LPG Explained... An Intro to LPG

[stradlater]

Not many of you guys know about the exactings of LPG and/or how an LPG system works. I know it's pretty common to know the specifics of an EFI system, but I thought I'd write a little bit about the components involved in a performance LPG system. I don't mean to be insulting anyone's intelligence, so please note this was written in a very simplistic fashion for those who know very little about LPG.

NB: There are newer LPG systems out there that involve a fully fledged ECU just like EFI. To my knowledge at the moment these systems aren't targetted at the performance market and hence are not to be covered here.

The Fuel

LPG - Liquid Petroleum Gas - is stored in a liquid form at both the service station and in the car. It's only at the final stages before it's burnt that it turns from a liquid into a gas. The components along the way in the LPG system convert it from a liquid into a gas and then meter exactly how much of the gaseous substance gets into the car.

A common mis-conception is that LPG is higher in octane. Whilst it is occasionally, it's highly variable and often dependant on how much you have left in your tank. The reality of it is that you can wind far more advance into an LPG powered car because the burn time of LPG is much slower than that of petrol. It's harder to ignite, so you can run higher advance with less chance of pinging. The flip side is that you have to have a high powered ignition system to properly ignite the pressurized gas when running high boost.

The Key Components

• Gas Tank
• Line(s)
• Lock off Valves
• Lock off Solenoids
• Lock off Relays
• Converters
• Mixer/Carby
• Balance Pipe
• Coolant Connections

Gas Tank

Acting just like a fuel tank, but pressurized, is the gas tank that can be mounted either in your boot or underneath the car. Looking much like a gas tank found around the house, the LPG tank has a high pressure connection to a fitting on the outside of the car where the LPG filler is connected to whilst at the petrol station.

The tank also has the connections to the fuel lines. Whilst most tanks will only have one connection given that there is only one fuel line, more performance oriented installations will have a tank with two connections to allow more gas to flow out of the tank. The tank also have a pressure gauge on it which acts as a fuel gauge and can be interfaced to a standard fuel gauge in the dash.

For each of the line fittings there will be a lock off valve. This valve exists to shut off the flow between the tank and the line(s). This valve remains shut by default unless opened by the lock off solenoid (discussed later).

All of these fittings tend to remain hidden under a nice little tank cover.

Lines

LPG lines are not as simple as your average fuel line. Given the extremely high pressures run in an LPG system to keep the liquid from turning into gas, the lines must and associated hose fittings and ends must be capable of resisting extremely high pressures.

There is some debate over the size of the lines. Some say it's better to run 1 larger line, but others say it's better to run 2 standard sized lines to keep the pressure and gas velocity up whilst maintaining the volume of liquid getting to the converters. Personally I went for two standard sized lines.

Lock Off Valves

On either end of the lines (tank end and converter end) are lock off valves. By default these valves force themselves shut and block the flow of liquid through the lines. The valves are opened by a Lock Off Solenoid.

Lock Off Solenoid

The lock off solenoid sits over any lock off valve. Once power is supplied to the lock off solenoid, it forces the lock off valve open and allows gas to flow through the lines and into the converteres.

Lock Off Relay

The lock off solenoid is supplied power by a Lock off relay. A Common lock off relay takes it's input from the ignition power and the coil negative (tacho) output. Upon the ignition switch first being switched on, the relay will power the solenoid. The valve will open.. Then when the engine is first turned over the relay will see the input from the tacho. It then knows that the engine is being turned over. as soon as the input from the tacho ceases, the power will drop and the lock off valve will close, thus protecting the driver from gas leaking in the event of a crash.

Converters

Once the liquid has reached the converters, it is then converted (as the name suggests) into gas. The Converter is basically a pressurized (lower pressure though) chamber that allows the liquid to expand into gas. Since this chemical process requires heat, to prevent the converters from freezing, coolant from the engine is run through the converters to heat them up. This also helps in keeping the engine coolant temperature down (added bonus!).

The Converter then stores the gas in said chamber until it's sucked out through a connecting line to the mixer/carby.

Mixer/Carby

Acting for the most part like a conventional carby, the mixer (NB: the words mixer and carby are being used interchangibly though in some standard non-performance oriented installations they may infact NOT be the same thing) meters the release of gas to mix with the incoming air flow. The carby contains a throttle butterfly and acts also like a throttle body on an EFI car in so much as it also lets air into the plenum.

As the air flow through the carby increases, so does the amount of gas being sucked out of the converter(s) and pushed into the plenum. This idea works fine at atmospheric pressure, but as soon as the pressures increase to above atmospheric pressure, the converters need to be balanced to the same pressure the mixer is seeing...

Balance Pipe

In order to ensure that the converters are working at the same pressure as the mixer, a balance pipe must be installed in the air flow somewhere. In standard non-performance non-forced induction installations, this tends to just be plumbed into the inlet tract commonly after the air filter. In a forced induction setup, this balance pipe would come out of the converter(s) and connect to somewhere in the inlet tract that contains the same pressure as is seen as the mixer. For example, in my installation the balance pipe is connected at the intercooler pipe just after the turbo outlet.

