So now we're looking at a four stroke combustion engine. And as you can see there are more parts associated with this type of engine than for the two stroke engine. Despite this, the suction compression ignition exhaust part of the combustion cycle that has not changed. Let's go through that right now. Then we'll look at some of the new components in greater detail. So let's push play.

The piston is moving downwards, and at this point we're drawing air into the combustion chamber. Zoom in. We can see that we have two valves that sees two items here and they are open. Let me back it up the moment the piston travels down, the valves open and we draw air in to the combustion space. zoom back out a little bit because See that at some point the piston is going to reach bottom dead center. The valves are now closed, and the pistons going to travel linearly back up again.

So we reached bottom dead center we can see that because the crankshaft is rotated all the way downwards here. So the piston now is going to return to top dead center. Just angle this correctly, we're going to compress the air. But notice this time I said air I didn't say air fuel mixture I just said air, the combustion space is full of air only. However, if we look very closely here, we can see that fuel is being injected into the combustion space. This is actually a fuel nozzle here and we're using the fuel nozzle to spray fuel is another So into the combustion space.

So let's zoom out again. So we've got a injector which is spraying fuel into the combustion space, we've got an oxygen which is contained in the air. Let's compress all of that, we'll get an increase in pressure and increase in temperature. And now our fuel is going to combust. So there we are top dead center, we have our control explosion that's indicated here by this red color. And then we'll get a massive increase in pressure and temperature and we're going to force that piston back down the cylinder liner goes all the way to bottom dead center.

And at this point, we're going to start to travel back up, push play again. And you can see the other two valves have opened. Those are exhaust gas valves, so they're allowing the exhaust gas to exit out of the combustion space. And the piston itself as it's moving up is pushing that exhaust gas out. Once we've completely removed the exhaust gas, it's time to repeat the cycle again. So, let's go down and we can count the number of strokes, suction, drawing air in opening up the air intake valves, compression, compressing the air, fuel injection ignition power stroke and then we will force the exhaust gas out of the combustion space and the cycle repeats.

So stroke one suction straight to compression, stroke, three power and then stroke for exhaust. If I'm honest, when I am teaching, I actually prefer the four stroke engine because it's quite easy to understand exactly what's happening. piston comes down draws air in, we compress it in jet fuel, it ignites control, explosion, expansion, power stroke, and then exhaust. If you can't remember all that just remember, suck, squeeze, bang, blow, suck, squeeze, bang, blow, thanks for speed off slightly, maybe I can do suck, squeeze, bang, blow, suck, squeeze, bang, blow and etc. So, that is essentially how a four stroke engine works. So the combustion cycle is the same, but the number of strokes is different.

And also how we let the air into the combustion chamber is different, and how we get the exhaust gas out. This type of engine may be either a diesel engine, or a petrol engine. It's not totally on you To have a petrol engine where the fuel is injected into the combustion space, there are four main methods of delivering fuel to the combustion space for petrol engines. One of them is in the poor, I believe one is sequential and the one here is direct. And there is one other as well. The reason that this is not a petrol engine is because we have no sparkplug.

There is only a fuel injector that signifies that this is a diesel engine. So if you ever looking at engines and you find spark plugs, you'll know immediately that it's a petrol engine. This type of engine that is the type that uses diesel fuel is known as a compression ignition engine. Because when we compress the air, we get the increase in temperature we need in order to get combustion. That's not the same for petrol engines. When we compress the air for petrol engines, even when we add the fuel we need to ensure that there is efficient combustion.

And in order to have this what we actually need is a spark plug, so we're supplying the source of ignition. For this reason diesel engines are cool compression ignition engines, and petrol engines are called spark ignition engines. Let's have a look at the valves and how they open and close. And also we can have a look at the fuel injector. We'll do the fuel injector first because it's relatively basic. can see on the opposite side, we've got a connection here.

That connection connects to our fuel injector. And as the name implies, we're injecting fuel into the combustion space. You can see it's got quite a pointy shape. It goes all the way down into the combustion space. We'll inject the fuel into the combustion space for spray nozzle. If we've been very precise here, we might even see tiny dots on the spray nozzle.

But it doesn't like it. So there will be tiny dots here right on the tip of the nozzle, and the fuel will be injected into the combustion space and it will be vaporized, it's going to be like a fuel cloud. That means it has very good contact with the air and that means we're going to get very efficient combustion. If we injected just a few drops of fuel, those drops are not going to have very good contact with all the air so we're not going to get efficient combustion. So we vaporize the fuel we make it into a diesel cloud and that means we have very good contact between the fuel and the air so we get very efficient combustion. Now we've got two air inlet valves and two exhaust valves at the moment We're just starting our suction stroke.

