# Conservation of Energy Question with Internal Combustion Engine -> linear tran rpm vs. exponential aerodynamic load

• posted

With a manual transmission vehicle, how does the car through one gear as the RPM increases linearly, an hence consumes fuel linearly while the load on the vehicle increases exponentially due to the drag. This would mean individual cylinders would have a wide variation of energy per explosion. Does this happen with elecronic fuel injection systems? Or rather, are there just loses interms of incombusted fuel and localized temperature variations in each cylinder.

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• posted

Well, based on your wording, I'm not sure what your question is here. As far as your message title is concerned though, energy is conserved through the engines heat transfer to the enviroment. It doesn't matter if you have a manual or auto trans. or what the drag is on the vehicle, you're moving the mass of the car...and eventually against a drag force . The explosion in each cylinder will result in work (Pv work) on the crank. Losses from unburnt fuel would be from poor valve timing, engine design..etc.

If you are trying to relate drag back to an engines ability to fire correctly, thats wrong. The drag just increases the amount of work the engine has to do. Not only does the engine have to accelerate the car, it has to work against the air now. Engines are not like motors where more instantaneous current will be drawn under load. IC engines bring in a mixture of gas and air, compress it, light it up and what results is work(Pv) onto the crank, which moves the car, F*D work. You can rev an engine to 3500rpm under no load and all that energy will be turned into useless heat(entropy). You can also rev it to

3500rpm, engage the tranny and have the engine do work on the car, which slows the rpms but consumes the same amount of fuel, fuel injected or not.

SoCal wrote:

• posted

For a constant throttle setting, you can suppose (not quite accurately) that shaft HP (and fuel consumption) is proportional to engine revs.

You can reasonably suppose that a car needs higher HP to go faster: due to a combination of linear drag and square law drag.

Given these two facts, you can easily deduce that acceleration decreases at increasing speeds. and fuel consumption increases with speed - both of which are non- linear.

Brian Whatcott Altus OK

• posted

Perhaps I should have worded it a bit differently. Manual was given as an example because of the lack of slippage.

But my question is really, does an individual explosion in cylinder generate a significantly different amount of energy and possibly use a different amount of fuel each explosion. Your answer suggests a constant amount of fuel per explosion. So then, there should be a tremendous range of MPG within a single gear? Do the exhaust temperatures fluctate wildly along with the load on engine cooling system.

This then leads me to believe variable-sized explosion should be implemented or most cars should have 2-4-8 cylinder activation.

• posted

No. All cylinders will fire with the same amount of fuel unless you are talking about a time based thing here with the engine changing revs(e.g. opening the throttle while looking at each cylinder intake). At a constant RPM, all cylinders will consume the same amount of fuel. There will be a big range in MPG while rev-ving up(increasing RPMs) through one gear because the engine and gear will have a peak efficiency at a certain point. For example, in first gear the peak efficiency would be moving the car at around 5 mph. Any faster of an engine RPM would do nothing except waste fuel, because the flywheel would want to spin faster than the driveshaft and the driveshaft would want to spin slower than the flywheel (compression braking), at this point if your foot was off the pedal, the cylinders would be getting an idle fuel-mixture.

In theory, the exhaust temperatures will change but it would be so fast I doubt there exsists a thermometer to measure it reliably relative to that small amount of time. Now if you were to redline your engine, a lot of excess heat would be generated and might exceed the cooling limits of your radiator. This is why police cars have not larger engines, but larger cooling systems than normal cars. I see what you're getting at with the firing order however I think the technology implemented would outweigh the benefits. BTW...What I'm telling you is based on my thermodynamics classes, experiences and reading. I'm sure an engineer who deals in engines could further clarify this stuff, but probaly not in the easy terms as I have tried to:) Aside from the basic thermodynamics books, I have encountered two good books on the subject of IC engines.

Pulkrabek, Willard W. "Engineering Fundamentals of the Internal Combustion Engine"

Ferguson, Colin R. Kirkpatrick, Allan T. "Internal Combustion Engines: Applied Thermosciences."

SoCal wrote:

• posted

Brian Whatcott wrote in news: snipped-for-privacy@4ax.com:

According to my mpg simulator, in any gear but first gear the aero effect is so strong that max mpg is achieved at minimum rpm. In first gear the aero effect is relatively small, so the engine efficiency map becomes more important, and optimum mpg is achieved at 10 mph or so. The best mpg in obtained in 5th gear, at minimum rpm, that is, 21 mph.

Note that these estimates use the engine efficiency map from a Prius, but the simulation assumes a manual gearbox.

Cheers

Greg Locock

• posted

Hmmm... In a Prius simulation, you mention...

1) Except in 1st gear, max MPG occurs at min rpm. 2) in first gear, best mpg occurs at 10 mph. 3) in fifth gear best mpg occurs at 21 mph.

I don't have comparitive data handy, but I recall reading that one or two cars have had best mpg at 30 to 35 mph. Don't know which though.

It would seem reasonable that low engine revs would favor better fuel efficiency: less engine drag, better cylinder filling but most of all, slower going over the road reduces air drag and wheel bearing drag and transmission losses.

On the other hand, low throttle openings induce bigger pumping losses.

Brian Whatcott Altus OK

• posted

Yes. That's what the throttle does. By restricting the intake airflow, you change the amount of air in the cylinder. Since most modern engines run in a very narrow air/fuel ratio band, the fuel scales linearly with the intake air volume, to first order.

There is. Any car that has an instantaneous MPG gauge will show that.

Yes. Any heat not removed as mechanical work and the cooling system comes out in the exhaust.

It is. That's what the throttle does.

That works too (Mercury Marine has had it in outboard motor engines for years) but a throttle is a lot simpler.

Tom.

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