btu's per hp

[provelle]

Is there a formula for figuring out how many btu's are consumed per horsepower. In other words, gas has 125,000 btu's per gallon, theoretically at 100 percent effiency how far would this take a 3500 vehicle rated at 200 hp. I realize friction among many other varibles come into play, just looking for ballpark, thanks

 

[patprimmer]

Your biggest loss is thermodynamic efficiency.

About 1/3 the total heat released by the fuel goes to the cooling system, about 1/3 to the flywheel and about 1/3 out the exhaust pipe.

Friction is relatively small with most being in the rings.

There is also a bit through windage in the crankcase and through driving the oil pump.

A little is lost in pumping air.

A little is lost to accelerating and decelerating reciprocating weight.

Other very variable losses are through ancillaries like alternator, power steering, air conditioning, water pump etc.

All of the above vary a lot with different engine size and fuel type.

Also, comparable products from various manufacturers vary to a lesser degree from individual design details.

For comparable engines, the main factors effecting thermodynamic efficiency are compression ratio, expansion ratio, cam timing, surface area to volume ratio of the surfaces contacting the charge during the compression and power strokes, coolant temperature and conductivity of the materials.

Regards

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[dcasto]

high effiency units run 7500 LHV (lower heating value) BTU/hp-hr older engines 10,000 btu/hr-hr. Turbines run 9000 to 13000 btu/HP-hr.

A 3500 lb vehicle would only use 200Hp runnig at 150+ miles/hr if it has a low drag coefficient.

200HP is 1,500,000 BTU/hr If Gasoline has 125,000 BTU/gal, 12 gallons/hr. In 1 hour, you could travel 150 miles on 12 gallons, 12+ miles/gallon.

 

[franzh]

But, a 3500 lb vehicle at 65 mph with a drag CF of around .32 on level ground may only require 40 bhp, and that is through wind resistance and to a lesser extent road friction. The rest is lost through pumping losses and engine thermodynamic loss (re: Patprimmer).

A 120 cid engine at WOT will pump about the same theoretic CFM as a 350 cid engine at 1/3 throttle opening, with only half the internal friction.

Just guessing the vehicle mileage through fuel BTU's is only a small fraction of the total equation.

Franz

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[provelle]

Thanks guys for the replys, I see I was a little brief in my question. dcasto answer seems to be based on me using a substained 200 hp (which is how I ask the question) per gallon of gas. This would have most four cylinders and some six cylinders at wot, so I can see why only 12 mpg. franzh answer is closer to what I was really asking.

My real question is, is it mathematically possible for the likes of Charles Pogue, Fisher, Smokey Yunich to get over 100mpg. Is there enough energy in a gallon of gas to carry a vehicle that distance?

 

[franzh]

Smokey Yunick's nickname of Smokey came from two sources: One of which was his everpresent cigar, the other from what he blew up others nethers! He was indeed a genius and his 100 mpg Chevy of the 1980's was pretty ingenious but never worked the same way twice. I saw it during one of my SAE workshops in Daytona in the late 80's, all it was is a sawed in half 2.8 liter GM V-6 (3 cyls), a turbocharger, and a real small carburetor with superheated gasoline (he called the process a homogenizer!).

Reality:
For a conventional vehicle to get 100 mpg, it would barely run over idle, a real tiny engine (swept area), stripped of all inertia bound weight, coasting downhill with a strong tail wind, and a huge flywheel. I don’t really see how.

Pogue and the Fish carburetor guru groups are nothing but charlatans and never proven outside of questionable unsubstantiated non verifiable third party anecdotes.

They were never bought out by the oil companies, the government didn’t "lose the patents", they didn’t magically "disappear". Take one of these units plus their most vocal advocate and install it under recognized emission laboratory conditions with third party non-biased witnesses and lets see the actual results.

Our engines today are approaching their limits for thermal efficiency, dont expect any significant improvements over what they are currently. Any improvements will be in parallel energy storage (hybrid, kinetic, etc).

Franz


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[patprimmer]

Fuel economy comes from optimised engine design (pretty well much already there), reduced weight and reduced drag.

