What is a diesel? Principle of operation, arrangement and technical characteristics of a diesel engine

Diesel engines are very common on passenger cars. Many models have at least one option in the engine range. And this is without taking into account trucks, buses and construction equipment, where they are used everywhere. Further, it is considered what is a diesel engine, design, operating principle, features.

Definition

This unit is a piston internal combustion engine, the functioning of which is based on the self-ignition of the atomized fuel from heating or compression.

Design Features

The gasoline engine has the same structural elements as the diesel engine. The scheme of functioning as a whole is also similar. The difference is in the processes of the formation of a fuel-air mixture and its combustion. In addition, diesel engines are more solid parts. This is due to approximately twice the higher compression ratio than that of gasoline engines (19-24 against 9-11).

Classification

According to the design of the combustion chamber, diesel engines are divided into variants with a separate combustion chamber and direct injection.

In the first case, the combustion chamber is separated from the cylinder and connected to it by a channel. During compression, the air entering the vortex chamber is twisted, which improves the mixture formation and self-ignition, which begins there and continues in the main chamber. Diesel engines of this type were previously distributed in passenger cars due to the fact that they were distinguished by a low noise level and a large range of speeds from the options considered below.

In diesel engines with direct injection, the combustion chamber is in the piston, and the fuel is supplied to the above-piston space. This design was originally used on low-speed motors of large volume. They were distinguished by a high level of noise and vibration and low fuel consumption. Later, with the advent of high-pressure fuel pumps with electronic control and optimization of the combustion process, the designers achieved stable operation at a range of up to 4500 rpm. In addition, increased economy, reduced noise and vibration levels. Among the measures to reduce the rigidity of work - multi-stage pre-injection. Due to this, engines of this type have been widely spread in the last two decades.

According to the principle of operation, diesel engines are divided into four-stroke and two-stroke, as well as gasoline engines. Their features are discussed below.

Principle of operation

To understand what a diesel is and what its functional features are due to, it is necessary to consider the principle of operation. The above classification of piston internal combustion engines is based on the number of cycles entering the working cycle, which are distinguished by the value of the angle of rotation of the crankshaft.

Consequently, the duty cycle of four-stroke engines includes 4 phases.

• Inlet. Occurs when the crankshaft is turned from 0 to 180 °. At the same time, air passes into the cylinder through the inlet valve, which is open at 345-355 °. Simultaneously with it during the rotation of the crankshaft for 10-15 ° the exhaust valve is opened, which is called the overlap.
• Compression. The piston, moving upward at 180-360 °, compresses the air 16-25 times (compression ratio), and the intake valve closes at the beginning of the stroke (at 190-210 °).
• Working stroke, expansion. Occurs at 360-540 °. At the beginning of the bar, before the piston reaches the top dead center, the fuel is fed into the hot air and ignited. This is a feature of diesel engines, distinguishing them from gasoline, where there is an ignition advance. The resulting combustion products push the piston down. At the same time, the combustion time of the fuel is equal to the time it is injected by the injector and lasts no longer than the duration of the working stroke. That is, during the working process, the pressure of the gases is constant, as a result of which the diesel engines develop a greater torque. Another important feature of such motors is the need to provide excess air in the cylinder, since the flame occupies a small part of the combustion chamber. That is, the proportion of the fuel-air mixture is different.
• Release. At 540-720 ° turning crankshaft open exhaust valve piston, moving upwards, displaces exhaust gases.

The two-cycle cycle is characterized by shortened phases and a single gas exchange process in the cylinder (blowdown) occurring between the end of the working stroke and the beginning of compression. When the piston moves down, the combustion products are removed through the exhaust valves or windows (in the cylinder wall). Later, the inlet windows for fresh air are opened. When the piston rises, all the windows are closed, and compression begins. A little before reaching TDC, fuel is injected and ignited, expansion begins.

Due to the complexity of ensuring the purging of the vortex chamber, two-stroke engines are only with direct injection.

The performance of such engines is 1.6-1.7 times higher than the characteristics of a four-stroke diesel engine. Its growth is provided by twice the implementation of working moves, but partially reduced due to their smaller size and blowdown. Due to the doubled number of working strokes, the push-pull cycle is particularly relevant in the event that it is not possible to increase the speed of rotation.

The main problem of such engines is blowdown due to its short duration, which can not be compensated without reducing efficiency by shortening the working stroke. In addition, it is impossible to separate the exhaust and fresh air, because of which part of the latter is removed with the exhaust gases. This problem can be solved by ensuring that the exit windows are outstripped. In this case, the gases begin to be removed before purging, and after closing the exhaust, the cylinder is supplemented with fresh air.

