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Old 06-10-2014, 06:12 PM   #603 (permalink)
aerohead
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hills/mpg

Quote:
Originally Posted by 3-Wheeler View Post
Ah,

Talking about counter-intuitive!!

Kinetic Energy = WV^2/2g

This is one form on an equation for moving a mass up a hill. Work out some numbers and one quickly sees that going up a hill faster simply takes more energy.

Since we are trying to save fuel, it makes sense that going up the hill slower takes less energy (think force times distance divided by time or F * D / T). However, taken to an extreme, going up a hill in first gear may use more fuel than going up in third for example.

I always watch the FCD in the Insight when going up hills to get a feel for what works best.

In the case of the hills by LaCross, going up in third was way better than a higher gear, and keeping the speed to around 30 mph.

Regarding engine cooling.... About 1/3 of the HP generated by an engine goes into the radiator as heat. If we go up the hill slower, use less energy, then it goes that there should be less heat as well, and less cooling requirements from the radiator. However, again taken to an extreme, going to slow lowers the air velocity through the radiator, and thus at some point would heat it up higher because of the slow speed.

In my case, the lower engine temperatures I saw told me that the speed, engine rpm, and gear worked, because engine temperature was lower than going up in forth, with more engine load.

Jim.
In an article about the old Mobil Economy Run,some Chrysler guys said that they pre-tested their car for every road grade that would be encountered on the run.
An optimum velocity was recorded for each grade,and a ball bearing inside a constant-radius ashtray bottom served as an 'inclinometer' for the team.Some other competitors used a stopwatch hanging from the rear-view mirror to gauge the road grades.
If you construct a graph of the speed/grade/velocity you find a relationship to a constant 'climb rate' of vertical feet per unit of time across the carpet graph that they want to maintain to keep the engine in a BSFC 'sweet-spot.'
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