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

 

[GregLocock]

We're talking about thermodynamic efficiency, not gallons per bhp hour.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dcasto]

Well, Greglocock, you are talking thermo, but most people like WGJ are on MPG. The fad is switching to diesel, but it isn't a one for one competition, its emotional and a mines better discussion. The biggest thing we can all do to lower energy used is DROP the weight.

As a side note, the US EPA has set a new standard for running the MPG tests that are posted on vehicles. The new test more closely mimics how people drive. The results are most cars will have a 8% reduction in MPG for city and 12% for highway. The high effiency vehicles ove 30 mpg will be hit 15% / 20%. The hybreds will get wacked 20%/30%. Again, hybreds are just a fad, you can't add 500 pounds of motors and batteries and get that much farther ahead.

 

[patprimmer]

The OP asked a Thermodynamic efficiency question with a hint that it related to fuel efficiency of his car.

He considered weight only, and considered maximum power. This indicates he knows very little about the subject, or meant to be very specific in looking at thermodynamics.

He did quote a BTU value for the fuel, so that should exclude all fuels not of that value.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SMOKEY44211]

Just for the record Henry "Smokey" Yunick (yes I borrowed my screen name from him) got the nick name from a track announcer reffering to a motorcycle he had raced that blew a lot of smoke from the exhaust. Regarding the hot vapor engine I saw two working examples of this method both were sawed off from a buick v-6 3.8liter. A 2cylinder version is in a vw rabbit that is in the storage area behind the pet store his daughter operates in Holly Hill FL. The other one is a 3cylinder version that he had in a Ford Fiesta for many years. That engine is in a storage warehouse for the Smithsonian institute. Smokey's grandaughter was still driving it around (original engine was put back in) as of may of last year. I had the privledge to speak at legnth with 2 gentleman who worked along with Smokey on some of his projects. One was an engineer who's career started at GM on the original rochester mechanical fuel injection system, worked for holley carburetor( major contributor to the deign and developement of the holley 4160) and recently retired from an R+D position at Crane cams. The other was a more hands on employee. Both of them along with several friends and family who drove vehicles with one of these engines in place report gasoline fuel milage figures in the 60 mpg range. The 3 cylinder version was capable of 200 hp. The intelectual property rights to the proccess were licensed to the Delorean Motor Co. and after the demise of that company Crane cams along with Smokey formed a company named Motor Tech that was going to develope and market it in kit form for retrofit in the aftermarket. Most of this information is public and fairly easy to obtain. In answer to the original question from this post I do believe the technology exists today for a gasoline fueled 3,500 lb vehicle with a drag coefficient of .40 to achieve 100mpg. I believe the biggest obstacle is that fuel prices are still low enough that it is not cost affective to offset the higher cost needed to manufacture such a vehicle.----Phil

 

[dcasto]

SMOKEY, your car would have to use less than 40 HP and the engine be better than 50% efficient, so its max speed would be 40 mph, it would not go up a hill to save its life. As for fuel price dictating car design, fuel in europe is $5.00/gallon and they are not at the level you predict, close, but not quite there.
Its not the fuel price, its, not anything we have control over. The US public wants a car that is bigger than their neighbors so if they collide, the neighbor will die and not him. Second, we want cars that will pull a 12,000 pound trailer (just in case we need to tow away or nieghbors car from a crash site), go 0 to 60 in under 6 seconds (just in case we have to evade our neighbors car), have every electric gadget known to man (because our neighbor does, at a cost of fuel), AC to chill us to the bone (never see the neighbors window down, heck the AC drops my milage by 10% when I run it). Its not the fuel, its peoples perception of what they think they need.

 

[EdDanzer]

I think a 3500 lbs car could get 100 mpg if you could regenerate 80%+ of the braking energy at 80%+ total efficiency, and you use multiple 4 stroke linear engines and are city driving in a city without many hills.
The reason for many linear engines is to have a longer power stroke than intake, then only operate the number required to propel the vehicle. No crankshaft engine will have the same efficiency at different loads and speeds. The power to the wheel varies from little to a lot depending on terrain, speed and if you are accelerating.

 

[patprimmer]

Vapourising the fuel might get greater combustion efficiency, preventing unburned fuel going out the exhaust.

That would help if that were a major source of engine inefficiency, however it is not. Virtually all the fuel is burned and turned into heat well before the end of the power stroke. This means that any improvement can only be small as there is not much room for improvement. Like I said earlier, in a modern petrol engine, about 1/3 goes to the cooling system. Vapourising the fuel won't change that very much. About 1/3 goes out the exhaust. Vapourising fuel won't change that at all.

Vapourising fuel might give a slightly faster burn rate. This will require less timing advance to avoid detonation. This later ignition will give less time at higher cylinder pressures, so a little less will go to the cooling system, also less power will be consumed pushing against cylinder pressure on the late compression stroke and all available heat will be generated earlier in the power stroke so more work can be extracted, but at this stage it will also put more heat into the cooling system. There might be one or two percent in it. A long way short of 100 miles per US gallon.

Smokey might have obtained very high MPG figures, but it was not down to the fuel heating. He had many other significant mods, not the least of which was half an engine. I strongly suspect he used smoke and mirrors as a diversion from the real causes. While everyone focused on the fuel heater, no one noticed other features that do work.

