Why does it use more gas to accelerate more rapidly?

[jamesqf]

Originally Posted by Old Mechanic

Why does it use more fuel to accelerate at higher rates?
You are doing more work to increase the inertial state of the vehicle.

No. From a pure physics POV (neglecting engine inefficiencies, etc) you're doing exactly the same amount of work to accelerate to a given speed: Work (physics) - Wikipedia, the free encyclopedia You're using a greater power, but applying that power for a shorter time: http://en.wikipedia.org/wiki/Power_(physics)

My Insight would use 12 times the fuel at maximum acceleration than it did at lower constant speeds.

Sure, but what about fuel consumption vs rate of acceleration? Then with the Insight, there's still another factor, which is that if you accelerate hard enough, the electric assist comes on, using energy that you'll (partially) recover in the next braking. But if your battery charge is below about 3/4, the engine will be charging the battery, and thus using more fuel...

[Piwoslaw]

Originally Posted by jamesqf

No. From a pure physics POV (neglecting engine inefficiencies, etc) you're doing exactly the same amount of work to accelerate to a given speed: Work (physics) - Wikipedia, the free encyclopedia You're using a greater power, but applying that power for a shorter time: http://en.wikipedia.org/wiki/Power_(physics)

I think we're getting closer to the answer.
Keeping engine efficiencies aside, since greater acceleration means more power, but for a shorter time, then we need to know if twice the power (let's assume twice the instantaneous fuel consumption) will result in half the time needed to accelerate? In theory yes, but after factoring in aero and rolling resistances?

[Maury Markowitz]

Originally Posted by drmiller100

The faster you accelerate, the more time you spend at "top speed", which means you burn more fuel simply because you are going faster.

In my case this represents perhaps 1 km over a 100 km one-way route, so the effect on overall would be perhaps 1% if this was simply a geometric effect.

Looking at the instantaneous gauge, mpg goes from about 65 cruising at hiway speed, to about 15 to 20 mph during acceleration.

Originally Posted by drmiller100

Second, the ECU computer in the car richens the mixture drastically above a certain point. It goes "open loop" and throws gas at the engine, because extra gasoline gives more power. You REALLY want to stay out of "open loop" for mileage.

Ahhhh, now *that* sounds like we might have hit on it...

Originally Posted by drmiller100

Third, an automatic transmission will stay in low gears longer, meaning more average RPM's, which means more gasoline burned.

Is this purely linear? IE, is 2x RPM and 1/2x the gearing to get the same end speed using twice as much fuel?

Maury

[ecomodded]

Quote: Why does it use more gas to accelerate more rapidly?

Because you are injecting a larger amount of fuel with more losses to heat, friction and the force of the rapid acceleration.

Simply put the more pedal you give it the more fuel it waste.
Much like doing 70 on the highway compared to doing 55 on the highway.
Use less hp and you will also use less gas

[jamesqf]

Originally Posted by ecomodded

Because you are injecting a larger amount of fuel with more losses to heat, friction and the force of the rapid acceleration.

But you are doing that for a proportionately shorter length of time, so - assuming your engine &c is equally efficient at all loads & RPMs - you should wind up using exactly the same amount of fuel.

[gone-ot]

Originally Posted by jamesqf

But you are doing that for a proportionately shorter length of time, so - assuming your engine &c is equally efficient at all loads & RPMs - you should wind up using exactly the same amount of fuel.

...theorectically, that would be true, but nothing is 100% efficient, so accumulating 'losses' cause our realworld results to be otherwise!

[kennybobby]

This is silly to think that a bunch of physickists and mathemagicians can't understand the difference between energy and power?

A 3200 lb car accelerates from rest to 60 mph (88 ft/sec).

The "work" or kinetic energy is .5mv^2 = 387,200 lb-ft regardless of how long it takes to get up to that speed.

But power is the time rate of the change in work or energy, dKe/dt.

So 0 to 60 in 5 seconds = 77,440 lb-ft/sec = 140 hp
and 0 to 60 in 20 sec = 19,360 lb-ft/sec = 35 hp.

How much more fuel does it take for your engine to make 140 hp versus 35 hp?

Look at the dyno data--the torque-speed curve and the volumetric efficiency-fuel consumption curve to find the actual number, but it will be at least a factor of 4.

BTW this is why stop and go traffic kills mileage--it takes so much more power to get up to speed than to just maintain a constant speed.

In this case if your mileage gauge normally reads ~40 mpg for the slow acceleration, then it will drop to ~10 mpg for the faster accel.

[serialk11r]

Originally Posted by kennybobby

This is silly to think that a bunch of physickists and mathemagicians can't understand the difference between energy and power?

A 3200 lb car accelerates from rest to 60 mph (88 ft/sec).

The "work" or kinetic energy is .5mv^2 = 387,200 lb-ft regardless of how long it takes to get up to that speed.

But power is the time rate of the change in work or energy, dKe/dt.

So 0 to 60 in 5 seconds = 77,440 lb-ft/sec = 140 hp
and 0 to 60 in 20 sec = 19,360 lb-ft/sec = 35 hp.

How much more fuel does it take for your engine to make 35 hp versus 140 hp?

Look at the dyno data--the torque-speed curve and the volumetric efficiency-fuel consumption curve to find the actual number, but it will be at least a factor of 4.

BTW this is why stop and go traffic kills mileage--it takes so much more power to get up to speed than to just maintain a constant speed.

In this case if your mileage gauge normally reads ~40 mpg for the slow acceleration, then it will drop to ~10 mpg for the faster accel.

35hp average is a pretty low number for a 3200lb car. Typical cars are geared so that cruising on the highway is less efficient than wide open throttle 100% load acceleration, at very high rpms they're about even.

[kennybobby]

3200 lbs, Cd = 0.36, Area = 20 sqft, Crr = 0.030

Cruising at 60 mph constant speed on flat asphalt road, the power loss due to aero drag and rolling resistance is about 25.7 hp. Drag force ~65 lbs, Roll force ~96 lbs.

[stillsearching]

Originally Posted by MetroMPG

On what? BSFC? There's a decent article about it on Autospeed.com: link

I already hit the thanks button but I wish I could repeatedly - that is the BEST article i've ever seen on BSFC, finally explaining part throttle BSFC (which I see so much more rarely discussed) and singlehandedly making alot of things "fit" in my mind about why things work the way they do.

Thank you x10