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Ethanol & Flex Fuel Tuning: Fuel Economy And Energy Content

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Fuel Economy And Energy Content

04.19

00:00 - Two of the common objections I hear to the use of E85 include its reduction in fuel economy along with the fact that ethanol has less energy content than gasoline.
00:11 Both of these points are correct, however they do need a little more examination before we can draw any real conclusions.
00:18 Firstly with regard to the fuel economy, yes we are definitely going to see a reduction here due to the fact we're burning more fuel to make the same amount of power.
00:28 Without getting too deep into the specific calculations right now, we're going to find that we'll end up requiring somewhere around about 40% more fuel volume when running on E85 compared to what we would use on gasoline.
00:44 This alone should be enough to tell you that your fuel economy is going to take a reasonable large hit.
00:50 For those of you who rate fuel consumption in litres per 100 kilometers, you will see this increase by approximately 35% to 40%, and if you prefer to use miles per gallon, then the fuel economy will reduce by somewhere around about 25%.
01:07 You can of course offset some of this reduction in economy by virtue of the fact that E85 is cheaper at the pump than regular gasoline.
01:15 But even after accounting for this, your overall costs are going to be higher when running on E85.
01:22 This should also tell you that you're not going to be able to drive as far on a full tank of E85 compared to what was achievable on pump gasoline.
01:32 We can now move on and deal with the energy content of the two fuels.
01:36 What I mean by energy content is the energy that's contained in a given mass of fuel.
01:42 This is referred to as the energy density of the fuel.
01:46 For gasoline the energy density is generally accepted to be around 46.4 megajoules per kilogram, while for E100 it's 26.4 megajoules per kilogram.
01:58 On this basis E100 contains a little more than half the energy of gasoline, and quite clearly we're much worse off.
02:07 This doesn't tell us the full story because we'll usually be running on a mix of ethanol and gasoline such as E85.
02:15 And then we also need to consider that to achieve a stoichiometric air fuel ratio on ethanol blends, we're burning much more of the fuel.
02:24 So let's see how that affects the overall energy content.
02:28 When we're dealing with a blend such as E85, we have a mix containing some ethanol and some gasoline, and hence the overall energy density is a combination of these two elements.
02:40 To complicate things somewhat, ethanol is blended by volume.
02:44 But when we're talking about an air fuel ratio, this is defined by mass.
02:49 We'll delve into these aspects a little further into the course, so don't worry too much.
02:54 For now all we need to know, is that the combined energy density of E85 works out to be approximately 29.9 megajoules per kilogram.
03:04 Now while 29.9 megajoules per kilogram is still much lower than that of gasoline, due to the different stoichiometric air fuel ratios of gasoline and E85, we find that we're burning around 49% more E85 by mass, compared to gasoline.
03:22 If we now take this into account, we'll find that we end up with a total energy content of around 44.9 megajoules per kilogram.
03:30 So yes E85 does have a lower energy content than gasoline.
03:35 But at the stoichiometric air fuel ratio it's only about 3%.
03:39 Which is much closer than what it would appear if you simply compare the energy contents of the two fuels.
03:47 The other aspect that's important to consider here though, is that we can often take advantage of the properties of E85 to optimise the ignition timing, and make more power and torque than what's possible on gasoline.
04:00 So the energy content of the fuel is just one consideration when it comes to ethanol fuel blends.