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Ethanol & Flex Fuel Tuning: Fuel Characteristics

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Fuel Characteristics

12.33

00:00 - To better understand what we need to change with our tuning to suit an ethanol blend of fuel, we need to understand the differences in the actual fuel properties.
00:09 These differences affect the amount of fuel that we need to deliver to the engine in order to maintain a consistent air fuel ratio.
00:17 I've already mentioned that when we're switching from gasoline to E85, we'll need to add around 40% more fuel by volume.
00:25 But now we're going to look at why.
00:28 The first difference is that ethanol has a vastly different stoichiometric air fuel ratio than pump gasoline.
00:35 The stoichiometric air fuel ratio defines the mass of air and fuel that we need to combine in order to achieve theoretically complete combustion.
00:45 In other words the mass of fuel that we need to add to the mass of inlet air on order to completely combust all of the available components.
00:54 When we talk about pump gasoline it has a stoichiometric air fuel ratio of 14.7:1 This means that for every gram of fuel, we need 14.7 grams of air to achieve complete combustion.
01:08 If we consider pure ethanol though, things are very different.
01:12 Pure ethanol has a stoichiometric air fuel ratio of 9:1 which means that now for every one gram of fuel, we need to supply nine grams of air.
01:23 Let's reverse things though because when we're tuning the ECU, it's not the air that we're controlling, it's the fuel delivery.
01:31 The different stoichiometric air fuel ratio, means that when we swap from gasoline to ethanol, we need to inject a much larger volume of fuel, to achieve the specific target air fuel ratio.
01:44 We need to also consider that the stoichiometric air fuel ratio will vary with the ethanol content of the fuel.
01:52 We've looked at pure gasoline with a stoichiometric air fuel ratio of 14.7:1 and ethanol at 9:1 however as the ethanol content moves from 0% to 100% the stoichiometric air fuel ratio also changes between these two limits.
02:09 To complicate matters though, while the ethanol blend is defined on the basis of volume, when it comes to the air fuel ratio, it's the mass of the fuel and air that's important.
02:21 Let's discuss fuel density and then we can come back to the stoichiometric air fuel ratio.
02:27 The concept of mass versus volume is a tricky one but in simple terms, if we have two containers of a fixed volume, and we fill one with feathers and the other with sand, most people can understand that the container filled with sand would weigh more.
02:43 This is because sand is denser than feathers.
02:47 In order to convert from volume to mass we need to take into account the density of the fuel.
02:53 And this is another important fuel characteristic that differs between ethanol and gasoline.
02:59 The density defines how much a given volume of fuel will weigh.
03:04 The density of gasoline is approximately 0.739 kilograms per litre, whereas the density of pure ethanol is approximately 0.787 kilograms per litre.
03:16 This means that if we had one litre of each fuel, ethanol would weigh more.
03:21 To add a further complexity to this, the density of the fuel will also vary with the fuel temperature.
03:29 The densities that I just mentioned, are measured at 25 degrees centigrade.
03:33 But as the fuel heats up or cools down its density varies too but not necessarily at the same rate.
03:41 For example gasoline has a fuel temperature coefficient of 0.0009 kilograms per litre per degree centigrade.
03:51 This means that for every degree that the temperature increases, one litre of gasoline would decrease in mass by 0.0009 kilograms, which is about a 10th of a percent.
04:05 Now that might not sound particularly relevant, but when you consider that the fuel temperature can easily fluctuate by as much as 60 to 70 degrees centigrade, this would affect the fuel mass by around 7%.
04:19 In comparison to gasoline, pure ethanol's density varies slightly more as temperature changes, with a temperature coefficient of approximately 0.001 kilograms per litre per degree centigrade.
04:33 This means that the mass of a specific volume of ethanol will vary slightly more than that of gasoline given the same temperature change.
04:43 So now that we've learned about the fuel density, we can take this into account to see how the stoichiometric air fuel ratio changes as the ethanol blend fluctuates.
04:54 Let's consider E85 which is probably the most common ethanol blend we're likely to use.
05:01 E85 as we know is a blend consisting of 85% ethanol and 15% gasoline by volume.
05:09 In order to find the stoichiometric air fuel ratio for this blend though, we need to work out what this is as a mass ratio.
05:19 To do this let's assume we have one litre of E85 fuel.
05:23 To account for the fuel density, we can multiply 0.85 which is the volume of the ethanol in one litre, by the density of ethanol, which we know is 0.787 kilograms per litre.
05:37 This gives us a mass of 0.669 kilograms of ethanol in one litre of E85.
05:45 Now if we take the remaining 0.15 litres which consists of gasoline and multiply this by the density of gasoline, which is 0.739 kilograms per litre, we get a mass of 0.111 kilograms.
