20% Drivetrain loss doesn't make sense to my brain.

Here it is...I think it may be in incomplete form...I seem to remember it being longer:

http://forums.tdiclub.com/showthread.php?t=75062


The whole concept of a fixed percentage drivetrain loss in evaluating chassis dyno tests is one of the most absurd things i've ever heard of in my experience as a powertrain engineer, and is the motivation behind this post.

First, a primer: energy (or in the case relevant to this discussion -- power) cannot be created or destroyed. Simple enough? Where then, does that power that is transmitted from the crank, to the wheels, and ultimately to the road, go? Most of it goes to friction and therefore heat.

Second, an illustration: For argument's sake, take a stock 90HP TDI, and we'll arbitrarily say that it has a 15% driveline loss. That means that the engine would be developing about 104HP (90/1.15) at the crank. The loss through the drivetrain was 14HP (104-90). Now, you do a bunch of engine mods without touching the drivetrain, and you now measure, say, 135HP at the wheels. Adding the customary 15% to refer back to the crank, you get 155HP, but the loss through the drivetrain is now 20HP, a difference of 6HP, WHEN NOTHING HAS BEEN TOUCHED THERE!

Do you now see the absurdity of this concept?

Firstly, may I submit that 2WD vehicles with manual transmissions have very good mechanical efficiencies, as evidenced by the fact that 2 quarts of non-pressurized, non-circulating oil is sufficient to keep the entire transmission cool and lubricated. In fact, to attach a number to it, manual transmissions are usually over 90% efficient, and many over 95%. That implies a loss through the transmission of between 5.3-11%. Even the best automatic transmissions with lock-up TCs achieve between 80-85% efficiencies.

Secondly, may I submit that contrary to popular (mis)conception, flywheel weights, rim weights/diameters and tire type (should) have very little contribution to the HP numbers on a rolling road dyno. Heavy flywheels and rims act as inertial dampers but do not destroy or create energy, nor transform it to heat, as would have to happen to if it is to result in a greater or lesser HP value on the dyno. Tires will shed energy in the form of heat by the simple contact with the ground and also though the flexing of the treads and sidewalls, but this amount is negligable in the scheme of things that it is generally ignored unless you are an engineer for an OEM, race car team or tire manufacturer. More on inertia in a moment.

Thirdly, I hope the above underscores that an accurate measurement of drivetrain loss cannot be overgeneralized. For one, it is not constant across the entire measurement range within a given run. In fact, friction increases roughly linearly with speed. In automotive engineering speak, this is quantified by a parameter called the FMEP (friction mean effective pressure), and although it's is not called that, it is manifested in many engine graphs you may read without even realising it. Frictional losses are different at 2000RPM to 4000 RPM, etc., etc. You cannot, therefore, equate the drivetrain loss of a car whose engine is turning at 8000RPM at the maximum rated power to one turning at 4000RPM, because on the basis of the RPM alone, frictional losses at 8000RPM are roughly double that at 4000RPM.

That said, yes, it's true: gear selection when performing a rolling road dyno DOES have an impact on HP, but it is not usually borne in dyno results, because the difference is small and within the inevitable variation from test-to-test and also measurement error.

Further, engine/driveline design considerations mean that there is a wide variance in frictional losses between different cars; the comparison of mechanical efficiencies between manual- and automatic transmissions have already been discussed above. Cars with AWD, automatic trannies, and large-displacement/many-cylinder engines will tend to have higher frictional losses than small-displacement, 2WD, manuals.

Finally, The importance of "motored" or coast-down tests in a dyno evaluation is important and needs to be stresssed, because that is what accounts for your true frictional losses and balances the inertial "ledger sheet" of the different driveline components, including the wheels and tires. The energy that is absorbed in the form of inertia in the flywheel/wheels/tires, etc. is accounted for ("given back," to oversimplify) in this coast-down, and when doing a street (i.e. butt) dyno, also accounts for the very important aerodynamic drag.