Coolant Pipes

As mentioned earlier, it is important to heat the converters to avoid them getting too cold and freezing. This is done via connecting coolant lines from somewhere in the engines cooling system to the coolant collector points on the converters. This is commonly done in line with the cabin heater core, seeing as the lines are of similar sizes and generally in similar areas.

Tuning

Tuning a LPG system is very similar to tuning a petrol system, only the metering is done on the mixer and not in an EFI computer. The car can be placed on a chassis dynometer and an o2 sensor placed in the exhaust. The jets inside the mixer can be changed to ensure optimal A/F ratios across the rev range. Experience shows this is best done by a professional..... *sheepish look*...

The ignition system on a LPG car must be reasonably powerful. Given that there is natively no ECU in a performance LPG system there are a number of different ways available to control the spark. The options and configurations possible there are another article all on their own.

Common Misconceptions

LPG cars are no good for turbocharging - Incorrect. In actual fact they tend to be better because of the interherent properties of LPG as discussed above.

LPG cars are harder on engines - Correct, atleast generally speaking. LPG cars do favour certain types of oils given the 'dry' nature of LPG itself. An incorrectly tuned LPG car will kill an engine. But then again an incorrectly tuned Petrol car will kill an engine too.

LPG is no good for performance - COMPLETELY INCORRECT. You've only got to take the number of turbo charged XF falcons running 250 crossflows, LPG and BIG turbos as proof. There are even a number of LPG VL turbos happening these days.. It's all about the tuning and the configuration.

BBQ gas is better than pump LPG - Correct. BBQ gas has a higher propane level and hence has a higher energy rating than typical pump LPG. Pump LPG tends to have a higher propane level during the winter months because it's colder and the propane prevents the LPG freezing in the lines when it's cold. If you do get a chance to fill up on BBQ gas, GO for it. You'll get more performance out of your car.




Well, that's the end, Atleast for now. I hope this has been of use to people, if not atleast mildly entertaining (It took me long enough to write!!!).
Please comment if you find this useful.

[joecoolmk2]

so in other words you can make your engine rev harder/quicker?

how much would it cost to set up?, is there a standard size tank or can you get bigger if you want?

would yobe better with two mixers or isn't this possible/sensible?

can you get manifold for lpg setups that will fit an 18R-G?

[joecoolmk2]

i just had a thought, has anybody done this with a 1uz?

do you reckon this would go well?

[oldcorollas]

I recently read an article talking about maintaining the quality of LPG (think it was on the EPA website??), and discussing how the auto LPG market can be used as a sink for the relatively unwanted butane.... but they say there should be a cap because it lowers the octane and people might complain


a few questions (if i may)..
what is typical tank pressure? i vaguely remember it only being around 50 or 70psi on a cold day?

typical convertors have an outlet pressure of a few psi? 3? 5?. are normal convertors ok when pressure referenced to boosted manifold, and so running outlet pressures of sayy 20+psi?

for the ECU based systems, the Keihin injectors run at around 16psi (iirc) and so require special convertors.. however the injectors are rated at 100kw each (bonus)..
but this brings the point of tank pressure... if convertor pressure is 15psi, boost is sayy 25 (not muckin around here).. thats 40psi already.. how much over line pressure does tank pressure need to be to maintain flow....

anyway, just want to provoke a few things i'm dead ken to run gas with injectors in the foreseeable future!!!

oh, and is it possible to fill up with pure propane anywhere?? ie for dyno/track days etc??
Cya, Stewart

[thomasbl]

"The reality of it is that you can wind far more advance into an LPG powered car because the burn time of LPG is much slower than that of petrol."

In addition to this point, a slower burn time does mean an increase in the production of heat in the combustion process, as do other factors in running LP gas. This, along with the dry nature of the fuel is what takes its toll on valve seals and seats. There are however ways of combatting this, and if the focus is on performance, there are very few things you can do to increase power without stressing your engine anyway. Worth keeping in mind for your daily driver cars on gas though.

Tom

[Cool1]

Great info
Whats the weight of LPG per liter in the tank compared to Unleaded fuels? Also what roughly is the weight of an LPG tank?

[Cool1]

On the dryness issue, would it be possible to design some kind of oil injection just like fitted to 2 stroke bikes/boats?

[thomasbl]

There are oil squirter and feeder kits available for some holdens i believe, designed to combat this issue, im sure you would not have to design something from scratch. This is second hand info however, from a guy that used to fit LP gas. I havn't seen them myself!

[Noofa]

got a flashlube kit on our Patrol with the 4.5 straight 6. it puts a preset amount into the system around 20 drops per minuite at 2000 rpm or somthing like that to protect the valves and seats from a higher level of heat with a slower burn, that is to run as a dual fuel car. the new falcons run a standard LPG 4 litre 6 with no additives, not sure on the go there tho.

