LPG +petrol

Submitted: Friday, Apr 14, 2006 at 09:44
ThreadID: 32883 Views:2660 Replies:3 FollowUps:5
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Now here's a teaser. We have heard of diesel engines being supplimented with LPG but has anyone considered doing the same with petrol engines???????
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Reply By: nickoff - Friday, Apr 14, 2006 at 11:39

Friday, Apr 14, 2006 at 11:39
Why bother. You would only make a petrol engine more complicated to use and service. A petrol engine can ignite LPG on its own, an as LPG is cheaper than petrol, the conversion is done on an either fuel soure.

With the diesel engines, ignition is not achieved from an outside source, IE spark plug, but from compresion ignition. Compress a gas to about 20 ATM's and its temprature rises about the ignition point of the fuel you are injecting. Diesel fuel, both Dino and Eco, have an ignition point of about 385 Deg C. LPG on the other hand has a flash point of about 500 Deg C. In the diesel engine, the diesel fuel ignites first, increasing the themprature in the combustion chamber to above 500 Deg C, and ignites the LPG, which continues to burn on the down stroke of the piston. As the piston decends, the gas charge cools, due to expansion, below the ignition point of the oil injected, thus you get an incoplete burn of fuel. The addition of small amounts of LPG, Usually between 10 to 30 percent, allow the diesel fuel to continue burning, thus completing the combustion process in the cylinder, delivering more power per gram of fuel burnt.

Adding LPG as a fumigation to petrol engine with cost to much, wuth too little gain.

AnswerID: 166998

Reply By: brd - Friday, Apr 14, 2006 at 19:54

Friday, Apr 14, 2006 at 19:54
Hi Ray

As i understand it, petrol engines are stociometric (I think I've spelt it wrong), whereas diesels are excess air engines. That is petrol engines match fuel and air in reasonably precise ratios resulting in little oxygen left over after the fuel burn. Diesels have still plenty of oxygen left...I've measured over 12% regularly in fuel consumption tests with mine trucks.

Based on this I don't think LPG will work as a boost fuel in petrol like it does in diesel.

AnswerID: 167054

Reply By: F4Phantom - Friday, Apr 14, 2006 at 20:25

Friday, Apr 14, 2006 at 20:25
Hey then why not just make a diesel engine which runs off petrol? meaning it ignites also from compression. Perhaps with petrol it is to hard to get the exact pressure, temp correct. Or perhaps it has something to do with emissions.
AnswerID: 167057

Follow Up By: tucon - Saturday, Apr 15, 2006 at 09:15

Saturday, Apr 15, 2006 at 09:15
Novel idea adding lpg to engine that is already using it's own fuel source, however if you are thinking of adding this idea to your own vehicle DON'T bother it will only cause heartache and ultimately a loss of money to your back pocket, lpg is a very volitile substance and can and does damage engine's .. to gain power and save money? does not work!! as more power requires more fuel (more money ) to be more fuel efficient less fuel less power (less money ) the 2 do not not equate , so we have to make a happy medium and ultimately be happy with what you have got..the best suggestion i can give is to maintain your engine to be the best it can be.. .. the tried and proven way's of better power and better efficiency comes in the forms of 1/ get the old exhaust out,get it flowing! extractors/ larger diameter exhaust systems ,mandrel bends,full flow mufflers, larger bore catalytic convertor if possible.. 2/ get it to breath !! open up air inlet system buy a K&N air filter, fit a snorkel,fit an aftercooler / intercooler , open up or fit larger bore throttle body's..
these things alone cost a little however they work !! they do not cause adverse reaction to the engine and do give you noticeable power and increased fuel economy without major modification
FollowupID: 422129

Follow Up By: F4Phantom - Saturday, Apr 15, 2006 at 09:31

Saturday, Apr 15, 2006 at 09:31
no thats not what i ment. what i mean is why cant you make an engine run off petrol but using compression ignition. btw i have a real diesel but at around or over 10:1 compression petrol engines ping, so they are igniting off compression so why the spark plugs?
FollowupID: 422130

Follow Up By: bombsquad - Sunday, Apr 16, 2006 at 14:04

Sunday, Apr 16, 2006 at 14:04
I can't remember the fugures, but I think it goes something lke this - petrol and lpg require about 550deg to ignite, wheras diesel only needs about 380. The lpg injection on diesels does not get hot enough on compression to ignite, but it is hot enough to ignite the diesel which in turn ignites the lpg. If you have a high enough compression to consistantly ignite petrol or lpg, your egt's will go through the roof, with engine parts through the bonnet soon to follow.

My figures are guestimates, but I think the logic is correct

Cheers andrew
FollowupID: 422318

Follow Up By: F4Phantom - Sunday, Apr 16, 2006 at 19:44

Sunday, Apr 16, 2006 at 19:44
yes. i actually thought pinging was when the compression of an engine is too high and the fuel egnites early trying to force the piston against the engine. This is why we have high octane fuel, as the high octane has ( i suppose) high egnition temp so the compression of the engines gas does not set it on fire. So from this line of thought you could have a diesel engine running petrolium. I would love to know the answer to all this. Funny, you think you understand hoe engines work!!
FollowupID: 422364

Follow Up By: Michael ( Moss Vale NSW) - Monday, Apr 17, 2006 at 21:33

Monday, Apr 17, 2006 at 21:33
Sadly i am old enough to remember Water Injection for petrol engines,,,, that was a total failure also!! Michael
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