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Old 29th November 2020, 10:11   #19
Mike Noc
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Rover 75 CDT Manual Connoisseur SE, Rover 75 CDT Automatic Connoisseur SE & a Freelander Td4.

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Always a friendly discussion with you Jon,

Yes diesel fuel does mix with air to burn, but not as in a petrol engine where the air and fuel is precisely measured, mixed and then ignited.

With a diesel the injected fuel mixes and ignites with air as it is injected - there is loads of excess air left if the engine is idling when very little fuel is injected and the same amount of air as would be there if you were injecting the maximum amount of fuel.

For this reason you can't run a diesel engine too lean - at idle they can happily run with an air fuel ratio of 120 to 1. Try doing that with a petrol engine.

That's also why you can run with the MAF disconnected too - on a petrol engine you need a precise measurement of the mass of air for the mixture, but on a diesel you just need to ensure that you have enough air available as you can never have too much.
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