Four-stroke engine duty cycle - features, diagram and description

2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16

Motorists should at least in general terms know how the engine works 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 works as follows. The fuel mixture enters the combustion chamber, then it is compressed under the influence of the piston. The mixture then ignites. This causes the products of combustion to expand, pushing against the piston and exiting the cylinder.

In two-stroke engines, one revolution of the crankshaft takes two cycles. A four-stroke piston engine completes a work cycle in two revolutions 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 takes place inmoment 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 motor was invented by Felicce Matoczi and Eugene Barsanti. But this invention was incredibly lost. Only in 1861 a similar unit was patented.

And the first usable engine was developed by German engineer Nikolaus Otto. The motor is named after the inventor, and the duty cycle of the four-stroke engine is also named after the engineer.

The main differences between four-stroke engines

In a two-stroke engine, the piston and cylinder pins, crankshaft, bearings and compression rings are lubricated by oil 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 oils and gasoline.

The advantages of the system are that the amount of carbon deposits on the mirror in the cylinders and on the muffler walls is much less. Another difference is that in two-stroke engines, a combustible mixture enters the exhaust pipe.

Engine running

Regardless of the type of motor, the principle of its operation is similar. Today there are carburetor engines, diesel, injection. All models use the same four-stroke engine cycle. Let's take a closer look at what processes work inside the motor and make it move.

A four-stroke cycle is a sequence of four working cycles. The cycle is usually taken as the beginning when a combustible mixture enters the combustion chambers. Although other actions take place in the engine during its flow, the designated cycle is one work process. For example, the compression stroke is not only compression. During this period, the mixture is stirred in the cylinders, the formation of gas begins, it ignites.

The same can be said about other stages of the engine. The most important thing here is that the different processes for a better understanding and simplification of the working cycle of a four-stroke engine are decomposed into only four cycles.

Intake

So, in the combustion chamber of the power unit, the 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 sucks in fuel. The intake valve is in the open position, and the exhaust valve is closed.

When the piston starts moving down, the volume above it increases. This is what causes the breakdown. It is approximately 0.071-0.093 MPa. Thus, gasoline 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. Forengines with a carburetor power system, this indicator will be from 0.64 to 0.74. The higher the coefficient value, the more powerful the motor.

Compression

After filling the combustion chamber with a combustible mixture of gasoline vapors and air, if the crankshaft rotates, the piston will begin to return to its lower position. The intake valve will begin to close at this point. And graduation will still be closed.

Working stroke

This is the third stroke of a four-stroke internal combustion engine. It is the most important in the operation of the power unit. It is at this stage of engine operation that energy from fuel combustion is converted into mechanical energy, which makes the crankshaft rotate.

When the piston is close to TDC, even during compression, the fuel mixture is forcibly ignited by the engine's spark plug. The fuel charge burns out very quickly. Even before the start of this cycle, the burnt gases have a maximum pressure value. These gases are the working fluid compressed in a small volume of the combustion chamber of the engine. When the piston starts to move down, the gases begin to expand rapidly, releasing energy.

Among all the strokes of the working cycle of a four-cylinder engine, this one is the most useful. It operates on the load of the unit. Only at this stage the crankshaft receives accelerating acceleration. In all others, the motor does not generate energy, but consumes it from the same crankshaft.

Release

After committinggases of useful work, they must leave the cylinder in order to make room for a new portion of the fuel-air mixture. This is the last stroke of a four-stroke engine.

Gases at this stage are under pressure significantly higher than atmospheric pressure. By the end of the cycle, the temperature drops to about 700 degrees. The crankshaft moves the piston to TDC by means of a connecting rod. Next, the exhaust valve opens, the gases are pushed into the atmosphere through the exhaust system. As for the 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 combustion products in the cylinder. Therefore, they are mixed with the fuel mixture during the next intake stroke.

Working order

The described steps constitute the operating cycle of a four-stroke gasoline engine. You need to understand 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 engines in completely different ways.

In any cylinder, the duty cycle of a four-stroke carbureted engine proceeds in this way. Each combustion chamber in an engine is needed to rotate a single crankshaft that takes 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. He is notchanges during the operation of the mechanism.

The implementation of the order of work is carried out by alternating sparks that come to the candles from the ignition system. So, a four-cylinder engine can run 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 a four-stroke carbureted engine or whatever works. The power supply 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 disadvantages, and in the case of injectors, these disadvantages are not in the system.

Diesel engines

The working cycle of a four-stroke diesel engine is the same sequence of processes as the cycle of a carburetor engine. The difference lies in how the cycle proceeds, as well as the differences in the mixture formation and ignition processes.

Diesel intake stroke

When the piston moves downward, the gas distribution mechanism opens the intake valve. A certain amount of air enters the combustion chamber. The temperature in the cylinder is about 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.

Diesel compression stroke

At this stage of work, the pistonin the combustion chamber goes up towards TDC. Both valves in the car's engine are in the closed state. As a result of the operation 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 supplied on diesel engines through nozzles installed on each cylinder. In winter, glow plugs are used. They preheat the cold mixture. This makes it easier for the engine to start in winter. But not all cars have such a system.

The gas expansion stroke in a diesel engine

When the diesel engine piston has not yet reached the top point by about 30 degrees on the crankshaft, the injection pump delivers high-pressure fuel to the cylinder through the nozzle. A value of 18 MPa is necessary so that the fuel can be finely sprayed and distributed throughout the entire volume in the cylinder.

Further, the fuel under the action of high temperatures ignites and burns quickly. The piston moves to the lowest point. The temperature inside the cylinder at this moment is about 2000 degrees. The temperature drops towards the end of the cycle.

Diesel exhaust

At this stage, the exhaust valve is open, the piston moves to the top point. Combustion products are forcibly removed from the cylinder. Then they go to the exhaust manifold. After that to workthe catalytic converter is turned on. Gases passing through it at high temperature are purified. Clean, harmless gas is already released into the atmosphere. On diesel vehicles, a particulate filter is additionally installed. It also helps to purify gases.

Conclusion

We have analyzed in detail how the working cycle of a four-stroke engine is carried out (it takes two revolutions of the crankshaft of the power plant). And the cycle itself includes many different processes.