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Understanding AFR: Special Circumstances

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Special Circumstances


00:00 There are a few situations where the numbers I’ve given so far won’t be suitable, particularly under full load.
00:06 It’s important to understand your particular engine so you can apply a sensible AFR.
00:13 Let’s discuss that in some more detail with a few examples.
00:18 First lets assume we have a naturally aspirated engine that has been turbocharged.
00:23 In this situation, often the peak boost will be kept quite low due to a high static compression, and possibly to limit the amount of stress placed on the engine components.
00:34 If for example we were running this sort of engine at 0.5 bar or 7 psi boost, I would still choose to use a relatively rich AFR of around 0.80 to help control combustion temperature in an engine not designed for boost.
00:51 Secondly, if you are tuning an engine fitted with a catalytic convertor in the exhaust, we do need to be a little careful about our target AFR, as the cat may be damaged if the exhaust temperature is too high.
01:05 In this situation, I will address this by targeting a mixture 0.02-0.03 lambda richer at high RPM.
01:14 I will use the numbers we have already discussed up until about two thirds of the engine rev limit and then smoothly taper the mixture richer towards redline.
01:24 This is the best we can hope to achieve without directly monitoring the exhaust gas temperature during operation.
01:33 Another example which is hopefully uncommon, would be a turbocharged engine fitted with no intercooler.
01:39 In this case, the intake air temp is likely to be very high and this can push the engine closer to detonation.
01:47 In this situation I would choose to run the engine richer to help reduce combustion temperature.
01:54 The actual AFR the engine responds to best will need to be found on the dyno, but it would not be uncommon to run as rich as 0.75 lambda.
02:05 Lastly we have been dealing solely with piston engines, and if you are tuning a rotary engine, we need to be aiming slightly richer than the targets we have discussed so far.
02:16 At wide open throttle, a naturally aspirated rotary engine will perform well with a target lambda around 0.85-0.87 and in the transition area, a turbocharged rotary would want to see a lambda around 0.90.
02:33 At medium boost we would want to be targeting around 0.78 lambda, and at high boost approximately 0.75 lambda.
02:44 You can see from this that you can’t just choose a fixed number for the lambda that you will tune your engine to.
02:50 You need to understand how the AFR effects the way the engine runs, and how the particular components on the engine may effect its requirements.
03:00 This will give you the knowledge to make informed decisions on what to do.
03:05 So by the end of this section you should understand the different operational areas of the engine and how to address each one.
03:14 You should also understand how to select a suitable AFR.