What is the working principle of a diesel engine? Differences between diesel and gasoline engines
Working principle of a diesel engine
The working process of a diesel engine is actually the same as that of a gasoline engine; each working cycle also goes through four strokes: intake, compression, power, and exhaust. However, because the fuel used in a diesel engine is diesel, which has higher viscosity than gasoline and does not evaporate easily, and its autoignition temperature is lower than that of gasoline, the formation of the combustible mixture and the ignition method are different from those of a gasoline engine.
During the intake stroke, the diesel engine draws in pure air. Near the end of the compression stroke, the diesel is pressurized by the injection pump to above 10 MPa and injected into the cylinder through the fuel injector, where it mixes with the compressed high-temperature air in a very short time to form a combustible mixture. Because the diesel engine has a high compression ratio (generally 16–22), at the end of compression the air pressure in the cylinder can reach 3.5–4.5 MPa, while the temperature can be as high as 750–1000 K (whereas in a gasoline engine at this point the mixture pressure is 0.6–1.2 MPa and the temperature reaches 600–700 K), far exceeding the autoignition temperature of diesel. Therefore, after being injected into the cylinder, the diesel mixes with air and immediately ignites and burns by itself in a very short time. The pressure in the cylinder rises rapidly to 6–9 MPa, and the temperature also rises to 2000–2500 K. Driven by the high-pressure gas, the piston moves downward and drives the crankshaft to rotate, performing work, and the exhaust gas is likewise discharged into the atmosphere through the exhaust pipe.
In a conventional diesel engine, the fuel is driven by the engine camshaft and delivered to each cylinder's combustion chamber by means of a high-pressure fuel pump. This fuel supply method must change with engine speed and cannot achieve the optimal fuel delivery at all speeds.
The common rail injection fuel supply system consists of a high-pressure fuel pump, a common fuel rail, fuel injectors, an electronic control unit (ECU), and some pipeline pressure sensors. Each fuel injector in the system is connected to the common fuel rail through its own high-pressure fuel pipe, and the common fuel rail provides hydraulic pressure accumulation for the fuel injectors. During operation, the high-pressure fuel pump delivers fuel at high pressure to the common fuel rail; the high-pressure fuel pump, pressure sensor, and ECU form a closed-loop system to precisely control the fuel pressure in the common fuel rail, completely changing the phenomenon in which fuel supply pressure varies with engine speed. Its main features are as follows:
1. Injection timing and fuel metering are completely separated; injection pressure and the injection process are controlled by the ECU in real time.
2. The injection pressure, injection start point, and duration for each cylinder can be adjusted according to engine operating conditions, thereby pursuing the optimal injection control point.
3. It can achieve very high injection pressure and can realize pilot injection of diesel.
Compared with a gasoline engine, a diesel engine has a low fuel consumption rate (on average 30% lower than a gasoline engine), and diesel is lower in price, so fuel economy is better; at the same time, the speed of a diesel engine is generally lower than that of a gasoline engine, and its torque is greater, but it is heavy, noisy during operation, costly to manufacture and maintain, and its emissions are also worse than those of a gasoline engine. However, with the development of modern technology, these shortcomings of diesel engines are gradually being overcome.
Differences between gasoline and diesel engines
A gasoline engine draws in a mixture of fuel and air and compresses it, then ignites the mixture with a spark. A diesel engine draws in only air and compresses it, then injects fuel into the compressed air. The heat generated by the compressed air ignites the fuel.
The compression ratio of a gasoline engine is 8:1 to 12:1, while that of a diesel engine is 14:1 and can even reach 25:1. Because a diesel engine has a higher compression ratio, it is also more efficient.
Gasoline engines usually use carburetion, in which air and fuel are mixed before the air enters the cylinder or port, or port fuel injection, in which fuel is injected before the intake stroke begins (outside the cylinder). Diesel engines use direct injection, in which diesel is injected directly into the cylinder.
The figure below is an animated illustration of the diesel cycle. You can compare it with the animation of the gasoline engine to see the differences:
Image courtesy of Baris Mengutay
Note that a diesel engine has no spark plug; it draws in and compresses air, then injects fuel directly into the combustion chamber (direct injection). It is actually the heat generated by the compressed air that ignites the fuel in a diesel engine.
The fuel injector is the most complex component of a diesel engine and has been the subject of extensive experimentation. This is because the injector's location may differ for each specific engine. The injector must be able to withstand the temperature and pressure inside the cylinder while atomizing the injected fuel into a fine mist. Achieving uniform distribution of the fuel mist circulating inside the cylinder is also a problem, so some diesel engines use special induction valves, pre-combustion chambers, or other devices to make the airflow swirl in the combustion chamber or to improve the ignition and combustion process.
Horsepower
For the definition of horsepower and its effect on engine performance, please see Horsepower and Its Applications.
A major difference between diesel and gasoline engines is the injection step. Most automobile engines (gasoline engines) use port injection or a carburetor rather than direct injection. Therefore, inside an automobile engine, all the fuel is injected into the cylinder during the intake stroke and then compressed. Compressing the fuel-air mixture limits the engine's compression ratio, because if the air is compressed too much, the fuel-air mixture will ignite by itself and cause knocking. A diesel engine compresses only air, so its compression ratio can be much higher than that of a gasoline engine. The higher the compression ratio, the greater the horsepower produced.
Some diesel engines include some kind of glow plug (not shown in the figure). When the diesel engine is cold, the compression process cannot raise the air to the ignition point of the fuel. The glow plug is an electric heating coil (think of the heating coil you see in an oven); it can ignite the fuel when the engine is cold, thereby starting the engine. Clay Blaisdell, an experienced heavy equipment technician, believes:
All functions of modern engines are controlled through the ECM's communication with a complex set of sensors that measure everything from R.P.M. to engine coolant and oil temperature to engine position (i.e., T.D.C.). Today's large engines rarely use glow plugs. The ECM detects ambient temperature and delays timing in cold weather, so the injectors inject fuel later. The more the air in the cylinder is compressed, the more heat is generated to assist starting.
Smaller engines and engines without advanced computer control use glow plugs to solve the cold-start problem.
Diesel
If you compare diesel with gasoline, you will find that they are different. They smell different. Diesel is heavier and more oily. Diesel vaporizes much more slowly than gasoline; in fact, its boiling point is much higher than that of water.
Because diesel is heavier, it evaporates slowly. Compared with gasoline, it contains more and longer carbon atom chains (gasoline is generally C9H20, while diesel is generally C14H30). Less refining is required to produce diesel, so diesel is cheaper than gasoline.
Diesel has a higher energy density than gasoline. On average, 3.8 liters of diesel contains about 155x106 joules of energy, while 3.8 liters of gasoline contains 132x106 joules. This advantage, combined with the higher efficiency of diesel engines, explains why diesel engines cost less per mile than gasoline engines.
