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Engine Building Fundamentals: Compression Ratio

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Compression Ratio

07.32

00:00 - The compression ratio of the engine is one of the most important design parameters to consider when selecting engine components.
00:07 It will affect the engine performance as well as how susceptible to knock or detonation the engine will be.
00:15 In this module, we'll discuss exactly what compression ratio is, and find out how we can accurately measure and calculate it.
00:24 To put it in simple terms, compression ratio is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of the stroke with the volume of the cylinder and combustion chamber when the piston is at the top of the stroke.
00:39 Let's say, for example, that with the piston at the bottom of the stroke, the volume was 500 cubic centimetres, or cc for short, and when the piston was at the top of the stroke, then the volume was 50 cc.
00:53 In this case, the compression ratio would simply be 500 divided by 50, which equals 10.
01:01 When talking about compression ratio, though, we are expressing a ratio, so we would say that the compression ratio is 10 to one.
01:10 Well that covers the basic concept.
01:13 There's a little more to it.
01:14 So let's find out what we need to know.
01:17 First, there's a few terms we need to understand in order to actually measure or calculate the compression ratio in the engine.
01:25 We'll start with the swept volume of the cylinder, which is simply the area of the bore multiplied by the stroke of the crankshaft.
01:35 This is the same way we calculate the capacity of a cylinder when we're trying to calculate engine capacity.
01:42 For an example, if we have a bore with a diameter of 100 millimetres and a crankshaft with a stroke of 100 millimetres, we can find the swept volume of the cylinder by multiplying the area of the bore by the stroke.
01:56 We will start by calculating the bore area, which is found using the formula pi multiplied by bore radius squared.
02:04 In this equation, pi is a constant of 3.142, and the radius of the bore is simply half of the diameter.
02:13 So now we can put some numbers into this formula, and we get 3.142 multiplied by 50 squared, which equals 7,855 square millimetres.
02:27 Now we can multiply this area by the stroke to find the swept volume.
02:32 Putting some numbers into the equation, we have our bore area of 7,855 millimetres squared, and the stroke which was 100 millimetres, so the swept volume becomes 785,500 millimetres cubed.
02:49 When discussing engine volumes, we normally express engine capacity in cubic centimetres, and we can convert from cubic millimetres into cubic centimetres by diving by 1,000 since there are 1,000 cubic millimetres in one cubic centimetre.
03:06 In this case, the swept volume of a single cylinder is 785.5 cubic centimetres, or cc for short.
03:16 Next we have the term clearance volume which represents the volume between the cylinder head and the top of the piston when the piston is at TDC.
03:26 In its simplest form, you could think of the clearance volume as the volume of the combustion chamber.
03:31 However, in a real engine, the clearance volume is comprised of a number of different measurements we need to make.
03:37 So let's find out what exactly makes up this clearance volume.
03:42 First of all, we have the volume of the combustion chamber, which as its name implies is just the volume of the cylinder head's combustion chamber.
03:51 This volume can either be measured or it can be taken from manufacturer's data.
03:56 However, measuring will give you the most accurate results.
04:00 Second, we'll also have the head gasket volume, which is the volume defined by the thickness of the head gasket and the head gasket's bore diameter.
04:11 Note that the head gasket bore diameter is frequently different to the cylinder head diameter too, so this does need to be confirmed.
04:20 We can calculate the head gasket volume in the same way that we calculated the swept volume of the cylinder.
04:26 Simply calculate the area of the gasket bore diameter, and then multiply this by the thickness of the head gasket.
04:34 It's important to note here that we're interested in the compressed thickness of the gasket too.
04:41 Next we may find that the piston doesn't exactly reach the deck surface of the block when it's at TDC, so there will be some volume here that we refer to as the deck clearance volume.
04:54 This may be a positive value if the piston sits below the deck surface at top dead centre.
05:00 However, if the piston protrudes from the desk surface at TDC, the deck clearance volume would become a negative value as the piston itself is now actually taking up some of this room.
05:12 We can calculate the deck clearance volume by measuring the deck clearance, and then multiplying this by the area of the cylinder.
05:22 Last, we also need to account for the dome, or the dish of the piston crown, which we refer to as the piston dome volume, and this can be measured or it can come from manufacturer's data.
05:34 In this case, if we have a dished piston, we will add the volume of the dish to the clearance volume.
05:41 However, if the piston uses a dome, then this will protrude into the clearance volume effectively reducing the volume, so we would reduce this value from the clearance volume.
05:52 For simplicity, we'll represent a dished piston volume as negative, and a domed piston volume as a positive value.
06:00 Now that we understand all the terms, we can calculate the clearance volume which will be the combustion chamber volume plus the head casket volume plus the deck clearance volume minus the piston dome volume.
06:15 Now that we have the clearance volume, we can look at the calculation for the compression ratio, which is the swept volume of the cylinder plus the clearance volume divided by the clearance volume.
06:27 It's common when selecting components for an engine project to select an off-the-shelf piston with a specific quoted compression ratio, and then assume that that's what we'll end up with.
06:39 In reality, though, it only takes a slightly different deck height or a different head gasket thickness to affect the final compression ratio.
06:49 At the same time, manufacturing tolerances may also mean that combustion chamber volumes vary from one cylinder to the next.
06:57 This is why actually measuring, confirming, and adjusting the compression ratio on each cylinder is an important part of performance engine blueprinting.
07:07 So the important parts to take away from this module are the concept of what the compression ratio actually means.
07:15 The ratio between the swept volume and the clearance volume to the clearance volume.