Air Ratio for LPG Combustion

The upper explosive limit (uel)is 9.5% and the lel is 2.5%, so, I'd say the ideal mix would be 6%.

Search for tables on the net.

My pickups run around 16:1 A/F ratio on avarage. The realistic ideal ratio is not actually fixed. Typical propane you buy has several other gases mixed in with it. Butane is often the second higher level gas in the mix. Because of this and some engine design variations you may see a range of 14:1 to around 17:1 working the best.

Idealy you would be best off tuning it under actual load conditions for best power. That will also be your best running mix rate as well. To set up my systems the old fashioned way I just lean them out until they bog and then richen them back up again until they get back their power.

That is true about the idealistic ratio not being accurate due to presence of butane in commercial "propane" or LPG. The number I gave is just a starting point.

If you adapt an electronic system with LPG injection and an oxygen sensor, it can self-correct and even adaptively learn (saved to memory) for changes in the fuel composition. This is very possible with advanced fuel systems (ECU, regulators, injectors) available today.

However, if you use a carburetor, the tuning process is quite different and more "seat of the pants" in nature.

For most hydrocarbons fuels the ratio is 14,7 it is usually useful for first calculations and estimations. If you found out its 16 so its mean that your mixture is poor and your engine inner elements are exposed to hot oxygen containing athosphere and should be destroyed.

You may want to brush up on your chemistry. Good grade gasoline has a stoichiometric ratio around 14.7:1. And I lean heavy on the AROUND part too!

Some fuels have more some less. Also peak power and efficiency A/F ratios for each fuel is not the same as their stoichiometric ratio either. Gasolines peak power and efficiency happens at at slightly rich condition around 12.5:1 - 13.4:1 just depending on the actual engines design and fuel mix.

Any other fuel has its own ratio.

Its an exactly that I said. I don't understand why you spring out from your trousers.

thank a lot guy.....if there any more information,,please let me know..thank

If you are going to be tuning the AF ratio to a different fuel, I would suggest that you use a 5 gas exhaust analyzer as your test insturment. Since LPG is about 10% shy on but per unit of measure, compared to gasoline, this means that you will need an additional 10% of LPG added to the air that is engering the engine. The most ideal AF ratio of any fuel is refered to as Lambda=1. This is what you want to look for on the 5 gas. Since LPG is about 10% short on BTU compared to gasoline, at 14.7 (lambda=1)you will have to use a AF ratio of 16.17 (lambda=1).

There's a thread on an alternative fuels web site discussing this topic which might be useful.

There are a number of issues floating around here that are confusing me slightly

Most importantly you need to be aware of the variation of your fuel. LPG is a mixture of Propane and Butane that varies from place to place round the world and throughout the year (the cooking lpg we are supplying for kuwait is 70% Butane in the summer 65% Butane in winter. In the northern US / Canada where its colder the Propane component would be much higher.

Although on a weight basis the Air / Fuel ratio is similar for both Butane and Propane (I get about 14.1 Butane, 14.3 Propane) butane is denser (between 10 and 15% depending on the ratio of iso to normal C4) so if you filled up with pure propane and then pure butane you would need to increase the air flow by 10 - 15%. Unless you have a reliable consistent you will need to have some exhaust m,easurement t correct the air flow to maintain the optimal ratio as the composition of the fuel changes.

Although LPG has a lower fuel density cf gasoline I dont see just adding more lpg as the answer. I would assume you just have to run the engine faster so that you consume 10% more lpg in the same time frame and so release the same energy but you would still need to maintain the AF ratio to get good combustion.

thank all for helping..know i get the ratio,,but i need to prove it by thermochemistry equation...

For propane,

C3H8 + 5O2 =3CO2 + 4H20

for butane

C4H10 + 13/2 O2 =4 CO2 + 5 H20

so to calculate the air ratio we use mass air/mass fuel..mass=no of moles x moculer weight...i try to calculate and i dont get,,i dont understand why ...can someone show me please? thank

MW C3H8= 44 g/mole

MW O2 = 32 g/mole

5*32/44 = 3.6

air is 23.14% O by weight

3.6/.2314 = 15.7

thank buddy..how about the butane?its also occur in LPG.did we just ignore it?

This is obviously home work. CR4 is not to place to get your home work done for you.

Say substance A is 70 % of the mix and substance B 30%.

A has specific weight of 2 and B of 3. What is the specific weight of the mix?

.7* 2 + .3 * 3 / ( .7 + .3) = 2.3.

Hope that gives you a hint.

its not homework..it about my research and in my report i need to state everything in detail...anyway thank all for helping..