5 ms·
I think you should reconsider "physically impossible". Sure, in some idealized situations, the statement you're responding to would be wrong, grievously wrong.
by mturmon 11y ago
I think you should reconsider "physically impossible". Sure, in some idealized situations, the statement you're responding to would be wrong, grievously wrong.
But the actual situation has two novel features: (1) the engine uses up power just by being on; (2) you must maintain a minimum speed in order not to block traffic.
If you put these constraints in, you can see that it's possible for a pulsed-acceleration strategy to win. You put in a pulse of acceleration to get up the hill, turn the engine off, and coast down, maintaining enough speed to keep up with traffic.
Now sure, without the minimum-speed constraint, you'd probably be better off flat, and possibly using a pulsed-acceleration strategy.
I haven't done the math, but I think it would be reckless to say it's "physically impossible" for all values of static energy use and slopes.
Take a look at the stack exchange answer the commenter above linked to.
- aianus 11y agoIf you're maintaining a minimum speed in order to not block traffic, you would have to downshift to climb the hill which would kill your efficiency. But you raise good points, maybe I jumped the gun on 'physically impossible'.
- TylerE 11y agoNot at all, in many vehicles. 6th gear in my car is quite long (Turns 1950RPM at 75MPH) and I have no problem maintaining speed on highway grades. Even if you do downshift that doesn't necessarily mean you're being less efficient either. Being a bit higher up the rev range may actually put you in a better spot on the efficiency curve for a given power output. http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_(BSFC)_Maps http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Cons...
- WalterBright 11y agoI read years ago that pulsing was the most efficient strategy. Engines have very different efficiencies at different rpms and power settings. The idea with pulsing is to try to operate it at its most efficient rpm and throttle setting, which of course rarely corresponds to the speed over flat ground you'd like to be travelling at. Hence pulsing.
- aianus 11y agoI'm getting motion-sick just reading that :)
- WalterBright 11y agoI've been known to do it when gas goes over $4 a gallon, but not when there are cars behind me.
- TylerE 11y agoYep, and this is why moderate hills are better. It's essentially constant speed pulse-and-glide. Just building and releasing potential energy rather than kinetic.