02-12-2012, 06:33 PM | #1 |
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Highway fuel economy
For the 35i, at what highway speed does the car get the best MPG/ fuel economy? I would like to set my cruise control to that speed. And yes I'm on the highway right now
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02-12-2012, 08:07 PM | #2 |
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The lower the speed the better. The car probably gets its best gas mileage between 30-45MPH. The energy required as a function of speed is a v^3 function. Double your speed and the energy required increased 8 times.
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02-12-2012, 08:44 PM | #3 |
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Going on cruise set at 75 mph for 4 hours I got 28 miles to the gallon. I was happy with that.
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02-13-2012, 08:32 AM | #4 | |
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I think that modern engines and fuel management systems are too pro-active in maximizing performance to reduce fuel economy to a two term equation. I just drove to and from Montreal (336 miles each way) and with 4 people on board and some light luggage I got 23mpg with an average speed of 70mph. The temp was -12C (10F) which didn't help matters. |
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02-13-2012, 10:13 AM | #5 | |
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Some losses are just a function of speed such as wind resistance. Since wind resistance is the really big hitter at constant cruise controlled higher "highway" speeds, the energy needed to propel the vehicle is indeed a function of the cube of the speed. At "highway" speeds there is essentially no braking or acceleration so windage is the only consequential energy loss. At lower speeds" windage is negligible. The big hit energy losses are due to breaking where all the vehicles kinetic energy is converted to heat. This energy is recoverable (regenerative breaking) and that is why hybrids do so well in traffic but no better than non-hybrids at higher speeds where brakes are not used very much. Of course, the weight of the drivers foot is also a factor as the vehicle's engine must abide by Newton's second law. That is, the force needed to accelerate a car (mass) is directly proportional to how quickly you want to accelerate it. Higher acceleration means greater force which means more gasoline. At a constant cruise controlled speed the ONLY energy needed is that necessary to overcome losses as the vehicles inertia (Newton's first law) will keep the car going forever if it was not for the losses. The actual biggest energy loss is due to the inefficiency of the thermodynamic cycle that governs the engine. Greater than 2/3 the energy extracted from the gasoline is peed away out of the radiator and engine block. However, there is some point that the engine is most efficient so, when considering the rpm for optimum engine efficiency, braking, acceleration, and windage, that is why there is some speed at which the curve peaks. This point is invariably well below "highway" speeds. Bearing losses exist but are negligible so are typically ignored. In other words, you can't F%$$ with mother nature when it comes to mpg. The faster you make the car go, the lower your mpg (unless, of course, you choose to drive in a vacuum). At highway speeds the aerodynamic slipperiness of the car is the dominant factor. At lower speeds the energy losses due to windage are essentially zip and braking/acceleration losses/inefficiencies dominate. A hybrid recaptures the low speed breaking losses but can't do squat for windage. Acceleration in a hybrid, is the result of a much more efficient energy conversion process (battery and electric motor vs an internal combustion engine). BTW, temperature is a double edged sword. At lower temperatures the air is more dense. This means higher windage but more oxygen per unit volume of air sucked into the engine. I have no clue how significant these factors are or which dominates. I do remember from many years ago a friends GTO seemed to accelerate better on cold days. Last edited by RhoXS; 02-13-2012 at 11:36 AM.. |
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02-13-2012, 10:46 AM | #7 | |
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02-13-2012, 03:17 PM | #8 | ||
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At low speed losses are (in approximate order of significance) - Heat lost in the engine - Braking / Accelerating - Vehicle accessories (A/C, steering pump, transmission pump, radio, lights, ect) - Tire rolling resistance - Aerodynamics At high speeds losses are (in approximate order of significance) - Heat lost in the engine - Aerodynamics - Tire rolling resistance - Vehicle accessories (A/C, steering pump, transmission pump, radio, lights, ect) Here is an interesting article Tesla Motors put together for their electric vehicles range versus speed. Relevant, but not perfect since it does not have a combustion engine! http://www.teslamotors.com/blog/road...ency-and-range |
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02-13-2012, 09:23 PM | #9 |
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Not so. Keeping he same engine conditions helps, but cruise control aims to kEep the same speed instead. On hilly routes in particular cruise control will do much worse as it accelerates, coasts, and brakes to keep the set speed.
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02-14-2012, 03:27 AM | #10 | |
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Back in the 50s and 60s there used to be a yearly long distant economy run that was always a feature article in Popular Mechanics. I think it was called the "Mobile Economy Run". They would take, if I remember correctly, a stock automobile and drive it across country and achieve some truly phenomenal average mpg numbers, much greater than anything normally achieved. I remember reading how they did it. They used very well practiced and disciplined drivers that held their entire bodies and accelerator foot extremely still. I read an article by one driver that even stated talking would reduce his economy numbers. If they slowed going up a hill they did not compensate by giving more gas. They anticipated stopping and used the brakes as little as possible, kept all the windows up for the least wind resistance, accelerated as slow as they reasonably could, etc. So, yes, using cruise control would not do as well here because the cruise control would add fuel and accelerate to maintain the preset speed. Of course, a "however" is now coming with respect to the cruise control. However, if you carefully watch the speed of the average driver without cruise control, you will notice the speed varies quite a bit over a band of a few mph. This constant speeding up (accelerating) and slowing down is noticeably less fuel efficient than the constant speed maintained by a good cruise control. So, unless you want to drive ridged still, not talk to your passenger (that might be a good thing), and have practiced maintaining complete control over your accelerator foot, cruise control will allow you to use less gas. |
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02-14-2012, 04:08 AM | #11 | |
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With engine speed you are balancing the inertia effects of the engine parts accelerating,the piston friction, and combustion efficiency with the heat lost through the cylinder walls. To wager a guess, I would imagine that the most efficient RPM for your engine to operate is slightly higher than what you might expect. The designers also had to balance noise and engine lifespan into the equation as well. |
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02-14-2012, 12:12 PM | #13 |
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Driving speed is every ones preference, minimize applying brakes to get best fuel economy. maintain distances and use brake only when needed. With this technique, I have even got 28 mpg in city driving.
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02-14-2012, 12:50 PM | #14 |
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If you drive a petrol 4x4 should you really bother about economy. If you did you would drive a Prius!
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02-14-2012, 02:55 PM | #15 |
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Here is my real world fuel mileage. I've had the car 5 1/2 months, driven 15269.2 miles and used 723.325 gallons of 93 octane and once in a while 92 octane. Overall 21.1 mpg. My best for a tank was 24.03, 364.6 miles, 15.172 gal. These are not estimates from the vehicle but calculated from the amount of fuel I had to replace in the tank. I don't drive slowly though.
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02-14-2012, 03:19 PM | #16 | |
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