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Practical Diesel Tuning: Timing Calculators

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Timing Calculators

06.08

00:00 - In the diesel tuning aftermarket, timing calculators are often used to help tuners speed up the process of adjusting injection timing maps.
00:06 These are available built into some of the tuning softwares such as those produced by EFI Live.
00:11 For other tuning packages, you may need to resort to third party calculators developed by other enthusiasts.
00:17 In short, a timing calculator is a spreadsheet or piece of software that can be used to automatically generate new injection timing values based on the changes that you've made to injection pulse width.
00:28 How this works is that you'll copy your desired fuel quantity, main injector pulse width, fuel pressure target table and start of injection tables into the spreadsheet and in turn the spreadsheet will be able to output a table showing what percentage of the injection pulse width is to be delivered before top dead centre.
00:43 It can be interesting to do this on a stock calibration to gain some insight into what the OE calibration engineers have done.
00:49 However you need to remember that OE tuning is almost never focused on performance or economy and emissions are the main driver in their tuning decisions around injection timing.
00:59 Once you've made appropriate changes to your desired fuel quantity and fuel pressure targets, you can then have the spreadsheet generate you a new injection timing table based on your desired injection requirements.
01:08 For example, you can request to have 50% of the fuel delivered prior to top dead centre and the spreadsheet will do the heavy lifting and tell you what start of injection is required to achieve that aim.
01:19 You can then copy and paste your new injection timing table or part of it into your ECU editor.
01:24 In general terms, injecting more fuel before top dead centre can show an improvement in economy, efficiency, performance, lower exhaust gas temperatures, while injecting more fuel after top dead centre or retarding the timing, gives us less noise, and lower NOx emissions and more heat in the exhaust.
01:42 So there's a tradeoff here.
01:43 It's also important to consider that you won't necessarily want to target 50% of injection timing throughout the entire table.
01:49 And the injection timing can still be manipulated based on operating conditions.
01:54 Every engine is different, so it's difficult to provide absolute values for injection timing that can be applied to every engine.
02:00 There are also a few cautions around using timing calculators as the optimal timing depends on the injector pulse width and the engine load but also on the amount of heat that's available in the combustion chamber to begin the combustion process.
02:12 What I mean by this is that a 50% injection timing target on a low compression engine or one that's fitted with a large turbocharger can be problematic because the lower heat in the combustion chamber means there's more of a delay between when you start injection and when the combustion starts.
02:27 This can give a significant rattle with aggressive timing tables.
02:30 As usual, the best way to confirm what is best for your engine combination is to use a dyno or get in the vehicle.
02:37 It's also important to understand that timing changes on an emissions equipped engine can get you into trouble quickly.
02:42 Since advancing the injection timing will dramatically increase the NOx output which can trigger fault codes on engines fitted with NOx sensors.
02:50 If you're going to use a calculator, some common sense needs to be applied to ensure that the numbers that the calculator produces actually make sense.
02:57 We also need to manually confirm that the timing values aren't becoming dangerous since these calculators can often produce timing values of 40 degrees or more at high RPM with long pulse widths.
03:07 In this instance, some manual changes may be required to the areas where timing is overly optimistic.
03:12 Lastly, I'd recommend some smoothing of the numbers in these tables as it's important to ensure we don't have significant jumps or steps in the injection timing table.
03:21 On this note, even in a stock tune, we'll likely find the tables are a little irregular and they can benefit from some smoothing.
03:27 I always believe it's better to have an understanding of how something works, rather than blindly taking something like a timing calculator and trusting the results.
03:34 This separates us from those who really don't understand what's going on and means we're in a much better position to tune our engine safely.
03:41 On this note, we'll go through a quick example of how the calculator works.
03:45 In order to calculate what percentage of the fuel is injected prior to top dead centre, we need pieces of information.
03:51 The engine RPM, pulse width, and injection timing.
03:55 To do this, we need to start by looking at the requested fuel mass for a current operating point.
04:00 Let's take the stock calibration for the Colorado for example.
04:02 Let's assume we're operating at 1600 RPM and the requested torque is 560 newton metres.
04:09 We can see in this table that the fuel quantity will be 55 mm³.
04:12 Next we need to know what the fuel pressure will be at that same point.
04:17 We can see from our target fuel pressure table that for this point we're actually interpolating between two sites.
04:22 I'll spare you the math but the target fuel pressure works out to be about 109.5 mPa.
04:27 Which for simplicity I'll round to 110.
04:29 Now we can go to our main injector pulse width table and we are again interpolating between two zones.
04:34 Again, sparing you the math, this works out to be a pulse width of 1699 microseconds or 1.699 milliseconds.
04:42 Again, for simplicity I'll round this here to 1.7 milliseconds.
04:45 Now we know the injector pulse width and the engine RPM so we can calculate how many degrees of crank rotation this will take up.
04:52 If we divide 120 by the engine RPM this will give us the duration of the engine cycle at that RPM.
04:59 In this case, 120 divided by 1600 equals 0.075 seconds or 75 milliseconds.
05:05 So we can keep the units the same.
05:08 What we know is that there are 720 degrees of engine rotation in a single engine cycle.
05:12 So if we divide 720 by 75 we get 9.6.
05:15 This simply means that the crankshaft will rotate 9.6 degrees every millisecond.
05:21 Now that we know this, it's easy to calculate the crank rotation we'll see for a given pulse width.
05:27 In our example, we can multiply 9.6 by 1.7 which equals 16.3 degrees.
05:33 It's pretty easy now to work out where to start the injection based on our desired injection percentage.
05:38 For example if we want to inject half the fuel prior to top dead centre, we would need to multiply our injection duration of 16.3 by 0.5 which would give us the result of 8.15.
05:50 This means we need to start our injection event 8.15 degrees before top dead centre.
05:54 If we actually check the stock timing map for this operating point we can see that all of the injection is occurring after top dead centre.