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Diesel Tuning Fundamentals: Injection Timing

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Injection Timing

06.38

00:00 - One of the most critical aspects of common rail diesel engine operation is the timing of the fuel injection events.
00:06 As we've already discussed, it's likely that there may be three or more separate injection events during a single engine cycle, and these need to be very accurately timed and controlled.
00:17 What we will be focusing on for this module however is the timing of the main injection event where the majority of the fuel is delivered into the combustion chamber.
00:26 Remember that once the fuel is injected there's a brief delay while the fuel vaporises and mixes with the air before the combustion process begins.
00:35 In this was the injection timing has the effect of defining where in the engine cycle the combustion will commence.
00:42 In a round about way the injection timing in a diesel engine has many similarities to the ignition timing in a gasoline engine.
00:50 What we're trying to do is select an injection timing so that we achieve peak cylinder pressure at a point in the engine cycle where we can take maximum mechanical advantage from it acting on the top of the piston crown and being transferred down through the conrod and finally into the crankshaft.
01:08 You'll remember that when discussing diesel combustion we'll often be studying the rate of heat released during the engine cycle, and what we're trying to achieve here is a 50% burn of the delivered fuel at a point a little after top dead centre in the engine cycle.
01:23 If we can achieve this aim then we'll achieve maximum torque from the engine, from the volume of fuel being delivered.
01:30 When we're talking about diesel injection timing we're going to be referencing the point when the injection first starts, which is referred to as SOI or start of injection.
01:42 In general terms for the main fuel injection event, this is likely to be in the vicinity of perhaps 10 to 15 degrees before TDC.
01:50 It's important to understand however that the injection timing is going to depend on both engine RPM and load, so we'll have a three dimensional table defining how the injection timing changes with regard to these parameters.
02:05 For an example of what this looks like, here's the stock injection timing table from GM Colorado.
02:11 You can see that the axes of this table are engine RPM and fuel quantity.
02:16 In particular we can see that the general trends in this stock map are that as the engine speed increases, the timing is advanced, or in other words, the injection starts earlier in the engine cycle.
02:28 Likewise as the amount of fuel delivered increases we also see the same trend.
02:33 If we consider a single injection event, we start with the ECU signalling the injector to open.
02:40 It's important to remember that since the injector is a mechanical device, there's always going to be some latency involved with the injector actually opening and the flow of fuel beginning.
02:51 After the fuel flow is initiated a portion of the injected fuel will be atomised and then becomes super heated and vaporised by the high cylinder temperature.
03:01 One of the aspects that's important to keep in mind is that there is a complex interaction between the injection timing, fuel pressure, and fuel pulse width.
03:09 And each of these parameters affects the others.
03:12 For example if we fix the injection timing and we double the injector pulse width, this will have the effect of injecting fuel for more of the engine cycle.
03:21 In turn, the combustion process takes longer to occur and the 50% burn point occurs later in the engine cycle which may be detrimental to engine performance.
03:32 If on the other hand we leave the injection timing fixed and we double the fuel pressure, this will mean that we need a shorter injector pulse width, if we want to achieve the same volume of fuel.
03:44 So in this case with more fuel pressure and a shorter injector pulse width, this has the effect of delivering the fuel more quickly and in turn the 50% burn point is advanced.
03:54 Hopefully you can see from these examples that it's not enough to consider your injection timing in isolation and as you alter the volume of fuel delivered and the injection pressure, you're inevitably going to need to also alter the injection timing to achieve optimal performance.
04:11 As we've just discussed with a typical injection timing table, we see a trend where as the engine RPM increases the start of injection is advanced or in other words occurs earlier in the engine cycle.
04:23 Let's see why that's the case.
04:25 A really handy calculation to always keep in the back of your mind is that we can quickly calculate the cycle time of the engine or in other words how long the engine takes to complete one full cycle by dividing 120 by the current RPM.
04:42 For example if we're at 1000 RPM, the cycle time would be 120 divided by 1000 which equals 0.12 seconds.
04:51 When dealing with small timeframes like this in an engine we tend to talk in milliseconds and 0.12 second is the equivalent of 120 milliseconds, since there are 1000 milliseconds in one second.
05:06 Now if we make the same calculation at 4000 RPM, we'll find that the cycle time is 120 divided by 4000 which equals 0.03 seconds which is also the equivalent of 30 milliseconds.
05:20 So this shows that at 1000 RPM, a full cycle of the engine takes 120 milliseconds, but when we increase the RPM up to 4000, it now only takes 30 milliseconds.
05:32 Let's look at that in relation to crankshaft degrees though to really make these numbers meaningful.
05:39 There are 720 degrees in a full cycle so at 1000 RPM the crankshaft is rotating six degrees every millisecond.
05:48 At 4000 RPM though the crankshaft now turns through 24 degrees per millisecond.
05:54 The reason we've just gone through these calculations is that if we consider that the injector latency and ignition delay remain relatively fixed, then we're going to need to start the injection event earlier as the engine RPM increases if we want to maintain a 50% burn a little beyond TDC.
06:12 Of course this again is a very simplified view of the process, but hopefully it explains why we see the injection timing advance with engine RPM.