Understanding AFR: Special Circumstances
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Special Circumstances
03.26
| 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. |
| 03:19 | We can now move on and start applying this to the engine and actually tune it. |