If you want 100 mpg, buy a moped. If you want 50 mpg, buy a Japanese micro car like a very small Dihatsu or Suzuki

Regards

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[GregLocock]

I worked on a car that got a genuine 80 mpg on highway, although to be honest it would not have passed modern crash regs or emissions.

Inside it was about as roomy as a Focus.

It was very light, had a high efficiency 3 cylinder, manual trans, and great aero.

Performance was pretty good due to the light weight and aero.

http://www.austin-rover.co.uk/index.htm?researchecvf.htm

Cheers

Greg Locock

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[crysta1c1ear]

Let me use metric units initially as it's easier.

There is a force resisting motion due to friction and air resistance, which is so and so many Newtons. Multiplying top and bottom by metres, the resistive force is so many N*m/m and N*m is an energy unit called Joules. So the resistive force can be measured in J/m or kJ/km.

What that means is the resistive force is just telling us the energy required to go a distance, and taking its reciprocal, the resistive force tells us how many km/kJ the vehicle is capable of.

In imperial units that is saying the (reciprocal of) the resistive force is just a measure of how many miles/BTU you can get.

So to get an MPG figure, now isn't difficult

Miles per gallon = miles per BTU * BTUs per gallon.

As others have said, the BTUs per gallon require some assumption of engine efficieny, eg BTUs per gallon for the vehicle might be 30% of the figure you gave for BTUs in the fuel.

I'm not going to do any calculations for force resisting motion, as what one person finds an acceptable vehicle might be considered a joke to somebody else. However its easy to estimate from frontal area, coefficient of drag, etc.

Volkswagen created a street legal prototype (the so called cigar car) which consumed 1 litre per 100 km, which is 235 MPG US. 100 MPG is achievable by combining
(1) an efficient engine
(2) a low coefficient of drag
(3) a low frontal area
(4) driving well

 

[crysta1c1ear]

I came across a calculation I have done some time ago, assuming a more efficient engine put in a Loremo and travelling at constant speed.

For all I know, it could be full of mistakes, but something in it might be useful to somebody.

Engine
0.35 lbs of fuel per horsepower hour
equals 0.21 kg per kilowatt hour
which gives [B]4.69870971 kWh / kg of fuel used.[/B]

Rolling resistance 
Mass               450 kilograms
Add Driver and multipy by gravity
Weight             5297.4 Newtons
Use Coefficient of rolling resistance just over 1%
Roll Force         55.6227 Newtons

Air resistance
Wind resistance  (1/2) * airdensity * velocity * velocity * frontalarea * coefficientofdrag
 eg Loremo       (1/2) *    1.2     *   27     *   27     * 1.1           * 0.20
Air force          94.9662307 Newtons

Add air resistance and rolling resistance together

Air force          94.9662307 Newtons
Roll force         55.6227 Newtons
Total force        150.588931 Newtons
                 = 0.0418302585 kWh / km
       which gives [B]23.9061396 km per kWh[/B]

Finally, let's assume a compact fuel, eg diesel, rather than a less dense fuel like petrol.

Fuel Density    [B]0.86 kg / litre[/B]

Fuel economy

Now we just need to multiply things together:
How good the car is at getting miles from energy
how good the engine is at getting energy from a mass of fuel
and how dense the fuel is.

                23.9061396 km per kilowatt hour
              * 4.69870971 kWh / kg 
              * 0.86 kg / litre
Fuel economy  = 96.6020889 km / litre
             ie 272.272246 UK miles per gallon

 

[GregLocock]

That's an incredibly efficient engine.

Also Loremo won't achieve that Cd in practice. A looks low as well - draw a box 3.3 feet square and sit two people side by side in it.

Crr is a tad optimistic, for grooved tires.

And your final drive would appear to be at least 15 % less inefficient than any I've seen.


But yes, that's the way to do it.


Cheers

Greg Locock

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[WGJ]

An excellent link to BL Greg.

Pat, I think that references to 50mpg (Imperial gallons?) only being achievable in 'micro cars' aren't all that fair.
The latest crop of diesel vehicles from Peugeot and Ford of Europe are proving that (shared engine technologies).
A c-class vehicle will achive 50mpg, no problem and CD class are approachng these levels.

