20-22mpg!

[SpeedingFirebird]

Originally posted by slow6:
option #3: neither ac, nor windows down

rhouse1, we have gears to help the engine
</font><blockquote>quote:</font><hr />Newton's Laws don't care about gears. It takes the same amount of energy to maintain this cruise speed (and level of momentum) regardless of what gear you're in.

newtons laws do care.
and sometimes its easier for the engine to stay at same speed by being in a higher gear than lower gear. </font>[/QUOTE]That was my point about momentum. Not the car's, but the motor's momentum.

[HokieBird]

Originally posted by SpeedingFirebird:
</font><blockquote>quote:</font><hr />Originally posted by dz:
While we're on this topic, is it better to run air conditioning in your car or is it better to have the window down?

I'm imagine having the window down increases drag by letting air into the car, but having the air condition on taxes the engine.

I'm not sure which one would be more efficient though :-/

Mythbusters did a show on this last year. They found the windows down to be more fuel efficient. </font>[/QUOTE]I think the areo dynamics of the specific car would have a major affect on that. Just curious what car did they use for their testing?

[SpeedingFirebird]

Originally posted by rhouse21:
Ok. This is from my brother who did his masters degree at the University of Michigan specializing in engine controls:

and I have a Master's degree from RPI and working on my Ph.D.

Originally posted by rhouse21:


Ok, let's not make this into battle of the engineers.

Nope, a friendly tech discussion among friends. :D

Originally posted by rhouse21:


So, if every pulse is burned at the optimum fuel/air ratio, the more pulses you have per second, the more fuel you have to pump into the engine to maintain emissions.

To use his words, “more revs = more pulses = more fuel.”

This completely discounts throttle position. More pulses with your foot OFF the throttle will not lead to more fuel consumed. Fewer pulses with WOT will burn more fuel.

You did ask a qood question before. To paraphrase, why have overdrive gears is higher RPMS are more fuel effecient? Good question. My thought there is to save the motor by running lower RPMS.

Originally posted by rhouse21:

If you don’t agree with my conclusions, sit down and work through the equations.

yuck. After I passed my quals, I vowed never to open my calc book again! [img]graemlins/barf.gif[/img]

Maybe BMW does have a trick. Based on that gauge and my intuition, higher RPMS = lower fuel consumption than lower RPMS.

[rhouse21]

True, there is a certain amount of momentum carried by the flywheel and the crank, but a good chunck of that gets eaten up by the effort needed to pump the pistons back and forth in the cylinders. Just as an experiment, park your car and rev your car to about 4,000 rpm. Now let your foot off or the gas and watch how quickly it falls back to idle. Now, note how long it takes to rev it back up. The speed that your engine gains and looses revs is all a result of the engine's internal drag and the rotational moment of enertia of your engines internal components which dictates how quickly your engine can gain "angular momentum" (rotational speed). The lighter the internal components, the lower their rotational moment of inertia, the faster the engine revs. As you probably know, this is why the big dogs do so much to cut down in the rotating mass in their engines and why and why my wife's TSX can rev to 7,000 rpm in about the same time my engine takes to get ro 5,600.

... And you're right. Higher Flywheel momentum probable do a little to help you maintain speed, but you have to burn so much energy to keep the revs high it's not worth it. Just to prove this point, try another experiment. Fill up your car and reset your trip computer. Now hop on the highway and cruise for about an hour one gear higher than you normally would so your car maintains a higher rpm than usual. Drive back to that gas station, fill up again, and compute your mileage. Tell me what you find?

[ September 15, 2005, 02:48 PM: Message edited by: rhouse21 ]

[SpeedingFirebird]

Originally posted by HokieBird:
</font><blockquote>quote:</font><hr />Originally posted by SpeedingFirebird:
</font><blockquote>quote:</font><hr />Originally posted by dz:
While we're on this topic, is it better to run air conditioning in your car or is it better to have the window down?

I'm imagine having the window down increases drag by letting air into the car, but having the air condition on taxes the engine.

I'm not sure which one would be more efficient though :-/

Mythbusters did a show on this last year. They found the windows down to be more fuel efficient. </font>[/QUOTE]I think the areo dynamics of the specific car would have a major affect on that. Just curious what car did they use for their testing? </font>[/QUOTE]Ford Exploders.

