DIESEL ENGINE - OPERATING PRINCIPLE

At first glance, a diesel engine almost does not differ from a conventional gasoline engine - the same cylinders, pistons, connecting rods. The main and fundamental differences are in the way the fuel-air mixture is formed and ignited. In carburetor and conventional injection engines, the mixture is prepared not in the cylinder, but in the intake tract. In gasoline engines with direct injection, the mixture is formed in the same way as in diesel engines - directly in the cylinder. In a gasoline engine, the fuel-air mixture in the cylinder ignites at the right time from a spark discharge. In a diesel engine, the fuel is ignited not by a spark, but due to the high temperature of the air in the cylinder.
The working process in a diesel engine is as follows: first, clean air enters the cylinder, which, due to the high compression ratio (16-24: 1), is heated to 700-900 ° C. Diesel fuel is injected at high pressure into the combustion chamber as the piston approaches top dead center. And since the air is already very hot, after mixing with it, the fuel ignites. Self-ignition is accompanied by a sharp increase in pressure in the cylinder - hence the increased noise and rigidity of the diesel engine. This organization of the working process allows the use of cheaper fuel and work on very lean mixtures, which determines higher efficiency. Diesel has a higher efficiency (35–45% for diesel, 25–35% for gasoline) and torque. The disadvantages of diesel engines usually include increased noise and vibration, lower liter power and cold start difficulties. But the shortcomings described relate mainly to old designs, and in modern ones these problems are no longer so obvious.
DESIGN.

PECULIARITIES.

As already noted, the design of a diesel engine is similar to that of a gasoline engine. However, similar parts of a diesel engine are significantly strengthened in order to perceive higher loads - after all, its compression ratio is much higher (16-24 units versus 9-11 for gasoline). A characteristic detail in the design of diesel engines is the piston. The shape of the bottom of the pistons in diesel engines is determined by the type of combustion chamber, so it is easy to determine by the shape which engine this piston belongs to. In many cases, the piston crown contains the combustion chamber. The bottoms of the pistons are above the top plane of the cylinder block when the piston is at the top of its stroke. Since the ignition of the working mixture is carried out by compression, there is no ignition system in diesel engines, although candles can also be used on a diesel engine. But these are not spark plugs, but glow plugs,
Diesel pistons and spark plugs
The technical and environmental performance of an automobile diesel engine primarily depends on the type of combustion chamber and fuel injection system.

TYPES OF COMBUSTION CHAMBER.

The shape of the combustion chamber significantly affects the quality of the mixture formation process, and hence the power and noise of the engine. The combustion chambers of diesel engines are divided into two main types: undivided and divided.
A few years ago, diesels with divided combustion chambers dominated the passenger car market. In this case, fuel injection is carried out not into the over-piston space, but into a special combustion chamber made in the cylinder head. In this case, two mixing processes are distinguished: pre-chamber (it is also called pre-chamber) and vortex-chamber.
Diesel combustion chambers
In the prechamber process, fuel is injected into a special preliminary chamber connected to the cylinder by several small channels or holes, hits its walls and mixes with air. Having ignited, the mixture enters the main combustion chamber, where it burns completely. The cross section of the channels is selected so that when the piston moves up (compression) and down (expansion), a large pressure drop occurs between the cylinder and the prechamber, causing the gases to flow through the holes at high speed.
During the vortex chamber process, combustion also begins in a special separate chamber, only made in the form of a hollow ball. During the compression stroke, air enters the prechamber through the connecting channel and intensively twists (forms a vortex) in it. The fuel injected at a certain moment mixes well with air.
Thus, with a divided combustion chamber, a kind of two-stage combustion of fuel occurs. This reduces the load on the piston group, and also makes the sound of the engine softer. The disadvantage of diesel engines with a split combustion chamber is: an increase in fuel consumption due to losses due to the increased surface of the combustion chamber, large losses for the flow of air charge into the additional chamber and the burning mixture back into the cylinder. In addition, start-up performance deteriorates.
Single chamber diesel engines are also called direct injection diesel engines. Fuel is injected directly into the cylinder, the combustion chamber is made in the piston crown. Until recently, direct injection was used on low-speed, high-volume diesel engines (in other words, on trucks). Although such engines are more economical than engines with separated combustion chambers, their use in small diesel engines was constrained by the difficulties in organizing the combustion process, as well as increased noise and vibration, especially in acceleration mode.
Now, thanks to the widespread introduction of electronic control of the fuel dosing process, it has been possible to optimize the combustion process of the fuel mixture in a diesel engine with an undivided combustion chamber and significantly reduce noise. New diesel engines are being developed with direct injection only.

POWER SYSTEMS.

The most important link in a diesel engine is the fuel supply system, which ensures the supply of the required amount of fuel at the right time and with a given pressure to the combustion chamber.

