Hi, Devil here. In this post, we're going to be looking at a four stroke diesel engine. We're gonna run through all of the main parts, and them I'm gonna show you exactly how it works. Now, if you're interested in a four stroke gasoline or petrol-fired engine, that's no problem. The working principle is pretty much the same as for the diesel-fired engine. So, before we go too far into explaining exactly how the four stroke engine works, let's have a look at some of the individual components.
What are the Components Of Four Stroke Diesel Engine?
- Engine Block
- CrankShaft
- Piston & Piston Ring
- Connecting Rod
- Main Bearing
- Cylinder Head
How Engine's Components Connect With Each Other?
As you can see, we have a piston. And the piston connects to a connecting rod. And the connecting rod connects to a crankshaft, that is this weird up and down shaped item here that stretches from the right side all the way along to the left side. The piston itself travels linearly up and down within the combustion space. You can see on the edge, that is one side of the combustion space. So, the piston will travel up and down. And every time it travels down once, we refer to that as a stroke. Although the piston itself is traveling up and down, the crankshaft rotates.
So, we're converting the up and down motion from the piston into rotary motion. Because of this, every down motion of the piston and every up motion of the piston translates to one revolution, that is one full rotation of the crankshaft. So, one stroke down, one stroke up, is one revolution of the crankshaft. We'll have a look at that in a moment. Let's just have a look at some of the other key components that we need to see in order to understand how this four stroke diesel engine works.
If we go up to the top, we can see we've got a series of valves. We have two valves. You can actually see them a little bit better if we look from underneath. These valves are used to allow air into the combustion chamber, and also to allow exhaust gas out of the combustion chamber. And in the middle, between these valves, is a tiny, small, sticky-out bit, this nozzle here. This is actually the end of a fuel injector. We call this the spray nozzle. And we're gonna spray fuel out of the tip of the injector, which is this section here. If I go up, we can actually see the injector. Trying to find a better angle. That is the injector. And it connects to the center of the cylinder, goes through the center of the cylinder, and will inject fuel into the combustion space. If we come around the side, we can actually see the fuel connection.
So, the fuel comes in through this pipe, and will connect to the fuel injector, as we see here. And the fuel is then injected into the combustion space. So that is our fuel injector, and those are our valves. Let's now have a look at exactly how this four stroke diesel engine works.
How Four Stroke Diesel Engine Works?
Now, there are four strokes per combustion cycle. That means, in order to fully end a combustion cycle, the piston is going to need to move down once, up once, down once, up once. That's four strokes. And each of those strokes has a purpose. Let's have a look at stroke one. We can see we've got a slight blue color here in this space. The blue represents air. I'll push. The piston is moving down, and you can see that these two valves have opened. We're now drawing air into the combustion space. Now, this air may come from a turbocharger, or it may just be atmospheric, where we draw air in. But as the piston moves down, it's creating a slight vacuum, and it's drawing the air into this space. We wanna completely fill the combustion chamber with air because air contains oxygen. There are three things we need for combustion. That is heat, oxygen, and fuel.
So, as we draw the air in, we're obtaining oxygen. That's the first part of the fire triangle. Let's allow the piston to move down to what we call bottom dead center. That is its lowest point of transit within the cylinder lineup. We are now at bottom dead center. We've come all the way down, and the piston is as close to the crankshaft as it's ever going to travel. That is one full stroke, and we call that the suction stroke or the intake stroke, because we're sucking air into the combustion chamber. Notice now that the piston has traveled all the way down, and correspondingly, on this section here, where it's connected to the crankshaft, that is also at its lowest point. Let's push, and we can watch the crankshaft rotating.
Now, we're gonna push the piston back upwards to what we refer to as top dead center. That's the farthest point away from the crankshaft that the piston is going to travel. The piston moves upwards, and we are compressing all of that air. The inlet valves have closed, and that's what's allowing us to compress the air. The inlet valves would have closed roughly when we reached bottom dead center. We compress the air as the piston moves up, and we're injecting fuel. You can see that there are some lines coming in here. This is supposed to represent atomized diesel fuel. We'll inject the atomized fuel into the combustion space. And because of the massive increase in pressure, there is also a corresponding increase in temperature. As we compress the air still further, the temperature is going to rise to such a point that the fuel will ignite. Now, because we are compressing air in the second stroke, we refer to this stroke as compression.
So, remember we had suction, that is stroke one, compression, stroke two, and now we're gonna move on to stroke three. We've finished compressing the air. We get a controlled explosion. The fuel has now ignited within the combustion space. That is represented by this red color, which signifies heat. We have a controlled explosion, and the piston is gonna be forced downwards. Because of this explosion, there is a huge increase in pressure and temperature, and we're going to force that piston down. And as the piston travels down, it's gonna complete one full stroke. We're now at bottom dead center again. We can see that on the crankshaft. And that stroke is referred to as the power stroke, because that essentially is the stroke that is allowing us to extract the energy that we've got from combustion and turn it into mechanical energy.
