2 Stroke Vs. 4 Stroke – Differences

Once, the 2 stroke engine was just as represented as 4 strokes are now, at least in the motorcycle world. However, 2 stroke engines have largely gone out of production as of roughly 2005, even though they still had a lot to offer. But what exactly do 2 Strokes have to offer if virtually 100% of cars and trucks use 4-stroke engines?

The main 2 stroke vs. 4 stroke difference is that in a 2-stroke engine, every time the piston goes up, it’s combustion stroke, while in a 4-stroke, it’s every second time. Also, a 2 stroke doesn’t have valves or camshafts making it simpler, lighter, cheaper, and more powerful.

However, it also burns oil, pollutes the environment, and has a much higher fuel consumption. So, in the 2 stroke vs. 4 stroke fuel consumption equation, the 4 stroke is the clear winner.

2 Stroke Engine Explained

When comparing 2 stroke vs. 4 stroke engines, the 2-stroke working principles come off as pretty simple but also very interesting to anyone who doesn’t know much about them. The main reason is simple that it doesn’t have a camshaft, valves, timing belt, or chain, nor does it precisely control the air and fuel intake in any way.

So, how does it do? Well, in a 2 stroke engine, the air-fuel mixture first enters the crankcase when the piston moves up instead of directly into the combustion chamber. Then, as the piston moves down, the air-fuel mixture passes through channels inside the cylinder wall and into the combustion chamber.

That means the cylinder wall has four or five holes near the top for air/fuel intake, plus one more for the exhaust. So, here is what the 2 stroke cycle looks like. Now, instead of me explaining how the two cycles or strokes work, here is a quick video animation.

Now, as you can see in this video, the intake has a valve, but it’s nothing more than a simple one-way valve controlled by the crankcase pressure. Meaning when the piston moves up, the crankcase is in a state of vacuum and pulls air through the intake. Then, as the piston moves down, the crankcase pressure is positive, and the valve closes.

Also, you will notice on top of the combustion chamber that the intake and exhaust strokes overlap. And that’s the biggest problem of two-stroke engines. Each time the burnt exhaust gasses go into the exhaust so does a part of the fresh air-fuel mixture. That’s not only wasteful but also bad for the environment.

Lastly, if the air/fuel mixture passes through the crankcase, where is the engine oil? And no, it doesn’t have a dry sump lubrication. All 2 stroke engines mix engine oil with fuel. Sometimes the driver puts oil into the fuel tank, and in some cases, the oil has a separate container and is mixed inside the carburetor.

But whatever the case, all 2 stroke engines burn oil together with fuel, so you constantly have to keep it topped up. And that, too, is wasteful and extremely bad for the environment. It’s also why 2 stroke motorcycles blow blue smoke out the exhaust and smell nice, depending on who you ask.

Pros of 2 Stroke Engines

Cons of 2 Stroke Engines

4 Stroke Engine Explained

The 4-stroke engine is by far the most common engine today when it comes to any type of vehicle. Moreover, it’s the only available engine in cars ever since the first one, with only a handful of exceptions. So, what does make the 4-stroke so good?

To start off, the 4-stroke engine has a power stroke every other crankshaft revolution. In other words, half as many power strokes as a 2 stroke engine. But there is a good reason for that. The 4-stroke engine uses precisely timed intake and exhaust valves to control the air/fuel mixture.

And while that complicates the engine, it also provides precise control over the air/fuel mixture resulting in a significantly more efficient engine than a 2-stroke is. And since the air/fuel mixture enters directly into the combustion chamber, the 4-stroke can have an oil sump. That eliminates the need to burn oil in the combustion chambers and, again, increases efficiency.

Now, the way everything works is that when the piston is at the top dead center, and the spark plug fires, all valves are closed, unlike in a 2 stroke where the intake and exhaust ports are always open. Then, the piston moves down, making the power stroke, and when it comes back up, the exhaust valve opens, allowing the burnt air/fuel mixture to escape. And that’s the one missing power stroke which is the defining factor when it comes to 2 stroke vs. 4 stroke power output difference.

After that, as the piston moves down, the intake valve opens and closes when the piston comes back up. That’s the compression stroke when the spark plug fires again, and the cycle is over. To help you understand that better, here is a video animation.

In the video, you can clearly see that there is no overlap between intake and exhaust. But you can also see just how many moving parts the 4-stroke engine has. Still, in cars and trucks, that extra weight is neglectable, and because cars have at least four cylinders, even the power loss is minimized.

On top of everything, the four-stroke is considerably more reliable, plus the running costs are incomparable with those of a 2 stroke. And that’s why 4-stroke engines have been virtually the only option in cars and trucks ever since 1895, only ten years after the first passenger car was made.

Pros of 4 Stroke Engines

Cons of 4 Stroke Engines

2 Stroke Vs. 4 Stroke Engine: Comparison

When comparing the 4-stroke and 2-stroke engines, the main thing to focus on is the number of power strokes per crankshaft rotation. For every crankshaft rotation, the 2-stroke has one power stroke, while a 4-stroke engine has one power stroke for every two rotations.

So, for example, let’s imagine we have one 4-stroke and one 2-stroke cylinder engine, and both are running at 500 RPMs. At 500 RPMs, a 2-stroke will fire the cylinder 500 times, while a 4-stroke will fire it 250 times because every other revolution is an exhaust stroke that doesn’t generate any power.

