Power to Weight versus Torque to Weight

[Bilbo]

Hi Paul

 

AT LAST "Torquexrpm IS power" no argument from me as you posted POWER is calculated.

 

I know you find this splitting hairs but I think its important to understand what happening from the basics.

 

1988 200 bhp, 146 ft lbs, 1700cc Cosworth BD? with Brooklands and Clamshell wings, Freestyle Motorsport suspension. Q 979 CGY

 

[Bilbo]

Hi Tony

 

"We arent talking about engine losses are we 'cos that get tricky"

 

Well measuring is not as it is just energy in against energy out. The differences is the losses, finding where is the losses occur and reducing them is the real problem.

 

Get back later on dyno's it been a long day at work writing/typing minutes and I not in the mood at present. I am going to relax

 

1988 200 bhp, 146 ft lbs, 1700cc Cosworth BD? with Brooklands and Clamshell wings, Freestyle Motorsport suspension. Q 979 CGY

 

[Paul Ranson]

AT LAST "Torquexrpm IS power" no argument from me as you posted POWER is calculated.

I still don't understand what you are banging on about.

 

Dynamometers measure power, how they do that is up to them.

 

Calculation might get you from a rolling road power measurement followed by a coast down test and the application of corrections to a nominal flywheel power figure.

 

Paul

[Peter Carmichael]

Hi Bilbo. I am with Paul on this one.

 

The losses thing is all to do with losses external to the engine rather than internal ones. If you measure (or calculate) power at the flywheel then all internal losses are accounted for because the flywheel is external to the engine. It is important that all ancillaries be attached to the engine and that it should be providing its own electrical power from its alternator for proper comparison.

 

I'll get back to this topic when work is quiet.

 

Highlander, glad we have spent the time working through our phase pf vigorous agreement.

[Bilbo]

Hi Paul and Peter

 

Guess its just a work thing really. When I witness test something I am required not just to know that something gave as a claimed overall output but make them prove how it was derived. When the test kit was last calibrated and by whom. Were they an accredited independent tester. Strangely often IF its actually related to the point of the test. Is the test a true demonstration of its use and related output.

 

To do this you need to start from what the actual input is and what the REAL output is that is being measured. Then what the kit that's measuring does with it and where errors can creep in. Often this can relate to many things in a complex machine in a test and a few sensors that measure the overall effect.

 

I did post earlier that the engine was just that and the car was a machine but it was just to make a point and simplify relative to this argument. Its obviously not true, the engine running is a complex machine thats to say a mixture of more than one machine.

 

So I always think in terms of why, how and if the results can err be smoothed out. Then I need to write up why I accepted the demo. After all its my signature on the report and (such as it is) my own reputation.

 

So I really look at what is measured, what is derived and how the final figure is arrived at, also how it can be influenced. Sometimes NOT very often with good reason. In one case following some sound tests that I arranged a manufacture withdrew a "Badged" product from the market. ❗ Then I had to arrange similar tests with another manufacturer who were spot on what they similarly claimed.

 

So as to dyno and RR where is the "screwdriver" inserted to correct the calibration ❗ This to me on a RR has normally been irrelevant I take the car in they give it back running very much better, me very happy. Thats to say the output reading is me driving the car.

 

Peter I think you remember that on one RR I went back and the method of measurement of one outputs has been amended. Not that I knew why it was just that something felt wrong relative to a previous RR. It was just about were error can creep in. Not claiming I knew why, it was the second RR owners skill that very quickly decided why. It also had nothing to do with him setting the car up as it was just a power run.

 

As to Peter's comment on having all the bits attached totally agree. I was just posting you can find what the internal losses are, just saying where they occur is difficult. Nothing to do with the engine set up just the internal resistances and as you post the effect for example of the alternator more output from that less engine output. I.E. Energy in is never the out where you want to use it. It was just intended as a throw away comment. If someone on list would like to explain where all the losses occur I love to read it.

 

In someway this is where an RR expert earns his money seem to remember Oilyhands had a RR that showed very poor results DW quickly diagnosed a bearing fault.

 

So just one more time what's measured is torque how you read its interpretation is often POWER BHP but it can be the resulant force you started with measuring in the first place. 😬 😬 😬

 

Why anyone needs to ague with this leaves me .

 

How as we started it is relative to driving a car is a different matter not one I started to question seems quite unintentionally I have sent the thread down a side track.

 

 

 

1988 200 bhp, 146 ft lbs, 1700cc Cosworth BD? with Brooklands and Clamshell wings, Freestyle Motorsport suspension. Q 979 CGY

 

 

Edited by - bilbo on 23 Sep 2002 22:10:50

[Peter Carmichael]

Bilbo, very glad to read a lot of sensible comments about witness testing. The rhetorical question remains: Which torque is it that the rolling road measures?

 

 

[NickAllen]

Now that PC and Highlander have made up I can post this :-

 

Highlander - wasnt that a film with Mel Gibson in it. supposedly factual but infact a complete fairy tale.

 

Well it made me giggle - No offence intended 😬 😬 😬

Nick *smile*

[Peter Carmichael]

You are thinking of Braveheart. Highlander was a frenchman pretending to be scottish and a scot pretending to be spanish, with comedy beheadings - pure Shakespeare...

[Highlander]

Yeah but still a classic ! Just don't talk about Highlander II or III.

 

Actually I think Mel's accent was marginally better than Christopher Lambert's. Also I think most of Braveheart is just as fictional. Historically speaking.

 

And who said we've made up !

 

Going back up the posts a bit, and off thread again, back to the CVT thing.

 

All other times you want to match road load with minimum losses. So you slide down the power curve and run at at WOT (minimum pumping losses) at low engine speed (where the power curve matches the road load).

---

 

This doesn't ring true to me. In steady state conditions, i.e. matching forward thrust to road load, drag etc, I would expect the CVT brain to back of the throttle /increase the gearing to a point where the engine torque matched the road load. This wouldn't be on the torque (power) curve as that is the edge of the engines 'performance' envelope attained running at full throttle.

Also if the engine was being run with the throlle wide open then engine would tend to accelerate until it reached it's maximum speed or the available torque matches the road load. Yes I agree that the latter is a strady state scenario, but at maximum vehicle speed (for the gearing available), whereas I'm interested in what's happening just cruising along.

Anyone care to comment?

 

Shields Up

[Peter Carmichael]

This doesn't ring true to me. In steady state conditions, i.e. matching forward thrust to road load, drag etc, I would expect the CVT brain to back of the throttle /increase the gearing to a point where the engine torque matched the road load. This wouldn't be on the torque (power) curve as that is the edge of the engines 'performance' envelope attained running at full throttle.

---

 

Why back off the throttle when you can increase the gearing to the point where you are lower down the power curve? The point is that running at full throttle is quite an efficient thing to do. Running at full throttle and low revs is more efficient than running at part throttle and higher revs. How much power do you need? Where is the most efficient place in the engine's operating envelope to get that amount of power?

[Morls]

"The point is that running at full throttle is quite an efficient thing to do. "

 

But surely lots of cammy engines won't do this unless 'in the band', I suppose if an engine built to work well at high rpm is not on the cam then it's ineficient anyway, but...

 

I just recall lots of (mainly motorbike) engines that wouldn't take WOT untill the engine is spinning at halfway through the rev range or more. Bikes with CV (like an SU in concept) carbs don't seem as bad but stiil accelerate harder when the throttle is knocked back a little.

 

Is this just poor set up?

 

Have I grasped the wrong end of the stick?

 

Who wants a DAF / Honda vision anyway?

 

 

 

Mark