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Ethanol & Flex Fuel Tuning: Blend Tables

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Blend Tables


00:00 - We've now discussed the key aspects that we're likely to need to change in a flex fuel system as the ethanol content fluctuates.
00:07 However we need to discuss exactly how this can be achieved.
00:12 Typically this is accomplished using what's referred to as a blend table.
00:17 What these blend tables do is define how the ECU will interpolate between a table tuned for pump gas, and a table tuned for ethanol as the ethanol content varies.
00:28 These blend tables may be included for fuel, ignition, boost, and cold start corrections.
00:35 A blend table will typically be a two dimensional table with ethanol content as the axis value.
00:43 The numbers in this table represent the blend amount which will vary from 0% to 100%, and we need to understand what these numbers mean.
00:52 For an example let's assume we're talking about a single cell in the ignition table and we've tuned table one for pump gas, and we have a value of 10 degrees.
01:03 We've then tuned table two for E100 and we've found the optimal timing was 20 degrees.
01:09 The blend table defines which value the ECU will apply or in other words how the ECU will interpolate between the values in these two tables.
01:20 With a value of 0% in our blend table, the ECU will use all of the value from the first ignition table, in this case 10 degrees.
01:30 On the other hand with a blend value of 100% the ECU would use all of the value from the second table which is 20 degrees.
01:39 The slightly trickier part is when we use blend values between 0% and 100% where the ECU will interpolate between the values in the two ignition tables.
01:50 For example if our blend value was now 50% the ECU would output an ignition timing value halfway between the value in each table, which in this case would be 15 degrees.
02:03 So we can understand how this blend value will work, let's look at how the ECU calculates the final value.
02:11 You'll remember our two ignition values are 10 degrees and 20 degrees.
02:16 The difference here is 20 minus 10 which is 10 degrees.
02:21 That's the amount the ECU can manipulate the ignition timing.
02:26 We can vary our blend table from zero to 100, which is a range of 100.
02:31 So let's divide 10 by 100 to find out how much the ignition will vary as we move the blend value by 1% 10 divided by 100 equals 0.1 This means as we increase the blend value from zero, each one percent change will increase the timing by 0.1 degrees.
02:53 Now if we have a blend value of 35% we can simply multiply 35 by 0.1 and we find that the ECU will add an additional 3.5 degrees which will give a total ignition advance of 13.5 degrees.
03:09 This is just the 10 degree value we had in the first ignition table, plus the 3.5 degrees that we've just calculated.
03:16 This principle remains the same regardless what the blend table is blending for.
03:21 Ignition, fuel, or boost for example.
03:25 The table is simply defining how much of the pump gas table or the ethanol table value will be used.
03:32 The advantage of using a blend table like this, is that we don't necessarily need to move from one table to the other in a liner way as the ethanol content changes.
03:45 For example we know already that the biggest improvement in knock resistance from ethanol will come as we move from E0 through to approximately E40.
03:55 This means that if we want optimal performance from the engine, we'll benefit from blending quite aggressively from the pump gas ignition map towards the ethanol ignition map, as the ethanol content increases from E0 to E40.
04:10 Above E40 it's likely we'll still want to increase the ignition timing, but not so dramatically, so the blend numbers would increase more slowly.
04:19 This will give us an ignition blend table that may look something like this.
04:26 When the ethanol content is zero, we have a blend value of 0% which means we're running solely on the pump gas ignition table.
04:35 This blend value jumps up to 70% by the time the ethanol content has reached 40%.
04:42 And then we see a linear increase up to a blend value of 100% at the point where the ethanol content reaches 80.
04:50 This means that any time the ethanol content is at 80% or above, the ECU is solely using the second ignition table values.
05:00 That example is just one way of using the blend tables and you can use them to manipulate the values as you like.
05:08 Another example would be to provide a much more conservative approach to the ignition blend, which would look something like this.
05:16 Here you can see that the blend value remains at 0% until the ethanol content reaches 20.
05:23 Above 70% ethanol the blend value is at 100% and we have a liner interpolation between these sites.
05:31 We can deal with the blend tables for boost control in a similar fashion.
05:35 However where it comes to the fuel blend table, this is a little bit different.
05:41 In this case we're talking about a situation where there are separate fuel tables for pump gas and ethanol in order to maintain a specific lambda target.
05:51 Theory would suggest that since the fuel properties vary in a linear fashion, we could just blend from one table to the other linearly as the ethanol content varies.
06:02 However we often find in the course of testing that this doesn't entirely hold true.