What is diesel? The principle of operation, device and technical characteristics of the diesel engine

2024 Author: Erin Ralphs | [email protected]. Last modified: 2024-02-19 11:42

Diesel engines are very common in 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. Next, we consider what a diesel engine is, design, principle of operation, features.

Definition

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

Design Features

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

Classification

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

In the firstIn this case, the combustion chamber is separated from the cylinder and connected to it by a channel. When compressed, the air entering the vortex-type chamber is twisted, which improves mixture formation and self-ignition, which begins there and continues in the main chamber. Diesel engines of this type were previously common in passenger cars due to the fact that they were distinguished by a lower noise level and a large rev range from the options discussed below.

In diesel engines with direct injection, the combustion chamber is located in the piston, and the fuel is supplied to the over-piston space. This design was originally used on low-speed high-volume engines. They were distinguished by a high level of noise and vibration and low fuel consumption. Later, with the advent of electronically controlled high-pressure fuel pumps and optimization of the combustion process, designers achieved stable operation at a range of up to 4500 rpm. In addition, increased efficiency, decreased noise and vibration levels. Among the measures to reduce the rigidity of the work is a multi-stage pre-injection. Thanks to this, engines of this type have become widespread 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.

Operating principle

To understand what a diesel engine is and what determines its functional features, it is necessary to consider the principle of operation. The above classification of piston internal combustion engines is based on the number of strokes included in the operating cycle, which are distinguished by the magnitude of the angle of rotation of the crankshaft.

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

• Intake. Occurs when the crankshaft rotates from 0 to 180°. In this case, the air passes into the cylinder through the intake valve open at 345-355 °. At the same time, during the rotation of the crankshaft by 10-15 °, the exhaust valve is opened, which is called the overlap.
• Compression. The piston moving up 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, extension. Occurs at 360-540°. At the start of the stroke, until the piston reaches top dead center, fuel is injected into the hot air and ignited. This is a feature of diesel engines that distinguishes them from gasoline engines, where ignition advance occurs. The resulting combustion products push the piston down. In this case, the fuel combustion time is equal to the time of its supply by the nozzle and lasts no longer than the duration of the working stroke. That is, during the working process, the gas pressure is constant, as a result of which diesel engines develop more torque. Also an 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 air-fuel mixture is different.
• Release. At 540-720° of crankshaft rotation, the exhaust valve is open and the piston moves up to displace the exhaust gases.

The two-stroke cycle is characterized by shortened phases and a single process of gas exchange in the cylinder (purge) occurring between the end of the stroke and the start of compression. When the piston moves down, the products of combustion are removed through the exhaust valves or windows (in the cylinder wall). Later, the inlet windows are opened to let in fresh air. As the piston rises, all windows close and compression begins. Just before TDC is reached, fuel is injected and ignited, expansion begins.

Due to the difficulty of purging the swirl chamber, two-stroke engines are only available with direct injection.

The performance of such engines is 1.6-1.7 times higher than the performance of a four-stroke type diesel engine. Its growth is ensured by twice as frequent implementation of working strokes, but is partially reduced due to their smaller size and blowing. Due to the double number of working strokes, the two-stroke cycle is especially relevant if it is impossible to increase the speed.

The main problem with these engines is scavenging due to its short duration, which cannot be compensated without reducing efficiency by shortening the stroke. In addition, it is impossible to separate the exhaust and fresh air, due to which part of the latter is removed with the exhaust gases. This problem can be solved by providing advance exhaust windows. In this case, the gases begin to be removed before the purge, and after the outlet is closed, the cylinder is supplemented with fresh air.

Besides, whenusing one cylinder, there are difficulties with the synchronism of opening / closing windows, therefore there are engines (PDP) in which each cylinder has two pistons moving in the same plane. One of them controls the intake, the other controls the exhaust.

According to the mechanism of implementation, purge is divided into slotted (window) and valve-slotted. In the first case, the windows serve as both inlet and outlet openings. The second option involves using them as intake ports, and the valve in the cylinder head serves as an outlet.

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 engines. This is due to the high requirements for the accuracy of fuel supply in terms of time, quantity and pressure. The main components of the fuel system - injection pump, injectors, filter.

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

Turbocharging

Turbines are widely used on diesel engines. This is due to the higher (1.5-2) times the pressure of the exhaust gases, whichspin the turbine, which avoids turbo lag by providing boost from lower revs.

Cold start

You can find a lot of reviews that diesel does not start at low temperatures. 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 placed in the combustion chambers, which, when the ignition is turned on, heat the air in them and work for another 15-25 seconds after starting to ensure the stability of the cold engine. Thanks to this, diesel engines are started at temperatures of -30 … -25 ° С.

Service Features

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

First of all, this concerns the fuel system of high complexity. 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 well as the engine as a whole, compared to gasoline of the same level.

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

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

As it wasnoted that these engines have cold start problems when diesel does not start at low temperatures. In some cases, this is caused by using the wrong fuel (depending on the season, different grades are used on such engines, since summer fuel freezes at low temperatures).

Performance

In addition, many people do not like such qualities of diesel engines as lower power and operating speed range, higher noise and vibration levels.

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

The small range of operating speeds is due to the longer ignition of the fuel, as a result of which it does not have time to burn out at high speeds.

Increased noise and vibration levels cause a sharp increase in pressure in the cylinder during ignition.

The main advantages of diesel engines are considered higher traction, efficiency and environmental friendliness.

Traction, that is, high torque at low speeds, is explained by the combustion of fuel as it is injected. This provides greater responsiveness and makes it easier to use power efficiently.

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

The environmental friendliness of diesel engines is explained by the fact that their exhaust gases have a lower content of carbon monoxide. This is achieved by using sophisticated cleaning systems, thanks to which the gasoline engine now meets the same environmental standards as a diesel engine. This type of engine was previously significantly inferior to gasoline in this regard.

Application

As is clear from what a diesel engine is and what its characteristics are, such motors are most suitable for cases where high traction at low revs is needed. 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 efficiency, urban models are also equipped with these motors. In addition, they are more convenient to manage in such conditions. Diesel test drives testify to this.