[-=DV=-]

excellent write up +rep, now do LPG injection

[mrshin]

I guess it was inevitable that I would pop up in this thread eventually...

On the topic of injection, while I am still playing with my own system, I have also just ordered a set of Keihin injectors to fit to a 1UZ, in the name of experimentation. I'll let everyone know how it goes in a few weeks when I've got all the bits.

[oldcorollas]

how many convertors did you end up getting (assuming you get their ones?) i vaguely remember they rated thm at about 200-300kw each.. which seemed a bit crap since thats just two or three injectors....

[river]

Hi,

If this is the same gas you use for your Barby, can you get a nozzle attachment fitted so you can fit a BBQ grille to your system? When you go out camping you automatically have some gas to cook with.

seeyuzz
river

[stradlater]

JoeCoolmk2 - I don't know about the engine revving harder/quicker, because the burn rate of LPG is slower than Petrol. Yes, you can wind in more advance, but this is accounting for the slower burn time. It's safe to say that there are clear performance benefits in using LPG, but the system must be tuned properly for it.

A prime example is my fuel system. RA23 with a 1g-gte. I've been able to use a much larger turbo with a relatively un-exctiing fuel system on a standard engine, and I can run much higher boost than my mates who all have petrol. They would have to go to higher extremes to be able to run the boost I run on their engines.

There is no 'standard' sized tank. THere are lots of tanks available, and you just get what you need to fit your car.

My setup cost about $2500. That's all parts included and the tuning. That's for a fuel system that will NEVER run out of head room on my engine/turbo setup. Infact what I have would probably comfortably service a 3L turbo.

Two mixers is ENTIRELY possible. I've seen it done on a great many V8's. It's just like running two throttle bodies, and it's definately a good idea.

I don't know if you can get manifold setups for an 18rg, I doubt it'd be off the shelf, but you may well find that a Gas Research Throttle Body(mixer) will bolt on to where your current throttle body bolts on. Mine did.

I have seen a 1uz (only photos) on gas, and it used two mixers, one either side of the plenum.

Oldcorollas - From my memory typical tank pressures are up around 90 to 110 mark. The 70psi your talking on a cold day can well be an issue, as you won't have enough back pressure to force the gas down the lines.

Your correct about the outlet pressure of the standard converters. Normal converters, (impco and the likes) designed for standard cars, don't really take boost that well. Performance oriented setups typically use an "Aussie B2" converter that is modified to have a stronger diaphram and pressure balance setup.

Line pressure is typically pretty darn high too. I can see your concerns, but given that they tend to keep the physical line size DOWN, the pressure stays up and things generally don't cause too much havoc.

To my knowledge you can get pure propane at a pump at those "SupaGas" places. that have it at a 'servo' style pump. Dunno the cost though.


Thomasbl - Absolutely correct. I've already burnt one set of valve stem seals out, given exactly what you've said, and I think I'm on my way to my second. You can however replace the valves and stem seals with stronger ones that will help protect against this.

Cool1 - Not sure of the weight mate, I'll have to check it out for you.

As for the oil injection, the 2 stroke motor bikes mix it with the fuel. It'd be a little hard unless you could really properly atomise a decent thin oil into the air/gas mix.

mrshin - I'm keen to know how the LPG injection works in a performance sence.

OldCorollas - I got two converters. That's if you were asking me.

River - Funny you should talk about the BBQ adaption. I had thought that very thing. It is very possible to do, but it's actually illegal to have a fitting on the tank that lets gas out any other way than through the supply lines... But hey, who am I to say. :-)

[mrshin]

You could adapt a GRA carb to an 18RGE manifold, if you wanted to.

Tank pressures can get even lower than that at times.

I think OC was asking me about converters actually... I've only ordered one for the time being, given that the engine will be staying fairly standard, but I also have another 1UZ I will be trying the system out on that will be getting a little more work done on it. B2 converters are horrid things, but then so are the others like them, and they're pretty much what you have to use. Pull one apart sometime and be horrified at just how crude and flimsy they really are. Much like all of your typical GRA/Impco/etc. LPG products really...

The weight of the fuel... I can look that up later on if you're keen.

Yes, some Supagas outlets sell propane at a similiar price to what other servos sell it for. Just not very common/convenient. However, another thing is that servo's that are out of town a bit often only get deliveries from one truck - i.e. the same truck fills up peoples household gas and the servos, meaning servos are selling propane.

As for the injection, well definately don't bother asking anyone in Australia about it from any kind of performance sense, it takes a lot of chasing elsewhere to get anything. However, hopefully I'll have some info to share with everyone in the coming weeks.

Yes, BBQ adaptions = fun. Just make SURE you use appropriate fittings etc. - LPG is NOT dangerous provided you treat it with a small amount of respect!