Efficiency of RWD vs. AWD at high speed

[Braymond141]

Here's a quick question.

When a car reaches it max speed based solely on resistance (meaning gearing would allow for more speed, but HP isn't high enough to push it anymore) what is happening? At this max speed is power said to be null? Or is there a better way to describe what happens to the power that can no longer push the car anymore.

 

[mgb_phys]

Yes it means the engine power is exactly matching the air+mechanical resistance - mostly air resistance (drag) as this goes as speed^2.

 

[Braymond141]

Yes it means the engine power is exactly matching the air+mechanical resistance - mostly air resistance (drag) as this goes as speed^2.

So power isn't really lost (after all the whp calculations at that speed)? Is there a proper term for this (kinda like terminal velocity of a falling object)?

 

[Mech_Engineer]

So power isn't really lost (after all the whp calculations at that speed)? Is there a proper term for this (kinda like terminal velocity of a falling object)?

It's called the vehicle's "drag-limited top speed." This is the terminology used in most by automotive enthusiasts. Basically, it is when frictional forces including air drag exactly equal the power output of the engine at full throttle for that specific rpm.

A vehicle's ability to reach it's drag-limited top speed depends on a host of factors, but mainly its gearing and any computer-controlled speed limiters. It is actually somewhat rare to find a car that can reach it's drag-limited top speed.

- In the case of "low" gearing, if the gearing is too low (high gear reduction) the vehicle's engine will hit maximum rpms before it is going fast enough for air drag to match the engine's power output.

- In the case of a speed limiter, many consumer vehicle's engine control computers will automatically cut fuel to the engine at a certain speed, so that you cannot accelerate past that pre-determined top speed. Most Mercedes and BMW's for example have a speed limiter set at 155 mi/hr. Some lower-grade consumer vehicles such as Toyota Camry's will have a speed limiter at about 105 mi/hr.

 

[Braymond141]

It's called the vehicle's "drag-limited top speed." This is the terminology used in most by automotive enthusiasts. Basically, it is when frictional forces including air drag exactly equal the power output of the engine at full throttle for that specific rpm.

Thanks!

- In the case of "low" gearing, if the gearing is too low (high gear reduction) the vehicle's engine will hit maximum rpms before it is going fast enough for air drag to match the engine's power output.

- In the case of a speed limiter, many consumer vehicle's engine control computers will automatically cut fuel to the engine at a certain speed, so that you cannot accelerate past that pre-determined top speed. Most Mercedes and BMW's for example have a speed limiter set at 155 mi/hr. Some lower-grade consumer vehicles such as Toyota Camry's will have a speed limiter at about 105 mi/hr.

Yup, but it's more specific than that. BMW and many car companies adjust the computer controlled top speed limit per the car's suspension or current regulations.

My 96 M3 has a limit of 137mph
An e36 325 has a 125mph limit
An E46 325 has a 117mph limit
An E46 330 and M3 has the 155mph limit you mentioned
I think 318's have a ridiculously low limit as well.

 

[Braymond141]

Still trying to find out what kind of drag equation that would apply to all the components of an AWD car.

 

[Greg Freeman]

Still trying to find out what kind of drag equation that would apply to all the components of an AWD car.

I seriously doubt you'll find something simple that gives you an answer that will give you something usable for the drag on the components. It will depend on the size/shape/design of what's in the car, lubrication, etc. I challenge anyone to prove me wrong because I want to know how it was figured out myself.

To be honest getting a very accurate answer will just trick you to make you think you know something to an extreme amount of precision about the car, but you really don't. Those 17% to 25% figures that you quoted and I'm surprised that you didn't mention when you first asked the question, and I also have no idea where they came from, are probably the best answer you will get.

Why?

-You don't know how accurate your vehicle drag coefficient is
-You don't know how accurate your speedometer is
-You probably won't be accurately measuring windspeed in any real world case
-You won't accurately know the air density (which will affect not only drag but the motor's ability to develop power)
-You won't know anything about rolling resistance
-You still don't have an accurate number for crankshaft horsepower
-The accuracy of the dyno is also something unknown (but you could probably get an accurate number for that because hopefully the guys that own the dyno keep it reasonably calibrated)


I'm not trying to be a killjoy or anything, but people in controlled environments where a lot more is known about the situation are happy to get an answer within 5% most of the time, with much simpler experiments. All the additional precision in the world for one aspect of the problem isn't going to make the whole thing more certain.

 

[Ulysees]

What about equations guys? The OP was asking for them. Frictional torque in drivetrain:

T_f = k_f S \omega^g

S = area of contact, therefore longer drivetrain (ie larger area of contact) produces more friction at a given angular velocity \omega.

Angular velocity:

\omega = k_t v,

k_t = transmission factor
v = speed of the car

Hence

T_f = k_f S (k_t^g) (v^g)

If the engines and air drags are identical, the car with the smallest T_f over the race wins.

If the transmission ratios were identical (meaning k_t factors identical), the winner would always be the car with the smallest surface S, ie the one with shortest drivetrain (the RWD).

But transmission ratios are different so the RWD wins at high speed (its transmission factor k_t is low, and also its frictional surface S is less due to the shorter drivetrain).

At low speed the other wins because drivetrain friction becomes less relevant, it is small compared to the engine torque. So at such speed it's the k_t factors of the engine-torque transmission that make a difference, and the AWD happens to transmit more torque to the wheels.