State of the art very small engine, light weight, good aero, hard small tyres, constant low speed, simple light weight transmission are the secrets to low fuel consumption

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dcasto]

The heat of vaporization for hydrocarbons is about 130 BTU/lb, The heat of combustion with the total recovery of heat back to 60 F, including the condensing of the water vapor formed is 21,000 BTU/lb, so vaporizing the fuel first will incease effiency by 130/21000 by .6%, can't argue thermochemistry. The recondensing of the water in the exhaust is about 2000 BTU/lb of fuel burned, so the water vapor alone has 9% of the thermal energy available. The exnaust it's self is where the majority of the energy goes. At a 700 F exhaust temperature, there is another 30% of the efficency lost.

I have a home heater that is 92% effcient, the exhaust of the heater leaves the house just 30 degrees warmer than the air intake to the burner, it condeses the water by preheating the air. So if we want cars that can achieve this kind of efficency, lets go back to the steam car, or put a combined cycle system where the exhaust gases are make steam that drives turbines to get energy.......

 

[GregLocock]

Which is perfectly feasible, technically, it just isn't worth doing financially. (yet)



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dcasto]

GregLocock, agreed. Can anyone agree that a 80% efficient internal combustion engine isn't possible unless we get "waste heat recovery" and that this type of recovery isn't practable unless we find some way to use it in a vehicle?

 

[SMOKEY44211]

dcasto I agree with you that waste heat recovery is the path to achieve that objective. I think turbo compounding will get about 60% of what's available from the exhaust but it will only be optimum at a specific rpm and load. I also think that steam driving a second turbine using heat from the cooling system and whatever is left over in the exhaust is the best method to do that. Packaging is somewhat cumbersome but doable. I have spent most of my spare time in the last 4 years and mega$$$ retro fitting an existing vehicle with the methods decribed. It works but I won't live long enough to offset the cost in fuel savings. Throw the economy of mass production into the mix and your probably looking at a ten year break even payback. I am not much of a profit but I believe that fuel prices will rise enough in about 5 years to make it economically viable.-----------Phil

 

[patprimmer]

A 10 year payback in an environment with 10% interest means the capital is never reduced let alone paid back.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dcasto]

How about a thermal generator to make electricity out of the exhaust, that might get 5% or so. Switch from compression AC to thermal AC, like a propane refrigerator.

 

[GregLocock]

Yup. Peltier devices after the cat have been demonstrated.

The trouble is that Carnot tells you what the max efficiency of the entire system is going to be, so until we can figure out how to extract all of the enrgy at high temperature, we are never going to see much better than 60%.

Of course, the reduction in fuel consumption is much greater in going from 20% to 40%, than 40% to 60%.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[marcdeluca]

Years ago I believed in the 200 mpg carburetor. Then I got into propane. I realized that burning propane was accomplishing the same goal; to burn fully vaporized gas. Propane has 30% less btus than gasoline. But I found that I didn't get any outstanding mileage with lp, in fact it was worse than with gasoline. On a modern engine with feedback electronics, lp power will get 20% less mileage than gasoline. That tells me that there is only about 10% room for improvement for gasoline mileage by vaporizing the fuel, not 500 or 1000%. Last year Bruce Crower of Crower Cam fame came up with a 6 stroke engine that I think is revolutionary. Instead of opening the exhaust valve on the 4th stroke, it compresses the exhaust gas. Around TDC, an injector injects water. The water hits the hot gases and vaporizes, providing a second power stroke. It exhausts on the 6th stroke. The engine needs no cooling system, and he says it provides around a 30% efficiency gain. He is able to recover almost all energy from the exhaust. A downside is that you now have to carry water as well as fuel as a consumable, and the water has to be distilled. Keeping moisture out of the oil would be a challenge on a cool running engine. Crower developed this on a one cyl diesel engine that he fitted with an ignition system and carburetor, then used the injection system to do the water. Of course he had to completely change the cam setup to run at 1/3 speed instead of 1/2.

 

[Lou Scannon]

I'm not much of a thermodynamicist, but the Crower 6 stroke cycle p-V diagram doesn't look like a winner to me. Anyone want to comment on the T-s diagram?

 

[dcasto]

Here is actual data with effiency from a spark ignition engine

Caterpillar G16CM34
6100 bkW @720 RPM
7671 BTU/bkW-Hr
7909 BTU/ekW-Hr*
44% Efficiency
0.5 Gm/BHP-Hr NOx
power output 5.9 MWe
cylinders & arrangement 16Vee
cylinder output 380 kW/cyl
bore 340 mm
stroke 420 mm
speed 720/750 rpm
fuel Natural Gas Methane > 70
Fuel Cons 7909 BTU/ekW

 

[r2800]

A rough estimation for brake specific horspower per imperial gallon is 45hp/gallon/hour

 

[ebola]

my friend, professional chip tuner was experimenting with 3 cyl.diesel vw lupo last year.He was driving at "ecology rallye" wich happens every year in Lithuania ant took second place.His lupo at 105 km/hour eats average 2.5 liter diesel no highway - country asfalt road.Car with all guts - full interior and ordinary tire pressures,driver alone.