LPG running lean - how will I know?

[RollingThunder]

Thanks for the feedback once again guys - very helpful!

I've now bought the car, its on the LPG register and runs soooo smoothly. I checked out the LPG changeover etc during the test drive and it seems A1. Its an OMVL system btw. No difference on petrol/LPG, just the lamp on the switch and the lovely feeling of running a high performance car for the cost of a diesel golf lol. Its got mega mileage, but it feels like a 50k mile car, tight and responsive, and looks mint. I'll post some pics after I've collected it at the weekend (waiting for the beginning of the month for a fresh tax disc).

I'll be giving it a thorough service shortly and will get it booked in for an LPG service at Mugglestones as they seem to have a good reputation.

Thanks again for your help, its genuinely appreciated.

[bongodave]

Originally Posted by RollingThunder

Thanks for the feedback once again guys - very helpful!

I've now bought the car, its on the LPG register and runs soooo smoothly. I checked out the LPG changeover etc during the test drive and it seems A1. Its an OMVL system btw. No difference on petrol/LPG, just the lamp on the switch and the lovely feeling of running a high performance car for the cost of a diesel golf lol. Its got mega mileage, but it feels like a 50k mile car, tight and responsive, and looks mint. I'll post some pics after I've collected it at the weekend (waiting for the beginning of the month for a fresh tax disc).

I'll be giving it a thorough service shortly and will get it booked in for an LPG service at Mugglestones as they seem to have a good reputation.

Thanks again for your help, its genuinely appreciated.

Glad it's all good and you're enjoying it mate. I bought my V70r awd 1998 LPG in December and love it. Have had to change the heater matrix, but that was cheap and cheerful enough. It's done 197000miles and drives exactly as you described yours.

Can I ask how much you paid for it? Mine was £1350, but similar to yours in that it looks and drives like well-under half that.

[capt jack]

Many congratulations RT

Hope you enjoy the new car!

Cheers

Jack

[Rversteeg]

Correct me if I am wrong, but as far as I know the combustion temperature of LPG will be lower on a lean mixture, contrary to petrol. So no risk of melting pistons. By nature the combustion temperature of LPG at optimum mix ratio is already higher than petrol, which explains the higher thermal load on valves. Fuel enrichement will not reduce the temperature as there is no cooling effect of evaporising petrol.

[Clifford Pope]

When tuning all kinds of engine I've always regarded the spark plug condition as the best guide.
I would have thought an engine that is running so hot as to burn out pistons would long ago have given a warning in the form of white, blistered, eroded or melted plugs if they had been checked.

It's an interesting theory that a weak LPG mixture does not run hotter (unlike petrol) so does not risk burning out plugs or pistons. Has someone a link to a reputable report stating that?

[Lpgc]

I am an LPG expert...

Regarding 'How will I know if the LPG is running lean on my V70'....

I don't know what type of system is installed on the V70. On an older V70 the MIL light not come on as long as the lambda signal flicks occasionaly. On a newer V70 the MIL light is more likely to come on if the mixture is incorrect at any point in the load / rpm range, even if the lambda signal flicks occasionally.


If it is a modern sequential system on a newer V70 then the MIL light should come on if the engine runs lean, but only during the petrol system's 'closed loop mode' operation. Closed loop mode is the usual mode, but from a cold start, on the over-run or on high engine loads (when you put your foot down) the engine runs 'open loop mode' (i.e. it ignores the lambda signal). It is possible for a vehicle to have the correct mixture on LPG most of the time, but to run lean when the driver puts his foot down. If the engine runs lean when the driver puts his foot down, this will do more engine damage and sooner than if the engine runs lean at part throttle. The way to check is to monitor lambda signal under such conditions.

If it is an older system (single point or multipoint none sequential) then the lambda signal might be emulated to prevent MIL light illumination. In these cases the only way to know if the mixture is lean is to connect to the LPG system or connect to the lambda probe with a voltmeter. It is unlikely one of these systems is fitted to a later V70, as later V70's use a type of lambda probe that isn't compatible with the lambda input on older LPG systems... But I owned a V70 with a multipoint none sequential system where the installer had fitted a second lambda probe specifically for the LPG system!

