Which is better between two-stroke and four-stroke motorcycle engines? Two-stroke and four-stroke comparison
Introduction to motorcycle engines
Motorcycle power units use internal combustion engines; people are used to calling diesel engines as diesel engines, and gasoline engines as gasoline engines. Because gasoline engines have the advantages of light weight, small size, low noise, low vibration, easy starting and low cost, motorcycles generally use gasoline engines as their power units.
The working principle of two-stroke motorcycle engine
The piston moves from bottom dead center to top dead center, it will complete the process of intake and compression, It belongs to the first stroke of the reciprocating motion of the piston. The piston moves from the top dead center to the bottom dead center, and it will complete the work process of combustion expansion (work) and exhaust, which belongs to the second stroke of the piston reciprocating motion. When the piston moves from the bottom dead center to the top dead center and completely closes the ventilation port and the exhaust port, the exhaust and ventilation process is terminated, and the fresh combustible mixture in the cylinder will begin to compress. At the same time, due to the upward movement of the piston, the volume of the crankcase under the piston gradually increases, which reduces the pressure in the crankcase and forms a vacuum degree. When the vacuum degree of the crankcase reaches a certain level, the reed valve is automatically opened and atomized by the carburetor. The combustible mixture is sucked into the crankcase. When the piston continues to move upward, when it is about to approach the top dead center, an electric spark is issued by the spark plug to ignite the compressed combustible mixture. At this time, the burning gas expands rapidly, causing the temperature and pressure of the combustion chamber to rise sharply, forcing the piston to move downward, and the piston performs useful work through the connecting rod and the crankshaft. When the piston moves from the top dead center to the bottom dead center, the pressure in the crankcase will increase as the volume decreases, and the reed valve will gradually close automatically. At this time, the combustible mixture entering the crankcase begins to be pre-compressed . When the piston goes down to the opening of the exhaust port, the exhaust gas is discharged into the atmosphere through the exhaust port, exhaust pipe and muffler. When the piston continues to descend to the opening of the ventilation port, the pre-compressed fresh combustible mixture in the crankcase enters the cylinder through the ventilation port, and drives the exhaust gas in the cylinder to be further discharged. This process is called the scavenging process. The engine thus completes a working cycle.
The working principle of four-stroke motorcycle engine
The first stroke ——- intake stroke: the piston is at a certain point before the top dead center When the crank angle is specified, the intake valve opens and the combustible mixture is sucked into the cylinder. When the piston moves from the top dead center to the bottom dead center, the exhaust valve is closed at a specified crank angle of the top dead center, and the volume of the cylinder above the piston increases, so that the cylinder forms a vacuum and the combustible mixture continues to pass through the intake air Door inhalation. When the piston travels to a certain crank angle after the bottom dead center, the intake valve is closed, and at this time, the intake process ends.
Second stroke——-compression stroke: The piston moves from the bottom dead center to the top dead center. When the intake process ends, the intake valve and exhaust valve are both closed. At this time, the combustible mixture in the cylinder begins to be compressed.
The third stroke——-combustion expansion work stroke: in the compression stroke, when the piston travels up to a certain crank angle before the top dead center, a spark will be emitted between the electrodes of the spark plug, which will be The compressed flammable mixture ignites. The burning combustible mixture causes the temperature and pressure in the cylinder to rise sharply, and the piston moves from the top dead center to the bottom dead center under the action of high temperature and high pressure air pressure, and drives the crankshaft to rotate through the connecting rod to perform useful work.
The fourth stroke —– Exhaust stroke: In the combustion expansion stroke, when the piston travels to a certain crank angle before the bottom dead center, the exhaust valve opens, and the exhaust gas passes through the exhaust valve. The valve begins to discharge. The crankshaft continues to rotate, and pushes the piston to move from bottom dead center to top dead center. Push the exhaust gas out of the cylinder. This exhaust process ends when the exhaust valve is closed at a specified crank angle after the piston travels to top dead center.
Advantages and disadvantages of two-stroke motorcycle engines
A. Advantages:
*Each 1 burst per turn, so the rotation is smooth.
*Doesn’t need a valve and has few parts, so maintenance is easy and cheap.
*The inertial force generated by the reciprocating motion is small. Low vibration and low noise.
*Compared to a four-stroke engine, more power at the same speed.
*More explosive than a four-stroke. Therefore, under the same volume, if the average effective pressure is the same, the power is 2 times (actually 1.7).
B. Disadvantages:
*The time of the intake and exhaust process is short, so the fuel loss is large.
*There is a port on one side of the cylinder wall, where the piston ring is easily worn.
*Because the exhaust port is on the cylinder, it is prone to overheating.
*Slow and unstable.
*The consumption of lubricating oil is high.
Advantages and disadvantages of four-stroke engines
A. Advantages:
*Intake, The processes of compression, expansion (burst) and exhaust are carried out separately, so the work is reliable, efficient, and stable. The speed range from low speed to high speed is large (500～1000rpm or more).
*There is no blow-by gas return loss like a two-stroke engine, and the fuel consumption rate is low.
*Stable operation at low speed, relying on lubrication system to lubricate, not easy to overheat.
*The intake process and compression process take a long time, and the volumetric efficiency and average effective pressure are high.
*The thermal load is less than a two-stroke engine. Don’t worry about deformation and ablation. It has a large displacement and can be designed as a high-power engine.
B. Disadvantages:
*The valve mechanism is complex, there are many parts, and maintenance is difficult
*The mechanical noise is loud
* Since the crankshaft explodes once every 2 revolutions, the rotational balance is unstable.
