perfect engine? and another OT q...

[Bryan8412]

Ok first question and the biggest. What is the percentage of power lost when it reaches the crank. To put it scientifically: If no energy was lost by sound, movement of mass, and the reaction of air and fuel was perfect with no elements remaining, how much power would you have at the crank?

now as a completely unrelated question but i didn't want to crowd the forum: I dont know if there are any "old school" car fans here but i love chevelles, especially 67's and 69's. But my question is when looking around at some of the other models later than 69 (i think 70+?) I noticed those wierd pin things that i *think* are used on carbon fiber hoods. So are the hoods carbon fiber or whats the deal? I would think it's too soon for it's era, but who knows. Also just how much did those old school hoods way, at least a couple hundred lbs right? And while we're on the subject, why are these devices nessecary when using a carbon fiber hood?

thanks in advance, by the way heres a pic of that hood on a 72 SS chevelle:

[ivymike1031]

The pins are safety hold-downs. They can be used in place of, or in conjunction with, the standard hood latch. There may have been some hoods that heavy, but I've never seen one. I'd have estimated more like 50-80lbs for a heavy one. I had a '78 cherokee for a while, with a tremendous hood on it, and I could remove and install the hood myself, so it couldn't have been all that heavy.

If you had an engine that was 100% efficient, you'd get 3 to 5 times as much power out of a typical gasoline engine. You'd get about 2 to 3 times as much out of a turbocharged diesel (modern diesels are more efficient than modern gasoline engines, by 50% or so).

An example: 2000 Honda Civic Hx, 1.6L engine
displacement 1.6L
peak power 115HP @ 6000rpm
peak torque 110 FT*LBF @ 4500rpm

Torque at 6000 rpm is 100.7 ft*lbf, so let's estimate that volumetric efficiency is about 100.7/110 = 91.5% at that speed. (VE tells you how much air is getting into the engine compared to how much could get in if it were turned very slowly)

At 91.5% VE, 6000 rpm, this engine will pump about 0.481 kg/s of air through it. With an AF ratio of 14.9, the engine will use about 0.032 kg/s of fuel.

If all of that fuel is converted to mechanical energy, with no heat going out the tailpipe, no heat going out the cylinder walls, no friction losses, no pumping losses, etc., you will get about 326 hp.

This estimate gives the Honda Civic Hx engine a very high efficiency, about 35.3%, which makes me suspect I did something wrong. Perhaps I was too pessimistic about the volumetric efficiency of the intake?

If I redo the same calculation for 4500rpm (peak torque), I get the following:

the baseline engine produces 110 ft*lbf at 4500 rpm, or about 94 hp.
if I assume 100% volumetric efficiency, then I get 35.5% again. If I assume 104% v.e., I get about 33.9%. My reaction: wow, that's high.

[Bryan8412]

Wow thanks for the response, thats amazing that so much potential power is lost. How much is lost on its way to the wheels? 20% or something? So that means that 65% is lost in inefficiency within the engine and 20% is lost transfering the energy to the reat wheels. So total when you open up the throttle on a 4cyc, you're only feeling 28% of the true torque that could have been produced, and 72% of your power is lost? (I took 100hp, subtracted 65%, then subtracted 20% from that and got 28hp and assumed matching torque)

EDIT: ok i read through it again and got that a little wrong, which i corrected, but let me get something clear, how can hp/torque = % of VE? For a percentage to be acheived, matching units must be delt with right? (this is my education from physics 1-2 junior year so please excuse my ignorance)

EDIT2: im a little slow sorry. ok so you estimated 110 as the torque at that RPM with 100% VE? that would make sense.

[ivymike1031]

yeah, I assumed that at the peak torque point, the engine was at about 100% v.e. Then I assumed that the torque produced vs rpm would be most significantly affected by breathing (% ve), so if the engine was still getting 100% ve at 6000, it would still be producing about the same torque (110 ft*lbf). I know from the engine specs I found online (at carpoint.msn.com) that the engine actually produces 100.7 ft*lbf at 6000rpm, so I figured that the v.e. had dropped by about 8.5%.

