Engine Displacement - Explained

Hello everybody, welcome back to Plain and Simple. Today we're going to be looking at how the engine sizes are calculated, and what the size of the pistons have to do with the size of the engine. Let's start with a little bit of theory. Okay, now that we're set up, class is in session. Pay attention. Whenever you hear sizes of engines, be it in cubic inches, liters, cc's, it's all given in a unit of volume. In the case of most cars, the volume, the size of the engine is given in liters. Liters is a metric unit for volume. You can have a 2.5 liter engine, 2.4, 5.0. In the case of a Mustang GT, you have a 5.0 liter engine. That's a 5 liter displacement. What does that mean? That's what we're going to get into today. Scaling down the size of the engine considerably from a Mustang's 5.0 engine, let's move down to a scooter's 50 cc engine. That's a change of unit, cc's versus liters. cc's is short for cubic centimeters. One liter is 1,000 cubic centimeters. Therefore, a 2.5 liter engine would be the equivalent, or the same thing as 2,500 cc or 2,500 cc engine. 2,500 cubic centimeters or 2.5 liters. Now we're looking at an engine that has a single piston displacement of 50 cc's. That's what we have here. This is the piston that came out of a scooter that had a displacement of 50 cc's, 50 cubic centimeters. What volume is that 50 cc's referring to? Let me explain that to you. This would be looking at a cutaway, a side view of a piston in an engine, a cylinder. In an engine, you have a piston, which is this right here. That's the piston that rides up and down inside of a cylinder. A cylinder, quite literally, is a cavity for the piston to move up and down on. That describes a cylinder. The volume of that cylinder is what determines the size of the engine. Now, that cylinder is described with the bore. The bore is the diameter of the piston or the cylinder. The stroke is the amount of travel that the piston does up and down inside of that cylinder. Now, if you have a disc, take this piston to be a disc, and you slide it, you displace it in and out inside of a cylinder, that describes a certain volume. That volume is the displacement of the engine. In the case of this particular engine, the 50 cc, you have a piston that has a diameter of 43.5 millimeters. The radius is half of that. The area of a circle, which we'll want to determine the area of this circle here, the top of a cylinder or piston. The formula for area is pi r squared. If you take the radius and you run it through the equation, you end up with an area of the piston, the top of the piston, of 1,486 squared millimeters or 14.86 centimeters squared. Now, if you take that, that is the area of this, the top of the piston. You take that and you multiply it by a stroke of 3.36 centimeters, multiply the area by the height, and that's the formula for the volume of a cylinder. There's the area of the piston multiplied times the height that that piston is moving up and down, which is the stroke. That gives you a volume on the combustion chamber here. This is the combustion chamber. This is where the air and fuel mixture gets compressed and gets burnt. This, the size of this right here, is what determines the capacity of the engine to burn air and fuel, therefore producing power. The bigger this combustion chamber is, the more power the engine will make. And as I was saying over here, with a piston area of 14.86, a stroke of 3.36 centimeters, you multiply one by the other and you get a displacement or a volume of this cylinder of 50 cubic centimeters. That is the size of a 50cc scooter engine. Now, this is in the case of a single-piston engine. The size of the engine is the size of that volume in the combustion chamber. This, for example, is the piston of a 150cc scooter engine. You can see that it's considerably bigger. It means you have three times the displacement still on a single piston. Now, this is a piston of a small Briggs & Stratton engine that we're going to look at soon that is slightly bigger than the 150cc scooter engine. I am not sure of the displacement of this one, but it's a little more. But you can see how the pistons are getting bigger. Now, all three of these cases are from engines that are single-piston engines. That means that whatever the displacement or the volume of that combustion chamber is, that is the size of the engine. Now, in the case of this guy, this is a piston that is a little bigger than all the other guys. There's the 50cc piston. This guy is from a Lycoming O320 air-cooled aircraft engine. The diameter of this piston is just over 5 inches, which is what I have described over here. This is the dimensions of this particular piston and its cylinder. The area of this piston, with a diameter of 5.1 inches, the area is 20.4 square inches. The stroke of this engine is about just under 4 inches. So we have a piston that's 5 inches across, traveling up and down inside of a cavity, which describes a cylinder, that is the cylinder of the engine. If you take this big of a piston, slide it up and down for 4 inches, that will describe a volume of space that is 80 cubic inches. Now, this is using a different unit from this side over here. This is all metric, centimeters, liters, cc's. This is described in inches. In aviation, everything is still using the standard measurements, English. So everything is described in inches and cubic inches. But it's a different unit of volume, but it's still the same principle. It's still volume. In this engine, we have a single piston volume in the combustion chamber of around 80 cubic inches. Again, that's the area of the piston multiplied by the height that the piston is moving, which is the stroke of the piston. That will give you the volume of that cylinder. In this particular case, 80 cubic inches. Now, this engine has more than one piston. So this engine is not an 80 cubic inch engine. In this particular engine, the O320 has 4 cylinders, 4 pistons. Therefore, the size of the engine would be this volume per cylinder multiplied times 4, because we have 4 pistons in that engine. If you multiply 80 times 4, will give you the total displacement of the light combing O320, which is 320 cubic inches. Make sense? Simple? Very good. The same going back to metric and more automotive applications. In the case of a 4-cylinder engine, that is a 2-liter engine, that means that you have a displacement, a per cylinder displacement of 500 cc. 500 cc per piston times 4 pistons. You have a total displacement of 2,000 cc. 2 liters. Now, I'm actually going to show you this right here. Right here. This is that old Briggs & Stratton engine that I mentioned earlier. What I want to do is... Here's the cylinder. You can see inside of it. That's the cylinder. This is the piston that came out of that. I want to connect the piston to the connecting rod and connect it down to the crankshaft. We're going to spin it so that you can see the piston moving up and down. This theory is nice and dandy. I love it. But it's much better when you can see what it is that we're talking about. Let's connect the piston with its connecting rod using its wrist pin. That pin is called the wrist pin. Imagine the piston being your hand and your arm being the connecting rod as the piston is moving up and down. The connecting rod is connected to the crankshaft which is spinning in a circle. That would be like you holding on to the piston moving up and down. Your wrist is the one that's allowing the movement. This would be the arm. That would be the wrist. This would be your hand. That pin is called the wrist pin. We're going to partially assemble this. Inside of here we have the crankshaft. As this spins, it moves the connecting rod all the way around. That moves the piston up and down. There's the top of the travel. Bottom of the travel. That there, that space in that cylinder, that is the size of the combustion chamber. That is what determines the size of that engine. If you take the area of that piston, which is what we calculated earlier, and you multiply it times its travel, how far it moves up and down, that cavity, that is the volume of that cavity. That is the displacement of the engine. That is how much air and fuel that piston will displace as it moves up and down. There you go. That's how you calculate the size of an engine. That is what the size of the engine, whenever you hear the size of the engine, that's what that means. Again, this is in the case of a single piston engine. Whatever that displacement is, that is the size of that engine. If you had two pistons, then this engine would have twice the displacement. It would be twice as big of an engine. If you had eight cylinders, going back again to the Mustang, you would have eight times that displacement. In the case of a V12 on a Jaguar, a Ferrari, etc., you have twelve times that displacement. You get the point. That movement of the piston up and down is what determines the displacement of the engine, multiplied times the number of cylinders. There you go. I hope it's simple. I hope you found it interesting. I hope it helps. I think this is all pretty interesting. You go from this to that, and everything works. You guys stay safe. Have a good time. See you next time.