measure hp from gas?

hey is it possible to calculate your theoritical hp through some equation involving how much gas is injected into your cylinders? i mean like btu's of gas times the highest ammount of gas you can inject into the cylinders times number of cilinders times rps. and convert this into horsepower? or even graph the numbers you get out at say 100 rpm intravals and get a graph that will roughly resemble your horsepower on your dyno graph?

In theory you can, so long as you know the 'volumetric efficiency' of the engine, internal friction (resistance to motion) and some measure of thermodynamic efficency.
Power is more dependent on airflow (the amount of oxygen) that can be drawn into the engine than on the fuel used.

Airflow in an engine is complicated and dependent on many factors such as the design of the intake and exhaust systems, valve size, engine revs etc.

Also, combustion efficiency comes into play. Some engine design are more thermodynamically efficient that others and are able to convert more of the fuel's combustion energy into useful motion instead of heat and/or partially burned hydrocarbons.
Therefore, you can have 2 engines consuming the same amount of fuel and air, but one produces much more power than the other.

There's a very easy rule of thumb.

That is each 1lb/air per minute through your engine can develop 10hp.
Your target air/fuel ratio will give you the fuel mass, that is converted to volume quite easily.

Of course this is for a petrol engine, diesels run leaner, they process a lot more air to develop the same power.

While you could estimate it using the amount of fuel consumed, the actual amount of chemical energy that any given engine is capable of converting to kinetic energy is wildly dependent on a few hundred factors.

A good example is my 1940 John Deere tractor which uses almost 13 times as much fuel to generate the same HP as my 1996 car

A good example is my 1940 John Deere tractor which uses almost 13 times as much fuel to generate the same HP as my 1996 car

Do you mean 13 times worse power/displacement?

No, I mean for the fuel consumption (as in lb/hp or L/hp) the tractor takes about 13 times as much fuel to generate the same HP. It has nothing to do with displacement.

The John Deere G was rated at 34.5 hp from 412.5 cubic inches at a redline RPM of 975. It has a VE of about 22%. My 1996 car with 349.8 cubic inches produces nearly 100 HP at that RPM with far less fuel and a VE of about 85%. The tractor uses ancient head/chamber technology to produce adequate drawbar HP from the technology of the day. My car relies on modern EFI atomization and head chamber advancements.

If I plow our field, I can get an average of 1.5 MPG from the tractor. If I tow my 3500-lb boat with the 96 car, I get 20 mpg. So, my results may not be entirely accurate to the split-hair, but the bottom line is that you can't estimate HP from fuel consumption. As soon as you say "X hp per gallon" you have to adjust it for the millions of engine combos on the planet, their compression ratio, their combustion efficiency, the effective atomization of their fuel induction, and the decade for which the fuel was designed.

The reason for asking about power/displacement is that I can't imagine an engine being 13x worse and still being able to be used. As compared to being in such a sad state it'd be useless.

Weren't the horses bigger then? I'm not sure exactly when the germans standardised 1hp din to be around 740 watts.

One has to consider the fact that horsepower is calculated from an engine's torque output at a given RPM. The actual equation is:

HP = torque*RPM/5,250

Ever wonder why graphs of an engine's HP and torque cross at exactly 5250 RPM?...that's where they are equal, the 5,250 terms cancel each other out.

So, comparing a tractor to a car is like apples and oranges. A tractor engine is designed to provide lots of torque at low RPMs, whereas the car is designed for fuel efficiency at lower RPMs for cruising at high speeds and low power demand.

The second major problem with this comparison is that tractor engines are typically diesel-powered. Diesel engines run at much higher compression ratios (~17:1) than gasoline engines (~9:1). With each cylinder compression stroke, there is much more energy being stored in a diesel just prior to fuel injection and detonation. This is what gives the diesel the torque advantage...more air squeezed into a smaller space, resulting in a denser charge. Try this with a gasoline engine and you risk igniting the fuel/air mixture before the spark happens (compressing a volume of air causes it to heat up considerably).