The best way to check all is OK if you don't know the type of LPG system would be to visit a good LPG installer who knows all the systems...

Simon
www.Lpgc.co.uk

[Lpgc]

Originally Posted by Clifford Pope

When tuning all kinds of engine I've always regarded the spark plug condition as the best guide.
I would have thought an engine that is running so hot as to burn out pistons would long ago have given a warning in the form of white, blistered, eroded or melted plugs if they had been checked.

It's an interesting theory that a weak LPG mixture does not run hotter (unlike petrol) so does not risk burning out plugs or pistons. Has someone a link to a reputable report stating that?

It is the exhaust valves you need to worry about with lean LPG mixtures!

You cannot tell if an engine is running rich or lean by looking at the plugs after running on LPG.

Simon
Lpgc

[classicswede]

You are only going to know if the engine is running lean by monitoring the lambda readings (provided it is working correctly) and watching the fuel trims. In closed loop mode the ECU will try and correct for lean or rich running by adjusting the injector opening times. If is reaching the correction limits then the ECU will normally throw the engine light and give a fuel mixture code.

As an installer what you are making sure you do not have is a fuel map that is causing the fuel trim to jump giving very lean or rich spikes. In older engines with open loop mode under WOT the mixture needs to be on the safe side of rich but does not need to be as rich as you will often see with petrol as rich LPG running provides no extra cooling.

Running lean on LPG will not cause a piston to burn out but can burn exhaust valves. What causes pistons to go with petrol is knock or pre ignition. LPG needs over double the temperature to ignite and will not self ignite and will tolerate more ignition advance. Very lean mix will just fail to ignite.

LPG and NO2 might be a different story

[foggyjames]

Sorry about the lack of contributions to this thread lately...been busy!

Welcome, Simon. You'll need to register as a trader. Someone will be in touch shortly about that.

Just one point, on what you said there, about measuring the lambda signal. If we're talking about a narrowband (i.e. above/below 14.7:1 AFR 'switch' - see here) sensor, there's little point in monitoring it when on-load. Once it goes "lean", you're already far too lean. I presume you're talking about 2000-onwards cars, which (as far as I'm aware) feature a wideband, which will report AFRs accurately across the entire likely range? I only mention that, as it's something that owners frequently get mixed up about (hence all the sales of £20 lambda "meters" for connection to a narrowband...which are about as much good as a chocolate teapot).

I'm far from convinced that a lean LPG mix will not lead to elevated combustion temperatures. As far as I'm aware, that's an inherent feature of any spark ignition engine, regardless of the fuel type. My long-held theory (although not based on any kind of research on LPG-equipped vehicles, I must add!) is that the poor reliability historically associated with LPG (including valve seat regression, burnt valves, melted pistons, toasted headgaskets and backfiring) are all caused by lean running, courtesy of fuel delivery systems unfit for purpose (IMO anything other than multi-point!) and a lack of competence on the part of the installers.

On that latter point, I talked to several installers when I was toying with installing LPG, and what I heard from many made me shudder. I decided I didn't want to entrust my engine to an industry that (on average) didn't seem to have a working knowledge of combustion theory! I should add that I am absolutely sure that there are plenty of LPG installers who know their stuff...but there are also plenty who don't, and on that basis, it's no wonder to me that you hear so many horror stories of ruined engines.

As a side-note, Innovate Motorsports (manufacturers of aftermarket wideband lambda sensors) claim that an overly-rich mix does not provide a cooling effect, and that there's no point in running richer than about 12:1 AFR...other than that if your EMS is a bit dodgy (hacked LH2.4, etc), richer is better than leaner!

cheers

James

[Lpgc]

Hi FoggyJames, thanks for the welcome! Would be nice to be registered as a trader on this forum.

