Volvo Owners Club Forum - View Single Post - LPG running lean

capt jack · Jan 24th, 2014, 13:55

I'm no LPG expert, but I think I disagree with James a bit about a non-OEM O2 sensor.

The stoichiometric mix for LPG is different from that that for petrol. For LPG it's 15.8 :1, for petrol 14.7:1.

Remember that the throttle pedal on a fuel injected engine works by controlling the amount of air taken in, not the flow of fuel.

The O2 sensor works by conducting a continuous catalytic reaction in the exhaust gases, and a voltage is generated by the reaction. This is measured by the monitoring the voltage generated by the catalytic reaction in around the O2 sensor. The voltage generated is related to the amount of oxygen present. So if the O2 sensor transmits a voltage change to the ECU then the ECU assumes that there's either too much oxygen (ie: lean) or too little (ie: rich) mixing with the fuel. There is a degree to which the fuel mixture is adjusted by the ECU to maintain the correct oxygen level in the exhaust stream.

It does this by adjusting the amount of air, not the amount of fuel. The fuel is constant. That is why the air measurement sensors - MAPs and MAFs etc are so critical

If the ECU reaches the limit of the air intake adjustment without correcting the oxygen level it will ping the lambda light and the code will be recorded.

The LPG installer sets up the LPG system to provide the same amount of fuel at any given time as would the petrol system, allowing for the difference in the stoichiometrics, so that all the car's ECU has to think about is managing the air intake. If the LPG supply isn't matched to what the ECU thinks that the petrol supply should be then the O2 sensor will ping the lambda light because it can't adjust the air intake enough to compensate for the perceived lack/oversupply of fuel.

So the system relies not only on the correct amount of air and fuel being delivered, but also on the O2 sensor working properly.

Also critical are the plugs - which have to be exactly right to ensure the fuel is burnt properly.

So the best LPG set-up for the engine is one which most closely matches the petrol set-up.

On that basis the best O2 sensor surely has to be the right one for the car running on petrol, as the LPG set-up is referenced to that. In that way, if the LPG mixture ain't right the car will quickly let you know by running badly at idle and under load, and by triggering the EML and a DTC.

Interesting debate, and I'd be happy to be proved wrong if I am!

Cheers

Jack

Jan 24th, 2014, 13:55	#8
capt jack VOC Member Last Online: Jun 3rd, 2024 21:22 Join Date: Jun 2006 Location: Selby, North Yorkshire	I'm no LPG expert, but I think I disagree with James a bit about a non-OEM O2 sensor. The stoichiometric mix for LPG is different from that that for petrol. For LPG it's 15.8 :1, for petrol 14.7:1. Remember that the throttle pedal on a fuel injected engine works by controlling the amount of air taken in, not the flow of fuel. The O2 sensor works by conducting a continuous catalytic reaction in the exhaust gases, and a voltage is generated by the reaction. This is measured by the monitoring the voltage generated by the catalytic reaction in around the O2 sensor. The voltage generated is related to the amount of oxygen present. So if the O2 sensor transmits a voltage change to the ECU then the ECU assumes that there's either too much oxygen (ie: lean) or too little (ie: rich) mixing with the fuel. There is a degree to which the fuel mixture is adjusted by the ECU to maintain the correct oxygen level in the exhaust stream. It does this by adjusting the amount of air, not the amount of fuel. The fuel is constant. That is why the air measurement sensors - MAPs and MAFs etc are so critical If the ECU reaches the limit of the air intake adjustment without correcting the oxygen level it will ping the lambda light and the code will be recorded. The LPG installer sets up the LPG system to provide the same amount of fuel at any given time as would the petrol system, allowing for the difference in the stoichiometrics, so that all the car's ECU has to think about is managing the air intake. If the LPG supply isn't matched to what the ECU thinks that the petrol supply should be then the O2 sensor will ping the lambda light because it can't adjust the air intake enough to compensate for the perceived lack/oversupply of fuel. So the system relies not only on the correct amount of air and fuel being delivered, but also on the O2 sensor working properly. Also critical are the plugs - which have to be exactly right to ensure the fuel is burnt properly. So the best LPG set-up for the engine is one which most closely matches the petrol set-up. On that basis the best O2 sensor surely has to be the right one for the car running on petrol, as the LPG set-up is referenced to that. In that way, if the LPG mixture ain't right the car will quickly let you know by running badly at idle and under load, and by triggering the EML and a DTC. Interesting debate, and I'd be happy to be proved wrong if I am! Cheers Jack Last edited by capt jack; Jan 24th, 2014 at 13:59.