Why does it use more gas to accelerate more rapidly?

[MetroMPG]

Originally Posted by Maury Markowitz

Anyone suggest a good book on this in general?

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

In the winter that goes down to about 42 mpg, which is also interesting.

It is interesting - and there are lots of reasons for it. Here are a dozen or so:

http://ecomodder.com/forum/showthrea...leage-220.html

[drmiller100]

Originally Posted by Maury Markowitz

Steady state gas milage is roughly due to two factors, the air resistance which goes with the square of speed, and rolling resistance which goes with the speed. So, roughly, milage is the fuel use at idle dividend by the cube of speed. Roughly. Then you add in things like hills and such.

The part I don't understand is acceleration. It would *appear* that the amount of energy needed to accelerate to a given speed is a fixed, it's simply 1/2*m*v^2. There's no "t" term in there for the time you take, nor an "a" for the acceleration. It's just initial vs. final velocity.

first, the wind resistance is cube of speed, but you get to divide by miles covered, so your MILEAGE is a function of speed squared.

Mileage and acceleration are indeed sort of unrelated, but they do tie together.

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

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.

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

Fourth, driving conditions can mean you burn more gas as a systems point of view. If you and I are next to each other, and the light turns green, and you rabbit out there and get up to 45mph , only to stop 1/4 mile on for the next red light, and I ease out, and my top speed is only 30 mph, you burned well over twice as much gasoline on that single section of road than I did. First, you accelerated to 45 which takes twice as much gas as it takes to accelerate at 30mph, and second, you spent time at 45 mph which burns twice as much gas as it does for 30mph if you look at only wind resistance.

Hope this helps

and yes, i'm also a math/physics major.

[jamesqf]

Originally Posted by capnbass91

It's physics, if it takes a certain amount of energy to get from point A to point B, it will take more energy to get from point A to point B faster.

But that's not what's happening here. The problem is getting to the same speed - say 60 mph - and then staying at that speed. So obviously (at least I hope it's obvious) if you accelerate 0-60 in 8 seconds, the point A at which you reach 60 is a lot closer to your starting point than the point B at which you reach 60 if you do the 0-60 in 16 seconds.

I must admit I've wonder about it myself, and whether it's merely the effects of the IC engine/powertrain. What would a pure electric do? Anybody have a Tesla and want to run some tests?

[MetroMPG]

Originally Posted by jamesqf

What would a pure electric do? Anybody have a Tesla and want to run some tests?

How about a Nissan Leaf? My Nissan Leaf Forum • View topic - Faster acceleration vs slower acceleration

[shovel]

I stated above that I always accelerate much more rapidly than basically everyone else on the road (this is 6 stoplights per morning and 6 stoplight per afternoon 2-3 days a week at least when commuting by car.. and the hundreds of thousands of other stoplight accelerations ever since I began driving in 1995) and have always returned better than EPA-listed mileage without using any deliberate hypermiling techniques.

All I do is get up to the speed limit fast, then stay there. I drew a crappy graph in mspaint to illustrate where I believe the difference lies:


Even if other drivers maintained the same maximum speed, they spend a much longer duration at acceleration-level fuel consumption, and a shorter duration at cruise level fuel consumption. I believe based on the feedback loop (with its necessary lag time) engines use to regulate combustion, a steady cruise rate will necessarily result in a more accurate and thus more efficient level of regulation. It's worth noting that this particular difference may not be relevant to electric vehicles because even if there is a feedback loop, it operates at the speed of light rather than at the speed of combustion gases.

Here is an example, not based on any actual measured data but just my guess at why greater acceleration may not actually be a big culprit, even if the same cruise speed is attained - even when that means maximum cruise speed is maintained for a longer duration on the car that gets up to that speed first. Pay attention to the relationship between the green region and the yellow region as illustrated:

[euromodder]

Originally Posted by Maury Markowitz

Sorry, this is not correct. There is no relation between force and fuel use.

There is, and it's called the engine

The acceleration is generated by the fuel, and more precisely by how (in)efficient the engine converts that fuel into acceleration.

[Mustang Dave]

I'm doing something wrong here. I accelerate @ 80-95% load and upshift @ 1500-2500 RPM, depending on conditions. I use cruise control EXTENSIVELY, and use it to engage DFCO @ lower RPM. I NEVER EOC, I RARELY P&G or DWL, and I NEVER draft another vehicle.
My lifetime average is over 47% over EPA combined. What am I doing wrong?

[rmay635703]

Originally Posted by Mustang Dave

My lifetime average is over 47% over EPA combined. What am I doing wrong?

Your driving a manual transmission.

Despite being a physicist the op has little understanding of how our engines behave real world, there are volumes past and present about how inefficient most engines are, how many losses there are in various drivetrains, how most engines run efficiently near only one high level load, etc, etc.

Simply put if you drive a manual transmission you can load up your engine to its most efficient level, which is usually somewhere between 75-90%.
It may be more or depending on auto/manual and whether your car richens up dramatically.

Anyway Your motor inevitably makes power most efficiently near its design limit (least losses VRS power created) so accelerating along that curve is ideal if you can do it and your automatic doesn't twart your efforts.

This means if you pulse at max efficient power and kill the engine you will get the best FE out of your platform.

If you instead drive at low load levels your motors power will be wasted on parasitic losses AKA, throttling losses, compression and other internal engine efficiency losses and more on friction than at a high load level where internal friction is less of a percentage of total engine power.

Cheers
Ryan

[thomason2wheels]

Let me throw one more variable in the mix. The a/f mixture under heavy acceleration is significantly richer than during cruise or light throttle acceleration. Thet could account for some of difference in the results achieved by the 2 techniques. Is this not true?

[user removed]

You control the power an engine produces by regulating the air delivery, which in turn regulates the timing and fuel delivery, except in Diesels which have no air delivery restriction.

Engines are most efficient when they have maximum air delivery at fairly low RPM, without full load enrichment. I remember a Dyno test of a 2.5 L GM engine. At 1500 RPM producing 20 HP the engine required a specific volume of fuel. When the load was increased to 50 HP (same RPM), the fuel requirement rose by 50%, while the power produced rose by 150%. That is the essence of best BSFC, you get more work out of the same amount of fuel by making the engine work harder but not at maxumum for that RPM.

This is the essence of pulse and glide. Long before the term "Hypermiler" was ever adopted this principle was known. Using the "cheaper" (fuel wise) higher load capabilities of engines people who were not constrained by traffic and speed used P&G to extend the range of their cars. It mostly started in WW2 (but was done before that) Use the higher efficiency load to store energy in your car as inertia, then kill the engine and coast.

80% load is the ideal load range, becasue it does not use enrichment. Lower RPM does not encounter the much higher friction losses of high RPM.

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

My Insight would use 12 times the fuel at maximum acceleration than it did at lower constant speeds. The secret is to minimise the fuel used in acceleration then use the additional inertia to travel considerable distance with little or no fuel consumption.

regards
Mech