2-stroke vs 4-Stroke Engine — Do you know the difference? This is a common question that our readers ask, especially those who are curious about how things work. Each engine type has their own advantages and disadvantages that will be clear by the time you finish this article.
Let’s get started.
Table of Contents...
- 1 2 stroke vs 4 Stroke Engine
- 1.1 What is An Engine Stroke?
2 stroke vs 4 Stroke Engine
I cannot count the number of times I’ve been asked the question: Which is better, a 2-stroke or 4-stroke engine? There answer is that either can be the best solution, depending on what the engine is used for. Smaller engines that run at a high RPM (revolutions per minute) tend be 2-stroke. Larger engines with a greater torque requirement at a lower RPM are usually 4-stroke. So the 2-stroke vs 4-stroke debate centers mostly around the application for which the engine is used.
This article is going answer the question in more detail. I’ll start with the basics, explain how these engines work and then cover the finer details. We need to start by understanding what an engine stroke or cycle is.
What is An Engine Stroke?
You’ve heard of engines described by their stroke, this can also be called a cycle. An engine works by means of a piston that moves up and down in a cylinder. This movement is caused by an explosion. Gas engines use an electric spark to ignite the fuel, generating heat which expands, causing movement of the surrounding air. In order for this explosion to force the piston downward, it needs to happen in a complete vacuum. The piston needs to be completely sealed from the environment.
VIDEO | 2-Stroke vs 4-Stroke Engine
During this process, the piston needs to take in air and fuel, ignite the fuel, then release the exhaust gasses, so that the cycle can begin over again. This is repeated thousands of time per minute. Hence the term Revolutions per Minute (RPM). For each full revolution of the crankshaft, the piston has to go from its highest position, Top Dead Center (TDC), to its lowest position, Bottom Dead Center (BDC), and then back to TDC. This covers a 360° rotation of crank shaft, or one revolution. At 3000 RPM, this cycle happens 3000 times per minute. So it requires precise timing.
This is the basic principle for any gas engine. The difference between a 4-stroke and 2-stroke engine is the way in which the compression, spark and exhaust combine their actions to achieve this.
How Does A 4-stroke Engine Work?
A 4-stroke engine separates each step of the combustion and exhaust process into four individual steps, or strokes.
VIDEO | An Indepth Look at 4-Cycle Engines
In order for fuel to enter the combustion chamber, just before the piston reaches TDC, a valve (or valves) open to allow a fuel and air mixture to be supplied from the carburetor or fuel injection system. Once enough fuel has entered the chamber, this valve closes to create a vacuum. This valve then closes, sealing the cylinder and forming a vacuum. This is followed by spark, generated by the spark plug. This in turn causes the explosion that forces the piston to move down. A second (exhaust) valve then opens to allow the burnt gasses to escape. At this time the vacuum is broken causing a decompression in the cylinder and the momentum of the crankshaft pushes the piston back to the TDC (top) position, ready to start the whole process over again.
The cycle of a 4-stroke engine can be simplified into the following 4 steps:
Stroke 1 : Intake
- The intake valve opens to allow the gas and air mixture into the combustion chamber.
Stroke 2 : Compression
- The intake valve closes, compressing the gas in a vacuum.
Stroke 3 : Combustion (or Ignition)
- The spark plug ignites the fuel mixture.
Stroke 4 : Exhaust
- The exhaust valve opens, forcing the combusted gasses out of the chamber.
How Does A 2-stroke Engine Work?
A two-stroke engine doesn’t use valves to allow fuel to enter and exhaust gasses to exit the combustion chamber. This simplifies things and the engine accomplishes the full cycle in only two strokes.
Instead of valves, a 2-stroke engine has intake and exhaust ports. These are simply openings in the side of the cylinder that coincide with a pre-calculated position of the piston. So this means that the movement of the piston is used to seal or open these ports.
The intake port is situated just below the TDC position. When the piston moves up from the BDC position, this port is open and allows the fuel mixture to enter the combustion chamber. When the piston moves past the port, the side wall of the piston blocks the opening. At the same time the spark plug ignites the fuel. The compression results from the piston closing the intake port, combined with simultaneous combustion. So the compression and ignition stroke happen as one.
VIDEO | 2-Stroke vs 4-Stroke Engine
The exhaust port is on the opposite side of the cylinder near the BDC position. As the piston approaches the lowest point (BDC), it passes the exhaust port. At the bottom of the cycle, the piston is no longer covering the exhaust port and the burnt gasses escape.
A 2-stroke only has an upstroke and a downstroke.
Stroke 1 : Intake and Ignition
- As the piston moves up, fuel and air are forced into the combustion chamber. This is accompanied by the spark. This happens just before the piston reaches TDC.