Diesel power system.

The high-pressure fuel pump (TNVD), taking fuel from the tank from the booster pump (low pressure), in the required sequence, in turn pumps the necessary portions of diesel fuel into the individual line of the hydromechanical nozzle of each cylinder. Such injectors open exclusively under the influence of high pressure in the fuel line and close when it decreases.
There are two types of injection pumps: in-line multi-plunger and distribution type. An in-line injection pump consists of separate sections according to the number of diesel cylinders, each of which has a sleeve and a plunger included in it, which is driven by a camshaft that receives rotation from the engine. Sections of such mechanisms are usually located in a row, hence the name - in-line injection pumps. In-line pumps are currently practically not used due to the fact that they cannot meet modern environmental and noise requirements. In addition, the injection pressure of such pumps depends on the crankshaft speed.
Distribution injection pumps create a significantly higher fuel injection pressure than in-line pumps and comply with current emissions regulations. This mechanism maintains the desired pressure in the system, depending on the operating mode of the engine. In distribution injection pumps, the injection system has one distributor plunger, which performs translational motion to pump fuel and rotational to distribute fuel to the nozzles.

These pumps are compact, characterized by high uniformity of fuel supply through the cylinders and excellent operation at high speeds. At the same time, they make very high demands on the purity and quality of diesel fuel: after all, all their parts are lubricated with fuel, and the gaps in precision elements are very small.
The tightening of legislative environmental requirements for diesel engines in the early 90s forced engine builders to intensively improve fuel supply. It immediately became clear that this problem could not be solved with an outdated mechanical power system. Traditional mechanical fuel injection systems have a significant drawback: the injection pressure depends on the engine speed and load conditions. This means that at low load, the injection pressure drops, as a result, the fuel is poorly atomized during injection, falling into the combustion chamber with too large drops that settle on its internal surfaces. Because of this, the efficiency of fuel combustion decreases and the level of toxicity of exhaust gases increases.
Only the optimization of the combustion process of the fuel-air mixture could radically change the situation. For what it is necessary to make its entire volume ignite in the shortest possible time. And here you need high dose accuracy and injection timing accuracy. This can be done only by raising the fuel injection pressure and using electronic control of the fuel supply process. The fact is that the higher the injection pressure, the better the quality of its atomization, and, accordingly, its mixing with air. Ultimately, this contributes to a more complete combustion of the fuel-air mixture, and hence the reduction of harmful substances in the exhaust. Well, you ask, why not make the same increased pressure in a conventional injection pump and this entire system? Alas, it won't work. Because there is such a thing as "wave hydraulic pressure". With any change in fuel consumption in the pipelines from the injection pump to the injectors, pressure waves arise that "run" along the fuel line. And the stronger the pressure, the stronger these waves. And if you further increase the pressure, then at some point an ordinary destruction of pipelines may occur. Well, there is no need to even talk about the dosing accuracy of the mechanical injection system.
Pump-injector
As a result, two new types of power supply systems were developed - in the first, the nozzle and the plunger pump were combined into one unit (pump-injector), and in the other, the high-pressure fuel pump began to work on a common fuel line (Common Rail), from which fuel is supplied to electromagnetic (or piezoelectric) nozzles and injected at the command of the electronic control unit. But with the adoption of Euro 3 and 4, this was not enough, and diesel particulate filters and catalysts were introduced into the exhaust systems of diesel engines.
A pump injector is installed in the engine block head for each cylinder. It is driven by a camshaft cam using a pusher. The fuel supply and drain lines are made in the form of channels in the block head. Due to this, the pump nozzle can develop a pressure of up to 2200 bar. The dosing of fuel compressed to such an extent and the control of the injection advance angle is handled by the electronic control unit, issuing signals to the shut-off electromagnetic or piezoelectric valves of the unit injectors. Pump injectors can operate in multi-pulse mode (2-4 injections per cycle). This allows you to make a preliminary injection before the main one, first supplying a small portion of fuel to the cylinder, which softens the engine and reduces exhaust toxicity.

common rail system.

