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Old 11th January 2007, 12:34   #1
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Default Lpg Higher Octane Than Petrol??

Hi guys, just yesterday some1 told me lpg is higher octane than petrol.

if this is true shouldnt we get better perfomance from lpg rather then petrol??

then why most ppl complaint abt subdued pick up n spped on lpg ???

AMit
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Old 11th January 2007, 13:06   #2
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whoa there Amit....there's a misconception about octane that u have that must be cleared.

"Octane" does not signify "energy content" of a fuel. It simply means the "compressability" of a certain fuel in a given combustion chamber.

In this regard, LPG can simply be compressed more before being ignited, unlike petrol which is slightly lower octane.

This also clears the other idea that a higher octane petrol in a given vehicle will increase power and mileage. In some cases, this is true, but not always.

The octane of fuel you should use depends on your petrol vehicle's compression ratio. For CRs of 9 and below, regular 87 octane is recommended. For 9-11 CRs, higher octanes like Premium, Speed can be used for enhanced power and mileage. For petrol vehicles with very hi CRs (like race applications), sky high octanes should be used.

Thus, using low octane fuel in a vehicle of hi CR will cause knocking and possibly detonation, since low octane is prone to combustion at lower compression.

Along the same lines, diesel engines have very hi compression ratios since they work with auto-ignition.
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Old 11th January 2007, 13:31   #3
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Right. Octane rating is actually indicator of resistance to auto-ignition. So a fuel with a higher octane rating can be compressed to a higher degree before it auto-ignites and causes knocking.
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Old 11th January 2007, 13:42   #4
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Ok get that but what octane is an lpg fuel ??? & if significantly higher than is car designed to run on it without any problem ??

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Old 11th January 2007, 14:45   #5
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Not sure exactly what you're asking. It's true LPG has a higher octane rating than gasoline, but at the same time it has a LOWER energy content, due to it's lower density. This difference can be overcome by a higher compression ratio.
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Old 11th January 2007, 15:19   #6
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Here is the link for lpg as fuel.......

http://www.go-lpg.co.uk/whatislpg.html

Octane rating of LPG is in the range of 110-112.........indeed higher then petrol but to get best out of LPG we need a higher compression ratio....octane rating denotes resistance to ignition...as compression ratio is related to power normally petrol car has 9 to 9.5 ratio......but again lpg is mostly used as a duel fuel so in petrol car with 9 to 9.5 ratio we are not utilising octane rating of lpg effectively.....

If we increase CR too much beyond 10 then we get knocking problem in petrol.......special lpg engine has higher CR to get best out of it....also it is slow burning fuel & has less energy content then petrol.......

This link gives more idea...http://en.wikipedia.org/wiki/Octane_rating

Enjoy....
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Old 13th January 2007, 05:14   #7
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So it seems to be like that LPG is a better option for expensive Indian cars which requires higher octanes although they are having knock sensor that detects knock and retards the ignition timing.

LPG also seems to be better buy in places where higher octanes are not available.
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Old 14th January 2007, 16:09   #8
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Right on, Mithun....so I'm guessing a car with a CR of 10 or higher would not face almost any power-loss by converting to LPG.

But I know my low-CR Ikon (8.4:1) would suffer drastically with such a conversion, even with a knock sensor......you can only retard ignition for so long.

as a side note, the cylinder head of an LPG-converted car can be milled to increase CR.

see u on another thread!
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