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Practical Diesel Tuning: Fuel Pressure

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Fuel Pressure


00:00 - Increasing the injector pulse width in order to supply more fuel to the engine is effective to a point.
00:05 However there are also some downsides that can result from excessive pulse width.
00:08 Increasing the pulse width at a given engine speed means of course that the injector will be open for more degrees of crankshaft duration and we can introduce issues if the injection begins too early in the engine cycle or extends too far into the power stroke.
00:22 If we retain the original start of injection point and extend the pulse width, then we'll be injecting further into the power stroke which can potentially result in more power but it also increases exhaust gas temperature, exhaust smoke and other tail pipe emissions.
00:37 Advancing the injection timing to counteract this or starting the injection event earlier in the engine cycle, has its own set of issues.
00:43 This will create cylinder pressure earlier in the engine cycle and if we end up creating cylinder pressure too high on the compression stroke or too soon, it can become counter productive, working against the piston.
00:55 In high output engines this may also exceed the mechanical strength of the components, resulting in connecting rod compression or even head gasket failure.
01:02 This isn't to say that we can't make changes to the injector pulse width.
01:05 However, there's a finite window we have to work within which will limit how much additional fuel we can supply for a given injector size.
01:13 All is not lost, we still have one more variable here we can control to effect fuel delivery and this is fuel pressure.
01:20 With the high pressure fuel pumps fitted to modern diesel engines, we have a lot of flexibility when it comes to fuel pressure.
01:25 We can control the fuel pressure with respect to both engine RPM and required fuel quantity.
01:30 The maximum fuel pressure will depend on the specific pump fitted to your engine.
01:34 It's not uncommon for fuel pressures to exceed 200 mPa or approximately 29,000 psi.
01:40 The effect of fuel pressure on fuel delivery through the injector is that as we raise fuel pressure, we'll deliver more fuel for a given pulse width.
01:48 This is exactly the same effect we see in a low pressure port injected gasoline engine but the pressures we're dealing with here are obviously significantly higher.
01:56 We can see this in effect if we look at a stock injector pulse width table for a 6.6 litre GM Duramax.
02:02 If we want to deliver a fuel volume of 55 mm³ at a fuel pressure of 50 mPa, then we'll need a pulse width of 1912 microseconds.
02:11 If we raise the fuel pressure to 100 mPa, then we can deliver the same fuel mass with a pulse width of 1203 microseconds.
02:18 And if we double this pressure to 200 mPa, the pulse width only needs to be 788 microseconds.
02:25 As an approximate guide, you can expect that if you double the fuel pressure, you'll see the fuel volume for a given pulse width increase by approximately 40%.
02:33 We can often use fuel pressure to our advantage at higher load where we may have sufficient headroom with the factory fuel pump to allow a moderate increase in fuel pressure.
02:41 This can allow us to inject more fuel while maintaining reasonable injector pulse widths.
02:45 If course generally we would increase both fuel pressure and pulse width but by increasing the fuel pressure, we can add more fuel than would be possible if we were to just manipulate the injector pulse width table on its own.
02:56 The tricky part when it comes to raising the fuel pressure is that not all fuel systems are created equal.
03:02 And while it's reasonable to expect that any factory system will have some built in headroom in terms of range of pressure you can work with above the stock set point, we can't for example say that we should always target 200 mPa or 29,000 psi on every engine.
03:17 What we need to do first is find out where the factory set point is.
03:19 We can do this pretty simply by logging the fuel pressure or looking at the fuel pressure targets in the stock ECU.
03:26 In reality logging gives us more information as particularly if you already have some problems with the fuel system, it may be that your fuel pressure isn't hitting the desired target in which case it's unlikely that you're going to have much luck commanding more fuel pressure.
03:41 We also need to understand that when it comes to high pressure fuel systems, flow and pressure are closely related.
03:46 What I mean by this is that we may have a stock tune that's comfortably able to maintain a fuel pressure target of 200 mPa under wide open throttle operation.
03:54 If the same pump was now supplying a heavily modified engine with larger injectors, providing a lot more fuel volume, then we may see the fuel pressure drop away.
04:04 This is because in order for the pump to make a specific fuel pressure, it needs to be capable of pumping a greater volume of fuel than the engine is consuming.
04:10 As soon as there is no longer an oversupply of fuel, the pressure will begin to drop.
04:15 So this is something you'll need to watch out for and although it's unlikely to be an issue on a stock or lightly modified engine, it's still important to log the truck and see.
