going slower uses more gas. what??

[aerohead]

Originally Posted by Isaac Zachary

That's incorrect. From a physical standpoint it's the same amount of energy to accelerate to a certain speed whether you do so over a long period of time or suddenly accelerate within less than a second to your desired speed.

The difference is how close or how far from optimal efficiency is the load putting onto your engine and what RPM's are you having to rev it up to to get that kind of power.

In the past, BMW conducted an experiment which indicated that 'jack-rabbit' starts could be more fuel efficient than long, protracted accelerations.
The issue was that ICE engines have their worst thermal efficiency during transient loading.
If one got up to speed judisciously, you could reduce the engines residence time at lower efficiency, spending more time at a higher thermal efficiency, for an overall gain.

[freebeard]

In the past, BMW conducted an experiment which indicated that 'jack-rabbit' starts could be more fuel efficient than long, protracted accelerations.

I lived my life that way during the 36HP decades; even the XFi seems to regain momentum easily.

[aerohead]

Originally Posted by freebeard

I lived my life that way during the 36HP decades; even the XFi seems to regain momentum easily.

I drove the 40-HP flat-out, and no one was the wiser!
Downhills were my friend.
The only Karmann-Ghia 'dune-buggy' in the El Paso area!

[Tahoe_Hybrid]

Originally Posted by Isaac Zachary

I'm glad you were able to fix it!

I was also going to mention that in my Avalon I get worse fuel mileage at 45mph than 55mph cruising on a flat road. But I get better fuel mileage at 65mph than 75mph.

As mentioned before the reason has to do with changes in engine efficiency vs. aerodynamic drag. The slower you go the less power you need to maintain your speed. But that usually makes your engine less and less efficient as your power needs get farther and farther from optimal for your engine.

Because of that there's a curve to your fuel mileage when cruising at a certain speed on flat ground. There'll be a speed that gets you your best fuel mileage. Faster or slower than that will get you worse fuel mileage.

The hack to overcome this is pulsing and gliding. By accelerating you can put your engine's RPMs and load much closer to optimal while driving slower overall, both of which give you better fuel mileage.

38-42mph is the optimal sweet spot

I got the AUX pump replaced recently this way i don't have to worry about needing to cycle off my A/C in the brutal summer it still stalls out from the accelerating from auto stop but still can manually start it by shifting to M4. sigh...

[Drifter]

Originally Posted by maanma

Speeding up slower (longer in time) will save energy. Doubling time is halving the energy needed.

Originally Posted by Isaac Zachary

That's incorrect. From a physical standpoint it's the same amount of energy to accelerate to a certain speed whether you do so over a long period of time or suddenly accelerate within less than a second to your desired speed.

It is a little more complicated than that. In a frictionless environment the rate of acceleration does not matter, but in the real world the faster accelerating car averages a higher speed and therefore encounters more wind resistance throughout its trip.

Example:

Car A accelerates from 0-60mph slowly over the course of 2 miles. Car B accelerates more briskly from 0-60 mph over 1/4 mile and then travels steadily at 60mph for the remaining 1 3/4 mile.

Technically they both expended the same amount of energy *accelerating* to 60 but car B averaged a higher speed over the 2 miles and therefore expended more energy overcoming wind & rolling resistance.


Of course it still may be that the engine is so much less efficient at light loads that you still burn less fuel consuming more total energy by accelerating briskly. This is true for my Prius - for years I wasted gas (and time) accelerating gingerly whereas I now accelerate right up to the "power" zone threshold and still use less gas for the trip.