Principles Of Internal Combustion Engines

Due to the different characteristics of gasoline and diesel, there are differences in the working principle and structure between gasoline and diesel engines.
Working principle of gasoline engine
A four stroke gasoline engine is a mixture of air and gasoline in a certain proportion to form a good mixture. On the suction stroke, the mixture is sucked into the cylinder. The mixture is compressed, ignited, and burned to generate heat energy. The high-temperature and high-pressure gas acts on the top of the piston, pushing it to move in a reciprocating straight line, and outputting mechanical energy to the outside through the connecting rod, crankshaft, and flywheel mechanism. A four stroke gasoline engine completes a working cycle within the intake stroke, compression stroke, power stroke, and exhaust stroke.
(1) Intake stroke
The piston moves from top dead center to bottom dead center driven by the crankshaft. At this point, the intake valve opens, the exhaust valve closes, and the crankshaft rotates 180 °. During the movement of the piston, the cylinder volume gradually increases, and the gas pressure inside the cylinder gradually decreases from pr to pa. A certain degree of vacuum is formed inside the cylinder, and the mixture of air and gasoline is sucked into the cylinder through the intake valve and further mixed to form a combustible mixture inside the cylinder. Due to the presence of resistance in the intake system, the gas pressure in the cylinder is less than the atmospheric pressure p0 at the intake endpoint, i.e. pa=(0.80~0.90) p0. The temperature of the combustible mixture entering the cylinder increases to 340-400K due to the heating of high-temperature components such as the intake pipe, cylinder wall, piston crown, valves, and combustion chamber wall, as well as the mixing with residual exhaust gas.
⑵ Compression stroke
During the compression stroke, both the intake and exhaust valves close simultaneously. 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 increase after compression. When it reaches the compression end point, the pressure pc can reach 800-2 000kPa, and the temperature can reach 600-750K.
⑶ Power stroke
When the piston approaches the top dead center, the spark plug ignites the combustible mixture, which releases a large amount of heat energy during combustion, rapidly increasing the pressure and temperature of the gas in the cylinder. The maximum combustion pressure pZ reaches 3000~6000 kPa, and the temperature TZ reaches 2200~2800K. The high-temperature and high-pressure gas drives the piston to move from top dead center to bottom dead center, and outputs mechanical energy to the outside through the crank connecting rod mechanism. As the piston moves downwards, the cylinder volume increases, and the gas pressure and temperature gradually decrease. When reaching point b, the pressure drops to 300-500kPa and the temperature drops to 1200-1 500K. During the power stroke, both the intake and exhaust valves are closed, and the crankshaft rotates 180 °.
(4) Exhaust stroke
During the exhaust stroke, the exhaust valve opens, while the intake valve remains closed. The piston moves from bottom dead center to top dead center, and the crankshaft rotates 180 °. When the exhaust valve is opened, the exhaust gas after combustion is discharged out of the cylinder under the pressure difference between the inside and outside of the cylinder, and on the other hand, it is discharged out of the cylinder through the squeezing effect of the piston. Due to the resistance effect of the exhaust system, the pressure at point r of the exhaust endpoint is slightly higher than atmospheric pressure, i.e. pr=(1.05-1.20) p0. The exhaust endpoint temperature Tr=900-1100K. When the piston reaches the top dead center, there is still a certain volume of exhaust gas in the combustion chamber that cannot be discharged, which 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 the intake stroke, compression stroke, power stroke, and exhaust stroke. Compared with gasoline, diesel has low Autoignition temperature, high viscosity and is not easy to evaporate. Therefore, compression ignition ignition (compression ignition ignition) is adopted for diesel engines, while spark plug ignition is adopted for gasoline engines.
⑴ Intake stroke
The working fluid entering the cylinder is pure air. Due to the small resistance of the intake system in diesel engines, the intake endpoint pressure pa=(0.85-0.95) p0 is higher than that of gasoline engines. The intake endpoint temperature Ta=300~340K, which is lower than that of gasoline engines.
⑵ Compression stroke
Due to the compressed working fluid being 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 ～ 5000 kPa, and the temperature at the end of compression is 750 ～ 1000 K, which is much higher than the Autoignition temperature of diesel (about 520 K).
⑶ Power stroke
When the compression stroke is approaching its end, under the action of the high-pressure oil pump, diesel is injected into the cylinder combustion chamber through the fuel injector at a high pressure of about 100MPa. After mixing with air for a short period of time, it immediately ignites and burns on its own. The pressure of the gas inside the cylinder rises rapidly, reaching a maximum of 5000 to 9000 kPa and a maximum temperature of 1800 to 2000 K. Due to the fact that diesel engines self ignite and burn by compression, they are called compression ignition engines.
(4) Exhaust stroke
The exhaust of a diesel engine is basically the same as that of a gasoline engine, except that the exhaust temperature is lower than that of a gasoline engine. Generally, Tr=700-900K. For a single cylinder engine, its rotational speed is uneven, the engine operates unevenly, and the vibration is high. This is because only one of the four strokes does work, while the other three strokes consume power to prepare for work. In order to solve this problem, the flywheel must have a large enough Moment of inertia, which will lead to the increase of the overall engine mass and size. The use of multi cylinder engines can compensate for the aforementioned shortcomings. Hyundai Motor Company mostly use four cylinder, six cylinder and eight cylinder engines.