 

Indeed both get very hot as mentioned above. Not uncommon to see trans temps in the mid 200s and rear axles north of the 150 mark. In fact I do not recommend putting your hand on the gearcase after a long trip..

By the way, you can convert horsepower to BTU pretty easily.

http://convert.web-max.ca/index.php?select=58

200 HP = about 8500 BTU

 

Really? Touch them next time you make a couple runs.

The guy lost me quickly. He doesn't understand, no one is saying it's a scientific fact, they're saying a good rule of thumb is to take 15-20% off. If he doesn't understand that thought then he's one of those worthless engineers that can't function in the real world where you have to actually apply the engineering.

 

Yup, "rule of thumb" still stands. Without testing both ways with multiple horsepower levels it is near impossible to figure out.

Want to actually learn from engineers on the subject, this is some of the best I have ever found concerning the subject.

http://www.eng-tips.com/viewthread.cfm?qid=74391&page=9

 

He doesn't mention that drivetrain loss does not increase with power output. What he's saying is that taking an arbitrary percentage across the board is inaccurate. And of course he's right about that. There's only one way to prove drivetrain loss, and that's to take your engine out of the car and dyno it separately. Not many people would bother.

 

Yes it does. The more power that is input into a transmission greater forces are applied to gear teeth, more forces are applied to the bearings, shafts and the thrust bearings. If there is no power loss for your typical drivetrain then, Why do transmission have a maximum HP & TQ ratings?

That is because the bearings, shafts, gear composition, and housing are made stronger because of the addition forces being exerted due to energy being lost during transfer.

 

Yup, says so right here in this tidy sentence.

 

So........Am I better off running a handful 'O Flatheads, then one big 'ol Chevy?
Additionally, since "a given period of time" has come up, in this whole mess, as far as measuring force, or work...than does RPM not factor in, as well...I would imagine the frictional losses of a high-revving power-maker to be greater than the low rpm one, over a given time period, due to the friction created during each revolution of the motor being multiplied...
But, then..."an object in motion..." comes to mind.
Conversely, it will be the larger displacement engine that has more resistance, initially at least, to moving...hmmm...
I'm confused...and intrigued.

 

wow this is deep! Too much to think about! I believe i read somewhere one time that a th400 takes something like 35 more hp away than a th350. And knowing that always made me wonder why anyone would ever run the th400 behind say a 300 hp small block. That 300 hp would likely do no harm to the lighter trans that gets you more power to the ground! My brain hurts!

 

Dang, you guys are smart.

Thanks to everybody that took the time to explain, in detail, why the loss "rule of thumb" is a percentage as opposed to a flat hp loss. I understand now.

 

I'm sorry for my double negative.

"He doesn't mention that drivetrain loss does not increase with power output."

In other words - you, he, and I agree that losses increase with power output.

 

I wonder what the difference between running Sythetic verses 90 weight gear lube would be ?

 

Great thread.

If you don't agree with DirtyThirty, keep re-reading til his words make sense. It's all there, you're just experiencing too much parasitic loss between the eyes and brain

 

They're not his words, they're an engineer's article, or at least he claims to be an engineer, as far as we know he is merely TDImeister on a forum. Nothing against DirtyThirty, just didn't want to think I was disagreeing with him. I do understand what the engineer is saying, but he's one of those engineer's that doesn't understand "rule of thumb" i.e. we don't have the time/resources/money/whathaveyou to investigate this so for the sake of argument we'll say that there is 15-20% parasitic loss. Nothing scientific, just that it has been established that for the most part that is the norm. So rather then dyno every single part we'll take it for granted that is the loss. No parasitic loss between my eyes and brain. It is a good rule of thumb. Now if you want to dyno your engine and give us the reading at the crank, then the reading on chassis dyno, feel free. But it won't be. Then can you break down everything for us, accessories, transmission, u-joints, differential, wheel bearings, which lubricants for each, and on and on.
The absurdity of thinking that everyone has the where-with-all to test and add all the individual losses up is ludicrous. So we will estimate.

