Fuel requirement formulas for LPG and Petrol.

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Fuel requirement formulas for LPG and Petrol.

Post by storm » Wed Jul 01, 2015 9:44 pm

Due to the discussion in this thread I decided to sit down and work out the mathematics of fuel flow requirements. If anyone has any questions, additions, or changes please feel free to post them. I did this more for my own benefit, cause I'm inquisitive and it helps to keep the old brain functioning, but thought it would help others work out what they need to support their LPG installations.

Here are some LPG fuel flow formulas so people can work out what their engine needs to make a predetermined amount of horsepower. These formulas are modified from similar formulas for petrol and as such I will post both LPG and petrol formulas so everyone can see the differences.

The formulas written without numbers look like this
Minimum GPH = (Max HP x BSFC) / weight of fuel in pounds by US Gallon

Minimum LPH = (Max HP x BSFC) / weight of fuel in pounds by litre

So for petrol the formula would appear like this
Minimum GPH = (Max HP x .5) / 6
Minimum LPH = (Max HP x .5) / 1.585

If you know the BSFC is lower or higher than 0.5 then you can adjust it to suit but most naturally aspirated automotive engines run between 0.4 and 0.5 so 0.5 gives you a safety margin. HAving said that a healthy "street" engine can have a BSFC of up to 0.55. If you are using forced induction you will need to change the BSFC to something like .75

OK now for the formula for LPG
Minimum GPH = (Max HP x BSFC) / 4.2
Minimum LPH = (Max HP x BSFC) / 1.1095

So for LPG at 0.5 BSFC you will need 0.4506534475 litres/hour to support 1 horsepower and in US Gallons you will need 0.119047619048 GPH to support 1 horsepower. These equate to 45 litres/hour for 100 HP or 11.9 US GPH for 100 HP. To work out what you will need at 0.5 BSFC just times the first set of numbers above by your Max HP and you have the minimum required fuel flow for your engine.

EDIT: I am making a list of known lock off valves and their flow rates. The information will be based on manufacturers specifications and links, if possible, will be provided for you to verify the information.

AMR Manufacturing. http://www.amrgas.com/lpg-petrol-lock-off-valves.
GL20 LPG Lock Off Valve: 120L/HR @ 4.5 MPA (working pressure 3.0 MPA)
GL30 LPG Lock Off Valve: 180L/HR @ ?? MPA (working pressure 3.0 MPA)
GL30W LPG Tank Lock Off Valve: (working pressure 3.0 MPA)

Using the formulas above the flow to Horsepower rating is (please note this rating is at manufacturers listed flow rate pressure which may be well above working pressure).
GL20=120 L/HR will support 266 Brake Horsepower (crank not wheels) with a BSFC of 0.5
GL30=180 L/HR will support 400 Brake Horsepower (crank not wheels) with a BFSC of 0.5

Please feel free to question, comment, or make suggestions.
Fuel flow requirements calculations viewtopic.php?f=5&t=1638

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