HP and BTU question

**TwoSeven** · 18th June 2006, 19:31

Ok, given a fuel flow rate of 36.68lb/hr I want to calculate the btu/hr and from that the final hp. I'm expecting a number above 77hp but dont seem to get it.

Anyone have an ideal formula to do it ?

Biggest problem I am having is finding correct values for btu of petrol and a formula that actually works.

**boomer** · 18th June 2006, 19:38

Forty Two..

**cowpoos** · 18th June 2006, 20:45

Originally Posted by TwoSeven

Ok, given a fuel flow rate of 36.68lb/hr I want to calculate the btu/hr and from that the final hp. I'm expecting a number above 77hp but dont seem to get it.

Anyone have an ideal formula to do it ?

Biggest problem I am having is finding correct values for btu of petrol and a formula that actually works.

depends sooo much on the efficiency of the engine....bettr to track down a HP rate using cfm of air flowed....as your engine is real just an air pump....not a fuel burner....

ps: why do you want to know this....it seems very much like fairy bullshit to me...

**TwoSeven** · 18th June 2006, 21:02

If you want v.e use 0.85 and if you need airflow, just assume 12.5:1 mix. - but those are not important to the formula, just modifiers.

I need the energy of the fuel in brake mean HP, hence the question

**cowpoos** · 18th June 2006, 21:16

Originally Posted by TwoSeven

I need the energy of the fuel in brake mean HP, hence the question

thats why I think it fairy bullshit for you asking....how efficient is your engine??? theres a massive difference between a OHV briggs and stratton and lambourghini V12....theres forces to drive....weight of engine parts....thick-ness of the oil...abient temp....shit loads of variables....

the end result is your gunna get a figure which is meaning-less !!! their will be different hp figures for two engines coming off the producton line next to each other...

**Motu** · 18th June 2006, 22:12

When I was a 2nd year apprentice I sat in a cold prefab in St Pauls St on tuesday nights and did calculations about btu's - I didn't have a clue what it was about then,nor do I now.But I remember a gallon of petrol weighs 1 lb,that was the only easy bit.....

**Ixion** · 18th June 2006, 22:24

Uh, treacherous thing, memory, isn't it , as we get older.

The calorific value of petrol (no idea what octane etc) is about 19000 BTU per pound weight, if that's any help.

However , the conversion from BTU/hr to watts is 0.2931. Which would give a figure of about 272 hp. Which sounds rather optimistic?

**bobsmith** · 18th June 2006, 23:05

Originally Posted by Motu

When I was a 2nd year apprentice I sat in a cold prefab in St Pauls St on tuesday nights and did calculations about btu's - I didn't have a clue what it was about then,nor do I now.But I remember a gallon of petrol weighs 1 lb,that was the only easy bit.....

Uhhhh a US gal of fuel weighs about 5.99lbs.. and british gallon isn't that much smaller....

**Ixion** · 18th June 2006, 23:12

Originally Posted by bobsmith

Uhhhh a US gal of fuel weighs about 5.99lbs.. and british gallon isn't that much smaller....

Imperial gallon is bigger than US one. 4.5 litres versus 4 litres roughly. Yanks are behind the times, in 1766 they opted to stick with the obsolete Winchester measurerments , instead of the later London ones.

A gallon (Imp) of water weighs ten pound. Petrol is lighter but not that much lighter. About 7 lb, roughly.

**ZeroIndex** · 18th June 2006, 23:15

Originally Posted by boomer

Forty Two..

by that, I gather you have recently watched the Hitchhikers Guide to the Galaxy

**bobsmith** · 18th June 2006, 23:22

Originally Posted by Ixion

Imperial gallon is bigger than US one. 4.5 litres versus 4 litres roughly. Yanks are behind the times, in 1766 they opted to stick with the obsolete Winchester measurerments , instead of the later London ones.

I say the americans and the brits are both fucking slow. the rest of europe knew that the metric is the only way to go... I can't believe seeing american textbooks that has values for things like gravitational acceleration and specific mass of water in imperical units.... Oh and the unit of mass is pounds and the unit of force is.... oh also pounds... WTF!!!!!!!!!

**bobsmith** · 18th June 2006, 23:29

Ok so I just looked up all the constants and:

Specific gravity of vehicle petrol = 737.22kg/m^3 = 0.73722kg/Litre
1 Imperical gallon = 4.54609188 litre
1 Kg = 2.20462262 pound

so:

1 Imperical gallon of fuel is:

4.546 * 0.737 * 2.2 = 7.4 lb (2s.f)

right... now everyone can go to sleep now...

**Ixion** · 18th June 2006, 23:32

Metrichumbug. Woz the deal with a system where everythings a kilosummit. What's wrong with ells and firkins and leagues and minims and perches anyway. Rot set in when they made us give up our real money and settle for this danged funny money. Never bought as much. Petrol used to be 3/4d a gallon then. Look at it now, all on account of meddling with the money and measures.

**Motu** · 18th June 2006, 23:41

The USA has actualy signed up for Systems International (suposed to be in French,but can't be arsed tooking it up) so in theory they are as metric as any other country in the world.But they will never force their populace to change to metric,that old free will thing again.They like having that last Imperial yoke apon their shoulders.Good grief...seven sixtyfourths and three thirthyseconds...talk about making it hard for yourself....