Typical Energy Expenditure %

[redpoint5]

Just curious if there is any ballpark figure out there about what percent of motive energy is spent for the average vehicle to:

Accelerate
Overcome rolling resistance
Overcome aero resistance

By motive energy, I mean not factoring in engine (in)efficiency, drivetrain losses, accessories, etc.

My guess is that acceleration accounts for less than 5% of motive energy as an average, perhaps much less than that even. Obviously this figure would be higher for those that spend more time in stop and go traffic, and less for steady state highway cruising. As an average though, what do you think it would be?

[oil pan 4]

There's highway, city and mixed tables for this.

I think there is a rolling resistance and drag calculator over on camaroz28 dot com.

[serialk11r]

You can roughly estimate this. For example with my FR-S, I was getting about 18-20mpg? while looking for parking in San Francisco. I recall estimating that rolling at 25mph with pulse and glide was getting me around 90mpg, and almost all of that consumption would be rolling resistance.

Engine on P&G that number drops to the 40s I think. But the average speed is really much closer to 10mph, so it's like 30mpg.

So I think it's fair to say acceleration is taking close to 40% of the energy in that stop and go scenario.

For long freeway drives, probably 80% air resistance?

[Vman455]

Originally Posted by serialk11r

For long freeway drives, probably 80% air resistance?

Not quite that much for the typical car, I suspect, but some bro-trucks might be more. In a Prius L Eco, at 70 mph you're using ~64% of the energy to overcome aerodynamic drag and the rest for rolling resistance. A 2020 Ram in its lowest drag trim, Cd = 0.36, is around 74% to aero drag at the same speed. At Cd = 0.45, the same truck is 78% to aero drag.

But the total resistive force acting on the last truck example, and therefore the work required to move it a given distance at a constant velocity, is two and a half times as much as the Prius. That puts things in perspective.

[redpoint5]

Right, the answer for an individual use case and vehicle is, it depends on the individual use case and vehicle. Just curious if this has been estimated across the entire US fleet? I think even 5% for acceleration alone is high.

One way to roughly calculate this on an individual basis would be to keep track of how much electricity a hybrid or EV regens over time.

[serialk11r]

Ah you're right, I overestimated drag vs. rolling resistance.

FWIW, Enterprise had a discount a few months ago on Tesla rentals, and I drove a Model S 70D up a fairly steep hill. The battery drained very very fast going up, with the miles remaining estimate going down something like 20 miles for only 4 miles covered, and then on the way down it barely gained any miles back.

Since the speeds involved were low, that tells you rolling resistance is still quite significant. That said, a permanent magnet motor powered car would probably do a little better on the same run.

[jakobnev]

What does it mean to spend energy on acceleration? When you have accelerated you still have that energy in kinetic form. Only after losing the speed do you know where it went. (rolling, aero, braking etc)

[RedDevil]

Or two black stripes on the pavement

[redpoint5]

Originally Posted by jakobnev

What does it mean to spend energy on acceleration? When you have accelerated you still have that energy in kinetic form. Only after losing the speed do you know where it went. (rolling, aero, braking etc)

Exactly. The energy not accounted for by rolling or aero resistance; ie braking.

[Grant-53]

If you drag out your physics text book or other technical articles there are formulas to calculate energy required in each case. The mass of the vehicle is listed on the vehicle or there are weigh stations. Drag is most easily measured in a coast down test.