ChalieKilo3

I guess it's news to me that basic physics and aerodynamics are "mumbo jumbo". So much for five years in college and two degrees in engineering.

I tried to explain the issue in as plain and clear language as possible but I'm not a trained teacher so I may be missing the mark. The core of my message is that no matter how sleek and aerodynamic (or not) an object is, the faster you push it through air or other fluid, the higher the drag and the more energy required. This drag at normal auto speeds is proportional to the square of the speed. Of course, up near the speed of sound things get a little more complicated but now we're really getting into "mumbo jumbo".

This was the whole point of the 55 mph limit imposed during the 1970's. If you get everyone to slow down from 70 mph to 55 mph you save an enormous amount of gas. While I certainly objected to this at the time on a whole host of other grounds, there's basic physics behind the argument that if we slow down the fleet of US cars, trucks, and busses, we'd save considerable amounts of fuel.

On your other point, fuel consumption is not simply proportional to engine speed. There's a little thing called torque that gets involved. Let's say you're driving on level ground in 5th gear. You'll run a specific rpm (proportional to speed in any one gear), have a certain gas consumption, and there will be a certain amount of torque produced by the engine to maintain this speed. Now you hit a long steady grade. To maintain the same mph the engine has to increase its torque with no increase in rpm (assuming you don't downshift). Power equals torque times rpm so with increased torque comes increased power from your engine. Either you or your cruise control will have to press down on the accellerator. A little more fuel will be squirted into the cylinder during each cycle. This produces the extra torque and power and increases the gas consumption. There's no way around it, you need to produce the extra power required to maintain speed up that grade, even at the same rpm. The reverse happens if you go down a grade.

As for the trip computer, no it's not a laboratory grade instrument. That's why I suggested running at least 40 miles or more at each test speed so you average out some of the errors. Its absolute accuracy compared to calculations from fill-ups is not great but the relative accuracy should be good enough to show the significant difference between a 55 mph and a 70 mph run. However, I'm afraid that the computer also uses electronic sensors and circuits that might also be defined as "mumbo jumbo" (as can be the computer I've writing this on and the computer you're reading this on).

If you don't try to understand the physics and don't want to run an actual test then there's probably no other way to show or explain this issue to you.

In real world situations I don't set my cruising speed based on mileage (unless I'm almost empty and have a long way to go to the next gas station). I run at what seems reasonable and comfortable for the existing weather and traffic conditions. However, as an engineer I derive some pleasure by understanding the forces that are around me as I cruise down the road.

 

LOL. I have just learned to deal with the crappy fuel economy. I'd rather run over hot coals, than drive the Max at 55mph for 40 miles. Eventually, some hot $hit would pass me, and I would have to hot-dog it and romp them, so there goes that idea.

I am going to KY in March, I am going to try and cruise 65mph all the way through on the interstate - 375 miles-, and I better make it on one tank, or else!

Fernando

 

I don't have any college degrees but I have sense and knowledge of how things work. Are you a design or aeronautical engineer?

The core of my message is that if you make an object a sleeker shape, it will cut through the air instead of trying to push it head on. There are examples all around us. Birds flying in formation, swing wing fighter jets, the shapes of aircraft. I could give you many more examples if needed.

That is part of the reason for the 55 mph speed limit. The other part is that it takes more fuel to drive faster. Wind resistance is one of the factors in fuel consumption just as aerodynamics are.

The way I understand torque as it applies to speed is as speed increases, torque decreases. You don't try to get a car unstuck in a high gear do you? At 70 mph, speed is more dominant than torque. At 5 mph, torque is more dominant than speed.

I'm trying to understand the physics in a simple way. Maybe you're just too far outside the box for me. You can throw a frisbee further if you throw it from the flat horizontal position than from the vertical position. It offers a smaller surface area to the air so that it glides easier. Maybe you can ask some of your design engineer buddies why they introduced aerodynamics to tractor trailers.I leave the designing and testing to the people that are better equipped to do it. I have some links to a few web sites if you as interested.

 

I think we spun away from your original comment so I've quoted it back again. Yes the engine demands more fuel at 80 mph vs. 70 mph. Why? The largest factor is the air resistance and the higher drag at higher speeds. Yes, there are some other loads that also increase, such as engine friction, rolling resistance, etc., and there are other loads like electrical and air conditioning that actually don't increase with speed. However the dominant force is the drag.

Reread my earlier posts. I'm not saying that the air resistance and drag is the same for a Winnebago as a Prius, Civic, or Corvette. Of course it's not. What I'm saying is that for the SAME CAR its drag is higher at higher speeds (by the square of the speed). If you compare 70 vs. 55 mph, it means you're going about 27% faster (70/55 = 1.27). Air resistance would be (1.27)(1.27) = 1.62 or 62% higher. That has a major impact on gas mileage, much more so than most people realize. No, your mileage is probably not going to be 62% worse since, as I mentioned, there are other loads that increase only linenarly and others that hardly increase at all. The average reduction in mileage will be a bit less than this. The comment about the Prius and the Winnebago illustrates that this RATIO of drag vs. speed remains roughly the same for any vehicle, NOT the absolute mileage.

I think we've probably beat this to death enough and hijacked the original question which drew me to this thread. It looks like a lot of folks are experiencing pretty poor mileage lately and I was hoping to hear of something that could be causing this that I've overlooked to keep my girlfriend happy with her Maxima. Along those lines I've ordered a couple cans of BG 44K and will run a can of that through each of our cars. If it's an injector problem that might help. I used it every couple years on my '92 Maxima to keep it running smooth for 100+K miles. If it's the ethanol that's replaced the MTBE in the fuel here in the Northeast there's not much I can do about that.