The air valves are open, but how? Well in order to ensure that the entire engine is timed correctly, we need a camshaft. This is a camshaft here the camshaft is used to open and close the air inlet valves open and close the exhaust gas valves and sometimes also to control when fuel is injected into the engine. The camshaft is usually connected to the crank shaft and we will use a chain or a gear to connect the crankshaft to the camshaft directly. The reason we do this is because we want the camshaft to remain in sync with the crank shaft. So as the crank shaft rotates, it's also going to drive the gears or a chain and it's going to make the camshaft rotate as well.

The response or the rotation of the camshaft is directly proportional to How much the crank shaft rotates and will actually gear the camshaft so that it completes one full revolution every time the crank shaft completes two revolutions. So let's take this as an example. This black item here is called our cam lobe can also call this a cam is pointing more or less upwards. We can see here that the crank shaft appears to be pointing a little bit off to the right if we're basing that on this section here. So it's coming along now crank shaft is rotating, done half a turn. We've done one complete turn.

And you can see now the lobe is pointing downwards. So it's 180 degrees that that camshaft is rotated. crank shaft is rotated 360 degrees. So there's a two to one gear ratio Let's let it continue. crankshaft rotates again. It's coming back around now another 90 degrees and we'll get round to our start position.

So we completed two full rotations on the crankshaft 720 degrees and only one on the camshaft. But in this way we can control when and how the valves open and close. The gearing is important because if we were to match the rotation of the camshaft to the crankshaft, the energy would work. Let's see what the camshaft is doing. We'll take an example is a low consider pushing this item up this long stick this is known as a push rod. It's coming up here to what we call a rocker arm.

It's causing the rocker arm to be pushed down. That's pressing down upon this section here. As the top of our valves go down. We've got two valves and the valves. They're open it back up a moment. So the valves here are closed.

Low comes around pushes to push right up. Because we push the push rod up, we actually cause the rocker arms pushed down onto the top of the valves. See that again. Push the push rod up, push the top of the vows down. The valves are now open because they've been pushed down. And if we go down again, we can see a cam lobe.

It's going to continue to rotate. Now the push rod is dropping back down again. That means our rocker arm is returning to its position. And the things that are making the rocker arm returned to its position are the valve springs. See shiny items here is one is another one in the back. Another one there.

Those of our springs springs on residual stressed items. In other words, if we place them under compression, they want to expand due to the residual stress they contain also known as tensile force. So if we back up again, you can see there, we're compressing the springs. And as soon as the cam lobe is no longer pushing the push rod up the springs return the valve back to the closed position. You can see that here. The valves have completely seated now and they have stopped air traveling into the combustion space.

Now, the exhaust gas valves are pretty much the same, same design, same components, same mechanism for opening and closing. If we wanted, we could actually go down here and we could use the push rod in the back and we could watch the entire process repeat again. But we're not going to do that. And the reason we're not going to do that is because there's one area that I want to show you first, which is slightly more interesting. In order that we get more air into the combustion space, and to ensure we flush out all of the exhaust gas, we're actually going to make the air inlet valves bigger than the exhaust gas valves. You can't really see it so much here.

Let me just see if I can get some sort of valve overlap. This is our power stroke. exhaust gas valves are open, the piston must be traveling up, it's forcing the exhaust gas out. What we'd actually have at this point is a little bit of crossover known as valve overlap where the air inlet valves open slightly and they allow air in and that ensures that we're flushing all of the exhaust gas out. We don't seem to have any overlap on this particular 3d model. So that's slightly inaccurate.

But imagine for a moment we had a slight opening of the air inlet valve I'd say around about now that is going to rush in will flush out all of the exhaust gas and that ensures that all of the exhaust gas has been removed from the combustion space or as much as possible, which means we are completely filling up the combustion space with new fresh air fresh oxygen, which we can use for combustion. There's no point having a little bit of exhaust gas hanging around in this section. Let's imagine for a moment the exhaust gas valves are closed, the air inlet valves open but this space here is still full of exhaust gas. So when we open the air inlet valve, we have not flushed out the exhaust gas that's in the combustion space. So that's why we have valve overlap a point at which the air inlet valves and the exhaust gas valves are all open at the same time.

So that is how a full stroke engine works. There are variations and slightly different designs, especially if we're using different types of fuel, but essentially the strokes involved with a combustion cycle that is suck, squeeze, bang blow. They're always going to be the same. I hope you now know how a four stroke combustion engine works. Let's go and have a look now at some of the differences between petrol and diesel engines.