The tractor's engine is doing work of plowing the field...try that with your car and it'll probably get even worse mileage since it is not geared to use that torque to cut through and turn the soil as efficiently as the tractor. You'd have to rev the engine up high to get the torque output needed to match the tractor.

BTW, the term "horsepower" really evolved at the turn of the century when cars were being compared to horses for productivity...they were even called "horseless carriages". Someone (I can't remember their name off-hand) way back when sat and timed a horse turning a mill stone and crudely calculated the work the horse did (Work=Force*Distance). Thus, that became "1 HP".

Finally, there's a difference in the energy content of gasoline versus diesel versus alternative fuels (E85, CNG, H2, etc.) diesel/biodiesel has the highest energy density, and methanol one of the lowest. This is due to diesel's high number of long-chain hydrocarbons (from 10 to 20 carbon atoms per molecule). Gasoline contains chains of four to 12 carbon atoms long. In short, the more atomic bonds present to break during the combustion process, the more powerful the bang will be. I could get into the strengths of the different types of bonds but that is outside of this scope.

And yes, 1 HP = 745.7 Watts = 745.7 Joule/sec

Chew on that...:eek7:

The reason for asking about power/displacement is that I can't imagine an engine being 13x worse and still being able to be used. As compared to being in such a sad state it'd be useless. Weren't the horses bigger then?
Horses were bigger then. The 34.5 HP that the Deere puts out is gross HP, so by today's standards its more like 30 HP.

Its a function of RPM, fuel atomization, weak spark from a tiny magneto, insanely heavy internal parts, (each piston is like a cast iron coffee can), highly inefficient chambers, low compression, and the need to be able to run on darn near any fuel you put in the tank.

Think about it... just since the mid 80s things have drastically changed. Back then you could buy a 307 V8 Olds that made a wheezy 140 hp and got 16 mpg. Now you can buy a Corvette with 505 hp and it gets nearly 30 mpg, and it does it while emitting less than a tenth the emissions - even without the benefit of EGR.

I'm not sure exactly when the germans standardised 1hp din to be around 740 watts.

Not sure when the Germans derived the DIN hp, but here is what Wikipedia says about the origin of the word "horsepower." :

The development of the steam engine provided a reason to equate the output of horses with the engines that could replace them. In 1702, Thomas Savery wrote in The Miner's Friend: "So that an engine which will raise as much water as two horses, working together at one time in such a work, can do, and for which there must be constantly kept ten or twelve horses for doing the same. Then I say, such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work..." The term "horsepower" was coined later by James Watt (http://en.wikipedia.org/wiki/James_Watt) to help market his improved steam engine (http://en.wikipedia.org/wiki/Steam_engine). He had previously agreed to take royalties of one third of the savings in coal from the older Newcomen steam engines (http://en.wikipedia.org/wiki/Newcomen_steam_engine).[2] (http://en.wikipedia.org/wiki/Horsepower#cite_note-1) This royalty scheme did not work with customers who did not have existing steam engines but used horses instead. Watt determined that a horse could turn a mill wheel 144 times in an hour (or 2.4 times a minute). The wheel was 12 feet in radius, therefore the horse travelled 2.4 × 2π × 12 feet in one minute. Watt judged that the horse could pull with a force (http://en.wikipedia.org/wiki/Force) of 180 pounds (assuming that the measurements of mass were equivalent to measurements of force in pounds-force, which were not well-defined units at the time). So:
http://upload.wikimedia.org/math/0/8/0/080770c45f43bee417ca10cf589328e9.png

wow thanks guys i got alot more information than i thought i would with such an obscure question. i appreciate the help

the octane rating of a fuel or its ability to reisist pre-iginition and detanation can also be a factor in depending on what kind of power you can get from an engine

with more octane you can use more compression, timing advance, a few more psi if your engine is forced aspiration with out the fear of destroying the internals of the engine granted you don't get over zealous with it