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/sci/ - Science & Math

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3258227 No.3258227 [Reply] [Original]

faster a car goes, the less fuel efficient it is

why? is it because of drag only?

>> No.3258239

Because it gets exponentially harder to accelerate

>> No.3258253

>>3258227
Higher drag, [usually] higher RPM, less torque, etc.

>> No.3258259

What the fuck, we had this exact same OP the other day and it was answered already. It's not even a funny troll.

>> No.3258263

I guess drag only, it is square of the velocity, so 2x the speed = 4x drag

>> No.3258264

Engineer here.

Number of reasons:

RPM increases. This means more fuel is combusted.

The parasitic loss increases from the driveshaft and transmission as more power is put into it.

Less torque at a higher rev range.

>> No.3258267

>faster a car goes, the less fuel efficient it is

Only in excess of about 55 MPH.

Below that is detrimental because you're wasting energy.

>> No.3258272

>>3258239
Your engine is revving higher to keep up with the ground, thus burning more fuel.

>> No.3258278

Think of it in terms of a rocket, to move it has to push mass in the opposite direction. To move faster it has to push the mass with a greater force. But gasoline only has so much energy per unit volume, so to push faster you have to burn more gas.

>> No.3258279

>>3258267
why is about 50 - 55 the sweet spot?

>> No.3258323

>>3258264

>Less torque at a higher rev range.

so, for electric cars, drag would be the only factor?`

>> No.3258345

>>3258279

Because that's the zone of the average engine's output vs. the energy requirement for speed.

>> No.3258349

>>3258345
but why

>> No.3258359

>>3258349

It's a consequence of the engine's design and the energy output of gasoline?

>> No.3258364

>>3258264


actually it depends quite a lot on the design and purpose of the engine/vehicle.


Drag is important, but drag ultimately just becomes an "equilibrium" factor that is taken into consideration when tuning the engine.

most vehicles are designed to operate at ~3500 RPM at maximum efficiency. the automaker simply chooses the final gear ratio to reflect the speed of the motorways in the given market (taking into account some assumption of drag under some generic weather conditions).

the speed at which the maximum efficiency occurs is both a "guiding" number, as well as a by product of this generic process of optimization.

>> No.3258379

I'm not sure though.

>> No.3258380

I have a sort of relevant question to cars.

Does anyone else get really bad migraines from dark interiors of cars or rooms? Whenever I go into a store with black walls and floors I come out with head pains for the rest of the day. Sorry if it's not >>>/fit/ but all they talk about is zyzz.

>> No.3258411

>>3258345
>>3258279
>>3258267


Automakers choose an RPM that will be most relevant under the largest number of scenarios.


they choose something around ~3500 RPM +/- 500 (for gas; much lower for diesel). it depends on the engine and the car (sports cars will be maximally efficient at a higher RPM, naturally aspirated differ from turbo/supercharged cars, and economy cars will operate at a lower RPM)

they then choose the gear ratios with some degree of consideration for the most likely speed at which the car will move when operated at the efficient RPM, as well as consideration for the intended use.

this is the most basic version. modern engines have the ability to operate at different tuning profiles depending on different sensor inputs so that the maximum efficiency may occur at a different RPM.

I was mostly referring to "old" simple designs like carbureted and simple EFI systems (with no variable valve timing or cam profile or complicated ECU).


Parasitic losses in the drive train are most relevant to accelerating the engine (revving). In fact, modern cars often have some mechanical/electronic system that changes the operation of the engine when the car is cruising at some high speed:

ultra lean burn, disabling some of the cylinders, variation in the intake/exhaust pipe geometry, variation of the cam lobe profile/timing, etc.

>> No.3258587

>>3258323

I guess so

>> No.3258604

Its typically because of drag. The power required to overcome drag increases with the cubed of velocity. If you were to assume the engines efficiency is uniform across the rev range, that translates to a decrease in efficiency with a 1/v^2 relationship. However, the engine plays some role as well. Peak (thermodynamic) efficiency is usually in the 2-3000 rpm range for 4-stroke engines of typical automotive size. This is also, however, when they produce the least power (this is why your car has an overdrive gear for the highway).

>> No.3260436 [DELETED] 

bump

>> No.3261786

Many answers in this thread are correct, i.e. basic engine operating parameters, energy output from fuel, drag, etc.

OP, if you're serious about finding out more about engines, then read this:

http://www.scribd.com/doc/6549503/Engineering-Fundamentals-of-IC-Engines-WW-Pulkrabek

It's a textbook on internal combustion engines. It does have lots of problems for you to solve, but it's more interesting because it contains facts and basic explanations to things. As opposed to being pure science, it provides lots of application and basic knowledge, too. Some interesting facts in there.

>> No.3263223

>>3261786
>OP, if you're serious about finding out more about engines, then read this:

thanks, I am

>> No.3263644

>>3258278
Actually, rockets get more efficient the faster they go.

>>3258279
Because that's what they're engineered for. They could be engineered for any sweet spot.

>> No.3263804

>>3263644
>Actually, rockets get more efficient the faster they go.

how so?

>> No.3263825

>>3263804

>reading physics book
>see that most rocket weight is fuel
>more fuel burnt = lighter rocket
>mind was blown

>> No.3263840

>>3263804
he's refering to the oberth effect, where the rocket does more work the faster it goes because the propellant exerts a force across a larger distance.
however, chemical rockets produce the same thrust no matter the speed, so the gain in energy efficiency is most notable in ion/plasma thrusters that require lots of power to operate

>> No.3263848

>>3263825

Thats how they initially got the physics term "rocket scientist", derived from "rocket physicist", but most people can't pronounce physics so they used the former term since (around) 1950's

>> No.3263884

>>3263840
yes

>> No.3264431 [DELETED] 

Can someone confirm if electric car only has drag or not plz?

>> No.3264444

so, for electric powered car, except drag everything else talked about is a non-issue for electric cars?

>> No.3264451

More speed = more gas consumption = less fuel efficiency. My God, it seems people just continue to become less intelligent as the years go by. I bet OP believes in evolution too! WHAT A THOUGHT!

>> No.3266223

bump

>> No.3266224

>>3258227
That's not technically true. At least, I think not. I doubt the efficiency of a car "approaches a maximum" as the speed goes to 0. The efficiency graph probably (?) has a peak greater than 0.

>> No.3266235

>>3266224
If the speed is too low, you're burning fuel for nothing. Maximum fuel efficiency depends on the car, but it mostly falls around 60 mph.

>> No.3266238

>>3266235
Indeed. Which itself is amazing, perhaps, to someone uneducated in the engineering and physical sciences as myself. I could buy that efficiency "reaches a peek" as speed goes to 0. Nevermind. I feel stupid for even posting in this thread.