When I drive on the highway in my sister's Volvo S60, I carefully watch the computer and see the instantaneous fuel economy. Its peak torque surprisingly is 1500rpm which at 55mph, is the speed I'm going at on the highway. Because it's at its peak torque at that speed, when climbing grades on the highway, I don't have to downshift. So I use cruise control and have the car maintain 55mph while in 5th gear w/torque converter locked up. So if I'm on a flat portion of highway, computer says something between 35mpg-42mpg, on a down grade it can go up to 99mpg especially if DFCO is into effect but when I go up a grade, the instantaneous MPG drops to usually about 27mpg but sometimes as low as 20mpg! So I'm wondering, since the car is maintaining its RPM and MPH, the only thing that can be changing is the throttle body, the ignition and fuel timing and amount, right?

So with the above in mind why would the fuel economy go down when going up a grade and improve on the flat or on slight decline? The things I can think of is more air is getting into the engine b/c throttle plate is fully open which means more fuel, the ignition timing is being retarded and the air-fuel ratio is going from 14:1 to 12:1 possibly.

Any thoughts on this? But what exactly is preventing the engine from getting the same economy on a grade compared to flat road? Only thing I can think of why the above happens is because the engine would knock otherwise??

 

You need more hp from the engine to go up a grade than down or flat- thus more air and more fuel. Just because an engine is operating at an rpm does not mean that at that given moment it is putting out a certain amount of power. It take a load on the engine to make the power possible at any given RPM to move the car- thus with the car moving down the highway on a flat or downward slope you only overcoming the coefficient of drag and static friction between the road and the tires- this amount of fuel that is taken to maintain the vehicles speed is relatively minimal, but as you increase speed you also increase the amount of air the vehicle is displacing and the rate that the friction between tire and road must be overcome- this also uses more fuel air- but at the same time if the car does this difference at a rate similar at the slower speed then it will not lose "mpg". Going up a grade you are putting a load on the motor greater than you had before when you were moving the car across the ground or downward slope- thus you need more power to overcome not only the friction of tire to road and drag, but also the weight of the car- (degree of slope x acceleration of gravity x mass of car) plus the weight of the car intially- this equals out to be greater than what it had taken to move the car on a flat plane


Long story short you go up hill your cars weight increases thus greater load on your motor needs more fuel and air- less mpg

Go down hill gravity works opposite for you and your cars inertia/weight overcomes its coefficient of static friction with the ground PINE DERBY style and the thing uses no more fuel than is needed to idle along at a given rpm.

Go to engineering school if you are truly intrigued by all of this- then you could map it out on your own!

 

or in layman's terms. try pushing a car uphill and try pushing a car downhill.

 

Actually don't you need more torque, not HP to go up a grade? I mean yes HP is a function of torque but I wouldn't want to have to rev my engine to redline when I could have an engine that has more low end torque in the first place. Anyways, your post doesn't elaborate HOW the car uses/recieves more power. I thought I covered the points in my post as to how the car could more use fuel and I was hoping I could receive some feedback on it but instead you came out with that long *** post which basically said you need more power to go up a grade and less on flat or going down a grade, but that doesn't evaluate the in between, how do you get more power when maintaining a given RPM and MPH?

 

load changes. you can hold 2000 rpm which will keep a certain mph unless your tires are spinning. when you go uphill, the load increases to maintain 2000rpm at whatever mph that is. when you go downhill, the load decreases to maintain 2000rpm at whatever mph that is.

 

I thought I covered that, we already know the load increases but just because the load increases, it doesn't mean it uses more energy. If you put a fan on full power and then slow the fan blade down, it's not like it's going to use more electricity than if you had no fan blade altogether, though you might end up spinning the motor far too fast since there will be no load.

 

well, if the load (going uphill) increases and you're throttle pedal (engine load) does not increase, you will not maintain the 2000rpm.

 

your gas mileage is not going down because of more air... well, it is because you're going to open the throttle plate - which in turn will up your manifold pressure - which in turn will require more fuel - which in turn your gas mileage goes down. all this of course applies only if your cruise control is engaged to maintain that 55mph at 1500rpm.

 

Yeah but that's assuming the ONLY reason why there is less power at partial throttle vs. full throttle is because of the air restriction, but if that were true, you'd be getting so much worse mileage at partial throttle vs. worse, but not a whole lot worse. Also the effects of partial throttle at a low RPM should be less than at a higher RPM because there is still sufficient time to fill the cylinders with air, so you can't necessarily say you're starving the engine of air at 1500rpm like you are at 4000 or 6000rpm.

 

to get back on topic, the change in throttle body is the load. the change in throttle angle changes MAP.

there is less power at partial throttle vs. full throttle because of MAP. why does a car on 18psi of boost make more than a naturally aspirated engine? MAP.