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

2024 Author: Erin Ralphs | [email protected]. Last modified: 2024-02-19 11:42

What is the difference between a two-stroke engine and a four-stroke? The most noticeable difference is the modes of ignition of the combustible mixture, which can be immediately noticed by the sound. A 2-stroke motor usually produces a shrill and very loud rumble, while a 4-stroke motor tends to have a quieter purr.

Application

In most cases, the difference is also in the main purpose of the unit and its fuel efficiency. Two-stroke engines ignite with every revolution of the crankshaft, so they are twice as powerful as four-stroke engines, in which the mixture ignites only every revolution.

Four-stroke engines are more economical, but heavier and more expensive. They are commonly found on cars and utility vehicles, while smaller two-stroke models are more common on applications such as lawn mowers, scooters and light boats. But a gasoline generator, for example, can be found both two-stroke and four-stroke. The scooter engine 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 a beat?

The processing of fuel in both types of engines is carried out through the sequential execution of four different processes, known as cycles. The speed at which the engine goes through these cycles is exactly what makes a two-stroke engine different from a four-stroke engine.

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

Four-stroke engine operation

In a standard four-stroke engine, the mixture is ignitedevery second revolution of the crankshaft. The rotation of the shaft drives a complex set of mechanisms that ensure the synchronous execution of a sequence of cycles. The opening of the intake or exhaust valves is carried out using a camshaft, which alternately presses the rocker arms. The valve is returned to the closed position by means of a spring. To avoid loss of compression, it is essential that the valves fit snugly against the cylinder head.

Two-stroke engine operation

Now let's see how a two-stroke engine differs from a four-stroke engine in terms of the principle of operation. In two-stroke engines, all four actions are performed in one revolution of the crankshaft, during the stroke of the piston from top dead center to bottom, and then back up. The release of exhaust gases (purge) and 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 train.

The place of the valves is occupied by two holes in the walls of the combustion chamber. When the piston moves down due to the energy of combustion, the exhaust channel opens, allowing the exhaust gases to exit the chamber. When moving down, a vacuum is created in the cylinder, due to which a mixture of air and fuel is drawn in through the intake port 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. The important thing is that in this type of engine, the mixture is ignited with every revolution, which allows you to extract more power from them, at least in the short term.

Weight to power ratio

Two-stroke engines are better suited for applications that require fast and sharp bursts of power rather than steady operation for long periods of time. For example, a two-stroke jet ski accelerates faster than a four-stroke truck, but it is designed for short trips, while a truck can travel hundreds of miles before needing a rest. Two-stroke engines make up for their short lifespan by having a low weight-to-power ratio: these engines typically weigh much less, so they start faster and reach operating temperature faster. They also require less energy to move.

Which motor is better

In most cases, four-stroke engines can only operate in one position, while two-stroke engines are less demanding in this regard. This is largely due to the complexity of the moving parts as well as the design of the oil pan. Such a sump, which provides engine lubrication, is usually found only in four-stroke models and is of great importance for their operation. Two-stroke engines usually do not have such a sump, so they can be operated in almost any position without the risk of sloshing oil or interrupting the lubrication process. For devices such as chainsaws, circular saws andother portable instruments, this flexibility is very important.

Fuel efficiency and environmental performance

Compact and fast engines are often found to produce more air pollution and use more fuel. At the bottom of the piston movement, when the combustion chamber is filled with a combustible mixture, some fuel is lost into the exhaust port. This can be seen in the example of an outboard motor; if you look closely, you can see multi-colored oily spots around it. Therefore, engines of this kind are considered inefficient and polluting. Although the four-stroke models are somewhat heavier and slower, they burn fuel completely.

Cost of purchase and maintenance

Smaller engines are usually less expensive, both in terms of initial purchase and maintenance. However, they are designed for a shorter service life. Although there are some exceptions, most of them are not designed for continuous operation for more than a few hours and are not designed for very long service life. The lack of a separate lubrication system also means 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 a lubrication system in gasoline designed to be filled 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 a breakdown (if you forget to add oil). The 4-stroke motor in most cases requires a minimum of maintenance and care.

Which motor is better

This table briefly describes the difference between a two-stroke engine and a four-stroke engine.

	Four-stroke engine	Two-stroke engine
1.	One power stroke for every two revolutions of the crankshaft.	One power cycle for each crankshaft revolution.
2.	We have to use a heavy flywheel to compensate for vibrations that occur during engine operation due to uneven distribution of torque, since the ignition of the combustible mixture occurs only every second revolution.	A much lighter flywheel is needed and the engine runs quite balanced as the torque is distributed much more evenly due to the fact that the fuel mixture is ignited with every revolution.
3.	Large engine weight	Motor weight much less
4.	The design of the engine is complicated due to the valve mechanism.	The design of the engine is much simpler due to the lack of valvemechanism.
5.	High cost.	Cheaper than a four-stroke.
6.	Low mechanical efficiency due to friction of a large number of parts.	Higher mechanical efficiency due to reduced friction due to fewer parts.
7.	Higher productivity thanks to complete exhaust gas removal and fresh mixture injection.	Reduced high performance due to mixing of exhaust gas residues with fresh mixture.
8.	Lower operating temperature.	Higher operating temperature.
9.	Water cooling.	Air cooled.
10.	Less fuel consumption and complete combustion.	Higher fuel consumption and mixing fresh injection with exhaust residue.
11.	Takes up a lot of space.	Takes up less space.
12.	Complex lubrication system.	Much simpler lubrication system.
13.	Low noise.	More noisy.
14.	Valve valve timing system.	Instead of valves, inlet and outlet channels are used.
15.	High thermal efficiency.	Less thermal efficiency.
16.	Low oil consumption.	Higher oil consumption.
17.	Less wear on moving parts.	Increased wear on moving parts.
18.	Installed in cars, buses, trucks, etc.	Used in mopeds, scooters, motorcycles, etc.

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