In addition, when using one cylinder, there are difficulties with synchronous opening / closing windows, so there are engines (PDP), in which each cylinder has two pistons moving in the same plane. One of them controls the intake, the other - the release.

By the mechanism of implementation, the purging is divided into a slot (window) and a valve-slotted one. In the first case, the windows serve both inlet and outlet openings. The second option assumes their use as inlet ports, and for the outlet is a valve in the cylinder head.

Usually two-stroke diesel engines are used on heavy vehicles like ships, diesel locomotives, tanks.

Fuel system

The fuel equipment of diesel engines is much more complicated than that of gasoline ones. This is due to the high requirements for accuracy of fuel delivery in time, quantity and pressure. The main components of the fuel system - fuel pump, injectors, filter.

The fuel supply system with computer-controlled (Common-Rail) is widely used. She injects it in two portions. The first of these is small, serving to increase the temperature in the combustion chamber (pre-injection), which reduces noise and vibration. In addition, this system increases the torque by 25% at low speed, reduces fuel consumption by 20% and the soot content of the exhaust gases.

Turbocharging

Diesel engines are very widely used turbines. This is due to a higher (1.5-2) times the exhaust gas pressure, which untwists the turbine, which avoids the turbojack, providing a boost from lower revs.

Cold Start

You can find many reviews that at low temperatures there is no diesel. The difficulty of starting such motors in cold conditions is due to the fact that this requires more energy. To facilitate the process, they are equipped with a preheater. This device is represented by glow plugs located in the combustion chambers, which, when the ignition is switched on, warms the air in them and works for 15-25 seconds after starting to ensure the stability of the non-heated motor. Due to this, diesel engines start at temperatures of -30 ...- 25 ° С.

Features of the service

To ensure durability during operation, you need to know what a diesel is and how to maintain it. Relatively low prevalence of the engines in question in comparison with gasoline is explained, among other things, by more complex maintenance.

First of all, this applies to a high-complexity fuel system. Because of this, diesel engines are extremely sensitive to the content of water and mechanical particles in the fuel, and its repair is more expensive, as is the engine as a whole compared to gasoline of the same level.

In the case of a turbine, the requirements to the quality of engine oil are also high. Its resource is usually 150 thousand km, and the cost is high.

In any case, on diesel engines, oil should be changed more often than gasoline ones (2 times according to European standards).

As noted, these motors have cold start problems, when diesel is not started at low temperatures . In some cases, this is caused by the use of unsuitable fuel (depending on the season, different grades are used on these engines, as summer fuel freezes at low temperatures).

Performance

In addition, many do not like the quality of diesel engines, such as lower power and range of working speed, a higher level of noise and vibration.

The gasoline engine does indeed usually outperform in performance, including a liter capacity, a similar diesel. The motor of this type has a higher and more even torque curve. The increased compression ratio, which provides a greater torque, forces the use of stronger parts. Since they are heavier, power is reduced. In addition, this affects the mass of the engine, and consequently, the car.

A small range of working speed is due to a longer fuel ignition, so that at high speeds it does not have time to burn out.

An increased level of noise and vibration causes a sudden increase in pressure in the cylinder during ignition.

The main advantages of diesels are higher tyagovitost, efficiency and environmental friendliness.

Tightness, that is, high torque at low revs, is due to the combustion of fuel as it is injected. This provides greater responsiveness and facilitates efficient use of power.

The economy is due to both low consumption and the fact that diesel fuel is cheaper. In addition, it is possible to use low-grade heavy oils as it because of the absence of strict requirements for volatility. And the fuel is heavier, the higher the efficiency of the engine. Finally, diesel engines operate on poor mixtures in comparison with gasoline engines and at a high compression ratio. The latter provides less heat loss with the exhaust gases, that is, greater efficiency. All these measures reduce fuel consumption. Diesel, due to this, spends it 30-40% less.

The environmental friendliness of diesel engines is explained by the fact that their exhaust gas contains less carbon monoxide. This is achieved through the use of sophisticated cleaning systems, so now the gasoline engine meets the same environmental standards as diesel. The motor of this type was previously much inferior to the gasoline in this respect.

Application

As is clear from what a diesel is and what are its characteristics, such motors are most suitable for those cases where high thrust is required at low revs. Therefore, they are equipped with almost all buses, trucks and construction equipment. As for private vehicles, among them such parameters are most important for SUVs. Due to the high profitability of these motors, urban models are also equipped. In addition, they are more convenient to manage in such conditions. Test drives diesel engines testify to this.