Most diesel engines today are mid-stroke, but there are also two-stroke diesel engines. Many components of a two-stroke engine are similar to a four-stroke engine. The difference is that the cylinders do not use standard valves to allow air and fuel to enter the engine. The following JVH will take you to learn more about what is a two-stroke diesel engine and what are the advantages and disadvantages compared with a four-stroke engine.
How a two-stroke engine works
As shown, the working principle of a two-stroke engine is shown. To describe how it works, consider both the upper and lower piston movements. When the piston moves upward, as shown in Figure (a), a compression effect occurs on the upper part of the piston, while a vacuum is created in the crankcase below the piston, and fresh air and fuel are sucked in through the reed valve. NOTE: Oil must be added to the air and fuel mixture at this point because there is no oil in the crankcase. The oil in the fuel acts as a lubricant. Oil can also be injected into the crankcase for lubrication. Usually, the oil and fuel mixture used is from 20:1 to 50:1 or even higher.
How a two-stroke engine works
Combustion occurs as the piston continues to move up on the compression stroke and finally fires, as shown in Figure (b). The burning gases push the piston down. As the piston moves further down, high pressure is created in the crankcase. In the picture, the reed valve is closed under pressure, sealing the crankcase area. The reed valve is for illustration only and will not be used on a two-stroke engine that is being considered for use in an automobile. It will use the position of the piston as a valve. When the piston reaches a low enough position in its stroke, the intake and exhaust ports will open as shown in Figure (c). Intake and exhaust ports are holes cut in the piston through which air and fuel can be drawn in and out.
When the intake and exhaust ports are opened, under the action of the pressure in the crankcase, a certain mass of air-fuel-oil mixture enters the combustion chamber. These mixtures can also drive exhaust gas out of the exhaust. As the piston begins to move up, the intake and exhaust ports close. Compression and work on the upper part of the piston continues, as does suction and low pressure on the lower part of the piston.
Advantages and disadvantages of two-stroke engines
There are several advantages and disadvantages to using a two-stroke engine, two of which are:
1. Two-stroke engines generally have good responsiveness because there is a power shock every lap. Therefore, the time required from the idle time (500r/min) to the rated speed (1500r/min) is shorter.
2. Two-stroke engines are generally lighter in weight than four-stroke engines, so there are fewer parts.
The disadvantage of two-stroke engines has to do with efficiency, they are less efficient than four-stroke engines for three reasons:
1. The movement of air and fuel is small. Air and fuel can only enter the cylinder for a short period of time. Because less air and fuel enter the engine, efficiency is reduced.
2. Poor combustion efficiency. Oil in the air-fuel mixture reduces combustion efficiency because the oil has poor combustion characteristics.
3. Low mean effective pressure (BMEP). Because the power stroke is shorter, the total pressure generated during the power stroke is reduced. When the exhaust port opens, the power stroke is over.
While many problems remain, efforts are also being made to improve efficiency, combustion, fuel injection, and scavenging (making air and exhaust gas movement easier).
