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Understanding AFR: Lean Tuning

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Lean Tuning


00:00 When it comes to analysing engine failures, one of the more common problems you will tend to see comes from melted or damaged pistons.
00:08 In most situations the cause is put down to a lean mixture resulting in a melted piston.
00:14 Now while a lean mixture certainly can result in a melted piston, more often the damage is actually misdiagnosed detonation.
00:23 As we have already seen, a lean mixture will result in a higher combustion temperature.
00:29 This effects all of the components in the engine. The piston, cylinder head, valves and spark plugs all run hotter with a lean mixture.
00:38 The most critical element effect though in my experience is on the knock threshold of the engine.
00:44 The higher combustion temperature can tip the tune over the edge and result in detonation which is more often the real culprit.
00:53 The severe pressure spikes and turbulence of detonation can strip away the boundary layer of gasses that protect the piston from the full heat of combustion.
01:03 Light detonation can result in a slight sandblasted appearance on the piston crown, but in severe situations the crown and side of the piston can be melted away entirely.
01:15 Given the appearance, its easy to see how this damage can often be diagnosed as the result of a lean mixture.
01:23 The reality is that with a proper understanding of the requirements, it is possible to design and tune an engine to run reliably with what would be considered a very lean air fuel ratio.
01:36 This requires special attention to the engine components.
01:40 In particular increased piston to bore clearances and a thicker piston crown are necessary to handle the extra heat and expansion.
01:49 Wider valve seats are also essential to help transfer heat back out of the exhaust valves and into the cylinder head.
01:57 Finally we would need to ensure that the ignition timing is set correctly to avoid detonation.
02:04 We have already seen however that running the engine at excessively lean AFRs will actually reduce engine power, so in general there isn’t going to be much desire to tune here.
02:15 One situation when this could be an advantage though, is in an endurance application where the reduced power can perhaps be traded off against reduced fuel usage.
02:27 The point of this module is for you to understand that approaching tuning with a single number fixed in your mind as a target lambda can be limiting and potentially dangerous.
02:39 We need to investigate and experiment to find out what the engine likes and where it will run best and to do this, we need to have a good understanding of how adjusting the air fuel ratio will effect the engine’s operation and our tuning.

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