Regards lambda probes and fuel mixture, without re-reading the thread I think I talked about both narrow and wideband (post 2000 V70) probes - Most LPG systems, be they sequential, single point closed loop or multipoint none sequential are not capable of reading wideband probes but in the case of sequential LPG systems this doesn't matter because these systems work as a slave to the petrol system (which provides the basis for the mixture correction). I commented how I owned a 2000 model V70 which had a multipoint none sequential system fitted when I bought it, but the installer had fitted a second pre-cat Lambda probe in the exhaust for the LPG system to read.

A lean mixture doesn't increase the average combustion temp, it may cause localised increased combustion temps but the most damaging effect of a lean mixture is caused by a combination of the slower burn, which can lead to hotter gas exiting the cylinder past the exhaust valve, and the fact that this gas will contain a higher amount of oxygen. Slower burn rates due to lean mixtures are the reason why vehicles are more likely to backfire with a lean mixture, because the cylinder is hotter when the valves overlap (hot gas in cylinder when inlet valve opens ignites mixture in manifold on petrol carb and single point LPG systems).

There is more power to be gained from going rich running on petrol than there is going rich with LPG. Liquid petrol doesn't take up as much volume in the manifold or inlet runners as LPG vapour does and any volume taken by fuel is lost to air, thus an overly rich LPG mixture lowers effective volumetric efficiency. Petrol has the effect of cooling the intake charge during it's atomisation and this cooling effect can in some cases raise volumetric efficiency even though there is an 'extra' volume of liquid in the inlet runner.. Liquid fuel doesn't take as much volume for the same mass a a vapour fuel. So far I've talked about volumetric efficiency but there is also burn speed to consider - it is no good just getting the mixture that will provide the most heat (engines produce power from heat so a hotter burn is actually good news, though localised heat isn't good news) if some of that heat isn't utilised because it exits the exhaust valve having done no work in the engine because this will waste that heat and reduce power, it will heat the exhaust valve as hotter gas exits past it and will heat the inlet valve during valve rock. So as rpm increases we can expect the mixture to need to get very slightly richer to increase the burn speed (less time for the heat to do it's work before the exhaust valve opens) to prevent lowered power and heated valves. Hotter valves are softer, there is the metallurgy of the metal used for the valves to consider regarding wear but also valve spin - on engines where the valves are spinning faster as they reseat we can expect more wear than on engines where the valve isn't spinning as fast (like slipping a clutch with higher revs) even with the same valve temp and same metallurgy. Lambda probes measure exhaust oxygen content so closed loop fuel systems are by default self compensating for different fuel stochiometric ratios. Fuel mixture for WOT conditions can is some cases be a compromise between maximum power, protecting the valves from a slow burn rate and/or lean mixture, protecting the cat from an overly rich mixture. The best way to tune WOT fueling with a narrow band probe can be to ensure a slightly rich mixture for starters, then increase the mixture until the point that a seat of the pants test can detect power is starting to drop slightly, then go for a mixture in-between those points. With a wide band probe the system will be closed loop almost all the time or all the time, so there isn't much you can do with the LPG tuning to effect near WOT mixture anyway without remapping the petrol ECU, but most remapped petrol ECUs go overly rich and might actually reduce power on LPG.

Regards the lambda probe manufacturers claims, the cooling effect would only apply to petrol anyway but it is accepted generally that an overly rich petrol mixture does provide a cooling effect that increases volumetric efficiency. What most people tend to forget when talking about cooling effects etc is that the engine is a heat cycle engine, it makes all it's power by using heat to expand air in the cylinders to increase pressure and push the piston - no good cooling the average heat in the cylinder during the combustion stroke as this will reduce power, but a lower volumetric efficiency will also reduce power. They also tend to forget that another reason for going rich is to increase burn speed, this is also utilised on petrol engines to reduce the chance of detonation, but LPG is less prone to detonation than petrol.

Simon
Lpgc