[SpeedingFirebird]

Originally posted by rhouse21:

... And you're right. Higher Flywheel momentum probable do a little to help you maintain speed, but you have to burn so much energy to keep the revs high it's not worth it. Just to prove this point, try another experiment. Fill up your car and reset your trip computer. Now hop on the highway and cruise for about an hour one gear higher than you normally would so your car maintains a higher rpm than usual. Drive back to that gas station, fill up again, and compute your mileage. Tell me what you find?

I think there are too many variables for that to be a controlled experiment.

I guess we will have to agree to disagree until we can find more data. [img]graemlins/stickpoke.gif[/img]

[rhouse21]

No problem. I'm married, so agreeing to dissagree is what I do best. :D

I actually did the "low-gear experiment" once, but not on purpose. I was driving to Michigan from Boston or DC (it was a few years back so I do remember which). Anyway, I pulled into a rest area in the middle of the night, filled up, hopped back on the highway, and set the cruise control. This was a flat road in the middle of the night with almost no traffic. As you probably know, we can actually increase and decrease our speed in 1 mph increments through the cuise lever, so I didn't have to touch the gas or the brakes for hours.

After a while I start to wonder, "Why are my revs and engine temp so high?" I looked around the cockpit and discovered that somehow, while in my state of coffee-supported-conciousness at the rest area, I had managed to shift right through Overdrive into 3rd and had left it there for at least an hour, maybe two. :eek: I immediately shifted back into OD, watched the coolant temperature fall, and cruised for the rest of that tank. Every other tank that trip was ~28-30 mpg. If I remember correctly, that one was ~25 mpg.

No, it wasn't the most scientific of experiments, but it made me a believer. I'll go back through my mileage spreadsheet and see if I can actually find the data for that trip, just to satify my own curiosity.

[aa35199]

Alignment! I had a busted tie rod end and bad ball joints, fixed every thing and had an alignment done. I jumped from 17.5 mpg freeway to 27 mpg freeway. In my case my steering was really shot (you could wiggle the the front wheels with your hands). Its not all engine related some times theres alot of resistance from a bad alignment or worn chassis parts (ball joints, tie rod ends, etc). take it in, also a fresh alignment will do wonders for your handling, and in my severe case it improved my acceleration greatly.

[slow6]

rhouse1, i sometimes cruise in a lower gear on accident dont realize it
auto

[j10stuckinmud]

i have heard that it is much more efficient to use the ac then roll down the windows.

[saltnvinegar6287]

Someone make a scan of injector pulse at a certain speed in 3rd gear, then multiply ionjector pulse by the 3rd gear RPM. Then go the same speed in 4th gear, and multiply those injector pulses by 4th gear RPM. Which uses more fuel per minuite?

3,000 RPM in 3rd or 2,000 RPM in 4th. Since you were going the same speed in each test, you dont have to worry about distance traveled, just fuel injected per minuite.

[Turbo V6 Camaro]

Originally posted by saltnvinegar6287:
Someone make a scan of injector pulse at a certain speed in 3rd gear, then multiply ionjector pulse by the 3rd gear RPM. Then go the same speed in 4th gear, and multiply those injector pulses by 4th gear RPM. Which uses more fuel per minuite?

3,000 RPM in 3rd or 2,000 RPM in 4th. Since you were going the same speed in each test, you dont have to worry about distance traveled, just fuel injected per minuite.

only problem with that is the VE% if its making more power from the same amount or less, or more from % of fuel minus % power made = more then even though its using more fuel that little more could be pushing a bit farther or it could be using more fuel but the motor is not working as hard there for its still better.....

[saltnvinegar6287]

whatever he said. Now i know who to go to when i get my tuner

[camaroman1181]

personally, ive found that my car gets better gas mileage when kept in teh power band. case in point. i do a lot of highway driving. i get 28-30 mpgs when i'm cruising at 85-90... i get 26-28 when cruising at 75-80.. my car is a 5 speed. at 80, i'm sittin at 2500 rpms.. at 90, its closer to 3000. i too have a little engineering degree and i understand all the math being thrown around...i'm just saying that i believe we are not taking all things into consideration with simply the math being used thus far. i've done this experiment with my car and have found that low rpm's dont always the good fuel economy make. you wont lug your car up a mountain in 5th instead of shifting into 3rd and say that you are saving gas. to keep it moving in 5th, you gotta have it to the floor, whereas in 3rd, you are in the power band of the engine and dont require the pedal to be down soo far... both cases you could be going the same speed but in 3rd gear, you are putting less gas into the engine.
just my 2 cents...

[saltnvinegar6287]

less gas per rotation, but im wondering if its enough where even with more rotations per minuite, you are still averaging less gas.