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EFI Tuning Fundamentals: Pressure Volume Cycle

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Pressure Volume Cycle


00:00 - The four-stroke principle is important, but we also want to look at what's going on in a little more detail.
00:05 To do this, we can analyse what's happening to the pressure in the cylinder as we move through the four strokes.
00:12 Graphing this pressure change gives us a little more insight into how the engine produces power and how this can be influenced by its mechanical design.
00:21 So let's have a look at a graph of the pressure in the cylinder now.
00:25 I'll reference this graph as we move though the four strokes of the engine cycle.
00:29 First we have the intake stroke, which is represented on this diagram by the line between points 1 and 2.
00:36 We already know that the intake stroke will result in a vacuum in the cylinder and we can see this in the diagram where the line drops below atmospheric pressure.
00:45 Once we reach point 2, we start the compression stroke.
00:49 You can see that the pressure in the cylinder smoothly increases as we move between point 2 and point 3 because the volume in the cylinder is reducing as the piston moves toward TDC.
01:00 At point 3, the spark plug ignites the charge and the fuel and air begins to combust.
01:06 This combustion results in a rapid expansion of the gases and the result is a quick increase in cylinder pressure as you can see in the graph between point 3 and point 4.
01:16 The amount of pressure produced here is the key to the amount of power that the engine can produce.
01:21 It's this cylinder pressure that acts on the top of the piston and is in turn transferred into the con rod and finally the crankshaft.
01:30 The more pressure we have available to act on the top of the piston, the more torque we can produce at the crankshaft and consequently, the more power our engine can produce.
01:40 Once the combustion begins, we move into the power stroke which is represented by the line between point 4 to point 5.
01:48 Here, the piston is moving back down the bore which results in a larger cylinder volume which produces a smooth reduction in cylinder pressure.
01:57 When we reach point 5, the piston is back at BDC and we can see there is a small amount of cylinder pressure caused by the hot exhaust gas.
02:06 When the exhaust valve opens, this pressure drops quickly to zero as the exhaust gas escapes into the exhaust system.
02:12 Between points 5 and 6, the piston returns to TDC, forcing out the remaining exhaust gases.
02:20 Now this is a good place to stop and talk about how the engine's compression ratio affects the shape of the pressure volume graph as well as the engine's performance in general.
02:30 All things being equal, a higher compression ratio will result in a higher peak cylinder pressure occurring and in turn, this means that the engine can produce more torque and power.
02:40 A less obvious result of increasing the compression ratio however, is that the expansion ratio, which is when the piston moves back down the cylinder during the power stroke, is also increased.
02:51 Let's look at another diagram now where we have two engines with different compression ratios.
02:57 For the sake of simplicity, we'll assume that despite these different compression ratios, the peak cylinder pressure is equal.
03:04 Let's assume our first engine has a compression ratio of 10:1 while the second engine has a compression ratio of 5:1.
03:13 What we find is that as the piston moves down the bore, the shape of the pressure graph looks very different between these two engines with the pressure dropping much faster in the engine with the higher compression ratio as the piston moves towards BDC.
03:27 This is because as the piston moves down the bore, the cylinder volume increases faster compared to the lower compression engine, giving the combustion gases more room to expand.
03:37 This has two additional effects.
03:41 Firstly, the exhaust gas temperature will be lower in an engine with a higher compression ratio.
03:46 This is because in order to reduce the temperature of the combustion gases, we need to allow them to expand.
03:52 Providing a larger expansion ratio achieves this aim and cools the combustion charge further than an engine with a lower compression ratio.
04:01 The other effect is that if we look at the pressure remaining in the cylinder when we do reach BDC, this is lower in the engine with the higher compression ratio.
04:10 This means that when the exhaust valves open, more of the cylinder pressure has been harnessed to provide power to the piston meaning less is wasted.
04:19 So let's recap this module and look at the key points I want you to remember.
04:24 Firstly, it's important to understand how the pressure in the cylinder increases smoothly during the compression stroke and then sharply as combustion takes place.
04:33 Remember, it's the peak cylinder pressure acting on the top of the piston that's responsible for creating torque at the crankshaft.
04:41 It's also important to understand how the compression ratio affects the peak pressure you'll see in the cylinder, as well as how the expansion ratio, which occurs during the power stroke, affects the exhaust temperature as well as the amount of power that can be extracted from the combustion gases.

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