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Ethanol & Flex Fuel Tuning: Boost Pressure

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Boost Pressure

05.15

00:00 - While we don't necessarily need to change boost pressure when tuning on ethanol based fuel, increasing the boost pressure is one of the areas we can normally expect to see large gains when moving from gasoline to ethanol.
00:13 Increasing the boost ties in very closely with the ignition timing that we've already discussed.
00:19 When tuning a turbo charged engine on gasoline, we normally find that we'll get to a point where as we increase the boost pressure, the additional heat and pressure inside the combustion chamber causes the engine to detonate or knock.
00:33 In turn this requires the ignition timing to be retarded to prevent the detonation occuring.
00:39 We then go around in a circle adding boost and reducing timing while often not making any more actual power.
00:46 This tends to define the maximum practical boost and power level that we can expect from a particular engine and turbo charger on gasoline.
00:56 Ethanol's properties lend itself perfectly to a turbo charged engine, and this is why its use is so prevalent in these applications.
01:05 Their high octane rating and the cooling properties of the fuel, when compared to gasoline is perfect for any engine running high boost pressure, a high compression ratio, or potentially both.
01:18 The higher octane rating means that the fuel is much less susceptible to knock and is more stable in the face of higher combustion temperature and pressure that we'll see at high boost pressures.
01:30 As well as the higher octane rating of ethanol we also benefit in a turbo charged application from the cooling effect that the fuel has on the combustion charge.
01:40 As we've discussed earlier the fuel's latent heat of evaporation means that it absorbs a lot of energy in the form of heat from the combustion charge when it undergoes a phase change from liquid to vapour.
01:53 The result is that the combustion charge temperature is greatly reduced in comparison to what we could expect to achieve on gasoline.
02:01 When we talk about knock or detonation, it's the heat in the combustion chamber primarily, that's responsible for causing this.
02:09 The heat comes from two sources.
02:11 As we compress a gas its temperature will naturally increase and hence when we either raise the compression ratio or increase the mass of fuel and air that we're compressing, which occurs when we raise the boost pressure, the combustion temperature will rise.
02:28 Also when we raise the boost pressure, the same thing occurs in the turbocharger.
02:32 As the turbo compresses the intake air, the air temperature increases.
02:37 The amount the air temperature is increased will depend on the ambient air temperature, the boost pressure, and the efficiency of the turbo charger.
02:46 So if you're pushing a small turbo charger very hard at high boost pressures, it's likely to be putting a lot of extra heat into the intake charge.
02:56 Usually this increase in air temperature will be dealt with to some degree by an intercooler.
03:01 However these are still only effective to a point.
03:05 And if we raise the air temperature going into the intercooler, we can still expect an increase in the outlet temperature.
03:12 It's the combined increase in air charge temperature, along with the increased mass of fuel and air being compressed in the cylinder, that will ultimately lead to a higher combustion temperature and the likelihood of detonation.
03:26 The cooling effect of the ethanol fuel helps reduce the combustion charge temperature and in turn moves the engine away from detonation.
03:35 This means that we can safely increase the boost level with much less concern for detonation.
03:41 Again I'll point out that detonation is still possible on ethanol fuels, so we do need to approach optimising the boost and ignition timing with care, and ensure that we're using quality knock detection equipment to properly detect detonation.
03:57 If we're tuning for an ethanol fuel using dual maps, we'd simply use a different boost target for the gasoline map and the ethanol map.
04:06 If we're tuning a flex fuel system however we'll typically have a gasoline boost target, an ethanol boost target, and then a blend table that defines how much boost the ECU should target as the ethanol content fluctuates.
04:21 Again we'll discuss how we can approach tuning this sort of system in the worked examples.
04:27 The important part to take away from this module is that ethanol fuels will typically allow much higher boost levels than what we could safely tune to on pump gasoline.
04:38 This is one of the key reasons why ethanol fuels are so popular on turbo charged engines.
04:44 It should go without saying here that we do need to give some consideration for the strength of the engine components and be sure that they're up to the task before really leaning on an engine and tuning for ethanol.
04:57 It's not unheard of to double the amount of power the engine was producing on gasoline as we move to an ethanol blend such as E85.