Car enthusiasts should know at least in general terms how the engine is arranged and works. Most cars have a four-stroke four-cylinder engine. Let's look at the duty cycle of a four-stroke engine. Not everyone knows what processes occur when the car is in motion.
General principle of action
The engine operates as follows. The fuel mixture enters the combustion chamber, then it is compressed by the piston. After this, the mixture ignites. This leads to the expansion of the combustion products, they press on the piston and exit the cylinder.
In two-stroke engines, one revolution of the crankshaft takes place in two cycles. The four-stroke piston engine performs a duty cycle in two rotations of the crankshaft. Engines are equipped with timing. What is this mechanism? This is an element that allows you to let the fuel mixture into the chambers and release combustion products from there. The exchange of gases is carried out at the time of a single revolution of the crankshaft. Gas exchange occurs due to the movement of the piston.
History
The first device, resembling a four-stroke engine, was invented by Felicce Matoci and Eugene Barsanti. But this invention is incredibly lost. Only in 1861 a similar unit was patented.
And the first usable engine was developed by an engineer from Germany, Nikolaus Otto. The motor received the name of the inventor, and the duty cycle of the four-stroke engine also bears the name of this engineer.
The main differences of four-stroke engines
In a two-stroke engine, piston and cylinder fingers, crankshaft, bearings and compression rings are lubricated by oil, which is added to the fuel. In a four-stroke engine, all components are installed in an oil bath. This is a significant difference. Therefore, in a four-stroke unit, there is no need to mix oil and gasoline.
The advantages of the system are that on the mirror in the cylinders and on the walls of the muffler the amount of soot is much less. Another difference - in two-stroke engines, a combustible mixture enters the exhaust pipe.
Engine operation
Regardless of the type of motor, the principle of its operation is similar. Today there are carburetor engines, diesel, injection. All models have the same four-stroke engine duty cycle. Let's take a closer look at the processes that work inside the motor and make it move.
A four-stroke cycle is a sequence of four work cycles. The measure is usually taken as the beginning when a combustible mixture enters the combustion chamber. Although other actions take place in the engine during its course, the designated cycle is one working process. For example, a compression cycle is not only compression. During this period, the mixture is mixed in the cylinders, gas formation begins, it ignites.
The same can be said about other stages of the engine. The most important thing is that different processes are laid out for only four clock cycles to better understand and simplify the working cycle of a four-stroke engine.
Inlet
So, in the combustion chamber of a power unit, energy conversion cycles begin with the combustion reaction of the fuel mixture. In this case, the piston is at its highest point (TDC position), and then moves down. As a result, a vacuum occurs in the combustion chamber of the engine. Under its influence, a combustible liquid absorbs fuel. The inlet valve is in the open position, and the outlet valve is closed.
When the piston begins to move downward, the volume above it increases. This causes a vacuum. It is approximately 0.071-0.093 MPa. Thus, gas enters the combustion chamber. In injection engines, fuel is injected by a nozzle. After the mixture enters the cylinder, its temperature can be 75 to 125 degrees.
How much the cylinder will be filled with the fuel mixture is determined by the fill factors. For engines with a carburetor power system, this figure will be from 0.64 to 0.74. The higher the ratio, the more powerful the motor.
Compression
After filling the combustion chamber with a combustible mixture of gasoline vapors and air, if the crankshaft makes rotational movements, the piston will begin to return to its lower position. The inlet valve at this stage will begin to close. And the graduation will still be closed.
Working stroke
This is the third cycle of the four-stroke internal combustion engine. He is the most important in the work of the power unit. It is at this stage of the engine's operation that the energy from the combustion of fuel is converted into mechanical energy, which makes the crankshaft rotate.
When the piston is in a position close to TDC, even during compression, the fuel mixture is forcibly ignited from the engine spark plug. The fuel charge burns out very quickly. Even before the start of this cycle, burnt gases have a maximum pressure value. These gases are a working fluid compressed in a small volume of the combustion chamber of the engine. When the piston begins to move downward, the gases begin to expand rapidly, releasing energy.
Among all the clock cycles of the four-cylinder engine, this is the most useful. It operates on the load of the unit. Only at this stage the crankshaft receives acceleration acceleration. In all the others, the motor does not generate energy, but consumes it from the same crankshaft.
