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  #1  
Old 06-11-2005, 04:18 PM
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Fuel economy question. Explain please.

OK explain this to me someone. You're going 70 mph, 2500 rpm, you get 25mpg. You're going 70 mph, 2500 rpm, headwind of 20 mph. Engine revs the same, same gear, same amount of air goes in the plenum, fuel economy goes down coupla mpg's. Why?

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Old 06-11-2005, 04:50 PM
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because the engine has to push more to overcome the wind resistance. i heard that something like 20% of the fule that rigs burn is overcomeing wind resistance.
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  #3  
Old 06-11-2005, 05:29 PM
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Quote:
Originally Posted by Meta6981
because the engine has to push more to overcome the wind resistance. i heard that something like 20% of the fule that rigs burn is overcomeing wind resistance.

How is the engine 'pushing more' if it is turning the same speed with and without the headwind?
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  #4  
Old 06-11-2005, 05:35 PM
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Engine speed doesn't really matter... there is more of a load on the engine. If you keep your car in neutral and keep it revved at 3000 rpm until it runs out of gas, it will run a lot longer than a car running on the highway at 3000 rpm. Same thing applies here. While it's turning at the same speed, it has to make more power to do so... which means use more fuel. I hope this makes sense.
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  #5  
Old 06-11-2005, 06:09 PM
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Quote:
Originally Posted by tkamiya
Engine can only suck in the amount of displacement of gas/air mixture per revolution.... So the only way it can consume more gas is to either increase the RPM or raise the ratio of gas in gas/air mixture.

A diesel engine gets all the air it needs.

However, the amount of fuel that it gets is governed by your right foot. If you are on a flat surface and are going 60 mph, you give it about 1/5 pedal, right?

Now, you head up a steep hill. In order to maintain 60 mph, you have to give it about 2/3 pedal, right? This puts more fuel into the combustion chamber and the engine provides more torque to push you up the hill.

Now, since it is an automatic transmission, there is the "slip" factor. The higher the load, the more the slip. So, going uphill, the rpm's will be a little higher than going on flat ground. This is necessary to maintain the speed of the vehicle.

If you had a manual transmission, the rpm's and the vehicle speed would never vary, provided you added fuel on the uphills and removed fuel from the downhills. This is exactly what a cruise control will do. It adds fuel, or backs off on fuel, depending on the load.
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  #6  
Old 06-11-2005, 06:59 PM
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Quote:
Originally Posted by tkamiya
Since the RPM is a measurement of engine revolution and there is only so much air/fuel mixture it can be taken into the cylinders, I can't see how fuel CONSUMPTION (not mileage) per revolution would be different if RPM is constant. UNLESS the mixture ratio changes accordingly...
In a gasser with a carburetor, the throttle plate varies the amount of air pulled through the venturi, and thus the amount of fuel and resulting fuel/air mixture at a given rpm. With fuel injection, the amount of air is varied, the mass of the air measured, and the correct amount of fuel is injected into either the throttle body or the intake manifold to achieve the proper fuel/air mixture.

'Light' throttle allows a small volume of air, full throttle allows the complete volume of the cylinder. Light throttle = less fuel = low torque, full throttle = max fuel = max torque. This is true at any rpm, and at any rpm there is a range of torque values that the engine can produce.

There is no air/fuel mixture taken into a diesel cylinder as such. Air, and only air (exhaust from the EGR, and manifold vapors don't really count) is drawn into the the intake manifold and through the intake valve.

Fuel is injected into the pre-combustion chamber. The amount of fuel injected varies at a given rpm, depending on the postition of the injector rack, which determines the quantity of fuel to be injected.

Now, in the case of a headwind, there is more force resisting the forward motion of the car at a given speed. If we ingnore the small 'slip' of the torque converter (modern transmissions have locking converters at highway speed) the rpm do not change but the 'load' on the engine increases. More fuel must be introduced to make more torque to overcome the load.

At any rpm there is an upper limit to the amount of fuel that can be injected and burned efficiently. If more fuel is injected, it is simply exhausted as incompletely burned fuel, or black smoke.

In a gasser too much fuel in the fuel/air mix will will show up as black smoke in the exhaust, but if the mixture is made even more 'rich' it will not ignite and combustions stops.

In a diesel, surplus fuel will also show up as black smoke, but it is almost impossible to inject enough fuel to stop the combustion process. Before that happens, huge amounts of smoke and even liquid raw fuel will run out of the exhaust...

