Horsepower % lost

[tholyoak]

This 15% number is always thrown around.

Let's think about it for a second. If I have a 100hp car with a 15% drive train loss. That means 15hp is lost to the rotation of components between the flywheel and the tires (friction heat etc), fine. Now I take the same car and drop in a 300hp engine, by this flat 15% loss method the components between the flywheel somehow now consume 45hp. Doesn't make much sense does it. I am open to suggestions as to how this is but it seems to me that the parasitic loss from the flywheel should be constant for a particular car and not a function of the amount of horsepower being produced.

My take is that the trend of comparing runs on different dynos is worthless, also trying to extract accurate flywheel hp numbers from chassis dyno numbers is also a pointless pursuit.

Todd

[Lucky]

Originally Posted by tholyoak

This 15% number is always thrown around.

Let's think about it for a second. If I have a 100hp car with a 15% drive train loss. That means 15hp is lost to the rotation of components between the flywheel and the tires (friction heat etc), fine. Now I take the same car and drop in a 300hp engine, by this flat 15% loss method the components between the flywheel somehow now consume 45hp. Doesn't make much sense does it. I am open to suggestions as to how this is but it seems to me that the parasitic loss from the flywheel should be constant for a particular car and not a function of the amount of horsepower being produced.

My take is that the trend of comparing runs on different dynos is worthless, also trying to extract accurate flywheel hp numbers from chassis dyno numbers is also a pointless pursuit.

Todd

The drivetrain you need to handle 300hp (really the torque is the determinant, but I'll go with hp since that is your example) will have about 3 times higher loss (versus the 100hp drivetrain) due to higher rotational inertia (heavier gears) and more friction (larger surface area on gears and bearings). Will it be a perfectly linear 15% at all hp/torque levels? No, but it is a pretty good rule of thumb that matches nicely with real-world data.

Originally Posted by Lucky

The drivetrain you need to handle 300hp (really the torque is the determinant, but I'll go with hp since that is your example) will have about 3 times higher loss (versus the 100hp drivetrain) due to higher rotational inertia (heavier gears) and more friction (larger surface area on gears and bearings). Will it be a perfectly linear 15% at all hp/torque levels? No, but it is a pretty good rule of thumb that matches nicely with real-world data.

Lucky, I follow your example, and would agree, but in Tholyoak's example the car is the same (without a beefed-up tranny). So he does pose an interesting question to ponder.

[Lucky]

Originally Posted by bmussatti

Lucky, I follow your example, and would agree, but in Tholyoak's example the car is the same (without a beefed-up tranny). So he does pose an interesting question to ponder.

Guess I should read better...

Even so, I believe this is easily explained by the following High School Physics formula for the friction force observed between sliding surfaces:

F = mu * Fn

Where:
F = the force due to friction (loss in a tranny, for example)
mu = the coefficient of friction between two moving surfaces
Fn (as in "normal force") = the force applied perpendicular to the two moving surfaces

The key parameter above is "Fn", which inside a tranny occurs where gear surfaces touch and shafts push sideways against bearings (to counteract the torque that is spinning the shaft). As this "normal" force goes up, friction loss (F) goes up in a linear manner. So yes, the climbing loss as more power is put through a tranny makes perfect sense.

Yes, rotational inertia remains the same, but as friction loss climbs the inertia becomes less of a factor.

[blue2000s]

Originally Posted by Lucky

Guess I should read better...

Even so, I believe this is easily explained by the following High School Physics formula for the friction force observed between sliding surfaces:

F = mu * Fn

Where:
F = the force due to friction (loss in a tranny, for example)
mu = the coefficient of friction between two moving surfaces
Fn (as in "normal force") = the force applied perpendicular to the two moving surfaces

The key parameter above is "Fn", which inside a tranny occurs where gear surfaces touch and shafts push sideways against bearings (to counteract the torque that is spinning the shaft). As this "normal" force goes up, friction loss (F) goes up in a linear manner. So yes, the climbing loss as more power is put through a tranny makes perfect sense.

Yes, rotational inertia remains the same, but as friction loss climbs the inertia becomes less of a factor.

Yup, the amount of friction is directly proportional to the amount of force. Rotational inertia is a very minor contributor to drivetrain losses.

If that small engine in the example is at full throttle making all of it's 100hp and the bigger engine is at part throttle, making the same 100hp, the losses in the system will be the same, but add more throttle to the big engine and friction goes up in the pistons, crank, gears, axle joints and tires.

[tholyoak]

Yes the frictional loss will increase, but do you really think it would account for a 30 hp loss in the two examples, I for one don't think so, as the amount of heat generated would be pretty extreme and lead to failure of parts in pretty short order.

Anyway, I think trying to extract flywheel numbers from rwhp numbers is pointless.

Todd

[blue2000s]

Originally Posted by tholyoak

Yes the frictional loss will increase, but do you really think it would account for a 30 hp loss in the two examples, I for one don't think so, as the amount of heat generated would be pretty extreme and lead to failure of parts in pretty short order.

Anyway, I think trying to extract flywheel numbers from rwhp numbers is pointless.

Todd

Yes, it will increase in direct proportion of the power (actually torque, but whatever) increase.

[Lucky]

Originally Posted by tholyoak

...the amount of heat generated would be pretty extreme and lead to failure of parts in pretty short order...

Believe it. That is exactly why there are trannies desiged for 300 HP and different ones for 100 HP.