Different Strokes: How Two- and Four-Stroke Engines Work
Are you team 2-stroke or team thumper?
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Have you ever wondered why you need to add oil to your chainsaw’s fuel supply? Or why some dirt bikes have a higher pitched exhaust note than others?
We’ve all been burdened with yard work or been behind the wheel of a car. But that doesn’t mean you’ve had the opportunity to dive into the specifics of 2-stroke and 4-stroke engines. In fact, you might not have known there was a difference until someone brought up 2-strokes in conversation.
That’s nothing to be ashamed of either. The fact of the matter is that most people only rely on 4-stroke engines. And even that is changing as electric cars, motorcycles, and yard tools become more prominent in daily life. But, if you’re going to get into anything engine-related, you’re going to need to know the difference. Don’t worry, The Drive is on the case!
Understanding the Engine Cycle
First and foremost, 2-stroke and 4-stroke are terms that refer to the length of the engine cycle. To better understand what these terms mean, we need to start by clearing your mind. Get rid of everything you know about piston-driven engines aside from the fact that you have a piston continuously traveling up and down within the combustion chamber.
Visualize that piston moving up and down. The power driving this motion comes from the ignition or combustion of the mixture, which sends the piston downward with tremendous force. Ignition is the primary goal, but fuel and air must first enter the chamber for that to happen. Then it must be compressed to maximize the amount of energy that’ll come from the burn. Once compressed, ignition occurs. But before the cycle can begin again, the exhaust from the spent mixture must exit the chamber.
The term “stroke” comes from the motion of the piston throughout the engine cycle. The corresponding number refers to the number of times the piston needs to travel within the chamber to complete all functions. Simply put, 4-strokes accomplish these goals with four strokes, while 2-strokes get it done in, you guessed it, two.
What Is a 4-Stroke?
The 4-stroke cycle first appeared way back in 1861 when a man named Nicholas Otto came up with the design. That’s why you may have heard it referred to as the “Otto cycle” on rare occasions.
This method dedicates one stroke to each of the four functions we discussed earlier. But that’s not the only defining characteristic of this type of engine. How fuel and air enter and exit the chamber is crucial to this design. That’s where the camshaft and poppet valves at the top of the combustion chamber come into play.
Let’s break down the 4-stroke cycle. In a 4-stroke engine, the cycle begins with the intake stroke. As the piston travels downward, the camshaft moves to the position that opens the intake valve. The vacuum from the downward motion of the piston draws the air and fuel mixture through that valve. Once the piston reaches the bottom of the chamber, it can no longer draw any air in, and the intake valve shuts.
After the intake stroke, the piston begins traveling up the chamber to compress the mixture. Once the piston reaches the top of the chamber, ignition occurs, driving it downward again with all valves remaining shut during the duration of both the compression and ignition strokes. After the ignition stroke, the exhaust valve opens up, allowing the piston to push the spent mixture through until it reaches the top of the chamber. The exhaust valve then shuts, and the cycle begins again.
What Is a 2-Stroke?
Funny enough, 2-stroke engines appeared shortly after 4-strokes when a man named Sir Dugald Clerk invented the first successful design in 1878.
Once you understand the fundamentals of the 4-stroke cycle, it’s much easier to get up to speed with 2-stroke engines. Before we can dive into how the 2-stroke completes its cycle, we need to eliminate a few parts that 4-stroke engines rely on.
2-stroke engines don’t have a camshaft, nor do they have valves, as you would find in a 4-stroke. Instead, they feature a sleeve valve system where two permanently-open ports exist adjacent to each other in the cylinder wall. These are known as the exhaust port and the inlet port. The piston itself works as the valve controlling flow to and from either port.
It’s important to note that the exhaust port typically sits higher than the inlet port because this configuration allows more time for the exhaust to escape in this cycle.
This layout allows the exhaust, intake, and combustion of fuel mixtures to occur during the same stroke. From the point of ignition, the piston begins to travel downward. Once the piston travels so far, it exposes the exhaust port allowing the spent fuel mixture to start leaving the chamber. As the piston continues, the position will begin to uncover the inlet port so that a fresh fuel mixture begins to enter. Once the piston reaches the bottom, it begins to travel back up as it continues to push out the remaining exhaust and compress the new fuel mixture until it covers the port and only works to compress the fresh fuel mixture. Ignition begins the cycle again.
A key component to this cycle is the reed valve in high-performance applications. This valve exists within the inlet port and allows the vacuum of the piston's downward stroke to pull fuel in but not push it back through when the piston creates pressure on the upward stroke.
A unique characteristic of two strokes is that the fuel coming also goes to the crankcase. The two-stroke engines use a total-loss lubrication system where the fuel is used to lubricate the rotating assemble. That is why you must add special 2-stroke engine oil to the fuel in these engines.
