Understanding AFR: Lean Tuning
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Lean Tuning
02.56
| 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. |
