HP and BTU question

[Ixion]

so how come the UK speed limits are still in mph? As are the speedos on Pommie vehicles? I blame the Welch. It's all Biff's fault.

[Kickaha]

so how come the UK speed limits are still in mph? As are the speedos on Pommie vehicles? I blame the Welch. It's all Biff's fault.

Everything is Biffs fault

[marty]

they can't change to metric - i'd have to buy a completely new tool box. at least i won't have to unless i work on those french fly-boy thingies...

[imdying]

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

Hush young pup! It was a great book long before it was an average movie...

[F5 Dave]

You mean Books: 5 in an increasingly misnamed trilogy.


So by the logic applied with measuring fuel; does this make my old Victor 3.3 I had as a teenager heinously powerful? as it used to suck like an Ethiopian hooker. But it used to go through almost as much oil, so. . . Does anyone know the calorific value of oil. I could add that to the fuel used & get a real big number.

Another revelation: getting a tune-up reduces power!!


Hey! This is a conspiracy!

[Ixion]

I think that what is calculated is the theoretical 100% efficiency horsepower. If you supplied that amount of fuel to a magically 100% efficient engine, you would get that horsepower.

As we all know, the internal combustion engine is horribly inefficient.

So in reality the actual horsepower , as measured on a dyno, will be much much less.The ratio between theoretical or actual being the overall heat efficiency of the engine .

In the example case , about 77 / 272 = 28% efficiency roughly. The "missing" 195 horsepower is lost through the zorst, as noise, as waste heat in the radiator, as friction etc.

Or maybe it doesn't mean anything. I'm not quite sure what is being measured here.

[Fooman]

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.

I'll do it in S.I. units and then convert to hp:

Flow rate = 36.68lb/hour = 16.63777 kg/hour = 0.004622 kg/sec

Calorific value of petrol ~ 43500 kJ/kg

Therefore 0.004622*43500 = 201 kJ/s = 201 kW = 269.6 hp

I'm not sure if the calorific value is the low (net) calorific value, but it shouldn't make too much difference. The difference between my answer and >77hp is either:

1. I cocked it up.

2. The overall thermal efficiency of the engine in question.

If you want to go the BTU/hp route, I get 18701 BTU/lb for petrol, converted from the value above ( 1 btu/lb = 2.3260 kJ/kg). Also, 1 BTU/sec = 0.70678 hp:

36.68 lb/hr = 685974 BTU/hr = 190.55 BTU/sec = 269.6 hp.

Sources: Bosch Automotive Handbook, 3rd ed.

Also, if you have access to Microsoft Excel, the "CONVERT" function can convert between a lot of imperial and S.I. units, e.g. =CONVERT(50000,"W","HP") will convert 50 kw into HP. You have to string a few together if there is no direct conversion (e.g. BTU/hr to kW has to go BTU/hr to BTU/sec to J/sec to J/sec*1000 to get kW)

Hope this helps.

FM

[Ixion]

Ok

So, two different people, different methods , different sources, come up with the same answer (difference between my 272 and your 269hp being the difference bewteen my 19000 btu/lb and your 18700 - prolly varies with octane, brand too). So , either that IS the answer, or we're answering the wrong question. Over to original poster.

[Fooman]

Ok

So, two different people, different methods , different sources, come up with the same answer (difference between my 272 and your 269hp being the difference bewteen my 19000 btu/lb and your 18700 - prolly varies with octane, brand too). So , either that IS the answer, or we're answering the wrong question. Over to original poster.

Yup. The real simple answer is 1 lb/hour of petrol = 7.35 hp ( x the thermal efficiency of the motor).

What's really interesting is this quote I found in the Bosch Automotive Handbook:

Calorific value of air-fuel mixture

The calorific value of the air-fuel mixture determines the engine output. Assuming a constant stoichiometric ratio, this figure remains roughly the same for all liquid gases and fuels (approx. 3500 - 3700 kJ/cubic metre).

Which sort of makes sense. To get more power, stick in more air/fuel, rather than a specific type of fuel.

FM

[bobsmith]

I think you two are on the right track. Referring to :

http://ffden-2.phys.uaf.edu/102spring2002_Web_projects/Z.Yates/Zach's%20Web%20Project%20Folder/EICE%20-%20Main.htm

for effiency of internal combustion engines:

thermal effiiciency: ~ 26%
Mechanical efficiency: ~94%
so Overall efficiency: 0.26 * 0.94 = 24%

So multiply 270hp by 24% = 65hp... much closer to 77hp

[ZeroIndex]

Hush young pup! It was a great book long before it was an average movie...

yes i did read the book.. I even have it on AudioBook format.. I happen to think the movie was quite a good attempt at the book.. just like Fight Club..

[imdying]

Oooh, audio book... and ahh, how big would that be..

[ZeroIndex]

Oooh, audio book... and ahh, how big would that be..

44MB.. it's 5 hours long.. I'll upload it onto a download site, and post the link on this thread and also PM you the link..

[Firefight]

just like Fight Club..

Bloody hell, you mean there's a book and a movie out about us ???

hear that Rendy, Paco and Co we are famous !!!


Fight Club of 22 rules !!!!


F/F

[ZeroIndex]

Bloody hell, you mean there's a book and a movie out about us ???

hear that Rendy, Paco and Co we are famous !!!


Fight Club of 22 rules !!!!


F/F

the movie left out some interesting stuff that is in the book/audiobook.. the audiobook is quite more involved, more anarchistic and in a more 'audibly' violent way than the movie.. it is 86MB and about 5 hours too.. if you ask nicely, I might upload that audiobook next..