LPG writeup

[prj]

I made another writeup, this time about LPG.

http://prj-tuning.com/?p=233

My last writeup was covered here:

http://www.s2forum.com/forum/showthread.php?t=55886

I hope you like it

[driftin jo]

what mpg could you expect to get on an S2 running LPG ?
I was told it was only about 2/3 of what you get on petrol, that would still make it
a lot cheaper as it is about £0.78 a litre compared to V power at £1.5 a litre.

[driftin jo]

Nice write up by the way

[s2_bo]

Nice write up

The system on my s8 was defiantly not done very well. It runs fine and dosent cut back to fuel at any point in the rev range providing the coolant is up to temperature. But bank one is running lean by ~10% according to the diagnostics when its on gas.
The ECU is constantly trying to compensate by adjusting the fuel trim every time it starts on petrol and then when it switches back to gas back the other way.

Im not that fussed as its been like it for a while but i'm sure it could be improved.

I get 350 motorway miles from 75l of gas.

[dgacampbell]

Very interesting reading PRJ!
This is something i have been looking into for quite some time for my own car.
The problem being it is KEIII-Jetronic 2.3E Audi 90.
Therefor i have to first convert my car to efi, to provide the injector pulses etc.
I have been looking into the likes of megasquirt and vems ecus but as yet, is out of my financial reach.
So i will be keeping a close eye on this thread as i find it fascinating!
thanks
Dan.

[Error404]

As before cracking writeup, thanks for sharing

[prj]

Glad it is of some help.
MPG should not suffer a lot. Marginally 5%, maximum 10%. This assumes of course that the timing is set up right.

The main issue is that timing is usually way late for LPG on cruise... That also burns out the exhaust valves.

P.S.
Bo, your consumption is too high on LPG imo.

[driftin jo]

I thought the mpg was less on LPG due to it having a much lower calorific value than petrol.
I take it this is largely overcome by proper tuning and extracting more of the energy from it in to motive power due to higher knock resistance ?

[prj]

I thought the mpg was less on LPG due to it having a much lower calorific value than petrol.
I take it this is largely overcome by proper tuning and extracting more of the energy from it in to motive power due to higher knock resistance ?

Actually the lower fuel economy is 90% due to crappy tuning... or well, lack of tuning.

The calorific value per kilogram is higher, but density is smaller.
So yes, there is a roughly 25% difference in density, but because it is a more knock resistant fuel, you can extract more energy out of it.

Of course it all depends on the engine compression ratio. On a lower compression ratio engine it is a lot less efficient.
If you can run MBT in a cruise cycle on the engine on petrol, then you will have a pretty much exact 20-25% difference (not all engines can though...)
However, if you are giving it some more than just cruise control driving, that starts to go down really quickly, if the timing advance is dialed in properly.
It starts to go down pretty quick as engine compression ratio increases as well.

The 2/3 you mention is really a worst case scenario where no tuning is done at all.

[merlin's S4]

A very interesting read. Thanks!!

[afterburner]

I'm currently thinking about making an arduino piggyback ECU which will have only one purpose - to monitor the pressure and temperature of LPG in a fuel injection rail and calculate compensation values based on that.

It will accept the injector pulse signals from the original ECU and then output new ones with the compensation applied.

The idea behind this is to use it when the LPG is the only fuel powering the car and the car is using its original ECU, but remapped for LPG. No petrol tank, no fuel pumps, no petrol fuel lines, no petrol injectors (LPG injectors in their place), only the evaporator, LPG rail, LPG injectors, tank, lines.

I like simple things and dual fuel systems are messy, that's why I like my cars having only one type of fuel.

If it works it could also be used with standalone ECUs like Megasquirt, because it has only one temperature input for fuel/ignition compensation, that's usually used for monitoring the intake air temperature. If you want to monitor a second temperature (LPG temperature) you have your hands tied. With a simple additional piggyback you can overcome this.

[prj]

You will need electrical heating for the evaporator then.
And it will need to be brought up to temp every time before you can start driving.

Unless you want to inject liquid LPG.

[Matas]

Very nice reading, thank you. So what exactly fuel consumption are on LPG for that V8 in urban?

[prj]

Very nice reading, thank you. So what exactly fuel consumption are on LPG for that V8 in urban?

Too early to tell...
Average computer consumption dropped quite a bit, but we will need to see via the "tank full - tank empty" method to get exact numbers.

[afterburner]

You will need electrical heating for the evaporator then.
And it will need to be brought up to temp every time before you can start driving.

Unless you want to inject liquid LPG.

A friend of mine did something interesting. He connected a second LPG fuel line to the tank that stays on top of it (I think he's using the tank filler line with a T connection). There's always some LPG in gas form there. So when he's doing a cold start he uses a valve to switch the inputs to the evaporator from liquid LPG to LPG in gas form. That's how he starts the engine and lets it work for no more than a minute. A minute is enough to get the water flowing through the evaporator hot enough for it to work with liquid LPG so then he switches to the liquid phase. We've tested it and it works in -20 degrees. How does this sound to you?