Stroke 2 : Compression and Exhaust
At the TDC position, the piston blocks the inlet port, sealing the combustion chamber. This compresses the gas and forces the piston downward. At the lowest point, the exhaust port is no longer obstructed by the piston, allowing burnt gasses to escape.
Mechanical Differences | 2-Stroke & 4-Stroke Engine
When looking at 2-stroke vs 4-stroke engines, the differences go beyond the basic combustion process. A four stroke engines has valves at the top of the engine, seated in the engine head. These valves are completely independent and need to be controlled in order to open and close at exactly the right time. This is known as valve timing. Valve timing is controlled mechanically by a timing chain or belt that drives a camshaft when the engine has more than one cylinder. This is assisted by hydraulic lifters that use the engine oil pressure to lift the valves.
Diagram of a Honda 4-stroke engine : GX120/160/200
The timing belt turns along with the crankshaft at the bottom of the engine. The belt then drives the camshaft. As it rotates, cams press against valve rockers or pushrods to open and close the valves. Overhead Camshafts (OHC) operate by means of valve rockers that make direct contact with both the cam and valve. Older engines often have cams at the bottom of the engine and use pushrods to control the valves which are situated above the engine.
Because a 2-stroke engine has a more simplified process of using the piston to open close the inlet and outlet ports, there is no need for these extra mechanical components. This has an additional effect. The inlet and outlet ports run directly through the cylinder wall. This means that oil cannot be circulated around the piston, like it can in a 4-stroke engine.
A 4-stroke engine has a completely sealed cylinder, the valves only open at the top, into the combustion chamber. So the oil that lubricates the engine is kept away from the combustion chamber. A 2-stroke engine, cannot do this. When the piston passes the intake port, the combustion chamber is open to the cylinder. This means that the fuel and oil cannot be separated. For this reason, 2-stroke engines use a different type of oil to lubricate the engine. The oil is combusted with the fuel and is, therefore mixed with the fuel. This can be done before pouring the fuel into the tank, or as the fuel is channeled into the intake port.
These basic mechanical differences between 2-stroke and 4-stroke engines affect the maintenance, operating procedures, and emission control for these engines.
Performance & Applications | 2-Stroke vs 4-Stroke
A 2-stroke engine has fewer components and is, therefore, lighter and more compact than a 4-stroke engine. This is preferable for applications when the operator has to hold the machine that the engine powers. Handheld power tools, like chainsaws and gas-powered garden tools, are a good example of the weight advantage provided by 2-stroke engines. Reducing the weight of the machine makes it easier to operate and hold at the same time.
A 2-stroke engine has lower compression and spins more freely. This results in a quicker throttle response. A 2-stroke engine will go from idle to top revs in a very short space of time. These engines will typically rev much higher than 4-stroke engines. The advantage to this is that 2-stroke engines accelerate more rapidly.
When using a recoil starter, the lower compression means that less effort is required to start the engine. The operator does not have pull against the resistance of a high-compression engine. To combat this, many 4-stroke engines use a decompression device to open the valves and lower the compression when starting manually. This means that the engine has to have additional mechanical components, increasing the weight and adding to maintenance and repair procedures. 4-stroke engines with an electric starter don’t need this, as the electric motor is powerful enough to overcome the compression.
The downside to the free-spinning, low compression attributes of a 2-stroke engine is that they don’t function well at low RPM. A 2-stroke engine has an optimal power band at higher revs. When an engine encounters too much resistance, the RPM will drop. A 2-stroke engine does not recover from a drop in the RPM too easily. This results in a drastic loss of power at low RPM and a greater possibility of engine stall.
A larger, heavy vehicle has more resistance to the engine power than a smaller one. This is of particular importance when the vehicle approaches an incline. Generators can also have a greater resistance, draining power from the engine. As the alternator increases the load, the RPM will drop and needs to recover quickly in order to maintain the constant RPM required for a stable electric current. Because of these limitations, 2-stroke engines are not the best for cars, trucks and larger motorcycles, the weight of these vehicles offer too much resistance to the spinning of the engine. They are also no ideal for generators as the alternator offers too much resistance.
While a 4-stroke engine is much more capable at lower revs, it cannot accelerate as quickly as a 2-stroke engine. Timing lag is a common phenomenon associated with 4-stroke engines. This is a delay in the valve timing advancing when the engine needs to accelerate. Electronic timing for the spark can advance easily, electricity moves very fast. The mechanical advancing of the valves takes time and this results in delayed acceleration. So 4-stroke engines are preferred when torque is required over a greater rev range, but not are not that good at rapid acceleration. Variable Valve Timing (VVT) has done a lot to combat timing lag. However, even with this technology, 4-stroke engines cannot compete with the rapid throttle response that you get from a 2-stroke engine.
