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Understanding AFR: Full Load N/A - Transition F/I

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Full Load N/A - Transition F/I


00:00 The next area to focus on is the zone labelled High Load N/A which is also the transition zone if you have a forced induction engine.
00:09 Let’s deal with a naturally aspirated car first.
00:13 This area of the map corresponds to 100 kPa which is the maximum load you are able to achieve in naturally aspirated form.
00:22 If the engine is operating here, it means the driver is at full throttle and applying maximum load to the engine so we need to target a suitable AFR to achieve maximum power and control combustion temperature.
00:35 After tuning thousands of naturally aspirated engines, I find that most will respond well with a target AFR of 0.90 lambda.
00:45 This isn’t set in stone and a little later we will look at how to evaluate how the engine’s performance changes when we alter the AFR to find out exactly what the engine wants to see for the best results.
00:59 Its important to understand with a naturally aspirated engine that at high RPM, the intake, air filter and throttle body may become restrictive.
01:09 This can have the engine vacuum drop down to perhaps 90-95 kPa at high RPM.
01:16 We need to account for this by ensuring that the AFR target is sufficiently rich as low as perhaps 85-100 kPa.
01:25 If we are dealing with an engine using a turbo or supercharger, we will access this same area of the load map at part throttle instead of full throttle.
01:34 This is why we call it the transition area, as we pass through it as the engine transitions up onto boost.
01:41 Since we can operate the engine in this area at only part throttle, the load on the engine isn’t necessarily that high, even though we may be at 100 kPa.
01:52 This is just one instance where we can see that manifold pressure doesn’t always correlate directly to the amount of load on the engine.
02:00 We still want to move the target AFR richer in this area than the lambda 1.00 we were using in the cruise area, but we don’t need to be as rich in this area as we were in a naturally aspirated engine because the load isn’t so great.
02:14 I generally aim for lambda 0.95 around the 100 kPa region as this gives a good compromise between power, economy and smooth response.

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