[Bryan8412]

ok thanks again for the response and the clarification. So if it is 3-5x as powerful if it were the same engine w/o energy losses, and we took the mean average to 4x as much on a 100hp engine, it should be producing 400hp at the crank. So if it were to ideally produce that, and at the wheels of the actual engine it lost 20% to come out to a total of 80hp, 80/400 is 20%. So roughly 80% of the engines potential power is lost!? That's amazing, i thought maybe half at most. That's incredible...

EDIT: Also, I know turbocharging helps reduce this by using the pressure of the exhaust gases to compress air, so why dont they, on naturally aspirated engines, harness the wasted power to say, charge the battery or electrical components? Is it because no power will be produced unless they go WOT? And if that is so, why not have it as a back up, this way when you're opening up the throttle, it uses the turbines to power the electrical components, and if the power drops to low the alternator kicks in? Or is the power gained by not useing the alt not large enough to offset the new power lost in the obstruction of the exhaust flow?

Once again, thank you for your patience with my ignorance...

[ivymike1031]

The second thing you mention, using a turbine in the exhaust to run a generator, is done on some turbocharged engines under development. It's one form of what's called turbo compounding. Another form of turbo compounding uses the turbine to help turn the crankshaft, so the power goes to the wheels. They're both primarily intended to increase the specific power output of the engine, but if they're implemented well they may also increase efficiency.

The biggest reason that comes to mind for not using turbocompounding on a naturally aspirated vehicle is COST. There would also be a performance consideration, but it could easily be overcome (in my opinion) by adding a small compressor to the intake. It wouldn't be a N/A vehicle anymore, but hey, if you're already buying 2/3 of a turbo, why not go all the way?

Still, the idea is not bad, and you may see it on production vehicles someday.

[Chris]

Yes, in total, the modern engine converts 20% of it potential energy into usable power. The rest is lost to friction, inertia, sound, heat, and a bunch of other stuff. Kinda sad

[Bryan8412]

thanks for all your responses, and yeah it is sad. but with innovations possibly coming to cars like what ivymike mentioned who knows maybe we'll get some of that power back. and i never thought of it helping rotate the crank and just give the power back (somewhat) thats a neat concept.

[SaabJohan]

Originally posted by Bryan8412
How much is lost on its way to the wheels? 20% or something?

If it's a 2WD losses are around 10-15%. If it's a 4WD losses are around 15-20%.

[texan]

Originally posted by SaabJohan


If it's a 2WD losses are around 10-15%. If it's a 4WD losses are around 15-20%.

That actually depends heavily upon the drivetrain in question, rather than just it's configuration. Some 2wd setups lose closer to 30% of the total power output from the crank. Of course the difficulty in quantifying these figures also comes in how you are attempting to measure drivetrain loss. An inertial dyno is about the worst way to estimate it, but is also the most common way of measuring power output at the wheels.


And ivymike1031, that figure you came up with is within 5% of true from what I have heard. Honda engines may be small and to my liking a bit low on torque output, but they do know how to make an engine efficient. 35% is the current high water point for production gasoline engine efficiency, and you can bet Honda has at least a few engines operating in that range.

[SaabJohan]

Originally posted by texan


That actually depends heavily upon the drivetrain in question, rather than just it's configuration. Some 2wd setups lose closer to 30% of the total power output from the crank. Of course the difficulty in quantifying these figures also comes in how you are attempting to measure drivetrain loss. An inertial dyno is about the worst way to estimate it, but is also the most common way of measuring power output at the wheels.


And ivymike1031, that figure you came up with is within 5% of true from what I have heard. Honda engines may be small and to my liking a bit low on torque output, but they do know how to make an engine efficient. 35% is the current high water point for production gasoline engine efficiency, and you can bet Honda has at least a few engines operating in that range.

The best way to go is by a static test, with a real braked dyno. Then some measure directly on the axles and some on the tires. When tested on the axles some 2WD cars even go under 10%. That a 4WD drivetrain is more complex isn't har to understand, and the more complex the higher the losses are (the same with the automatic gearbox). But the numbers I mentioned is just a rough gudelines for standard cars.