 

^^
That statement is exactly where the rubber meets the road.

Still doesn't change the actual physics behind it, which can be fun to try and wrap your mind around. Yeah, that's right I said fun.

 

Yes it can be fun. I'll hopefully be testing some things I did with the rubber on the road and the strip this month. One of the best things that can be done is write down what you do, and do one thing at a time. Sadly I do not have the luxury of spending that much time on my personal stuff these days.

 

Just turn the dam engine sideways and hook the crank to the axle shaft through a clutch and eliminate all the parasitic losses!!

Who cares! No-one ever has been able to build a perpetual motion machine, that means no-one can build a transmission or rear axle assembly that will not be 100% efficient. It is physics, unless you get cosmic energies to change and Sir Issac Newton to rewrite the laws of physics, it can't be done. So just deal with it and estimate the losses by the rule of thumb 15-20 %

 

Redline brand oils say the synthetic reduces power loss and temperature.Probably a small amount like one or so percent.
Some one mentioned trannys get hot ...With a manual transmission it's hard to tell if the tranny gets hot from power loss or more from having the 200 degree engine in front of it radiating heat.
The drag on the engine of the generator,water pump and oil pump likely exceeds the driveline power loses.

 

I care.

It's a lot easier to save horsepower than to make more. But I do see where you're coming from.

 

Factor in tire and wheel weight.

I have an engine that made 400+ @ the rear wheels with a late model 10 bolt and aluminum wheels with small tires.
We figured out that the smll rear probably wasn't going to hold up if we got traction. So we built a 9" changed ratio from 4.10/1 to 3.26/1 and went to steel wheels when we went with the stickier tires.

Back to the dyno just for grins. Lost nearly 10 ponies. Go figure.

There is really no real way to figure parasitic loss unless you can establish a base line and to do that you really need to establish your baseline prior to putting the mill in the chassis.

A dyno is just a rough estimate and a usefull tool for tuning. The real test is what you feel in the seat of your pants. If you stab the loud peddle and you feel wet in the seat of your pants, then you done good.

 

I care, if that's okay with you.

I'm not trying to build a perpetual motion machine, or even lessen the current drivetrain drag. I am, though, interested in exploring "norms" that have been taken for granted even though they may or may not be correct. Seems like there are alot of other guys that are as well. And several with the brain power to think it through and explain it to us laymen. Being a Mechanical Engineer, I'd think you would be interested in such a discussion. But if not, there are plenty of other threads on the HAMB.

 

Naturally there will be some heat escaping the radiator and engine and exhaust, flowing under the car and heating up the trans. Radiant heat does not explain the total heat that can be measured. It's also foolish to think that even a manual transmission in direct drive does not create heat. It still has bearings and gears in motion, while those may not be under load they still generate heat. Still the input and output bearings are under load and generating heat.

Literally depends on the situation, but generally speaking, your oil pump is the largest parasitic loss of power. Hence the usage of lower weight oils and better filter media.

 

isn't some of the horsepower lost in the conversion of horsepower to torque through the gearing? in that case it's not really a loss,but a conversion of one to the other, simple higher rpm changed to lower rpm through gearing slower speed increased torque ,speaking in terms of a basic manual transmission, gets more complicated when a torque converter in involved

 

And there are lots of odd little forces you might not think of...look at the manual trans, perhaps. Obvious contact force between gear teeth...then the sliding friction of the meshing of the spirals...then, guess what, spiral gears generate axial thrust forces. More power forces the gear harder against whatever stops them from moving endways... that force can be enough to warp the case, perhaps break it. There is a lot going on in a complex train of spiral and offset gears, all of it gnerating drag and heat on the HP.

 

because you forget you convert horsepower to more torque through gearing, thats what actually moves you, torque is a measure of twisting force, the more torque you start with the more you have after the gearing.

