Propane Ignition Characteristics

[pbloore]

Hi All,

I'm new to this forum having found it by accident on a web search. I've been interested in LPG as a fuel since converting my V8 Land Rover Discovery 18 months ago. I've been having a lot of problems with (I think) the ignition system. Above approx 4000 RPM the lambda sensor switches lean and stays there casuing control problems with the fuelling. I have two theories currently

1) this is caused by the spark being too weak and causing a partial burn resulting in residual oxygen in the exhaust. This could be due to the spark advance unit I've fitted reducing the primary current in the coil or the LPG mixture requiring higher energy levels.

2) the spark advance unit is applying too much advance at higher revs resulting in the burn not being completed.

The ignition system fitted is the stock distributor with single coil and electronic breaker (with dwell control). In addition I have fitted an ignition modifier box which advances the spark on LPG.

WOuld anyone be able to offer any help on my problem.

Regards

Paul

 

[mer250c]

Hi Paul,
I am no expert by any means on this matter but I just finished a conversion to LPG from CNG due to refueling stations closing. My Van is a 5.2L Dodge with a multiport fuel injection system used with CNG previously. I have been running LPG at 50 psi at the fuel rail and had some of the same problems you are having. My problem turned out to be a bad O2 sensor causing improper F/A mixture. Replaced the sensor and the engine is running fine. When I was having the F/A mixture problem, it did at times appear to be missing due to ignition but was not the problem in my case. I have been told that a stock ignition MAY not be enough to fire a propane engine properly but the ignition that I am using appears to be a stock Dodge ignition and seems to be working fine for now. Franz is often involved in this forum and has far more knowledge than I concerning Propane and might be able to help you. I hope what little info that I gave you helps you in your search for the gremlin......

Steve

 

[pbloore]

Hi Steve,

sorry for the slow reply - I've been a bit tied up over the Christmas break.

I first suspected that the O2 sensor was playing up, but when I checked the performance on gasoline I found it worked without any problems. In the end I did replace it but it made no difference to the problem. I've also tried better quality ignition leads which also has had little effect.

Until I can work out an alternative I'm stuck with the ignition system I have now, but if I have time I'm going to try and build a new coil drive unit and see if I can optimise the spark energy.

Thanks for the reply

Paul

 

[franzh]

While ignition can indeed give the problems you describe, it seems like you are encountering a loss of fuel. If your O2 sensor is toggling lean and rich at lower speeds and flat lines lean at higher engine loads, that would tend to confirm the problem. Since you did not elaborate on what type of fuel management system you are using, I cannot offer any specific advice (not really the point of this forum anyway).

I would check the fuel flow, filter, max-load adjustment, electronic fuel management controls, and so on. If you have the means to slightly enrich the mixture to test, that would be a good way to start. I would also recommend a fuel pressure test (again, fuel system specifics prevail here).

Franz

 

[pbloore]

Hi Franz,

Fuel pressure checks out OK (it's a continuous gas injection system) and I have tried letting the control system keep richening up the mixture. This results in the power reducing quite dramatically if I let it consistent with an over rich mixture.

I fitted some higher spec ignition leads which seems to have had a very small effect in moving the engine speed at which the problem occurs up approx 200RPM.

I think my basic problem is that the stock ignition system is not that great anyway. On the 3.9 V8 Discovery it consists of a single distributor with an electronic coil triggering module which controls the dwell. I've fitted a modifier box from BRC to it which purports to add 12 degrees to the base timing when running on gas, but based on tests I've done recently it seems to do this by switching the coil off early. If I'm right this will drop the spark energy by 25% or so. Unfortunately I can't run without this box at the moment because the engine speed pickup on the LPG fuelling ECU can cope with an unfiltered signal.

In the next wek or so I hoping to try a little in line filter to clean up the signal from the coil so I can take the modifier box out of the circuit. As a slightly longer term thing I've just ordered some parts to build up my own ignition box which shouldn't suffer this problem.

Thanks for the suggestions.

 

[pbloore]

Thanks for the suggestion. I have not found much information around regarding optimising spark plugs for LPG operation - the difficulty as well is that the engine can run either gasoline or LPG which makes things tricky.

I've now changed vehicles and have just had a sequential vapour phase injection system fitted which I'm experimenting with. The new vehicle is much more difficult to modify with regard to ignition timing because it is all driven by the fuelling ECU (Bosch) rather than a distributor. There is an option with the LPG system to modify the crank signal to achieve the effect of advancing the ignition timing so I'm going to try that (carefully!).