The second major difference is that the 4-stroke has a lot more moving parts, which is always a disadvantage for a number of reasons. However, a 2-stroke consumes oil with fuel, while in a 4-stroke, you just need periodic oil changes.

Because of those differences, if money or pollution was no object, the 2-stroke would always be the better choice, even when it comes to passenger cars. Under such circumstances, the only application where the 4-stroke would be the better option is semi-trucks, and that’s mainly because of their low-end torque.

However, because the 4-stroke makes 8.5 times fewer carbon emissions, is 50% more fuel efficient, more reliable, more durable, and cheaper to run, it will always be the better choice for wide consumer use. The only thing a 2 stroke has going for it is the power-to-weight ratio. So, when weight is a significant priority and top-end power, the 2-stroke is always better.

Applications of 2 Stroke Engines

It’s probably not a well-known fact that the first ever passenger car, the 1885 Benz Patent-Motorwagen, was a two-stroke. And even though there aren’t a lot of cars that came after with a 2 stroke engine, there are a couple of them, most notably, older Saabs and Trabants.

Another lesser-known fact is that there are 2 stroke diesel engines as well. Those were mainly used as power generators, train, and boat engines. And when we say boat, we mean a full-size ship with an engine the size of a house.

Still, the most important two-stroke applications are those where the weight of the engine is crucial. And those include planes, outboard marine engines, motorcycles, and tools. But today, at least in Western countries, the only place you will find a 2-stroke engine is hand-held or manually operated tools. For example, chainsaws, lawnmowers, trimmers, water pumps, small generators, etc.

However, at one point in time, 2 stroke diesel engines were also very popular in semi-trucks and military vehicles. The main reason for that, even though the 2-stroke doesn’t make much torque, is a low cost of production (cheaper to buy) and lower weight. When it comes to semi-trucks, the lower the weight of the semi, the more weight can go into the trailer, and more money can be made.

2 Stroke Applications

Applications of 4 Stroke Engines

As you will notice after we make the list of applications, there isn’t a single exclusively 4-stroke application. However, you won’t find some of the 2 stroke ones. Now, the main reason for that is the weight of a 4-stroke.

For example, if you are working with a chainsaw, you are carrying the engine in your hands, so weight is extremely crucial, which is why you won’t find 4-stroke chainsaws.

Still, even though the applications overlap, it’s important to mention that 4-stroke is much more common. As we already mentioned, 99.99% (probably more) of cars have a 4-stroke engine. The same goes for semi-trucks, and starting with the year 2005, it applies to motorcycles, outboard motors, UTVs, etc.

So, even though you can find a 2-stroke engine in virtually all applications, in a lot of them, you can count the specific models on the fingers of one hand, making 4-stroke considerably more popular.

4 Stroke Applications

FAQs:

Q: What is the main difference between 2-stroke and 4-stroke engines?

The main difference between 2-stroke and 4-stroke engines is that a 2-stroke has one power stroke per crankshaft revolution, while a 4-stroke has one power stroke per two crankshaft revolutions. That means a 2 stroke engine has double power strokes (combustions) at the same engine speed.

Q: How do I know if I have a 2-stroke or a 4-stroke?

There are a couple of ways to tell if you have a 2-stroke or a 4-stroke, and the first one is to look for an oil dipstick; if the engine has one, it’s a 4-stroke. The second is to look at the fuel cap, and if you see that it has a fuel/oil ratio (40:1 or similar), it’s a 2-stroke. Also, if the carburetor or intake is attached to the cylinder head, it’s a 4-stroke, and if it’s attached to the crankcase, it’s a 2-stroke.

Q: Does 2 stroke need oil?

Yes, 2 stroke engines also need oil, but the lubrication system is different. In a 2-stroke engine, the oil is added to the fuel tank, or there is a separate oil tank, and the engine uses an oil pump to mix the oil with fuel. Also, 2-stroke engine oil is not the same as 4-stroke oil.

Q: Are 2-stroke engines still legal?

Yes, 2-stroke engines are still legal, but manufacturers are no longer allowed to produce or develop new ones for certain applications, including motorcycles with engines bigger than 50cc. However, you can still find new 2-stroke snowmobiles, outboards, and UTVs.

Q: Why are 2 strokes banned?

2 Strokes are banned primarily because of high exhaust gas emissions. A 2-stroke engine makes 8.5 times more CO2 emissions than an equivalent 4-stroke, plus it burns oil, which adds a whole new set of harmful exhaust particles. Moreover, the fuel consumption of 2-stroke engines is almost double of the 4 strokes.

Q: Why did motocross switch to 4 strokes?

Motocross switched to 4 strokes because 2-stroke production motorcycles were banned, and that happened because of high exhaust gas emissions, low efficiency, and high fuel consumption. And while that’s not a big deal for race bikes, all motorsport races are justified by the technology development that’s later transferred to production vehicles. And since road-going 2 strokes are banned, the racing technology for them becomes obsolete.

Which is better, a 2-stroke or a 4-stroke engine?

Nowadays, with turbocharging, variable valve timing, direct injection, and other technologies, the 4-stroke engine can make just as much power as a 2-stroke with much better efficiency. So, in modern times the 4-stroke engine is better in every conceivable way, especially for cars and trucks.

However, for hand-held tools like chainsaws, trimmers, and even lawnmowers, the 2-stroke engine is much better even today. The main reason is their low weight and high power output, plus the fact that you can turn the engine upside down if you want, and there will be no issues with lubrication because it doesn’t have an oil sump.

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