Fortunately, I seldom encounter roads like that. Most of the local hilly roads have hills that are short and can safely be coasted down without the car’s speed running away. When there is a long grade, it’s usually not steep, usually just enough to maintain the road’s speed limit.
Steep downhills where brakes must constantly be used to keep a safe speed will kill you gas mileage just about as bad as stop and go traffic, but moderately hilly roads that can safely be driven without using brakes will often get better mpg’s than level roads.
Thanks for the interesting post above, @B.L.E . Yes, the Bernoulli equation says the vacuum force is proportional to the velocity squared multiplied by the air density. I didn’t know that an airplane’s indicated airspeed is actually biased on the low side on purpose. I hope that information never comes to be personally useful to me … lol …
Problem being, is the fuel in the carb is metered proportional to the drop in pressure due to Bernoulli’s effect. This is the same effect as causes “lift” on an airplane wing: accelerated local airflow resulting in reduced pressure.
The PROBLEM is that Bernoulli’s principle says pressure drops proportional to 1/2[rho]v^2. Where [rho] is, essentially, air density. This means, if pressure halves, and you therefore double airflow (thus velocity in a fixed venturi)…you’ll wind up rich, as the “squared” variable is going up, and the “first power” is going down proportionally; thus, if the air density is halved, the mixture will richen by the square root of 2.
Yep, that’s why carbs are so mechanically complex. Lots of systems trying to get the correct air/fuel ratio for a given combination of altitude, temperature, load, engine temp, speed, etc. Throw emissions controls on top, you’ve got a REAL mess!
Not so much “on purpose” as “not messing with a good thing.” The lower the density of the air, the lower impact it has on things it hits. Since the wing uses the air to keep the plane in the sky, as the air gets less dense, the wing develops less lift.
So if the plane needs to be going 70mph at ground level in order to fly, it needs to be experiencing the same pressure from the air at 20,000 feet in order to keep flying, but since the air is less dense, the plane needs to be moving at a faster actual speed. Translating to speed-over-the-ground would get confusing to the pilot who would have to remember what stall speeds were at various altitudes, barometric pressures, and then figure out how fast the wind was blowing (a tail wind reduces lift) and by the time he figured all that out he’d crash. So leaving the pitot tube to do what the pitot tube does naturally is the best solution.
(yes, fellow plane geeks, I know about CAS, but that gets more into the weeds than necessary here )
Leaving the air speed uncorrected for altitude simplifies the pilot’s job, but makes the navigator’s job more complicated, although today I presume they use GPS for that.
Bernoulli’s equation is actually very similar to the roller coaster equation. On a roller coaster, potential energy + kinetic energy is constant, what you lose in potential energy (elevation) you gain in kinetic energy (speed).
In fluid flow, what the fluid gains in kinetic energy, it loses in head. You need “rho” or the density of the fluid to convert head into pressure.
Head is how high up a standpipe the pressure would elevate the fluid.
The output pressure of centrifugal pumps is usually given in feet of head instead of psi because the head will be the same regardless of the fluid density.
Yes, that was true for carbs, but for FI engines, they say stick with the mfgr’s octane, even at high altitudes. (That can be harder to find in Denver.)
I use a Scangauge and noticed an improvement on steep hills, also. That could be part of the MPG improvement. Going up a steep hill I noticed MPG dropped from 35 to 20 MPG, but down the backside it jumps to 80 MPG… or 185 MPG if I coast.
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