Engine Cylinder Design and Function - Part 2

Displacement
Volume and stroke




Everybody has heard of a 1.8 litre engine but what does that mean? Simply put it is the internal volume of the of the engines cylinders. This applies to both single and multiple cylinder engines.

In engine speak it’s the total volume dedicated to combusting the engines designated fuel, be it diesel, petrol or whatever. 

However, as usual it’s a bit more complicated than that. They’re 2 factors that determine the volume of a cylinder. These measurements are the bore and the stroke.

Let me be perfectly clear about this – only the bore and the stroke change the volume of the cylinder. It has nothing to do with the length of the connecting rod, what valves your using or even the height of the piston.

So lets get maths bit out of the way. To measure a cylinder it, as stated previously a circle with some height to it. To start with you work out the area of a circle and then multiply that value with the length of the cylinder:

Cylinder volume =  π x r x r x h

π = 3.142

r = the radius of the circle

h = the height of the cylinder

If you have the bore size (lets say 62mm for example) then the radius is just half that which in this case would be 31mm.

So taking our example a cylinder with a bore of 62mm and a stroke of 57.6mm would be as follows;

3.142 x 31 x 31 x 57.6  = 173921.0112 mm cubed

That’s a massive number, but this is mm cubed, and they’re tiny to convert this number into cc (cubic centimetres) we just shift the decimal point over 3 places so we get 173.9210112 cc or 174cc 

So you can see how helpful these 2 numbers are, with the bore and stroke values you can work out the engine capacity. The values are always given for 1 cylinder even in multiple cylinder engines. This means if you have a V6 engine then the total volume is the same as above but multiplied by 6.

In our example above that would be 174 x 6 = 1044 cc

Which in the real world would be classed as a 1.1 litre engine.

So all this seems pretty easy, which it is but all of this bore and stroke lark only tells you half the story. These numbers are the volume of the cylinder ONLY. And more to the point not the physical cylinder, just the volume that the piston travels within. To understand we need to look at an example of the physical engine volume.

The real engine
Volume - clearance and swept 


There are 3 different states when considering the internal volume of any piston engine. These are total, swept and clearance volumes. We’ll start off with the smallest which is the clearance volume.

The clearance volume is pictured below and illustrates how this volume is contained. When the piston is at TDC (top dead centre) the remaining space above is the clearance volume. This is also the combustion chamber volume, where all the compressed gases are contained.

 

The image below is the clearance volume in relation to the entire volume of this cylinder. As you can see this volume only exists when the piston is at TDC. 

This neatly brings us to the next volume which is the swept volume. This is what your measuring when you take the sum of the bore and the stroke. Now you can see why this value isn’t the actual volume but close enough.

  
The only remaining volume is the total volume. And as you’re already worked out this is the volume of both the clearance and swept volume.