Hondas last F1 engine in the late eighties had an effiency of 32%, and thats a racing engine which is almost 15 years old.

[replicant_008]

One more thought generated by someone mentioning giving power back to the engine...

Flywheel batteries are composed of flywheels circulating in a vacuum casing attached to a DC motor/generator - these are very efficient. Electrical energy is transferred to the flywheel by the motor and then back to energy by taking current off and turning the DC motor into a generator (in fact some solar powered satellites use this technology).

In theory, if you directed the engine power generated during braking or idling and transferred it to the flywheel it would theoretically be available for use when the engine was under duty. With an appropriate auxilary drive system you could use the flywheel power as a source of energy - it could be used for simple stuff like driving accessories (possibly the valvetrain reducing frictional losses) or even supplementing the engine for acceleration purposes.

Either GM or Chrysler had a look at this concept for a Le Mans car but pulled the plug before it went too far. I think the gyroscopic inertia could be a problem in a vehicle with rapid directional changes but that's something to think about....

[Porsche]

Originally posted by ivymike1031
The second thing you mention, using a turbine in the exhaust to run a generator, is done on some turbocharged engines under development. It's one form of what's called turbo compounding. Another form of turbo compounding uses the turbine to help turn the crankshaft, so the power goes to the wheels. They're both primarily intended to increase the specific power output of the engine, but if they're implemented well they may also increase efficiency.

The biggest reason that comes to mind for not using turbocompounding on a naturally aspirated vehicle is COST. There would also be a performance consideration, but it could easily be overcome (in my opinion) by adding a small compressor to the intake. It wouldn't be a N/A vehicle anymore, but hey, if you're already buying 2/3 of a turbo, why not go all the way?

Still, the idea is not bad, and you may see it on production vehicles someday.

Isn't this form of turbo compounding found on aircraft engines? Turboprops as there known?

Or is it a somewhat backwards principle, since a Turboprop (and all jet engines) just use one shaft connected to both the Compressor fan and turbine? So really a turbo prop just uses this same shaft to turn the prop? I guess turboprops are just really big 'open' superchargers then, and have nothing to do with turbo compounding, dut jet engines do use turbo compounding.

Sorry if I've confused anybody, I know I sure am.

[texan]

Originally posted by replicant_008
One more thought generated by someone mentioning giving power back to the engine...

Flywheel batteries are composed of flywheels circulating in a vacuum casing attached to a DC motor/generator - these are very efficient. Electrical energy is transferred to the flywheel by the motor and then back to energy by taking current off and turning the DC motor into a generator (in fact some solar powered satellites use this technology).

In theory, if you directed the engine power generated during braking or idling and transferred it to the flywheel it would theoretically be available for use when the engine was under duty. With an appropriate auxilary drive system you could use the flywheel power as a source of energy - it could be used for simple stuff like driving accessories (possibly the valvetrain reducing frictional losses) or even supplementing the engine for acceleration purposes.

Either GM or Chrysler had a look at this concept for a Le Mans car but pulled the plug before it went too far. I think the gyroscopic inertia could be a problem in a vehicle with rapid directional changes but that's something to think about....

It was Chrysler that tried to develop the flywheel generator eletric racer. Rosen Motors has what's generally considered the best automotive flywheel design, but the technology involved to make it work (and remove gyroscopic effects) makes it very expensive (as a result the company went under). And there's still the remote possibility of failure, where even with a great containment shield all hell breaks loose. Check here for more info.

[texan]

Originally posted by Porsche



Isn't this form of turbo compounding found on aircraft engines? Turboprops as there known?

Or is it a somewhat backwards principle, since a Turboprop (and all jet engines) just use one shaft connected to both the Compressor fan and turbine? So really a turbo prop just uses this same shaft to turn the prop? I guess turboprops are just really big 'open' superchargers then, and have nothing to do with turbo compounding, dut jet engines do use turbo compounding.

Sorry if I've confused anybody, I know I sure am.

Turboprops are just jet turbines spinning props rather than directly providing thrust. Slow and low flying isn't very efficient with direct thrust turbines, but they are the only power generation present turning the prop so it definitely isn't any type of compounding.