As for petrol engine technology development, what about the introduction of direct injection and the possibility of increasing the performance of pistons and combustion chambers by ceramic coatings?


Bill

 

[GregLocock]

I am continually reminded that the trick is not to get the guys who are doing 40 mpg, to do 60 mpg, but the neat trick would be to get the guys who are doing 10, to do 20.

And that is, to be honest, easy.

Cheers

Greg Locock

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[patprimmer]

Sorry Bill

I forgot about the recent developments of light weight high speed diesels in Europe. We don't see many of them here.

Last time I was in Europe (about 10 years ago) I had a Reno 2 litre diesel. It was a quite slow of the mark, a bit noisy, but performed OK on the autobahn. It seemed to use as much fuel of a more expensive fuel as something like a Suzuki Swift of the era.

I guess things have changed.

Regards

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[SomptingGuy]

The odd thing is that fuel tank sizes seem to be fairly standard across all engine types. This means that a modern small diesel vehicle has an awesome range between refuels.

I often think that the best way to market diesel vehicles in the USA would be to focus on this attribute. If the weather is nice, fill your car up. If it's really cold, wait until a warm day.

 

[WGJ]

A bit off-topic now, but, the big change in diesel vehicle acceptability has really come since the Fiat/Bosch introcution of small-engine common-rail diesel injection.

Calibrators are now able, using electronic control of turbo and fuel injection & timing, to dial-in fat torque curve, giving the driver a big-engine feel. These torques are also enabling quite high gearing for cruising. A 30mph/1000rpm top gear is not uncommon; accel is still capable due to the torques.
Check this, the writer notes a top speed of 124mph and 47mpg imperial:

http://uk.cars.yahoo.com/car-review...mondeo-tdci-diesel-euro-iv-range-1003354.html

Bill

 

[crysta1c1ear]

Hi Greg

I used the manufacturers quoted figures for the mass, area and drag for the Loremo, vieable at

http://www.loremo.com/daten_en.php.

If you cannot get two people side by side in that area, try putting them one behind the other like in a Jetcar (pictured above), and the area might still be achievable.

Point taken that there are no transmission losses in the calculation.

I used engine figures from a press article by John Mortimer.
A google search on
"lb/hp-hr" "city car" mortimer
will throw up a link for verification, but I think the article itself is pay-to-view.

The coefficient of rolling resistance was from a thread on eng-tips - if I remember rightly. I tried to take the lowest street legal figure achievable, so the figure is intended to be low: if the aim is best milage one has to push the limits of area, drag, mass, etc.

If I redo a similar calculation sometime I'll put some transmission losses in: gearbox and final drive. I guess I should find a reference for the rolling restiance or change the figures too, as you're given a hard time on eng-tips if you cannot support what you are saying, l.o.l!

 

[provelle]

Thanks again guys. I've been reading alot on vaporizing gas for fuel mileage and in theory this sounds good. Also there are many patents vaporizing gas suggesting this works. Knowing how inefficent a carburator is and how much fuel is wasted out the exhaust I thought there might be something to it.

On the other hand I knew propane had less btu's, but it is vaporized before it reaches the engine and these people sure aren't getting great milage. Looks like I'll be building that Joe cell after all, just kidding.

 

[GregLocock]

Prius tires Crr is .011. Other car tires up to 0.015

Prius engine efficiency average 33%, max 37%. You should be able to beat that with a diesel.

yes, I know Loremo are making some amazing claims. I like the idea, and even with more realistic numbers in it still makes sense. The Cd is achievable on a prototype, but will need a fair bit of stuff that is not realistic in a light production machine.

You have reminded me that Cd and A is a bit of a misnomer. In cars the surface area is really what matters, frontal area is not the most important parameter on a well deisgned car. That is, I can get lower overall aero with a larger A sometimes.






Cheers

Greg Locock

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[dcasto]

All you Deisel fans, remember, diesel has 15% more BTU's gallon. For fuel vaporizers, switch to propane, its already vapor if carburated into an engine. and it has 20% less energy than gasoline per gallon.