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What about fuel types & gas mileage?

297 messages,  Last post on Jan 20, 2013 at 2:20 PM

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What is this discussion about? Engine, Fuel System

Messages Page 26 of 31

#246 of 297 Fuel Mix by imidazol97

Jul 11, 2007 (1:19 pm)

I thought the computer kept the mixture the same. The air valve in the throttle body lets more or less fuel mixture into the intake manifold to give more power.
 
What is the difference in air resistance at 900 feet altitude compared to 0 feet?

#247 of 297 Re: Gas Mileage [calisteel] by elias

Jul 11, 2007 (1:22 pm)

Replying to: calisteel (Jul 10, 2007 5:29 pm)
thanks calisteel. your point about altitude is well-taken.
also, it explains why I've been seeing such lousy mpg while driving in the Marianas Trench.

#248 of 297 Re: Gas Milage [calisteel] by shipo

Jul 11, 2007 (8:16 pm)

Replying to: calisteel (Jul 10, 2007 5:29 pm)
"ALTITUDE will determine how lean or rich your car will run. Higher altitude will provide better gas mileage if your car is tuned properly for the higher altitude."
 
Ummm, no. Modern fuel injected cars measure the weight of the air coming into the engine and provide the exact same amount of fuel per pound of air at all altitudes and all temperatures and all humidities. Fuel economy as measured by the conventional Brake Specific Fuel Consumption (BSFC) measurement won't change one iota regardless of any of the above "Density Altitude" factors.
 
"This is why airplanes fly at the highest altitude they are capable of. When flying a piston engine aircraft, you manually lean the fuel mixture after reaching cruise altitude, you leave the fuel mixture at full-rich while climbing."
 
Ummm, no again. GA aircraft can extend their range at altitude simply because air resistance against the airframe is lower at altitude. Said another way, I take out say a Skylane and set it up as lean as possible to cruise at 100 knots in level flight at sea-level, and then measure the hourly fuel flow. I then take that plane up to say 17,000 feet and lean out to the exact same hourly fuel flow setting, that plane will have a true airspeed of 137 knots. That, and only that, is why aircraft get better economy at altitude.
 
FWIW, I've seen anecdotal evidence that suggests that cars will see some gains in economy at altitude (but no where near as dramatic as with aircraft due to the rolling friction of the wheels), especially turbo-charged cars.
 
Best Regards,
Shipo

#249 of 297 Re: Gas Milage [shipo] by Mr_Shiftright HOST

Jul 12, 2007 (9:21 am)

Replying to: shipo (Jul 11, 2007 8:16 pm)
I would think you'd need to also advance engine timing at altitude to maintain the same fuel mileage. But perhaps the re-calibration of the fuel mixture negates that necessity?

#250 of 297 Re: Gas Milage [Mr_Shiftright] by shipo

Jul 12, 2007 (9:22 pm)

Replying to: Mr_Shiftright (Jul 12, 2007 9:21 am)
General aviation engines have fixed timing and zero ability to adjust it one bit from where the factory set it and where the FAA certified it.
 
That said, stoichiometric is stoichiometric is stoichiometric. Given the proper air and fuel ratio, the mixture will burn exactly the same regardless of whether the aircraft is at a density altitude of sea-level or 17,000 feet.
 
Best Regards,
Shipo

#251 of 297 Re: Gas Milage [shipo] by Mr_Shiftright HOST

Jul 13, 2007 (7:58 am)

Replying to: shipo (Jul 12, 2007 9:22 pm)
I think that pre-computer, automobiles required a bump in timing (timing advance) to compensate for the carburator's inability to adjust proper fuel mixture at altitude.

#252 of 297 Re: Gas Milage [Mr_Shiftright] by shipo

Jul 13, 2007 (8:31 am)

Replying to: Mr_Shiftright (Jul 13, 2007 7:58 am)
Hmmm, thinking back to the 1970s (when I turned a wrench to make a living), I don't remember tweaking the timing to help cars deal with altitude, that said, I do remember rejetting any number of carburetors.
 
Best Regards,
Shipo

#253 of 297 Re: Gas Milage [shipo] by Mr_Shiftright HOST

Jul 13, 2007 (5:14 pm)

Replying to: shipo (Jul 13, 2007 8:31 am)
Well the bump in timing was great for cars that were going up 5,000 feet for a few days--you wouldn't want to re-jet just for that.

#254 of 297 Re: Gas Milage [Mr_Shiftright] by shipo

Jul 13, 2007 (10:07 pm)

Replying to: Mr_Shiftright (Jul 13, 2007 5:14 pm)
Ahhh, got it. If one of the cars I worked on in San Diego was only heading up to the 5,000 foot mark for a few days, I wouldn't bother touching the car, however, the bump in timing doesn't sound like such a bad idea. The only cars I rejetted were the ones that were heading up into the Rockies or the Sierras to stay (or at least to stay for a considerable length of time).
 
Best Regards,
Shipo

#255 of 297 Re: Gas Milage [shipo] by Mr_Shiftright HOST

Jul 14, 2007 (7:23 am)

Replying to: shipo (Jul 13, 2007 10:07 pm)
Oh advancing the timing definitely works--dramatic improvement in fact.

Messages Page 26 of 31

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