Release
After the gases have done useful work, they must exit the cylinder to make room for a new portion of the fuel-air mixture. This is the last cycle in the duty cycle of a four-stroke engine.
Gases at this stage are under pressure significantly higher than atmospheric. The temperature at the end of the measure drops to about 700 degrees. The crankshaft through the connecting rod moves the piston to the TDC. Then the exhaust valve opens, the gases are pushed into the atmosphere through the exhaust system. As for pressure, it is high only at the very beginning. At the end of the cycle, it decreases to 0.120 MPa. Naturally, it is impossible to completely get rid of the combustion products in the cylinder. Therefore, they are mixed with the fuel mixture at the next intake stroke.
Operating procedure
The described steps comprise the duty cycle of a four-stroke gasoline engine. It must be understood that there are no strict correspondences between cycles and processes in piston engines. This is easily explained by the fact that during the operation of the power unit the phases of the gas distribution mechanism and the state of the valves will be superimposed on the movements of the pistons in different motors in completely different ways.
In any cylinder, the duty cycle of a four-stroke carburetor engine proceeds in this way. Each combustion chamber in the engine is needed to rotate a single crankshaft receiving the force from the pistons.
This alternation is called work order. This order is set at the design stage of the power unit through the features of the camshaft and crankshaft. It does not change during the operation of the mechanism.
The implementation of the operating procedure is carried out by alternating sparks that enter the spark plugs from the ignition system. So, a four-cylinder engine can operate in the following orders - 1, 3, 4, 2 and 1, 2, 4, 3.
You can find out the order in which the engine cylinders work from the instructions for the car. Sometimes the order of operation is indicated on the block body.
This is how the working cycle of a four-stroke carburetor engine or any other proceeds. The power system does not affect the principle of operation of the unit. The only difference is that the carburetor is a mechanical power system that has certain shortcomings, and in the case of injectors, there are no such shortcomings in the system.
Diesel engines
The duty cycle of a four-stroke diesel engine is the same process sequence as the carburetor motor cycle. The difference lies in how the cycle proceeds, as well as in the differences in the processes of mixture formation and ignition.
Diesel intake timing
When the piston moves down, the gas distribution mechanism opens the intake valve. A certain amount of air enters the combustion chamber. The temperature in the cylinder is approximately 80 degrees. In diesel engines, the power system is significantly different from gasoline carburetor engines. For example, the hydraulic resistance in them is lower, and the pressure rises slightly.
Compression cycle in diesel engine
At this stage of operation, the piston in the combustion chamber is directed upward to TDC. Both valves in the car engine are closed. As a result of the piston, the air in the cylinder is compressed. The compression ratio in a diesel engine is higher than in gasoline engines, and the pressure inside the cylinder can reach 5 MPa. Compressed air heats up significantly. Temperatures can reach 700 degrees. This is necessary to ignite the fuel. It is fed on diesel engines through nozzles mounted on each cylinder. In winter, glow plugs are involved in the work. They preheat the cold mixture. Thus, the motor starts easier in winter. But such a system is not on all cars.
Gas expansion stroke in a diesel engine
When the piston of the diesel engine has not yet reached the top point by about 30 degrees along the crankshaft, the high-pressure fuel pump through the nozzle delivers high pressure fuel to the cylinder. A value of 18 MPa is necessary so that the fuel can be finely atomized and distributed throughout the volume in the cylinder.
Further, the fuel ignites under the influence of high temperatures and quickly burns out. The piston moves to a lower point. The temperature inside the cylinder at this moment is about 2000 degrees. By the end of the measure, the temperature decreases.
Diesel engine exhaust
At this stage, the exhaust valve is open, the piston moves to the upper point. Combustion products are forcibly removed from the cylinder. Next they go to the exhaust manifold. After that, the catalytic converter is turned on. Gases passing through it under high temperature are purified. Clean, harmless gas is already entering the atmosphere. A diesel particulate filter is optionally installed on diesel vehicles. It also contributes to the purification of gases.
Conclusion
We examined in detail how the four-stroke engine operates (it takes two turns of the crankshaft of the power plant). And the cycle itself includes many different processes.