LPG burning temperature vs petrol

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i860
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LPG burning temperature vs petrol

#1 Post by i860 » Mon Dec 10, 2018 10:00 am

Hi
I could find any info about the burning temperature of the LPG in the cylinder vs petrol.

Found this but I was under the impression that LPG temperature when burning is higher than petrol.

https://www.acemechanics.com.au/does-lp ... ine-orcar/

Gilbertd
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Re: LPG burning temperature vs petrol

#2 Post by Gilbertd » Mon Dec 10, 2018 6:07 pm

I seem to recall that many years ago someone (I think it was Rossko) posted on here that the combustion temperature of LPG was about 20 degrees higher than petrol but can't find it now. But of course then you need to take into account the cooling affect of vaporising the gas so that will reduce the coolant temperature so a car shouldn't run appreciably hotter on LPG than on petrol.
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Re: LPG burning temperature vs petrol

#3 Post by i860 » Fri Dec 14, 2018 1:30 pm

So 20 degrees above petrol is not so bad, I was under the impression that is way higher than that.
In my case when I start to drive the car hard on LPG my water temperature is going down 10-15 degrees, good and bad.
Bad because my thermostat opens at 100 degrees and the ECU will try to compensate with a rich mixture to achieve the right temperature, higher consumption??
Good
My vaporization will cool the water, water will cool the engine, engine oil and transmission oil so a win, win when driven hard.

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Re: LPG burning temperature vs petrol

#4 Post by Gilbertd » Fri Dec 14, 2018 5:59 pm

What car? A stat that opens at 100 degrees seems particularly high, most cars run at 90-95 anyway so have a stat that opens at 85-90.
'96 Saab 900XS, AEB Leo, sold
'93 Range Rover 4.2 LSE, Lovato LovEco, sold
'98 Ex-Police Range Rover 4.0, Singlepoint AEB Leo, my daily motor
'97 Range Rover 4.0SE, eGas multipoint, a project.....

Proud to be a member of the YCHJCYA2PDTHFH club.

i860
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Re: LPG burning temperature vs petrol

#5 Post by i860 » Fri Dec 14, 2018 7:04 pm

Hi
Is a Mercedes slk 350.

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Re: LPG burning temperature vs petrol

#6 Post by Gilbertd » Fri Dec 14, 2018 10:04 pm

Weird, my other half has a C180k and that is also listed as having a 100 degree thermostat yet according to the gauge it runs at a steady 85-90 degrees. I would have though that would cause the stat to be constantly opening and closing. The stat on my Range Rover opens between 85 and 88 so the car runs at a constant 90-95 degrees.
'96 Saab 900XS, AEB Leo, sold
'93 Range Rover 4.2 LSE, Lovato LovEco, sold
'98 Ex-Police Range Rover 4.0, Singlepoint AEB Leo, my daily motor
'97 Range Rover 4.0SE, eGas multipoint, a project.....

Proud to be a member of the YCHJCYA2PDTHFH club.

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Re: LPG burning temperature vs petrol

#7 Post by LPGC » Sat Dec 15, 2018 10:15 pm

You could find the burning temperature of petrol and of LPG on the net easily but the figures might not be representative of actual temperatures in an engine cylinder.

An engine only makes power at all because heat from burning fuel expands air in cylinders which increases cylinder pressure which pushes the piston... more pressure in the cylinder sees the engine produce more torque/horsepower and more heat produces more power. You control the amount of total heat the cylinder sees with the accelerator pedal. Burning X amount of fuel in a cylinder can only produce Y amount of total heat. If this heat is released in a short time (with a relatively quick burn) the peak temperature will be hotter than if the heat is released over a longer time (with a relatively longer burn). The total heat a cylinder sees is rarely a problem (unless the heat causes detonation, which it won't with LPG in an engine designed to run on petrol because LPG's anti detonation properties, RON, is higher than that of petrol) but heat that valves see can be a problem. Exhaust valves see more heat when the burning takes too long because under these conditions the fuel will still have been burning closer to the time that the valve opens... but under these conditions the peak cylinder heat will have been cooler than if burning took place in the usual shorter time that sees burning complete by the time the valve opens... or where did the extra heat come from? Leaner mixtures slow the speed of the burn so can produce hotter exhaust temperatures because the fuel was still burning more recently when exhaust valves opened than with richer mixtures. Exhaust gasses from leaner mixtures also contain more air (containing oxygen...) High heat with excess oxygen is what allows cutting torches to cut through metal, without the excess oxygen a cutting torch wouldn't work nearly so well, we can prevent excess air (oxygen) in exhaust gasses by running correct or rich mixture which greatly reduces the amount of air (and oxygen) in exhaust gasses.

