So here we are. Now we're looking at a two stroke engine. Notice that we have the same parts as before, we've got our piston connecting rod and crankshaft. The area surrounding the crankshaft is known as the crankcase. So that is this section around here. You can see we've got some labels here makes it a little bit easier to understand what's going on.

We've got air and fuel fuel is usually indicated with an orange or amber color. For purposes we've used blue as air. The reason that we have half of the air or blue and half of it orange is because we've actually got an air fuel mixture. That means that the fuel is mixed in with the air. I'll explain to you how that works in a moment. And here we have exhaust gas indicated by a black arrow.

So, what's happening? Let's play the animation a moment. Again, you see the piston moving down, and we're gonna see it moving back up in a moment, the crank shaft is rotating, and there are arrows popping up every now and again to show us what's occurring. This is a two stroke engine. So let me walk you through exactly how it works. Two stroke and four stroke engines are internal combustion engines.

That is because combustion occurs internally inside the engine. It actually occurs within the cylinder liner. That is this space here, and we also refer to it as the combustion space. In order to have combustion we need three things oxygen, heat, and fill. These three things are used to make a fire triangle. Where are we going to get the oxygen from?

Well, we get the oxygen from outside the engine, specifically just ambient air, we can draw the oxygen in with the air through this space here. This is known as our inlet port. We can see now that the air is rushing in, and we have some fuel that is mixed with the air. So we'll mix the fuel in with the air before it comes into the crank case. Now, I know he said we're going to be talking about diesel engines early on in the course. But I want to mention here one important thing.

The type of fuel used with two stroke engines is almost always technical. It's highly unlikely that you're ever going to encounter a two stroke engine that uses diesel. The only time that I've ever seen this is on very large container ships, oil tankers, very large ships, they will use diesel fighters. Two stroke engines. So if you want to take a very general approach, just say two stroke engines are petrified only. So the fuel is mixed in with the air, it is drawn into the crankcase, it flows into the crankcase, and then it is going to sit within the crankcase space.

But we can see that there is another transfer port over here. So we have our inlet port, we can get the air and the fuel coming in, fill up the crankcase and then it's going to travel out of our transfer port is going to come up here, and then it's coming into this section here. So it's coming into a cylinder liner. Notice over we're not going to get the air and fuel up into the combustion space because the piston is blocking that. So let's see what happens when the piston comes down. The piston comes down It covers up the air and fuel inlet.

So we're no longer going to get air and fuel coming into the crankcase. And when the piston travels down, it's actually compressing the air fuel mixture in the crankcase, and it's squeezing it out of the transfer ports. See, it's coming up here. And just round about here, it's starting to escape into the combustion space. So it's coming into the cylinder liner. As the piston continues downwards, it's going to reach the bottom of its transit.

And that's the point we actually call bottom dead center. Let me just highlight that because that is a very common term that you should definitely get to grips with. Let me just show you that again. piston coming down. Shut off the air fuel mixture or the air inlet port. Bottom dead center, the piston does not travel any closer to the crank.

Chef where it is now. So we'll call that bottom, that center will be the See, the opposite of BDC is TDC, which is top dead center, I can play that again. And let's just have a quick look because we can see top their center as well. The piston is as far away from the crankshaft as possible that is top dead center. So now we know those terms. Let's try it again.

Here's a piston coming down. We're heading towards bottom dead center. oppose the animation. And now you can see that the transfer port is completely uncovered. The Pistons gone all the way down, it's compressed the air fuel mixture in crankcase, it's essentially squeezed that out into the cylinder liner. And now the pistons going to come back up again.

So now we've got a few and the thing that we need now is a source ignition. So remember the fire, triangle, air, fuel and heat all of the things we need for combustion, I should actually say specifically here we need the oxygen that is contained within the air for combustion, so not just the air itself, so perhaps I should call that oxygen, heat and fuel. But we've got the oxygen which is contained in the air, we've got the fuel because we just drew all of that in through the transfer port. And now if we compress all that, whenever you compress a gas, its temperature increases as each volume decreases. Not only that, but as you compress the gas, its pressure will increase as well. So just remember that compressing a gas pressure increases, temperature increases, volume decreases, and then when we get to top dead center, round about here We're going to use a spark plug.