So, the fuel itself contained chemical energy, and when we combusted the fuel, we took that chemical energy and we transferred it into heat and pressure. And then we use this heat and pressure to drive the piston downwards. And in doing so, we converted the energy into mechanical motion. Now, if we could do that, if we could isolate the power stroke without needing the other three strokes, then we would have a very efficient engine. But unfortunately, we can't do that. Now, the combustion chamber is completely full of exhaust gas. We've burnt all of the fuel, and what we have is a space that is full of exhaust gas. Now, we can't just inject fuel into the combustion space at this point, because there's not enough oxygen within the combustion space for the fuel to combust.
So, we need to remove all of this exhaust gas from the combustion space. In order to do that, we'll open exhaust gas valves. Notice, we use these two valves to allow air into the combustion chamber. Now, we're going to use the other two valves in order to get the exhaust gas out of the combustion chamber. So let's watch that occurring now. We completed our third stroke, the power stroke, and now the piston's gonna travel back up from bottom dead center to top dead center. The piston is moving upwards now. Notice the exhaust gas valves have opened. And the exhaust gas is being pushed out past these two valves and into the exhaust gas manifold. Here comes the piston, completes its stroke, reaches top dead center. The exhaust gas has completely or almost completely exited the combustion chamber. And that means we've completed our fourth stroke, and that was our exhaust stroke.
So, we've completed now, one combustion cycle. Suction, compression, ignition, and exhaust. That is essentially how a four stroke combustion engine works. Or if you wanna be really specific, then you can say that is how a four stroke internal combustion engine works. If you can't remember the name of each of the strokes, suction, compression, power, and exhaust, also known as suction, compression, ignition, and exhaust, then try suck, squeeze, bang, blow. Suck for suction, squeeze, compression, bang for power or ignition, and blow for exhaust. Suck, squeeze, bang, blow.
How to Remember Four Strokes of Diesel Engine?
Personally, I find it easier to remember that, although over time, you'll gradually shift towards suction, compression, ignition, exhaust, or suction, compression, power, exhaust. Those are the four strokes associated with a four stroke engine.
Is only We Use Diesel as a fuel in Four Stroke Engine?
Notice that we've used diesel as the fuel in our example. Although it is possible on some engines that you can also use petrol gasoline and you can also have direct fuel injection, although petrol and gasoline engines typically can inject from other areas. I believe there's four different modes of fuel injection for petrol and gasoline four stroke engines, one of them being in the ports on the air intake side, one of them being direct injection, and two other means of fuel injection.
Why Four Stroke Diesel Engine is Relatively Simple?
The four stroke diesel engine, though, is relatively simple because we inject the fuel directly into the combustion space and the diesel engine is a compression ignition engine. That means, we compress the air in order to ignite the fuel, whereas a gasoline or a petrol engine is a spark ignition engine. And although we compress the gasoline or petrol to air mixture, we use a spark plug with petrol and gasoline engines in order to time the combustion correctly. But for diesel engines, this is not required. And so, diesel engines do not have spark plugs. So just remember, diesel engines are compression ignition engines, and gasoline or petrol engines are spark ignition engines because they use a spark plug.
How the Engine's Valves Open And Close On Time?
Now, as you may have noticed, four stroke engines use a lot of components. And all of these components need to operate, they need to open and close and move at the correct time in order that the engine can function correctly. In order for this to occur, we're going to use a cam shaft.
A cam shaft is this section here. And typically, it will run down the side of the engine, or perhaps the top of the engine. And the camshaft will control when we inject fuel into the combustion chamber, when the intake valves open and allow air into the combustion space, and when we open and close our exhaust valves and allow the exhaust gas out of the combustion space. So, the camshaft is a very important part of the engine because it essentially controls all of the timing of the components. The camshaft itself will also be driven directly from the crankshaft, using either a chain or gears. And that ensures that as the crankshaft rotates, the camshaft rotates as well, and the timing of the camshaft is directly linked to the crankshaft.
Let's have a look at an example. You can actually see that we've got a cam lobe here. That is this black section. And if we back it up slightly, we can see that as the piston is moving and the crankshaft, the camshaft is also rotating. And let's see what happens when the cam lobe comes and pushes up on the push rod. The push rod is this entire rod here. So, it comes from the bottom, stretches all the way up. And we go to what's called our rocker arms. That is this section here. But let's go down, we'll follow the movement. We can see the piston is at bottom dead center. We've completed the power stroke, and that means soon, we're going to need to open our exhaust valves. How are we gonna do that?