How Does Engine Cycle Impact Engine Performance?
Engine cycle plays a significant role in performance, but neither is universally superior to the other.
Just for kicks, let’s start by talking about the clear advantages of the 2-stroke. Because it has a much faster cycle, it can produce a lot more power without increasing displacement. In some cases, the power output is more than double that of a 4-stroke engine of similar size. 2-strokes do make peak power at much higher RPM, but the shortened cycle allows the engine to reach high speeds in a much shorter period, effectively increasing throttle response.
That’s not all in terms of performance. 2-strokes are much lighter than 4-strokes. 2-strokes don’t have a valvetrain, which is the most significant contributor to weight savings. So not only is a 2-stroke more powerful, it can weigh nearly 50% less than its 4-stroke equivalent, making for a significantly better power-to-weight ratio.
Another benefit to the 2-stroke design is that it can run in any orientation because fuel delivery isn’t dependent on being gravity-fed, which is ideal for a number of applications.
If 2-strokes are capable of creating more power, then why don’t we see them more often? There are two significant reasons for that. For one, they aren’t as durable as a 4-stroke, nor are they exactly emissions-friendly. Another factor is that they aren’t as easy to control for the average motorist.
Essentially, the relatively high durability and less-pollutive nature are why we tend to favor the 4-stroke for multi-cylinder engines we use in cars, trucks, and other large street-driven vehicles.
The high-power output of a 2-stroke and the ability to make high power without increasing the size and weight makes it the clear choice for other applications. Applications such as high-performance off-road vehicles and lawn equipment that rely on both a low power to weight ratio and need an engine that can run in an orientation favor 2-strokes.
It’s important not to rule out 4-strokes in either of these segments, though. The durability and low-end torque of these engines have their advantages. For one, the vehicles or utility equipment that rely on 4-strokes are easier to maintain. As with cars, the low-end torque is more manageable and requires less skill to control safely, making for a far more enjoyable experience for riders and operators.
That doesn’t mean there aren’t exceptions to the rule though. “Back in the day”, you might even find industrial diesel engines utilizing the 2-stroke cycle.
Engine Terms You Should Know
Stroke comes from the motion of the piston in the combustion chamber. Either cycle is self-explanatory. The corresponding number refers to the number of times the piston needs to travel to complete the four functions necessary for an engine to run.
Poppet valves are the valves used in 4-stroke engines. At the very least, these engines will have two valves per cylinder. The intake valve allows fuel and air to enter the chamber, and the exhaust valves allow the spent mixture to exit.
Another vital component of 4-stroke engines. This is the component responsible for opening the valves in the combustion chamber. It is linked to the crankshaft and must be timed correctly for the valves to open and close at the right moments.
2-stroke engines rely on ports that work with the piston to create a sleeve valve system. As the piston travels past these ports, they allow spent fuel to exit the chamber and a fresh mixture to enter.
A reed valve is present on the inlet port in high-performance 2-stroke engines. This simple valve system allows fuel to enter the chamber due to the vacuum created by the piston traveling downward but don’t let fuel be pushed out on the piston’s upward stroke.
FAQs About 2 Stroke and 4 Stroke
You’ve got questions, The Drive has answers!
Q: What’s the Difference Between 2-Stroke and 4-Stroke?
A: The primary difference 4-stroke engine completes all engine functions with four strokes, while a 2-stroke completes them in two. There are also some significant differences in the components used and oiling of these engines.
Q: Are 2-Stroke Engines Bad for the Environment?
A: 2-stroke engines are a significant contributor to vehicular pollution. Because they use ports instead of valves, unburnt fuel can leave the chamber and raise emissions. That is why they are typically limited for use in off-road vehicles and small-engine applications.
Q: Why are 2-Strokes Faster?
A: 2-stroke engines complete all functions more quickly than 4-strokes. They also have fewer parts, making them lighter. The increased engine speed and a better power to weight ratio both contribute to the enhanced performance of vehicles that feature these engines.
Q: Why are 2 Stroke Engines Not Used in Cars?
A: The primary reason we don’t see 2-stroke engines used in cars is due to their emissions. On top of that, they typically aren’t as durable or easy to control as a 4-stroke.
Q: What Happens if You Put 2-Stroke Gas in a 4-Stroke Engine?
A: Oil is added to 2-stroke fuel for applications such as dirt bikes because the fuel doubles as a lubricant for the engine. A 4-stroke engine can burn that fuel, but it can cause issues. It can damage the pump and filters, which is why you shouldn’t mix the two.
Learn more about 2-stroke and 4-stroke engines with this video.
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