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Ethanol & Flex Fuel Tuning: Cold Start Performance

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Cold Start Performance

08.06

00:00 - One of the largest problems with the use of ethanol fuels particularly in colder climates is with its cold start performance.
00:07 These cold start issues relate to the ethanol's lack of volatility or in plain english its ability to easily be vaporised.
00:17 Remember that in liquid form fuel isn't easy to combust, so for good cold start performance, a fuel's ability to be vaporised is important.
00:26 We've already discussed the different ethanol concentrations of pump E85 and how this will vary depending on the season.
00:34 Regardless of the volatility of E85, we're still going to need to make some changes over and above what we've already made to our fuel delivery in the cold start areas of our map, and we'll discuss these now.
00:48 In general the three main changes that we tend to need with ethanol based fuels is an increase to the cranking enrichment, the post start enrichment, and then the actual warm up enrichment.
01:01 Most ECUs will offer separate control of these three areas although they do tend to interact slightly.
01:09 We'll cover these in detail in the worked examples section of the course, however here we'll discuss broadly the approach to these tables.
01:18 Just like on gasoline it's not possible to properly calibrate the cold start side of your ethanol tune until the rest of the fuel delivery has been optimised.
01:28 The cold start tables typically work as a modifier to the main fuel table values and hence we need these to be correct first.
01:37 For this reason we address the cold start tuning after the main tune has been completed.
01:43 You also need to be realistic with your expectations.
01:46 If you live in an area where ambient temperatures regularly reach zero degrees centigrade or below, starting your engine with ethanol percentages over approximately 50% is likely to be challenging.
02:00 This after rule is one of the issues we face with ethanol.
02:05 Usually the start up operation will begin with cranking enrichment which is provided while the engine is deemed to be cranking on the starter motor.
02:14 During this area of the engine's operation the air speed is very low and additional fuel can improve the engine's ability to start quickly.
02:22 Insufficient fuel during crank will result in the engine cranking for an extended period before starting and it also may start roughly or not catch and fire at all.
02:34 On the other hand though excessive fuel here can result in the spark plugs becoming fouled and the engine failing to start.
02:42 The tricky part of tuning the cranking enrichment is that we can't check the air fuel ratio on a wide band meter and tune using this.
02:51 During cranking when the engine hasn't started, unburnt fuel and oxygen are passing through the exhaust system, so regardless how much additional fuel we add, the wide band is always going to read lean until the engine is actually running correctly on all cylinders.
03:07 It's really a case of starting the engine and testing the start up performance or how quickly and easily the engine starts, before making a change to the cranking enrichment and then testing again to see if the starting performance is better or worse.
03:23 Of course this is still the exact same technique we use when tuning the engine on gasoline.
03:30 Since we'll normally be starting our ethanol tuning with a calibration that is correct and optimised for gasoline, we need some idea of what the engine may need when we transition from gasoline to ethanol.
03:43 Through my own testing I found that a good place to start with our cranking enrichment on E85 for example, is to multiply the numbers for gasoline by a factor of 1.5 hence adding 50% to the cranking enrichment.
03:59 50% is actually unlikely to be enough of an increase to achieve clean starting on E85, however it's always better to start your tuning with less fuel, and slowly increase the enrichment rather than start with too much and risk fouling the spark plugs.
04:16 We can then test the start up performance and make some changes and assess the effects.
04:22 What I'm looking for is fast clean starting, although it's important to understand that even under warm ambient conditions the starting performance on E85 is usually a little slower than on gasoline.
04:35 So you need to be realistic about what can be achieved.
04:40 How I approach this is to test the start performance, make a change by adding an additional 10% to the cranking enrichment and then test again.
04:49 You should notice that as you get the enrichment in the ball park, the starting performance will improve.
04:55 It's not essential to have the cranking enrichment value 100% perfect.
05:00 And you'll find that the engine will start well over a reasonable range of cranking enrichment values.
05:06 To avoid the risk of fouling the spark plugs, I try to remain on the leaner side of the range of values that gives good performance.
05:14 Once the engine is deemed to be running, the cranking enrichment will be eliminated and the engine will move into a post start enrichment and then finally the warm up enrichment tables.
05:25 These table are a little easier to adjust because with both of these we have the opportunity to reference the measured air fuel ratio and make changes based on this.
05:36 Again we'll normally be starting with an engine that is tuned for gasoline, so we should have good post start and warm up enrichment tables that do an acceptable job on gasoline.
05:47 I usually find that the biggest required change for ethanol fuel is to the cranking and enrichment tables.
05:53 You may find that the post start and warm up enrichment tables need a small increase over what gave good results for gasoline, however I'll usually begin simply by applying the same gasoline post start and warm up tables across to the ethanol tables, and then optimise to suit.
06:12 Again the process of optimising these tables is identical to how we would deal with them on pump gasoline.
06:20 Achieving good cold start performance is difficult and time consuming regardless of the type of fuel you're tuning on.
06:27 As I mentioned, we can't hope to tune these tables until the rest of our tune is complete.
06:33 And then we need to let the engine completely cool before we can address the cold start tuning.
06:39 Depending on your ambient temperatures you may only get one to two opportunities per day to test the cold start performance as the engine will retain some amount of heat in the block for many hours after running.
06:53 If you're tuning a flex fuel system, you'll also need to define how these cold start parameters change with regard to ethanol content.
07:02 In this situation we'll usually have a set of cold start maps for gasoline, another set for ethanol, and then a blend table which defines how much of the ethanol cold start maps the ECU will use as the ethanol content changes.
07:17 This will ensure good starting performance as the ethanol content fluctuates.
07:23 Fortunately we can usually get acceptable results for the flex fuel system by simply performing a linear interpolation between these two tables as the ethanol content changes.
07:35 So at the end of this module, the key points to understand are that cold start performance can be problematic on higher blends of ethanol, and particularly if your ambient temperatures are very low, it may be impossible to achieve satisfactory cold start performance.