LPG conversions

cowgirl · 22-07-2008, 04:36 PM

Im thinking about getting my car converted to gas and wanted to find out a bit more about it before i decide to get it done. If anyone here has done it how have they found it? and what are some good reliable places that do it?

thanks

Jonchilds · 22-07-2008, 05:17 PM

I've had 2 cars on LPG, both falcons (1995 and 1998)

It's great - costs about 50-60% of running on petrol but range can be somewhat limited and hard to find in country towns (just use petrol then).

However, you NEED to take care of your ignition system etc otherwise it will start backfiring. Easy enough to change the spark plugs every 10,000km.

Would do it again...

Klink · 22-07-2008, 05:21 PM

mmm you have a small 4 cylinder car dont you CG?

cowgirl · 22-07-2008, 05:23 PM

yes?
can they be converted?

Ferris · 22-07-2008, 05:28 PM

Sure can.

Ps: Jonchilds, backfirings got nothing to do with ignition system condition.

If you want a detailed blow-by-blow of what a backfire is, and what causes it, ask away, I'll post up a lengthy essay.

Jonchilds · 22-07-2008, 05:57 PM

Happy to be told I'm wrong Ferris - been trying to sort out an issue with mine at the moment where it backfires when accellerating from idle on VERY cold mornings. The LPG fitter keeps hassling me to 'fix' my ignition system to sort this problem out.

An essay would be excellent educational value for myself and others...
(I understand backfiring is the air/fuel not yet in the cylinders igniting)

Ferris · 22-07-2008, 06:27 PM

Yes, you're correct, a backfire IS the air/fuel charge in the intake manifold igniting. But the reason WHY it is igniting is pretty simple.

the lpg goes in liquid form from the tank, thru the supply line, up into your engine bay, and into a regulator. The regulator allows the lpg to depressurise, changing state in the process. This process robs heat from it's environment, which is why regulators/converters are heated by the cars coolant system. It stops them icing over (due to the moisture in the air).

The lpg (now a vapour) gets sucked into the intake manifold by a vacuum restriction (on fumigation systems, not injection) that is upstream of the intake port. So LPG and air mix in the intake manifold, before being drawn into the combustion chamber when the intake port is open.

When the air/fuel ratio is correct, you'll get a pretty complete burn of the fuel in the combustion chamber. (it's never 100% perfect) The flame front originating from the spark plug ( the point of ignition for this process) propogates throughout the entire mix in a timely fashion, leaving no fuel unburnt by the time the exhaust port flushes the combustion chamber, and the intake port re-opens, admitting a fresh charge of air and fuel.

Now, when the air fuel ratio is too lean, the flame front takes TOO LONG to propagate throughout the chamber. The air/fuel mixture in the chamber burns so slowly that there is STILL FUEL BURNING IN THE COMBUSTION CHAMBER WHEN THE INTAKE PORT OPENS AGAIN. This ignities the incoming homogenous mix of air and fuel, which is sitting in the intake manifold. What occurs next is what you know as a backfire.

So to put it shortly and bluntly, backfires are caused by too lean mixtures.

There are only 2 ways to alleviate backfires.

1. Have a properly tuned fuel system.

2. Have the fuel introduction point as close to the intake port as physically possible.

I'm happy to elaborate further on LPG if required. I know too much.

Jonchilds · 22-07-2008, 06:31 PM

Thanks Ferris - shall richen up the idle slightly and see if this goes away. Cold mornings would indeed make the lean problem more pronounced.

Cowgirl - do it if you got the cash/boot space.
However, I understand post-2004 vehicles require a sequential injection system (may be wrong/money grabbing pitch by salesman) which costs significantly more.

Ferris · 22-07-2008, 06:36 PM

Yeah, sequential injection systems are pretty good, but you can still make 'em backfire. It's just a lot less severe, and not as destructive. The injection points for the LPG system are drilled as close as possible to the intake port. This reduces the incoming air/fuel charge in the intake manifold if and when backfiring occurs. You just get alot of loud popping, like a popcorn machine.

Backfiring doesn't occur as often with petrol vehicles because the petrol is introduced into the combustion chamber as a liquid (although very finely atomised by the petrol injector nozzle, it's still liquid droplets). Liquid doesn't burn. It's vapourised by heat, and then ignited. So the process that LPG undergoes in the regulator, all occurs in the combustion chamber on petrol vehicles. The same change of state, and the inherent robbing of heat.

This is why LPG vapour systems run hotter than their petrol counterparts. There's no change of state process occuring in the combustion chamber on LPG.

cowgirl · 22-07-2008, 06:38 PM

got an 03 car. should have room in the boot, only have a sub taking up space in there atm but am happy to take it out. And with the govt rebate thing should help with the costs. and as the car is technically my dads... i may try get him to pay for it, or atleast go halves with me

Ferris · 22-07-2008, 06:39 PM

If you have a spare wheel well in the boot, you can get whats called a Toroidal tank that will sit where the spare wheel normally is.

Make and model of car Cowgirl?

cowgirl · 22-07-2008, 07:00 PM

hyundai accent

Ferris · 22-07-2008, 07:01 PM

Plastic intake manifold, innit? Anyone know?

Stormrider · 22-07-2008, 07:30 PM

can some one please xplain to me what the term "atomised" means? It just sounds like "atomization's" cousin and likely to be used by the next "firepower" scam

Ferris · 22-07-2008, 07:37 PM

atomised. to be broken into very tiny parts.

In the context I was using it, it means the petrol is still a liquid, even though it has been reduced to tiny tiny droplets, which may APPEAR to the human eye, to be vapour, it isn't.

Rob · 22-07-2008, 07:48 PM

Hey Ferris - Great info in the thread!!

I have one of these petrol guzzling 2002 Mitsubishi Challengers - Have been thinking about getting a gas converison done.

Would you go completely gas, and not worry about petrol? I have heard that a full conversion is better than a dual fuel, as the tune is meant to be better. Is that right?

Any other advice would be sweet

Cam · 22-07-2008, 07:50 PM

With a 1.5L motor, how quickly do you anticipate breaking even on the install?

Ferris · 22-07-2008, 07:54 PM

Hmm, mitsi challenger....

The thing is, you can run a dedicated system in such a way that you don't have to make compromises in the variance in ignition spark timing and the calorific values between 2 fuels. You can tune properly for 1 fuel, not a compromise across both.

You'd need to ensure the valve train can take operation in a dry environment, as LPG doesn't have the lubricating effect petrol has, as it's not a liquid when it enters the combustion chamber.

Edit: oh you asked for an opinion. Um, personally, on a car with a knock sensor, I'd go dual fuel. On an old pre-96 bucket, I'd dedicated gas it. If you go dual fuel, make sure you run the petrol thru the car at least once a month. It's a good idea to keep the petrol side of things in use, so the fuel doesn't dry up, gum things up and harden fuel lines.

EDIT No 2: Cam, tbh, I've been outta the game too long to know what installs and rebates are these days. Gimme some cost+rebate figures, and I'll see if I can do the calc for you. A BALLPARK hazy guess I'd assume 5 years on a 1.5L.

Racer · 22-07-2008, 08:30 PM

You get $1000 from the state government and $2000 from the Feds, these are provided only if the car is for private use.

Rob · 22-07-2008, 08:36 PM

Cheers mate

whats the job worth if there is 3 grand in rebates there?

Is the waiting list for these conversions months long still?

ANy good / recommended fitting centers around?

How fucked is it going to be if the gas prices get jacked up once we is all hooked? Can that happen? Would they dare do that?