What is the difference between a two-stroke engine and a four-stroke engine? A comparative analysis

What is the difference between a two-stroke engine and a four- stroke engine ? The most noticeable difference is the modes of ignition of the combustible mixture, which can be immediately noticed by the sound. A two-stroke engine usually produces a piercing and very loud hum, while a four-stroke motor is characterized by a more quiet purr.

Application

In most cases, the difference is also in the main purpose of the unit and its fuel efficiency. In two-stroke engines, the ignition occurs at each revolution of the crankshaft, so they are twice as high as the four-stroke ones, in which the mixture ignites only through a revolution.

Four-cycle engines are more economical, but harder and more expensive. They are usually installed on cars and special equipment, while on devices such as lawn mowers, motor scooters and light boats, more compact two-stroke models are more common. But a gasoline generator, for example, can be found both push-pull and four-stroke. The scooter motor can also be of any type. The principle of operation of these engines is basically the same, the difference is only in the way and efficiency of energy conversion.

What is tact?

Fuel processing in both types of motors is carried out through the consecutive execution of four different processes, known as cycles. The speed with which the engine passes through these bars is exactly what a two-stroke engine differs from a four-stroke engine.

The first measure is injection. In this case, the piston moves down the cylinder, and the inlet valve opens to let the air-fuel mixture into the combustion chamber. Next is the compression stroke. During this stroke, the intake valve closes, and the piston moves up the cylinder, compressing the gases there. The stroke of the working stroke begins when the mixture is ignited. In this case, the spark from the candle ignites the compressed gases, which leads to an explosion, the energy of which pushes the piston down. The last tact is release: the piston rises up the cylinder, and the exhaust valve opens, allowing the exhaust gases to exit the combustion chamber so that the process can start again. The reciprocating motion of the piston rotates the crankshaft, the torque from which is transmitted to the working parts of the device. This is how the combustion energy of the fuel is converted into translational motion.

Operation of a four-cycle engine

In a standard four-stroke engine, the ignition of the mixture occurs on every second revolution of the crankshaft. Rotation of the shaft drives a complex set of mechanisms that ensure synchronous execution of the sequence of measures. Opening inlet or exhaust valves is carried out with the help of a cam shaft, which alternately presses on the rocker arms. The valve is returned to the closed position by means of a spring. To avoid loss of compression, it is necessary that the valves fit snugly against the cylinder head.

Two-stroke engine operation

Now let's see how the two-stroke engine differs from the four-stroke engine in accordance with the principle of operation. In two-stroke engines, all four actions are performed per revolution of the crankshaft, during the stroke of the piston from the top dead center to the bottom, and then back up. The exhaust gas (blowdown) and the fuel injection are integrated into one cycle, at the end of which the mixture ignites, and the resulting energy pushes the piston down. This design eliminates the need for a valve mechanism.

Place the valves occupy two holes in the walls of the combustion chamber. When the piston moves down due to the energy of combustion, the discharge channel opens, allowing the exhaust gases to leave the chamber. When moving downward in the cylinder, a rarefaction occurs, through which a mixture of air and fuel is drawn inward through the intake channel located below. When moving up the piston closes the channels and compresses the gases in the cylinder. At this point, the spark plug fires , and the entire process described above is repeated again. It is important that in engines of this type, ignition of the mixture occurs at each revolution, which allows them to extract more power from them, at least in the short term.

Weight to Power Ratio

Two-stroke engines are better suited for devices that require fast and sharp bursts of energy, rather than a uniform operation for a long time. For example, a hydrocycle with a two-stroke engine accelerates faster than a four-stroke truck, but it is designed for short trips, while a truck can travel hundreds of kilometers before it needs rest. The low duration of the two-stroke operation is compensated by the low mass-to-power ratio: such engines usually weigh much less, so they are quicker to start and reach operating temperature. To move them also requires less energy.

Which motor is better

In most cases, four-stroke engines can only work in one position, whereas two-stroke engines are less demanding in this respect. This is largely due to the complexity of the moving parts, as well as the design of the oil pan. Such a pallet providing lubrication of the engine is usually present only in four-cycle models and is of great importance for their operation. Two-stroke engines usually do not have such a pallet, so they can be used in virtually any position without the risk of splashing oil or interrupting the lubrication process. For such devices as chain saws, circular saws and other portable tools, this flexibility is very important.

Fuel efficiency and ecological component

It is often found that compact and fast engines more pollute the air and consume more fuel. At the lower point of the piston movement, when the combustion chamber is filled with a combustible mixture, some fuel is lost, getting into the outlet channel. This can be seen in the example of a outboard motor; If you look closely, you will see around it multicolored oily stains. Therefore, engines of this kind are considered inefficient and pollute the environment. Although four-cycle models are somewhat heavier and slower, but in them the fuel is burned completely.

Cost of purchase and service

Smaller engines are usually less expensive, both in terms of initial purchase and maintenance. However, they are designed for a shorter lifetime. Although there are some exceptions, most of them are not designed for continuous operation for more than a few hours and are designed for a not very long service life. The absence of a separate lubrication system also leads to the fact that even the best motors of this type wear out relatively quickly and become unusable due to damage to moving parts.

Partly due to the lack of lubrication system in gasoline, intended for pouring into a two-stroke scooter engine, for example, it is necessary to add a certain amount of special oil. This leads to additional costs and hassle, and can also cause damage (if you forget to add oil). Motor 4-stroke in most cases requires a minimum of maintenance and care.

Which motor is better

This table briefly describes how a two-stroke engine differs from a four-stroke engine.

It also shows the positive and negative qualities of each of these two types.