 

Yes,very little at low speeds.Running old farm tractors where your legs straddle the transmission I don't remember the tranny getting warm during normal use although these trannys have deep reduction gear ratios.And plowing snow with PU trucks the differentials stay snow covered after hours of use.Sure,it's 20 degrees outside,but the snow doesn't melt.
So...from my unscientific observations it might seems the power loss is greater at higher speeds with more heat generated? That's if heat and power loss are one in the same.

 

The transmission heat question can be answered by a O/T answer. Fuller transmissions have a spec on who needs trans coolers on big trucks. Air flow under the truck, engine power, weight being hauled consistently, speed of vehicle (quarry slow vs over the road). Why? In a manual transmission, the gears meshing together and friction between them from power source and load will generate heat. A trans low on oil will french fry the main drive gear (what the input shaft spins and drives everything else in the trans) and the two gears that run against it. Excessive engine rpm will cause the same thing in these trannys (125 mph across the desert in an 18 wheeler).
ELpolacko is mostly correct on a manual transmission generating heat. In direct, the power flow is straight through with no load against the other gears. But the lower ratio gears are spinning at a high rate of speed, being spun by the countershaft, causing power loss, load and heat generation even though they're not being used.
Some more of the 15-20%. The best explanation I could think of.

 

One last tidbit on why heat is generated.
Let's say you have an 80k lb over the road truck needing to take off on good hard concrete. The truck has 11R22.5 tires, 3.70 gears. It takes 9700 lb ft of torque at the driveshaft to make that truck move out of its tracks. The engine might be a 475hp Detroit with the capability of making 1700 lb ft of torque. How many have you heard the turbo whistling while it was climbing a hill or passing you? The engine has to work harder to make the vehicle move 60 mph than to take off in low gear.
This is a heavy large example, but the rules still apply in a lighter vehicle.

 

There is no real world conversion of horsepower to torque in your driveline. Horsepower is an artificial term that references the rate of torque rise and engine RPM. And yes, that's REEL simplified.

This one's tough to get your brain around!

 

Here you go - Real World Actual Test Numbers. Same motor with accessories and then installed in the car. Muncie 4 speed and a 12 bolt rear.
<TABLE cellSpacing=3 cellPadding=2><TBODY><TR><TD vAlign=bottom rowSpan=2></TD><TD align=middle colSpan=2>Flywheel </TD><TD align=middle colSpan=2>Rear Wheel</TD><TD align=middle colSpan=2>Difference</TD></TR><TR><TD>TQ</TD><TD>HP</TD><TD>TQ</TD><TD>HP</TD><TD>HP</TD><TD>%</TD></TR><TR><TD>2,250</TD><TD>435</TD><TD>187</TD><TD>367</TD><TD>158</TD><TD>29</TD><TD>18</TD></TR><TR><TD>2,500</TD><TD>448</TD><TD>213</TD><TD>378</TD><TD>179</TD><TD>34</TD><TD>19</TD></TR><TR><TD>3,000</TD><TD>467</TD><TD>267</TD><TD>392</TD><TD>223</TD><TD>44</TD><TD>20</TD></TR><TR><TD>3,500</TD><TD>454</TD><TD>303</TD><TD>398</TD><TD>265</TD><TD>38</TD><TD>14</TD></TR><TR><TD>4,000</TD><TD>428</TD><TD>326</TD><TD>371</TD><TD>283</TD><TD>43</TD><TD>15</TD></TR><TR><TD>4,500</TD><TD>384</TD><TD>329</TD><TD>327</TD><TD>280</TD><TD>49</TD><TD>17</TD></TR><TR><TD>5,000</TD><TD>329</TD><TD>313</TD><TD>263</TD><TD>251</TD><TD>62</TD><TD>25</TD></TR><TR><TD colSpan=4> </TD><TD>Avg.</TD><TD>42.7 hp</TD><TD>18.3%</TD></TR></TBODY></TABLE>