While the crankshaft rotates, the camshaft pushes up the push rod. Let's go and see what effect that has on the rocker arms and the valves. Notice that when you push up the push rod, the rocker arm is pushed down. And we're actually going to open the exhaust valve. Let's spin around here. So what's happened?
I'll back it up slightly again. So, the cam lobe comes around. We push up on the base of the push rod. We've pushed up on the base of the push rod. That has pushed our rocker arm down. What we've actually done is pushed both of the valves down as well. And that means that the valves are now open. If we wanna close the valves, what we have to do is continue the crankshaft rotation, which continues the camshaft rotation as well, and the lobe will rotate downwards. And at that point, the connection here is not fully correct. Normally this push rod will be rounded at the end. It makes loading and unloading of the push rod a lot more efficient, and you're less likely to get cracks.
So, let's just assume, for the moment, this is slightly rounded. And now, we have allowed the push rod to drop back down, which means our rocker arm has changed positions. And then springs, specifically these springs here, here's one and here's the other, these valve springs have pushed up against the rocker arm, and the valves have moved back to the closed position. Let's spin around and see now that the exhaust gas valves are now closed. So we're using mechanical energy from the power stroke. We're transferrin' it to the crankshaft, which is also transferred to the camshaft. We're pushing up the push rod, pushing down the rocker arm, opening the valves, and then we're using springs to return the valves back to their closed position. Now, as you might imagine, if these springs get worn and tired, then they're not going to have enough tensile strength in order to return the valves back to the closed position. So that's not good, because if the valves aren't seating correctly, then we're going to get an inefficient engine. Now, why will that occur?
It will occur because if the valves are, say for example, they are like this, slightly open, when the piston comes up to compress the air, some of that air will escape. And that means we're not gonna get what we call a P-max, or a pressure max within the combustion space. And all of that leads to wasted energy. Some of the air will escape, means some of the oxygen has escaped. That means we're not gonna be able to combust the fuel efficiently, and we're also not gonna get the large pressure we need within this combustion space in order to push the piston down and obtain maximum efficiency.
So, it's very important that all of the valves, not just the exhaust gas valves, but also the intake valves, seat correctly and that they are pressed up tightly against the top of the combustion space. Now, notice earlier that I said the camshaft is also responsible for timing of the fuel injection, and when the fuel injectors open and close. Now, you'll notice here that we have two push rods. One is for the two air inlet valves and one is for the two exhaust gas valves. But there are no push rods for the fuel injector.
Why there in no Push rod for fuel injector in Engine?
The reason is because this particular type of fuel injector is not open and closed mechanically. It's actually a fuel injector from a common rail diesel engine. And what that means is we have an electrical valve. Lemme spin around, see if I can locate it. Here is the electrical connection on the top. And this is actually what they refer to as a solenoid valve or an electromagnetic valve. That means when we supply electrical current to the top of the fuel injector, it is going to open the fuel injector. And when we remove the electrical current, then we are going to close the fuel injector.
So, that is a common rail fuel injector. We don't actually need to connect the fuel injector to the camshaft. And this is quite a recent advancement in diesel engine technology. Now, you might be wonderin', why would we do that?
Why don't we just connect the fuel injector to the camshaft?
And then that way everything's driven from the camshaft and it's all a lot easier. We don't need to use any electrical circuits. Well, that's how things used to be. The reason that it's changed and we're now usin' common rail fuel injection is because it's more efficient. We can better control when we start injecting fuel into the combustion space and when we stop injecting fuel into the combustion space.
It's important to realize that four stroke engines use a lot more components than two stroke engines. Small, two stroke engines do not requires valves. They do not require push rods, they do not require springs like those here or rocker arms or a camshaft, or any of that. The downside is, for two stroke engines, they have a very high power to weight ratio, but they're not very efficient. Four stroke engines are more efficient, but they weight more. It is possible, however, to have very large two stroke diesel engines, and they actually do have these on large merchant navy ships.
In this particular instance, large merchant navy ships, they will use the two stroke engine because they have space to build additional components for the exhaust gas valve and for the fuel injection, et cetera. And that allows them to increase the efficiency of the two stroke engine. These modifications, however, would not be possible for small engines, such as common two stroke engines like for lawnmowers or for motorbikes, or leaf blowers. So, keep in mind, four stroke diesel engines are used for small, medium, sometimes moderately large applications. But when you get to very large applications, you will not use a four stroke diesel engine, you'll actually use a two stroke diesel engine.
So, I hope you found that this post is informative and interesting, and I really do hope you understand how a four stroke diesel engine now works. You can also like or share the post on social media. It really does help us out. Thanks very much for your time.
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