Best Regards,
Jim

Last edited by Jim H; 06-11-2005 at 07:13 PM.
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  #7  
Old 06-11-2005, 08:10 PM
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How did you know you were in a head wind and how did you know the head wind was constant (direction and velocity) through out the journey?
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  #8  
Old 06-11-2005, 08:52 PM
laurencekarl
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Quote:
Originally Posted by Kuan
OK explain this to me someone. You're going 70 mph, 2500 rpm, you get 25mpg. You're going 70 mph, 2500 rpm, headwind of 20 mph. Engine revs the same, same gear, same amount of air goes in the plenum, fuel economy goes down coupla mpg's. Why?
Think of it this way. There is a bar attached to a spindle like they used to use to grind wheat or something. And you are at the end of this bar pushing. Now lets say that nothing is attached to the spindle it is quite easy to push the bar one revolution per minute. Now attach a heavy weight say a 200LB rock to the spindle. Now to maintain that same 1 revolution per minute you have to push much harder or input more energy for the same number of revolutions. In a car the force (equivalent to the mass attached to the bob*g) you are experiencing going up a hill is an increase in the acceleration of the earth i.e. is when you are flat you mainly experience gravity or the earth's acceleration of the mass of your car directed through the center of mass of the car straight towards the ground which does not affect your forward motion. When the car is at an angle to the direction of the earth's gravitational field (acceleration) then it does affect your forward motion in addition to the previous resisistors to motion (friction). Torque is measured in energy and to maintain the same number of rotations you have to have more torque or more energy which in this case is extracted from petrol.

Torque which is defined as radius (or distance from the application of force to the center of mass of the object being acted upon) X (cross product not multiplication) Force you are exerting. In the above case r = the distance from your hands on the bar to the spindle, Force = the acceleration you have to apply to the bar * the mass applied to the spindle.

Torque is measure in units of energy (kg m^2 s^-2 = Joule)
The equations are here:

http://scienceworld.wolfram.com/physics/Torque.html
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Old 06-11-2005, 10:08 PM
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Quote:
Originally Posted by Kuan
OK explain this to me someone. You're going 70 mph, 2500 rpm, you get 25mpg. You're going 70 mph, 2500 rpm, headwind of 20 mph. Engine revs the same, same gear, same amount of air goes in the plenum, fuel economy goes down coupla mpg's. Why?
with the headwind, you now need a wider throttle opening to maintain the same speed. Your intake manifold pressure will now be higher, and this means that more air/fuel is now going into the engine.

you can see this in your fuel economy gauge.
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  #10  
Old 06-11-2005, 11:16 PM
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Quote:
Originally Posted by bobbyv
with the headwind, you now need a wider throttle opening to maintain the same speed. Your intake manifold pressure will now be higher, and this means that more air/fuel is now going into the engine.

you can see this in your fuel economy gauge.
It appears you are speaking about a gasser. Gassers have throttles and gassers regulate air and fuel.

So, if it is a gasser, the intake manifold vacuum will be lower at wider throttle openings. All gassers (exception with turbo gassers) operate with intake manifold vacuum under all conditions.

If you are referring to a turbocharged diesel, there is no throttle and you add more fuel via the rack in the injection pump to maintain the same speed. Now the intake manifold pressure will be higher because more air is being forced into the engine by the turbo.
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  #11  
Old 06-11-2005, 11:19 PM
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Kuan,
Thanks for asking. And thanks to those who answered. This has been most informative.
'More pedal to produce the same engine response in the same gear into a headwind'
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  #12  
Old 06-13-2005, 03:18 PM
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Isn't it the same as installing identical engines in cars weighing, say, 2000 and 4000 lbs respectively? The latter will produce fewer miles per gallon.
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Old 06-13-2005, 05:25 PM
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I used to fly airplanes with recip engines. With constant speed props, the engine rpm could be maintained pretty closely. Push up the throttle and the manifold pressure goes up. Fuel consumption goes up to match. Cars work the same way, except we usually talk about manifold vacuum going down vs. pressure going up. Same difference.

The original question didn't specify gas or diesel, but the idea's the same. More pedal, more fuel.
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  #14  
Old 06-13-2005, 05:48 PM
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Quote:
Originally Posted by cscmc1
Isn't it the same as installing identical engines in cars weighing, say, 2000 and 4000 lbs respectively? The latter will produce fewer miles per gallon.
to make this comparison, we assume that the two cars are identical (i.e., body, running gear, tires, etc) except in weight.

assuming both cars are accelerated with the same throttle opening, the heavier car will take longer to accelerate to the desired speed than the lighter car. At any point in time, the lighter car will have covered more distance than the heavier one - hence it will have more miles for a given gallon of fuel.

if you accelerate both cars at the same rate (assuming this is possible), the heavier car will require a wider throttle opening than the lighter one, and will use up more fuel for the given distance.

if you measure the fuel economy of both cars once they have reached the desired speed, their fuel consumption per distance should be the same.

however, this will not be EXACTLY the same, because the heavier car will have more internal friction than the lighter one, due to higher loads on the wheel bearings. Although this will tend to increase the fuel consumption (i.e. wider throttle opening to maintain the speed), this difference is almost negligible.

however again, all other things being equal, the heavier car will sit lower than the lighter one, and will have a smaller frontal area than the lighter one due to less tire frontal area. The drag coefficient would also probably be better in the lowered car, due to less turbulence created by the air flowing under the car. A smaller frontal area and drag coefficient will tend to lower fuel consumption for the heavier car in this case (i.e. smaller throttle opening to maintain the speed)

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