04:23 Next I want to jump into relief valves.
04:25 The high pressure fuel system is fitted with a pressure relief valve or spiller valve that's designed as a safety mechanism for the fuel system to prevent excessive that may potentially result in component failures.
04:35 The set point for this relief valve will ultimately limit the maximum pressure you can run.
04:40 However if your engine is fitted with a mechanical relief valve, you don't actually want the pressure valve to open under normal circumstances.
04:45 When a mechanical relief valve opens it will result in a significant drop in fuel pressure rather than just nicely capping the pressure at the relief valve set point.
04:54 When the fuel pressure drops like this under wide open throttle operation, the injector pulse width will be increased dramatically in order to maintain the commanded fuel rate and this results in the injection event being extended well past TDC where it can result in excessive heat and smoke output.
05:10 These mechanical relief valves also require a substantial decrease in commanded fuel pressure before they can shut again and resume normal operation.
05:18 So we need to be mindful of the pressure relief valve and make sure that our fuel pressure targets leave sufficient head room that we aren't in danger of accidentally opening the relief valve.
05:27 We'll know this is happening in our datalogging because we'll see a sharp drop in fuel pressure from our target.
05:32 Example would be going from 23,000 pounds of rail pressure, straight down to 14,000 pounds of rail pressure.
05:40 It's a very characteristic drop of a relief valve opening.
05:43 Many modern diesel engines now use an electronic relief valve which offers a lot more control by the ECU.
05:48 In this sort of valve, there's no real issue with running the fuel pressure right on the point that the relief valve operates.
05:54 In this sort of fuel system, the fuel pressure is controlled by both an electronic pressure regulator and then the electronic relief valve.
06:01 Both of which are operated by the ECU via PID control algorithm.
06:04 If you're dealing with a popular engine where there's a solid knowledge base on enthusiast forums, then this is probably a pretty good place to start to find out what others are running in terms of fuel pressure.
06:13 Or at least find out where the limits of the factory fuel pump may be.
06:16 Beyond this, trial and error is the next best option, but of course we want to approach this carefully and log our results.
06:23 I suggest making small adjustments of approximately 5 mPa or 725 psi for each iteration of adjustments so you can assess the effects and make sure the pump is able to keep up with your demands.
06:35 When we're looking at the fuel pressure target table, we'll find we have a three dimensional table with RPM on one axis and fuel quantity on the other.
06:42 This is important because as we can see on this sample table, the fuel pressure isn't fixed, rather it changes as the fuel quantity requirements change.
06:49 This is because we have a relatively narrow range of injector pulse width we can work with.
06:54 But of course our fuel requirements at idle are dramatically different to what we need at wide open throttle.
07:00 In this example we can see that in the idle and cruise areas of the table, the fuel pressure targets are relatively low, only in the region of 30 - 60 mPa.
07:09 On the other hand, at higher RPM when the requested fuel quantity is in the region of 120 - 130 mm³, we can see the target increases to 200 mPa.
07:19 When we're making changes to the target fuel pressure, we're primarily going to be focusing on the areas we're operating when fuel demand is at its highest and we can largely ignore the rest of this table.
07:30 The reason for this is if we make large changes to the fuel pressure in the low fuel demand regions of this table, we're unlikely to see any advantage from doing so.
07:38 And in fact the ECU may struggle to control the injectors and provide an accurate fuel quantity.
07:44 It also takes more power from the engine to drive the mechanical fuel pump when it's producing more pressure so we're costing ourselves some engine performance as a result.
07:52 Finally as we increase the fuel pressure at light loads, we're likely to end up with more noise, vibration and harshness from the combustion process.
07:59 What I suggest doing is logging your fuel quantity versus engine RPM during full throttle operation.
08:04 So you can highlight the regions of the fuel pressure table that you're operating in.
08:07 You can then make changes to these areas only.
08:09 Of course we also want to blend those or smooth those into the completed table so we don't have any unrealistic steps in it.
08:16 As we can see here, it's likely you may have a range of pressure target tables available based on altitude and operating mode.
08:22 So it's advisable to make the same magnitude of changes to all the available maps.
08:26 The last point to cover here is that when we're targeting increased fuel pressure, you also need to be aware of any potential limits on fuel pressure in the ECU.
08:34 These can easily override any changes you're making in the target fuel pressure tables and leave you wondering why you're not getting the additional fuel pressure you're commanding.