The Common Rail power system has been used in serial diesel engines since 1997. Common Rail is a method of injecting fuel into a combustion chamber at high pressure, independent of engine speed or load. The main difference between the Common Rail system and the classic diesel system is that the injection pump is designed only to create high pressure in the fuel line. It does not perform the functions of metering the cyclic fuel supply and adjusting the injection timing. The Common Rail system consists of a reservoir - a high pressure accumulator (sometimes called a rail), a fuel pump, an electronic control unit (ECU) and a set of injectors connected to the rail. In the ramp, the control unit maintains, by changing the performance of the pump, constant pressure at the level of 1600-2000 bar at various engine operating modes and at any injection sequence in the cylinders. The opening and closing of the nozzles is controlled by the ECU, which calculates the optimal moment and duration of injection, based on data from a number of sensors - the position of the accelerator pedal, pressure in the fuel rail, engine temperature, its load, etc. The nozzles can be electromagnetic or more modern - piezoelectric. The main advantages of piezoelectric nozzles are high response speed and dosing accuracy. Injectors in common rail diesel engines can operate in a multi-pulse mode: during one cycle, fuel is injected several times - from two to seven. First, a tiny, only about a milligram, dose arrives, which, when burned, raises the temperature in the chamber, and then comes the main "charge". For a diesel engine with compression ignition, this is very important, since in this case the pressure in the combustion chamber increases more smoothly, without a “jerk”. As a result, the engine runs softer and less noisy, and the amount of harmful components in the exhaust is reduced. Multiple fuel supply in one stroke simultaneously provides a decrease in the temperature in the combustion chamber, which leads to a decrease in the formation of nitric oxide, one of the most toxic components of diesel exhaust gases. The characteristics of a common rail engine are largely dependent on injection pressure. In systems of the third generation, it is 2000 bar. In the near future, the fourth generation of Common Rail with an injection pressure of 2500 bar will be launched into the series. since in this case the pressure in the combustion chamber increases more smoothly, without a “jerk”. As a result, the engine runs softer and less noisy, and the amount of harmful components in the exhaust is reduced. Multiple fuel supply in one stroke simultaneously provides a decrease in the temperature in the combustion chamber, which leads to a decrease in the formation of nitric oxide, one of the most toxic components of diesel exhaust gases. The characteristics of a common rail engine are largely dependent on injection pressure. In systems of the third generation, it is 2000 bar. In the near future, the fourth generation of Common Rail with an injection pressure of 2500 bar will be launched into the series. since in this case the pressure in the combustion chamber increases more smoothly, without a “jerk”. As a result, the engine runs softer and less noisy, and the amount of harmful components in the exhaust is reduced. Multiple fuel supply in one stroke simultaneously provides a decrease in the temperature in the combustion chamber, which leads to a decrease in the formation of nitric oxide, one of the most toxic components of diesel exhaust gases. The characteristics of a common rail engine are largely dependent on injection pressure. In systems of the third generation, it is 2000 bar. In the near future, the fourth generation of Common Rail with an injection pressure of 2500 bar will be launched into the series. Multiple fuel supply in one stroke simultaneously provides a decrease in the temperature in the combustion chamber, which leads to a decrease in the formation of nitric oxide, one of the most toxic components of diesel exhaust gases. The characteristics of a common rail engine are largely dependent on injection pressure. In systems of the third generation, it is 2000 bar. In the near future, the fourth generation of Common Rail with an injection pressure of 2500 bar will be launched into the series. Multiple fuel supply in one stroke simultaneously provides a decrease in the temperature in the combustion chamber, which leads to a decrease in the formation of nitric oxide, one of the most toxic components of diesel exhaust gases. The characteristics of a common rail engine are largely dependent on injection pressure. In systems of the third generation, it is 2000 bar. In the near future, the fourth generation of Common Rail with an injection pressure of 2500 bar will be launched into the series.

TURBO-DIESEL.

An effective means of increasing the power and flexibility of a diesel engine is turbocharging. It allows you to supply additional air to the cylinders and, accordingly, increase the fuel supply during the working cycle, resulting in an increase in engine power. The exhaust gas pressure of a diesel engine is 1.5-2 times higher than that of a gasoline engine, which allows the turbocharger to provide effective boost from the lowest revs, avoiding the failure characteristic of gasoline turbo engines - "turbo lag". The absence of a throttle valve in a diesel engine makes it possible to ensure efficient filling of the cylinders at all speeds without the use of a complex turbocharger control scheme. On many cars, an intercooler of charge air is installed - an intercooler that allows you to increase the mass filling of the cylinders and increase power by 15-20%. Supercharging allows you to achieve the same power with an atmospheric engine with a smaller displacement, which means reducing engine weight. Turbocharging, among other things, serves as a means for the car to increase the "altitude" of the engine - in high mountainous areas, where atmospheric diesel lacks air, boost optimizes combustion and reduces harshness and power loss. At the same time, the turbodiesel has some disadvantages, mainly related to the reliability of the turbocharger. Thus, the turbocharger resource is significantly less than the engine resource. The turbocharger makes strict demands on the quality of engine oil. A faulty unit can completely disable the engine itself. In addition, the own resource of a turbodiesel is somewhat lower than the same atmospheric diesel due to the large degree of forcing.
The progress of diesel engines today has two main goals: increasing power and reducing toxicity. Therefore, all modern passenger diesel engines are turbocharged (the most efficient way to increase power) and Common Rail.