kPa, and the temperature TZ reaches 2 200～2 800K. The high-temperature and high-pressure gas pushes the piston to move from the top dead center to the bottom dead center, and outputs mechanical energy to the outside through the crank connecting rod mechanism. As the piston moves down, the cylinder volume increases, and the gas pressure and temperature gradually decrease. When it reaches point b, its pressure drops to 300～500kPa and the temperature drops to 1,200～1500K. During the power stroke, the intake valve and exhaust valve are closed, and the crankshaft rotates 180°.
⑷ Exhaust stroke
During the exhaust stroke, the exhaust valve opens, the intake valve is still closed, the piston moves from bottom dead center to top dead center, and the crankshaft rotates 180°. When the exhaust valve is opened, the combusted exhaust gas is discharged out of the cylinder under the action of the pressure difference between the inside and outside of the cylinder on the one hand, and discharged out of the cylinder by the displacement of the piston on the other hand. Due to the resistance of the exhaust system, the pressure at the exhaust end point r is slightly higher than the atmospheric pressure, that is, pr=(1.05～1.20)p0. The exhaust terminal temperature Tr=900～1100K. When the piston moves to the top dead center, there is still a certain volume of exhaust gas in the combustion chamber that cannot be discharged. This part of the exhaust gas is called residual exhaust gas.
The working principle of a four-stroke diesel engine
The working principle of a four-stroke diesel engine is the same as that of a gasoline engine. Each working cycle is also composed of intake stroke, compression stroke, power stroke and exhaust stroke. Compared with gasoline, diesel has a low self-ignition temperature and a large viscosity that is not easy to evaporate. Therefore, diesel engines use compression end compression ignition (compression ignition ignition), while gasoline engines are ignited by spark plugs.
⑴ Intake stroke
The working fluid entering the cylinder is pure air. Due to the low resistance of the diesel engine’s intake system, the end-point pressure of the intake pa=(0.85～0.95)p0, which is higher than that of the gasoline engine. The end temperature of the intake air is Ta=300～340K, which is lower than that of gasoline engines.
⑵ Compression stroke
Since the compressed working fluid is pure air, the compression ratio of diesel engines is higher than that of gasoline engines (usually ε=16-22). The pressure at the end of compression is 3000-5000kPa, and the temperature at the end of compression is 750-1000K, which greatly exceeds the auto-ignition temperature of diesel (about 520K).
⑶ Work stroke
When the compression stroke is nearing the end, the diesel fuel is injected into the cylinder combustion chamber through the injector at a high pressure of about 100MPa under the action of the high-pressure oil pump, and it ignites and burns immediately after being mixed with air in a short period of time. The pressure of the gas in the cylinder rises rapidly, up to 5000～9000kPa, and the highest temperature reaches 1800～2000K. Since the diesel engine ignites itself by compression, it is called a compression ignition engine.
⑷ Exhaust stroke
The exhaust of a diesel engine is basically the same as that of a gasoline engine, but the exhaust temperature is lower than that of a gasoline engine. Generally Tr=700～900K. For a single-cylinder engine, the speed is uneven, the engine works unevenly, and the vibration is large. This is because only one of the four strokes is used for work, and the other three strokes are strokes that consume power to prepare for work. In order to solve this problem, the flywheel must have a sufficiently large moment of inertia, which in turn will increase the mass and size of the entire engine. The use of multi-cylinder engines can make up for the above shortcomings. Modern cars mostly use four-cylinder, six-cylinder and eight-cylinder engines.
Post time: Jun-08-2021