Where are you measuring engine/water temperature? As the reducer changes LPG from liquid to vapour phase there is a cooling effect but it's not usually enough to see the engine run cooler on LPG than on petrol. On most modern vehicles there is no direct connection between engine coolant temp sensors and the gauge on the dash, the ECU sits between them and can make the dash gauge say what it wants (whatever the manufacturer programmed into the ECU). Most vehicle ECU's are programmed to show minimal temp changes on the dash gauge unless the engine seriously overheats (then the gauge suddenly goes sky high). Mercs are not usually the main culprits for this, the dash gauge on Mercs is usually accurate by modern standards. My ML320 temp gauge usually reads a bit lower when I'm booting it than it does when it's been sat idling for a good while - All engines run cooler water temp in driving conditions (including booting it up to a point) than when sat idling regardless of running on petrol or LPG.

Most LPG ECU's default gas temperature compensation (fuelling mix compensation) is a bit exaggerated. If an ECU can accurately read vapour temperature there is little need for reducer temperature compensation but it can be handy on installs that are set to switch to LPG at relatively low temperature on engines that gradually lean off cold start enrichment (choke effect) as the engine warms.
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i860
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Re: LPG burning temperature vs petrol

#8 Post by i860 » Sun Dec 16, 2018 10:32 am

Thank you for taking the time and fully explain it.
Since I convert my cars on LPG I am paranoid not to burn the valves.

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Re: LPG burning temperature vs petrol

#9 Post by Gilbertd » Sun Dec 16, 2018 11:22 am

Burnt valves has very little to do with combustion temperature, it's due to LPG being a dry fuel with no additives to prevent valve seat burning. Obviously if it does run very hot then it will still happen but even on petrol if the mixture is lean causing higher combustion temperatures it will still happen, keeping the mixture correct, if not slightly on the rich side, will do more to help than anything else. In the old days, lead was added to petrol to act as a lubricant for the valves and seats but that was removed years ago when unleaded fuel came out instead. At the time, there were numerous charts produced that showed what would run on unleaded without problem, what needed the ignition timing retarding and what couldn't run on it at all. Unleaded has an additive that does some of the work that the lead used to do but it doesn't work as well. What differentiated the different vehicles was the grade of steel used for the valves and seats. Things like the original A series BL engine couldn't run on unleaded because the valve seats were cut into the cast iron of the head which wasn't hard enough to cope without the lead. Most engines, especially those with alloy heads, have steel valve seats pressed into the head casting and it is the grade of steel used for those seats that make the difference. Some manufacturers use steel that is easily hard enough to cope without any lubricant, some use steel that is hard enough to cope with unleaded with the additive so need a Flashlube or similar system installing to use LPG. Mostly you will find that older designed engines have hard valve seats and that includes most European designed engines. So you will find that things like Saabs, Volvos, BMW, Mercedes and the good old 1960's designed Range Rover V8, don't burn valves and seats. Anything made in Japan will have valve seats that are only just hard enough to cope with unleaded so suffer on LPG without Flashlube (one test on a Honda engine showed burnt seats after just 10,000 miles on LPG). The biggest exception to this is modern Ford engines which have valve seats made from a grade of steel that is marginally harder than butter so suffer at a frightening rate. Considering manufacturers regard 100,000 miles as the lifetime of an engine these days, they will last that long but not much longer.
'96 Saab 900XS, AEB Leo, sold
'93 Range Rover 4.2 LSE, Lovato LovEco, sold
'98 Ex-Police Range Rover 4.0, Singlepoint AEB Leo, my daily motor
'97 Range Rover 4.0SE, eGas multipoint, a project.....

Proud to be a member of the YCHJCYA2PDTHFH club.

i860
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Re: LPG burning temperature vs petrol

#10 Post by i860 » Sun Dec 16, 2018 6:01 pm

On my slk 350 i did 30k miles on lpg so far. Every year I do a compression test via DAS and I am pleased to say that nothing changed. Also, I have a stage 1 remap, 2nd cat deleted and straight pipe to dissipate the heat =))
https://imgur.com/a/APkZI
https://imgur.com/a/r8KXX

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Re: LPG burning temperature vs petrol

#11 Post by LPGC » Sun Dec 16, 2018 8:52 pm

It wouldn't be of much benefit to dissipate the heat from your exhaust system, that leads to less power not more because the cooling exhaust gas then becomes denser and it's velocity decreases leading to more pumping work for the engine. But deleting cats can increase bhp because it lowers back pressure and pumping work.
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i860
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Re: LPG burning temperature vs petrol

#12 Post by i860 » Mon Dec 17, 2018 9:41 am

I was joking, I did that for the sound.
Standard the exhaust is quite so doing that it makes it louder. I start by removing the baffle but it wasn't much louder so I went with cutting the bends in the back boxes and put flex pipe in there.
https://imgur.com/a/7qAyb9b

Cold start
https://www.youtube.com/watch?v=jwPfgjLyqiA

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