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I have converted my car to LPG, and i've been tuning it today with a wideband lambda controller. It runs pretty okay, but i'm not yet satisfied.

The weird thing is, it accelerated best when the lambda value was tuned to about 0.78. That figure seems a bit too rich for me, so i'm wondering whether the measurement is off. People on the internet claim it doesn't matter for the oxygen sensor what fuel is used, but I beg to differ.

To my understanding, a wideband lambda sensor consists of a (narrowband-like) oxygen sensor and an oxygen pump. The pump is used to keep the lambda value inside the oxygen sensor at 1, and the needed electrical current for that, is translated to the actual lambda value of the exhaust gases.

My point is, LPG has a stoich ratio of about 15.5, and petrol 14.7. So LPG needs more oxygen. If then the oxygen excess or shortage is translated to a lambda value, that value may be different for LPG than for petrol. After all, every LPG 'molecule' needs more oxygen molecules to react with than a petrol 'molecule'. So the same pump current and same oxygen supply can compensate less LPG than it could compensate petrol.

Simplistically thinking, i'd say the actual lambda value given by the sensor differs as much as the stoich ratio of both fuel differs; 15.5/14.7 = 1.054. That means my measured lambda value of 0.78 translates to 1.054*0.78 = 0.82. And that's roughly the best ratio to maintain for maximum power, for petrol at least. I see this as a confirmation of my little theory, but i'm curious if there's someone else that can disprove or confirm it.

  • What “oxygen pump”? – Solar Mike Jul 3 at 18:16
  • @SolarMike It's an integral part of the wide band sensor system. Maybe "Oxygen pump" isn't the perfect name for it, but it is an electromechanical pump and its purpose is to keep the O2 concentration at the measurement point constant. The wide band lambda value is then calculated from the flow rate through the pump. – alephzero Jul 3 at 18:55
  • @alephzero so there is an oxygen separator or it is air that is pumped through? – Solar Mike Jul 3 at 19:19
  • @SolarMike It's a membrane that 'pumps' oxygen from outside air in and out the internals of the sensor. Only the oxygen from the air gets through. Bosch themselves refers to it as pumping, (they talk about pumping current and such) so I thougt i'd stick with that terminology. – Bart Jul 7 at 10:15

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