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EFI Tuning Fundamentals: Injector Deadtime

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Injector Deadtime

08.21

00:00 - Fuel injectors are obviously a pretty vital part to the performance of any EFI system.
00:05 But I believe they're also one of the least understood.
00:08 In this module, we're going to have a look at the injector in a bit more detail and find out what we need to know in order to be able to do a better job of tuning the engine.
00:17 You may have heard the term dead time used by tuners or injector suppliers, and in this module, we'll find out exactly what that term means.
00:26 Since a fuel injector is a mechanical device, it can't respond instantly to the signal from the ECU.
00:34 The internal valve has some mass to it, so there's also some inertia to overcome before the injector can open and start flowing fuel.
00:41 Likewise, there's also a slight delay between the ECU signalling the injector and the injector actually closing and cutting off the fuel delivery.
00:50 You can think of the dead time as the difference between the length of time the ECU signals the injector to open and the time the injector is actually flowing fuel.
01:01 This dead time is very small, perhaps less than one millisecond, but it's an important part of the injector operation and it can have a big impact on our tuning.
01:10 So we need to understand it and know how to account for it.
01:14 Depending on the ECU you're tuning, this lag or delay may be referred to as injector dead time, injector latency, injector battery compensation or offset.
01:25 The name isn't as important as the concept though, and in my experience, this is the part of tuning that 90% of tuners just simply don't understand.
01:34 The graph you can see here shows us the flow from an Injector Dynamics ID1000 injector under various battery voltages and pulsewidths.
01:44 The interesting part of this graph is that the injectors won't actually start flowing anything until they're supplied with a pulsewidth of around one millisecond at 16 volts.
01:54 If the battery voltage drops down to eight volts, then it takes over three milliseconds before the injector can actually start supplying fuel.
02:02 Now first of all, this shows the effect of varying battery voltage on the injector operation.
02:08 If you can remember back to the module on electronic fundamentals, I said that voltage is our electrical pressure, and this shows that electrical pressure in action.
02:17 A higher voltage offers more electrical pressure to get the injector open.
02:22 As the battery voltage drops, we have less voltage available to drive the injector, so the dead time increases.
02:28 While the injector dead time varies with battery voltage, it will also depend on the amount of fuel pressure too.
02:34 This is because the fuel pressure is working against the internal valve in the injector, forcing it closed.
02:40 So as the fuel pressure increases, it becomes harder to open the injector.
02:45 If the fuel pressure has increased too far, the injector may actually not open at all.
02:50 While we've just discussed that the injector dead time is affected by fuel pressure, it's actually the differential fuel pressure that's the important part, so let's talk about what that term means.
03:01 If you think of the way the injector is installed in the engine, we have the fuel rail on one side of the injector, and in most cases, the other end of the injector is going to be mounted in the inlet manifold.
03:13 This means that we have fuel pressure acting on one side of the injector, and we have manifold pressure acting on the other.
03:20 What we really need to know is the differential pressure acting on the injector, as this is the critical part when it comes to how much fuel it can flow as well as the dead time values.
03:31 The differential pressure is simply the difference between the pressure in the fuel rail and the air pressure in the inlet manifold.
03:38 So now that we know our injectors don't operate instantly, and this delay will also depend on battery voltage and differential fuel pressure, let's look at how this affects our tuning.
03:50 Many tuners feel that this doesn't matter since they can account for this delay when they're tuning the ECU.
03:56 They change the numbers in the fuel map until they get the right air fuel ratio, then they don't need to worry about injector dead time.
04:03 While this can get the engine tuned, it's going to result in an unstable tune and an air fuel ratio that may fluctuate.
04:11 Let's look at an example to illustrate this.
04:13 Here we have two ECUs, one is accounting for the injector dead time while the other isn't.
04:19 Let's say that we need two milliseconds of injector opening time to achieve an air/fuel ratio of 14.7:1, and the injector has a dead time of one millisecond at 14 volts.
04:31 If we have an ECU that is correctly accounting for dead time, the ECU will provide a pulsewidth of three milliseconds.
04:38 This includes the actual two milliseconds pulsewidth we need, as well as one millisecond injector dead time.
04:44 If we were tuning an ECU with no dead time compensation, we would need to enter an injector pulsewidth of three milliseconds in the fuel table, even though the end result is that the injector is only opening for two milliseconds.
04:58 Now let's consider what happens when the ECU applies a 10% correction due to a change in air temperature.
05:04 If the ECU is accounting for dead time, then the 10% correction is only applied to the amount of time the injector is actually open, which is two milliseconds.
05:14 So the correction will be 10% of two, which is 0.2.
05:18 If the ECU wasn't accounting for dead time though, the 10% correction will be applied to the entire injection pulsewidth of three milliseconds, which would result in a correction of 0.3 milliseconds.
05:30 You can see that when a compensation is applied on an ECU without dead time compensation, the correction won't work correctly.
05:38 In this example, the additional fuel supplied by the ECU with no dead time compensation is 50% greater than what we actually needed.
05:46 The result is that the ECU can't properly maintain a stable air/fuel ratio.
05:51 The next problem comes when the battery voltage fluctuates.
05:54 Going back to the graph, we can see that between 16 volts and eight volts, the dead time varies by almost two milliseconds.
06:02 If the ECU isn't compensating for injector dead time, the air fuel ratio will fluctuate wildly as electrical load varies.
06:10 This can happen when you switch on the headlights or the engine fan turns on, for example.
06:15 One aspect that isn't always obvious is that inaccuracies in dead time compensation will be most apparent at idle and low load when the injector pulsewidth is shortest.
06:25 This is because at this point, the injector dead time is a large percentage of the overall pulsewidth being sent to the injector.
06:32 So any error will have a large effect on the amount of fuel being delivered.
06:37 At full throttle and high RPM, the dead time has much less effect as it becomes a smaller percentage of the total injector pulsewidth.
06:44 What this means is that if your injector dead time isn't correct, it will be more obvious at idle and low load and have less effect at wide-open throttle and high RPM.
06:55 If you do make changes to the injector dead time after the engine has been tuned, you can also expect to see it affect the air fuel ratio, which will require the tune to be corrected.
07:05 For the reasons I've just mentioned, the effect on the air fuel ratio will be most noticeable at low injector pulsewidths and less significant at high injector pulsewidths.
07:15 Now that we know what dead time means and how it can affect the tune.
07:19 The question remains as to what we can do about it.
07:21 First up, we need an ECU that can properly compensate for injector dead time values.
07:27 These days, every good aftermarket ECU includes dead time tables, so this shouldn't be a problem The next hurdle though is getting the data to enter into these tables.
07:38 This isn't the sort of data we as tuners can accurately generate ourselves, so we need to rely on the injector supplier to provide it.
07:46 This is one reason why you should buy from a reputable supplier who can offer this data at the time of purchase.
07:53 If you have an unknown set of injectors or you're using the factory injectors fitted to an engine, then you may be able to have the injectors tested and characterised by a fuel injector specialist.
08:03 So to summarise, the concept of injector dead time is important.