Does an oxygen sensor's lambda value represent the same when running on LPG?

Question

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.

Answer 1

I don't think this is entirely correct.

You have to understand that lambda sensors operate post-combustion oxygen amounts.

They don't care at all about the post-combustion hydrocarbon amount, or whatever the relative amounts were pre-combustion.

The oxygen amount is heavily dependent on the stoichiometry of the gases in the cylinder. If there's exactly the amount of oxygen needed for combustion, you get a particular oxygen amount in the exhaust. If there's little bit too much oxygen, you get a massive increase in the oxygen amounts in the exhaust. If there's little bit too little oxygen, you get a very slight decrease in the oxygen amounts in the exhaust.

You want to be operating near the knee of the curve, the portion where increase in oxygen amounts pre-combustion would cause a massive increase in the oxygen amounts in the exhaust. Most likely the engine is operating under closed loop control, where it increases the oxygen amounts (i.e. decrease fuel amounts) in intake until it sees that massive increase of oxygen amount in exhaust, then back off and repeat, ad infinitum.

What could cause an optimal lambda sensor reading to differ could be flame temperature. A different flame temperature could cause a difference in the amount of oxygen that is bound to nitrogen oxides. Usually in modern cars there's a catalytic converter, so this means the correct oxygen amount post-cat is the amount where all oxygen is used for burning all hydrocarbons exactly and no oxygen is left (because it could be bound to nitrogen in that case). Pre-cat, however, the correct reading could differ based on the flame temperature. If the flame is very hot, then more oxygen is combined with nitrogen, so the optimal amount of oxygen left as diatomic oxygen is smaller.

However, most likely to achieve what you want, i.e. optimal performance on LPG, you should either reprogram the ECU or swap the ECU to one that has been designed for LPG right from the start.

Note that the oxygen sensor isn't reading the lambda value. It's reading the oxygen amount in the exhaust. The lambda value is then somehow calculated in the ECU based on injected fuel amounts and the amount of oxygen that's bound to fuel. Unless the injectors have been specifically designed to inject an amount of fuel that needs exactly same amount of oxygen as gasoline injected would need with the same control signal, it's possible the ECU calculation for lambda value is off due to the fact the oxygen sensor isn't measuring lambda but it's measuring oxygen amounts.

For acceleration, rich mixture could be good but for emissions it definitely ain't good. But it doesn't mean the mixture is rich, it's possible the ECU is calculating the lambda value incorrectly because the injectors look different to it.