The bulk of diesel engines are of the same design as existing gasoline engines, but with reinforced components to handle the higher pressure.
Fuel is supplied to the engine by a high-pressure fuel pump and metering device, the pump is usually mounted on the side of the cylinder block. An electric ignition system is not required for diesel engines.
The main advantage of diesel engines over gasoline engines is their lower operating cost. This is partly due to the greater efficiency of the diesel engine, due to the high compression ratio, and partly due to the lower price of diesel fuel – although at the moment the difference in fuel prices is changing, so that the advantage of diesel cars will diminish.
The main disadvantage of the diesel engine is its low performance compared to gasoline engines of the same power.
To solve this problem – it is possible to increase the size of the engine, but this will lead to a significant increase in the weight of the car.
Now, however, major car manufacturers are adding turbochargers to their engines to make them competitive in terms of performance.
How diesel engines work
A diesel combustion engine works a little differently than a gasoline engine, they have the same basic components and operate on a four-stroke cycle. And the main difference is how the fuel is ignited and how the engine power is regulated.
In a gasoline engine, the fuel-air mixture is ignited by a spark. In a diesel engine, ignition is achieved by compression of the fuel mixture. In modern diesels the compression ratio is 20:1, while the gasoline engine has 9:1. Such pressure in the cylinder heats the air to a temperature at which the fuel ignites spontaneously.
Gasoline engines consume different amounts of air per intake stroke, the exact amount of air depends on the degree of throttle opening. In a diesel engine, the same amount of air is supplied through an unregulated intake valve at any speed.
Diesel engine cycles
- Intake stroke – when the piston moves downward, at that moment the intake valve is open, air is sucked in, the moment the piston reaches its effective end point, the intake valve closes.
- Compression stroke – the piston begins to move to the top of the cylinder, driven by the energy of the heavy flywheel and other pistons, while compressing the air to twenty times its original volume, creating a pressure of about 40 bar.
- The working stroke – when the piston reaches the top of its stroke, a precisely metered amount of diesel fuel is injected into the combustion chamber. The heat from the air compression, which is about 550 °C, immediately ignites the fuel-air mixture, causing it to burn and expand, while the pressure in the cylinder increases to 150 bar and the temperature rises to 2500 °C. This causes the piston to move downward while rotating the engine crankshaft.
- Exhaust Takt – As the piston moves up the cylinder, the exhaust valve opens and allows exhaust gases to exit the engine through the exhaust pipe. At the end of the exhaust cycle, the cylinder is ready for fresh air.
Diesel engine design
Because of the higher loads, the main components of a diesel engine are significantly reinforced compared to gasoline engines, while still performing the same functions.
The walls of the cylinder block of a diesel engine are generally much thicker than those of a gasoline engine block, with greater metal rigidity to provide additional strength and absorb increased loads. In addition, the solid block parts, help to reduce noise levels more effectively.
The pistons, connecting rods, crankshaft and bearing caps must be made more powerful than their gasoline engine counterparts. The design of the cylinder head differs in that fuel injectors are installed in it, and because of the shape of the combustion chamber and vortex chambers.
In order for a diesel engine to run smoothly and efficiently, fuel and air must be properly mixed. Fuel and air mixing problems arise because of the different moments of air and fuel injection into the engine cylinder, where air and fuel are introduced at different points in time during the cycle and must be mixed within the cylinders.
During engine operation, air and fuel are introduced at different moments of time and during different cycles, as they must be mixed inside the cylinders, from this comes the problem of mixing the air-fuel mixture.
There are two main methods of fuel injection:
- Direct injection – where the fuel goes directly into the combustion chamber;
- Indirect injection – when fuel is injected into the pre-chamber (vortex chamber).
Most diesel engines use indirect fuel injection because it is the simplest method, in which the fuel spray mixes well with the highly compressed air in the combustion chamber.
The indirect injection engine has a small spiral precombustion chamber (vortex chamber) into which the injector injects fuel before it reaches the main combustion chamber itself. The vortex chamber creates turbulence in the fuel so that it mixes better with the air in the combustion chamber.
The disadvantage of this system is that the vortex chamber becomes part of the combustion chamber. This means that the combustion chamber as a whole is irregularly shaped, which causes combustion problems and makes operation difficult.
The direct injection engine does not have a vortex chamber into which the fuel is injected – instead, the fuel flows directly into the combustion chamber. The shape of the chamber is simpler, but it is more difficult to mix air and fuel, and the combustion of such a mixture produces a characteristic diesel “knock”.
Engineers must pay very close attention to the design of the combustion chamber in the piston head to ensure sufficient turbulence.
Engine speed control
Since a diesel engine does not have a throttle valve like a gasoline engine, the amount of intake air is always the same, at any rpm. The engine speed is regulated solely by the amount of fuel injected into the combustion chamber, with more fuel in the chamber, combustion occurs more aggressively, due to which the piston speed increases and the engine speed increases accordingly.
The accelerator pedal is connected to the engine injection system metering device, not to the throttle butterfly, as in the gasoline engine.
The diesel engine is still stopped by turning the ignition key, which disconnects the solenoid valve fuel supply to the injection pump, disconnects the fuel metering and distribution unit.
Diesel engines stop faster than gasoline engines because they have a higher compression ratio, which exerts a stronger engine deceleration.
Starting a diesel
Like gasoline engines, diesel engines are started using an electric starter, which starts the cycle of compression ignition. However, when it is cold outside, the diesel is difficult to start, simply because the compression of cold air does not reach the necessary temperature at which fuel is ignited.
To eliminate this problem, glow plugs are installed in the cylinder head. These are small electric heaters, powered by the car battery, which are turned on for a few seconds before starting the engine.
Glow plugs are used to preheat the cylinder head and engine block during a diesel engine cold start. They look like short spark plugs and are connected to the electrical system of the car. Once the power is applied, the heating elements inside heat up very quickly.
The glow plugs are activated either by an auxiliary position in the ignition switch or by a separate switch. As soon as the engine starts and rpm is above idle speed they are automatically switched off.
The fuel used in diesel engines is very different from gasoline. It is less refined, making it heavier, more viscous and less volatile.
In very cold weather, diesel fuel can thicken or even solidify. This is compounded by the fact that diesel fuel can absorb small amounts of water, which can freeze.
Any thickening or freezing of the fuel can block the fuel lines and injectors and thus prevent the engine from starting.
As a preventive measure, you can use a fuel additive, which is poured into the fuel tank before refueling, although modern diesel fuel already contains an additive and does not cause any problems with starting the engine, at outside temperatures below 12-15 ° C