That is this item here to create a spark fact we may even see that can see it here. We're creating the spark and we are going to ignite that air fuel mixture. So now we're going to burn the fuel and the oxygen in the air we have everything we need for combustion, oxygen, heat and fuel. And we even have a source of ignition. So the spark from the spark plug ignites the fuel within the air fuel mixture. Remember, it's very hot in here now we've got quite a high pressure as well.

We get a controlled explosion, and we get combustion. And because this is happening internally, we have internal combustion. In other words, an internal combustion engine is a large explosion. When we have that large explosion, we back it up slightly. explosion huge increase in pressure, huge increase in temperature, and we're going to force that piston downwards. As the piston goes downwards or travels towards the crankshaft, we're going to uncover an exhaust gas discharge port.

That is this port here you can see now we're discharging the exhaust gas. The reason we're discharging the exhaust gas is because we've burned all of the fuel and we've taken all of that chemical energy and we're turning it into mechanical energy, which is rotating the crank shaft. So within a very short space of time, we've managed to transfer that energy to the crankshaft. We'll get rid of the exhaust gas that will just come out of our exhaust gas manifold or tailpipe, and the process is ready to repeat to see that happening again, piston comes down and we can also See, we are now allowing air and fuel into the combustion space again. And we can compress all that and as it moves upwards, we're going to cover up the transfer port. If we zoom in here, you can actually see that a little bit later, we cover up the exhaust gas port.

Normally the exhaust gas port would be slightly lower down so we don't get too much waste, specifically air and fuel that's discharged out of the exhaust gas port when it shouldn't be. But now the remainder of the air and fuel will again be compressed. We'll get ignition and then the piston will move back down again. Now we've talked about bottom dead center and top dead center. Let me introduce you to a couple of other terms. The reason this engine is called a two stroke engine is because whenever the piston travels from top dead center to bottom dead center, just do that right now.

That is your first To as one stroke when we travel from bottom dead center to top dead center that is referred to also as a stroke. So whenever we go from TDC to BDC or BDC TDC, we complete one stroke. Now, you may have noticed that we complete an entire combustion cycle in two strokes, the piston went down and then up and then we repeated the cycle again. All internal combustion engines contain four main parts to each of their combustion cycles. Let's go through each of those parts right now. The part that we're in now, or coming up to is known as suction, sometimes referred to as intake.

If we zoom in, we can see that we're drawing air into the combustion space. So we'll call that intake or suction part of the cycle that is stage one. Once we have sucked in the air and the fuel, we go to our next stage which is compression. We're now compressing the air in the fuel that is stage two. Once we get to top dead center, we get a controlled explosion and we start the power part of the combustion cycle. Finally we uncover the exhaust gas port and we get to the exhaust stage of the combustion cycle.

So, suction, compression, ignition, exhaust, suction compression, ignition exhaust. You can also say suction compression, power exhaust or intake, compression, power, exhaust, whatever you want to say. Those are the four stages that make up every sentence No combustion engine cycle. The only difference between a two stroke and four stroke engine is that a four stroke engine uses one stroke per part of the cycle and a two stroke engine does not. So let's go through it suction, compression, ignition exhaust. And we can repeat that and we can even speed it up.

And that is essentially what is happening within a two stroke engine. The reason that two stroke engines are only used for small engines is simply because they're not as efficient as four stroke engines. They have only half of the strokes and that's true that can be seen as a good thing because we want to repeat the combustion cycle as much as possible. Unfortunately, they actually waste quite a lot of areas. fuel, especially during this changeover period, when the piston covers up the air and fuel inlet, and then later covers up the exhaust discharge outlet, so we have a bit of a loss there with the air and fuel traveling out of the exhaust port. But the power stroke itself is also incredibly small.

We look here, ignition expansion, exhaust, the power stroke is very small, it is literally only let's go back up to top dead center. Here it comes. A power stroke is more or less from this point where my mouse is now, down to here. As soon as we uncover the exhaust gas port, the pressure in the combustion space drops dramatically. And essentially we've extracted as much energy out of the power stroke as we're going to get. With four stroke engines, it's possible to make the power stroke a lot longer.

And you can also do time when air and fuel is let into the combustion space much more precisely, and when exhaust gas is let out of the combustion space. So all of these factors make the four stroke engine more efficient than the two stroke engine. This is despite the fact that the four stroke requires twice as many strokes per combustion cycle, and four stroke engines have many more components compared to a two stroke engine. So I hope you now know how a two stroke engine works. Let's now go and have a look at how a four stroke engine works.