Because 4-stroke engines take longer to accelerate, 2-stroke engines are preferred for high-performance motorcycles and power boats. Though this can only be applied if the motorcycle, or boat, is not too heavy. A lightweight vehicle powered by a 2-stroke engine has better acceleration, as long as the RPM remains high enough to keep the engine running optimally in the high-rev power band.
Maintenance & Repairs | 2-Stroke vs 4-Stroke
A 2-stroke engine requires more frequent basic maintenance. This is because the lubricating oil is mixed with the fuel. One reason for this, is the increased carbon produced during combustion. The carbon from the burnt oil collects on the spark plug. This requires more frequent cleaning of the spark plug. Oil residue also collects in the carburetor and this requires that you clean the carburetor more often than you would a 4-stroke engine.
The user also has to mix the oil and gas at the correct ratio. If there is too much oil in the fuel mixture, combustion will not be very efficient and the carbon levels inside the chamber (and exhaust system) will be excessive. This reduces engine performance and will require more frequent cleaning of the spark plug and the carburetor. Conversely, too little oil in the fuel mixture will result in insufficient lubrication. This will cause the engine overheat, the consequent damage can be detrimental to the engine life.
A 2-stroke engine may require more frequent basic maintenance, but these procedures are easy to perform and inexpensive. Routine maintenance on a 4-stroke engine may only need to be conducted once a year, or after certain amount of hours (or miles), depending on which happens first. While this is less often when compared to a 2-stroke engine, it is more complicated and a little more expensive. The sump oil in a 4-stroke engine needs to be replaced, along with the oil filter.
Because modern 4-stroke engines use hydraulic lifters to operate the valves, particular attention needs to be paid to the engine oil level, and the type of oil used. Both the oil pressure and viscosity affect how hydraulic valve lifters perform.
The oil level needs to be checked regularly because the valve operation is dependent on the correct oil pressure. If the oil level is too low, or the oil pump is faulty, the oil pressure will be lower than it should be. Low oil pressure causes the valves to malfunction and can easily cause engine failure, or serious damage to the engine. If the oil pressure is too high, because the oil sump has been overfilled, the engine will also sustain damage. Repairing engine valves is a complicated and expensive procedure, so it is critical that a 4-stroke engine always operate at the correct oil pressure, using the correct type of oil.
When moving beyond routine maintenance to long term maintenance and repairs, 2-stroke engines are both easier and cheaper to keep running. Because of its simplicity, a 2-stroke engine is much easier to strip and reassemble. It takes less time and requires less skill to repair a 2-stroke engine. Fewer components, like no valves or oil pump, means that there is less to go wrong. With prolonged use, valves will need attention. Valve stem seals and shims wear over time, as do the valves themselves and the valve openings in the engine head. Removing and repairing valves is complicated and may require specialized engineering.
For DIY engine repairs, looking at 2-Stroke vs 4-stroke, the 2-stroke engine is going to be the simplest least expensive option. Though a 2-stroke engine will generally produce more heat and this is likely to shorten engine life.
Emission & Noise Levels | 2-Stroke vs 4-Stroke
Air pollution from internal combustion engines is hot topic and emission standards are constantly being reviewed. The Federal Environmental Protection agency (EPA) and California Air Resources Board (CARB) are the two most important agencies governing the emission levels for gas, propane, and diesel engines in the USA. In order to meet these standards, engine manufacturers have to take measures to reduce harmful exhaust emissions.
Because 2-stroke engines produce higher levels of toxic emissions, they need more complicated emission control devices. These are primarily part of the muffler to contain exhaust gasses before they reach the atmosphere. This adds to the cost of the machinery that utilize 2-stroke engines. Emission control mechanisms fitted 2-stroke engines also need to be replaced more frequently and usually have less favorable warranties than 4-stroke engines. As the EPA and CARB introduce stricter regulations in the future, there is talk that the 2-stroke engine may not be able to meet these standards. This is one of the major concerns surrounding the use of 2-stroke engines.
When it comes to 2-stroke vs 4-stroke noise levels, the 2-stroke engine doesn’t fare to well either. The two stroke action makes for a noisier engine. This can be of great concern in urban areas. Not only are loud machines a cause for irritation in residential neighborhoods, some local authorities have noise restricting legislation to prevent urban noise pollution. These regulations can make certain engines unusable in these areas because of high noise levels. As 4-stroke engines are quieter, it is less likely that these engines will be as much of a problem in terms of high noise levels.