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Practical Diesel Tuning: Injection Timing

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Injection Timing

07.48

00:00 - The third part of the fuel delivery system that we need to consider when tuning is the injection timing.
00:05 As we discussed in the diesel tuning fundamentals course, the injection timing has a major effect on the engine operation as this will dictate where in the engine cycle combustion begins.
00:14 It's likely that there may be three or more separate injection events during a single engine cycle.
00:20 However what we'll be focusing on for this module is the timing of the main injection event where the majority of the fuel is delivered into the combustion chamber.
00:27 Once the fuel is injected into the combustion chamber, there's a brief delay while the fuel vapourises and mixes with the air before the combustion process begins.
00:35 In this way, the injection timing has the effect of defining where in the engine cycle combustion will commence.
00:40 In many ways the injection timing in a diesel engine has many similarities with the injection ignition timing in a gasoline engine.
00:47 What we're trying to do is select an injection timing so that we can achieve peak cylinder pressure at a point in the engine cycle where we can take maximum mechanical advantage from it acting on the piston crown, being transferred down through the conrod into the crankshaft.
01:01 When we're discussing diesel combustion, we'll often be studying the rate of heat release during the engine cycle and what we're trying to achieve here is a 50% burn of the delivered fuel at a point somewhere a little after top dead centre in the engine cycle.
01:15 If we can achieve this aim then we will produce maximum possible torque from the engine for a quantity of fuel delivered.
01:21 With 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.
01:28 In general terms, we're likely to see timing numbers anywhere from several degrees after top dead centre to 20 plus degrees before top dead centre.
01:37 Since the optimal timing will depend on the volume of fuel being delivered and the engine speed, we'll need a three dimensional table defining how injection timing changes with regard to those parameters.
01:48 For an example of what this looks like, here's a stock injection timing table from a GM L5P Duramax engine.
01:53 You can see that the axis of this table is engine RPM and fuel quantity.
01:58 In particular we can see that the general trends in this stock map are that as engine speed increases, the timing is advanced or in other words, the injection starts earlier in the engine cycle.
02:09 Likewise as the amount of fuel delivered increases, we see the same trend.
02:13 Let's consider a single injection event.
02:15 We start with the ECU signalling the injector to open.
02:19 It's important to remember that since the injector's a mechanical device, there's always some latency involved with the injector actually opening and the flow of fuel beginning.
02:27 After the fuel flow is initiated, a portion of the injected fuel will be atomised and then become superheated and vaporised by the high cylinder temperature.
02:35 One of the aspects that's important to keep in mind is that there's an interaction between injection timing, fuel pressure and fuel pulse width and each parameter affects the other.
02:44 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.
02:53 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.
03:03 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 injection pulse width if we want to achieve the same volume of fuel.
03:13 So in this case with more fuel pressure and a shorter injector pulse width, this has the effect of delivering fuel more quickly and in turn the 50% burn point is advanced.
03:23 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 alter the injection timing to achieve optimal performance.
03:37 As we've just discussed, with a typical injection timing table, we see a trend where the engine RPM increases, the start of injection is advanced.
03:45 Or in other words, occurs earlier in the engine cycle.
03:47 Let's see why that's the case.
03:50 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.
04:03 For example, if we're at 1000 RPM, the cycle time will be 120 divided by 1000 which equals 0.12 seconds.
04:11 When dealing with small time frames like this, in an engine, we tend to talk in milliseconds and 0.12 seconds is the equivalent of 120 milliseconds since there are 1000 milliseconds in one second.
04:23 Now if we make the same calculation at 4000 RPM, we find that the cycle time is 120 divided by 4000 which equals 30 milliseconds.
04:32 So this shows that at 1000 RPM a full cycle of the engine takes 120 milliseconds.
04:36 But when the increase the RPM to 4000, it now only takes 30 milliseconds.
04:41 Let's look at that in relation to crankshaft degrees though to really make these numbers meaningful.
04:47 There are 720 degrees in a full cycle so at 1000 RPM, the crankshaft is rotating six degrees every millisecond.
04:55 At 4000 RPM though, the crankshaft now turns through 24 degrees per millisecond.
05:01 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 the 50% burn at the same time point.
05:15 Of course, this again is a simplified view of the process but hopefully it explains why we see injection timing advance with engine RPM.
05:23 We also need to consider that if we are fitting a larger set of fuel injectors then this will also have an impact on optimal injection timing.
05:30 This is because for a given fuel quantity and injection pressure, a larger injector will be able to deliver this fuel in a shorter duration.
05:37 This has the effect of advancing the 50% burn point in the engine cycle.
05:41 Injection timing also has a serious impact on emissions at the tailpipe along with noise, vibration and harshness.
05:47 So from an OE standpoint, injection timing is very much a compromise and neither performance or economy are really on the top of their list.
05:55 What this means for us is that there's potential gains in engine performance by altering the injection timing.
06:00 Particularly if you live in a part of the world where emissions compliance isn't a consideration or you're in a competitive atmosphere.
06:06 Of course, injection timing does need to be approached with some care since advancing the timing too far can end up creating excessive cylinder pressure which can easily damage your engine.
06:16 Broadly speaking, advancing injection timing may provide an increase in power and torque while reducing EGTs.
06:22 However this will also come at the expense of an increase in NOx emissions.
06:26 Modern emissions equipped diesel engines are fitted with NOx sensors in the exhaust so making changes to injection timing, particularly in the cruise and light load areas of the map is likely to trigger a fault code from the ECU.
06:38 Advancing the injection timing may also come at the expense of an increase in diesel combustion noise which probably isn't desirable.
06:46 I'd be out of line if I said that every engine will get best results by advancing the timing by two degrees.
06:51 Every engine is different and we need to test and find out what our particular engine wants and this is where a quality dyno really is essential.
06:58 I'll give you a rough rule of thumb for setting maximum timing, you can take the pulse width and divide by 100 to get a reasonable starting point for the maximum injection timing at 3000 RPM at high load.
07:10 If you have a 2000 microsecond tune then we can expect the maximum timing at 3000 RPM under heavy load to be about 20 degrees.
07:18 This will give you a good starting spot.
07:21 It's important to mention that excessive injection timing can be dangerous.
07:24 Similar to excessive exhaust gas temperature.
07:27 It's difficult to give a specific maximum injection timing that will be safe for every application.