06:02 Now if we add these two masses together we can find the overall density of one litre of E85.
06:09 0.669 kilograms plus 0.111 kilograms gives us a total mass of 0.78 This means that the density of E85 is 0.78 kilograms per litre.
06:26 Now that we know the total mass and the mass of ethanol we can work out the mass ratio by dividing the mass of ethanol by the total mass.
06:35 This is exactly the same as how we work out the ethanol ratio except now we're using mass instead of volume.
06:42 In this case 0.669 kilograms divided by 0.78 kilograms, equals 0.858 which can be expressed as 85.8% This means that a blend of 85% ethanol by volume, or E85 as we would refer to it, contains a blend of 85.8% ethanol by mass.
07:07 So you can see there's a subtle but real difference between the volume blend of E85 and the mass blend.
07:14 Now let's take the stoichiometric air fuel ratio of ethanol which is 9.0:1 and multiply this by the mass blend we just worked out of 0.858 This gives us a value of 7.72 Now if we do the same for the gasoline component we need to multiply 14.7 by the remaining percentage, which is one minus 0.858 or in other words 0.142 This gives us a value of 2.09 If we now add 2.09 and 7.7 we get a final value of 9.81:1 This is our stoichiometric air fuel ratio for a blend of 85% ethanol by volume.
08:02 If the process we've just worked through here is a little too much for you, I've attached an Excel spreadsheet to this module, which will allow you to see how these calculations work in more detail, and to quickly work out the stoichiometric air fuel ratio for any ethanol blend.
08:20 If we look at the difference between the fuels based solely on their stoichiometric air fuel ratios, this means that when we move from gasoline to E85, we need to supply approximately 49% more fuel by mass.
08:35 When we take into consideration the higher density of ethanol though we find that the additional volume of fuel the injectors need to deliver isn't quite as high as our mass calculation works out.
08:47 Accounting for this, we'll find that the additional volume of fuel that we need to supply as we move from gasoline to E85 is actually closer to 40%.
08:57 To throw one last spanner into the works though, we'll also find that a given fuel injector tends to flow a slightly reduced volume of fuel on E85, in comparison to gasoline.
09:10 This is because the two fuels have a different viscosity.
09:13 In general this effect is most often ignored in tuning flex fuel vehicles, although some ECUs do offer the ability to trim the injector flow as the ethanol concentration changes.
09:25 The problem is that we currently don't have easy access to this sort of data, which makes it hard to incorporate.
09:32 So what is the relevance of all this? And how much do you actually need to understand? The aim of this course is to give you the correct information so that you understand how the fuel you're using will affect your tuning.
09:47 The actual application of this data will depend on the ECU you're tuning and how much control you have over the fuel delivery.
09:55 For example many ECUs completely disregard all of the fuel characteristics entirely, and we simply define how much more fuel the ECU is to deliver as the ethanol content changes.
10:08 This many sound quite a simplistic approach given the characteristics I've just mentioned, however in reality this method still does a perfectly good job.
10:18 An alternative option is to consider the actual stoichiometric air fuel ratio of the fuel relative to ethanol content.
10:26 And some ECUs go as far as to define how each aspect of the fuel changes with ethanol content.
10:33 The difference with these approaches, is that if we ignore the fuel's characteristics, then we need to do all of the work of telling the ECU how to vary the injected fuel volume as the ethanol content changes.
10:45 And this will require two completely separate and very different fuel maps.
10:50 On the other hand if the ECU knows what the fuel characteristics are, it can do most of the hard work in the background and the resulting air fuel ratio should be relatively consistent as ethanol content changes without the need for independent maps.
11:06 The reality is that even ECUs that properly account for the changing fuel characteristics will still see some error creep in as the content changes, and for this reason you'll still often need a trim table to correct any errors that the main fuel equation doesn't completely account for.
11:25 Remember at this point we're only focusing on the air fuel ratio and there are also aspects such as the ignition timing, cold start calibration, and boost control to consider too.
11:37 But we'll look at that a little further into the course.
11:41 As I've already mentioned you don't need to be a chemist in order to tune an engine on ethanol blended fuels.
11:48 Everything I've just discussed really is just there to explain why we need to inject so much more fuel when we switch from gasoline to ethanol, which is one of the key aspects of ethanol or flex fuel tuning, and hence is important to understand.
12:05 The key points to take away from this module, are that the stoichiometric air fuel ratio and fuel density both vary as the ethanol content of the fuel changes.