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Diesel Tuning Fundamentals: Diesel Combustion Process

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Diesel Combustion Process


00:00 - Now that we have a broad understanding of how the diesel engine operates, we need to get into a little more detail, specifically around the combustion process and how that proceeds.
00:10 What we're going to do is investigate the actual combustion process or in other words, look at what happens inside the combustion chamber from the time the fuel is injected.
00:19 One of the key operating principals behind a common rail diesel engine is that the fuel system operates at incredibly high pressure.
00:27 It's not uncommon for example for the fuel pressure to be in the region of 220 megapascals which is about 32000 psi for those who like to work in imperial units.
00:39 This fuel pressure is required for several reasons.
00:42 First of all the fuel is being injected at or very close to TDC, which is where the pressure inside of the cylinder will be close to its peak.
00:50 Forgetting for the moment about the result of the combustion that's about to begin.
00:55 In order to be able to inject fuel when the injector opens we need a large pressure differential across the injector or in plain english we need the fuel pressure to be much higher than the cylinder pressure.
01:07 The next consideration is that we only have a relatively small window in which to inject whatever volume of fuel we've decided the engine needs for its current operating point.
01:17 In order to get the required fuel into the engine quickly and not have the injector open for an excessive amount of time, we again need very high fuel pressure.
01:26 The effect of fuel pressure on the injector flow can be quite dramatic in a diesel engine where the fuel pressure can vary from less than 40 megapascals to greater than 200, and in approximate terms, if we double the fuel pressure we'll see the injector flow increase by around 40%.
01:45 Looking at this another way, if we double the fuel pressure, this would mean that the injector needs to be open for about 30% less time in order to deliver the same volume of fuel.
01:57 In the process of tuning a diesel engine it's not essential for you to make these calculations, so I don't want you to worry too much about them.
02:05 Rather than the specifics, it's the concept that's important for you to be able to understand.
02:11 The last consideration is that for the most efficient combustion, we ideally want very finely atomised droplets of fuel to be delivered into the combustion chamber.
02:21 The smaller the individual fuel droplets are, the easier they'll vaporise and the faster ignition of the fuel will occur.
02:28 Using very high fuel pressure, coupled with small holes in the injector nozzle, helps achieve this aim.
02:35 An important concept that's used when we're analysing the combustion in a diesel engine is the rate of heat release.
02:41 This is a method of analysing how the energy of the fuel is released as the combustion progresses.
02:48 As the fuel is combusted, this in turn affects the pressure inside the cylinder, which is what acts on the top of the piston and produces torque at the crankshaft.
02:58 One of the key metrics with diesel combustion is the point in the engine cycle where we've achieved 50% burn, or 50% heat release, and this has a significant impact on the engine's performance.
03:11 The rate of heat release isn't directly measured but rather is calculated from the pressure inside the cylinder.
03:18 Unless we're working in a well instrumented laboratory, it's unlikely that we'll be in a position to measure this ourselves, however it's important to understand this concept as it explains some of the fundamental aspects of the diesel engine's characteristics.
03:33 So with this in mind, let's look at the actual process that occurs inside the cylinder.
03:40 On the graph you can see here we've plotted the rate of heat release inside the combustion chamber relative to the position in the engine cycle.
03:48 We have the injection event shown between the point SOI, which stands for start of injection, and EOI which stands for end of injection.
03:57 We have several stages of the combustion process noted on this graph, and we're going to talk about each one in turn.
04:05 As the piston nears TDC, the fuel injector is opened by the ECU and the high pressure fuel will be delivered into the combustion chamber.
04:14 As the atomised fuel droplets enter the cylinder, there's a delay of several crankshaft degrees of rotation before the combustion process begins.
04:23 This is known as the ignition delay.
04:26 During the ignition delay we have both physical and chemical processes which are occurring simultaneously.
04:33 The physical processes consist of the liquid fuel being atomised, and then the atomised fuel droplets going through a phase change from liquid to vapour due to the high heat in the combustion chamber.
04:45 While the chemical processes on the other hand include pre combustion reactions between the fuel, air and residual gas mixture in the cylinder.
04:54 After vaporising and mixing with the air, combustion begins since the temperature in the combustion chamber is above the auto ignition point of the diesel fuel.
05:04 The actual ignition delay period varies from one engine to the next, as well as also being dependent on the load and RPM the engine's operating at, but it's likely to be in the region of 0.4 through to 1.0 milliseconds for a modern turbocharged diesel engine.
05:21 Once the combustion process is initiated, we move into the premixed or rapid combustion phase which is where combustion of the fuel that's already mixed with air during the ignition delay period, burns rapidly.
05:34 This results in a large and rapid increase in the heat release inside the cylinder over just a few degrees of crankshaft rotation which consequently also results in a sharp rise in cylinder pressure.
05:46 If the amount of fuel mixing with air during the ignition delay period is large enough, the resultant pressure spike is what causes the distinctive knocking sound that diesel engines are well known for.
05:58 Once the fuel and air that is premixed during the ignition delay period has being consumed, we move into the mixing control combustion phase, where the combustion rate is controlled by the rate at which the mixture becomes available for burning.
06:12 There are several processes involved here, however the rate of combustion during this phase is mainly controlled by the mixing process of the fuel vapour, and air.
06:22 This results in a more gradual and controlled heat release and hence pressure inside the cylinder.
06:28 Heat release may reach a second peak during this phase that's lower than the peak reached during the premixed combustion phase, and then the heat release will decrease as the phase continues.
06:38 The final stage of the diesel combustion process is the late phase combustion where heat release continues but at a lower rate into the expansion stroke.
06:47 There are several reasons for this late combustion phase.
06:50 A small fraction of the injected fuel may not have been combusted so far, and some of the fuel energy is also present in the form of soot and fuel rich combustion products and these can still be released in this late stage of the combustion process.
07:06 One of the aims with tuning a diesel engine is to control the combustion process in order to achieve a 50% burn point a little after the piston has moved past top dead centre.
07:18 If we can achieve this then we'll also achieve peak torque from the engine.
07:22 In a way you could liken this to the process of optimising the ignition timing in a gasoline engine.

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