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Diesel Tuning Fundamentals: Fuel Pressure

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Fuel Pressure


00:00 - As we discussed in the module on injection timing, there's a significant interaction between fuel pressure and fuel injector pulse width which is important when it comes to understanding the complete diesel combustion process.
00:13 In particular when it comes to improving or increasing the output from a typical diesel engine, one of the key changes we need to make is to increase the fuel volume being delivered to the cylinder.
00:24 One of the obvious ways of doing this is to increase the injector pulse width so that the injector is simply open for longer and more fuel can be supplied.
00:33 This is effective to a point however it also has the effect of extending the combustion time as it takes longer for the fuel to be delivered and hence combust.
00:43 This also has the effect of delaying the 50% burn to a point later in the engine cycle.
00:51 The net effect is that we may not end up realising the performance gain that the extra fuel volume should have offered.
00:58 We'll also see a higher exhaust gas temperature as a result of the combustion process continuing later in the engine cycle, and we may also end up with exhaust smoke being produced.
01:10 We can combat a lot of this by raising the fuel pressure which in a common rail diesel engine is often easy to do within reason.
01:19 By raising the fuel pressure we have the ability to now supply more fuel through the injectors for a given pulse width.
01:25 In plain english we can now supply more fuel volume without the need to alter our injection pulse width or at least we can minimise the increase in pulse width that will be required.
01:36 In this way fuel pressure is our friend when it comes to improving diesel engine performance and we want to be very careful that we use it to our advantage.
01:47 In general we want to use as much fuel pressure under high load operation as we can within the bounds of the fuel system's capability.
01:55 You will remember that in the module on fuel system fundamentals we learned that the common rail system is fitted with a pressure relief valve which essentially provides the upper limit for the fuel pressure.
02:06 That being said we'll often find that once this valve opens, the fuel flow must be reduced significantly before it will close again.
02:14 This means that we want to retain a margin between our target fuel pressure and the relief valve at all times.
02:21 When it comes to fuel pressure, we're going to normally have a three dimensional target table relative to fuel volume or load and engine speed that the ECU will use to provide closed loop control of the high pressure fuel pump in order to achieve the desired pressure.
02:38 It's important to understand that as the required fuel volume increases, so too will our target fuel pressure.
02:45 At low fuel volumes though we will need less fuel pressure.
02:49 This is because we're only asking it for a small volume of fuel and this requires a short pulse width.
02:56 As the fuel pressure increases, the pulse width in turn becomes smaller, and we can get to a point where the injector doesn't open reliably and the volume of fuel delivered becomes inconsistent and unreliable.
03:08 By reducing the fuel pressure in these low fuel volume areas of the target table, we increase the injector pulse width back into a range where the injector can operate reliably.
03:19 If we're dealing with a factory calibration, the general procedure is to leave the low volume fuel pressure targets largely untouched and focus instead on raising the fuel pressure in the high load high fuel volume areas of the target table close to the maximum pressure that the fuel system can sustain.
03:36 A word of caution however is that in some instances increasing the fuel pressure will result in the fuel spray impinging on the bowl of the piston which can result in poor fuel mixing and soot formation and potentially even lead to cracking of the piston.
03:53 In the aftermarket it's common to fit modified or upgraded fuel pumps and relief valves to allow the fuel pressure to reach levels that weren't achievable in the stock system.
04:04 This needs to be considered on a case by case basis and will depend on the specific engine that you're dealing with.
04:11 In some rare instances we may actually see a decrease in performance as the fuel pressure is increased as a result of the parasitic losses involved in driving the mechanical fuel pump.
04:22 In other words, as we aim for higher fuel pressure, the pump needs to work harder and the pump ultimately is driven by the engine so the power loss associated with driving the pump is increased.
04:35 We'll now have a look at a practical demonstration to help reinforce the relationship between fuel pressure and injection pulse width.

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