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CAUTION: throttle-stop testing

While I applaud any effort towards improved efficiency, I experienced a nagging concern, with respect to this method.
After many weeks of looking, I finally located a duel citation germane to this topic.
Rather than say a single word about it, I'll leave it up to members to interpret the data:
1) SAE Paper 810184
2) SAE Paper 830304

 JulianEdgar 01-13-2021 10:39 AM

Neither paper appears relevant to throttle stop testing, so you’ll need to explain the point you’re trying to make.

 Vman455 01-13-2021 11:03 AM

For those not wanting to google--
1) "Formulae for the Tractive-Energy Requirements of Vehicles Driving the EPA Schedules" (1981)

A comprehensive analysis of the tractive-energy requirements of the EPA Urban and Highway driving schedules has been made. The results are compact formulae in which the specific tractive energy required to negotiate a schedule is correlated primarily with the ratio of effective aerodynamic frontal area to vehicle mass; the tire rolling resistance coefficients are additional variables. The formulae coefficients permit easy determination of the proportions of the Input energy required for overcoming the aerodynamic and rolling resistances, and for supplying the net powered increases in vehicle kinetic energy, respectively. The specific energy dissipated by braking has also been formulated.

2) "Tractive-Energy-Based Formulae for the Impact of Aerodynamics on Fuel Economy Over the EPA Driving Schedules" (1983)

A fuel consumption analysis has been made to determine the impact of changes in aerodynamic drag on fuel economy over the Environmental Protection Agency (EPA) driving schedules. It is based on the tractive energy required by vehicles to negotiate those schedules — specifically, on the fraction that is required to overcome drag. Formulae for this energy fraction that are applicable to any automobile have been previously derived. In conjunction with empirical inputs on closed-throttle fuel rate and the fuel consumption fraction for engine and vehicle accessories, an expression for the aerodynamic influence coefficient relating any percentage reduction in drag to the corresponding attainable percentage reduction in on-road fuel consumption has been formulated. The simple formula is used to show the effect of drag changes of various magnitude on EPA Urban, Highway, and Composite fuel consumptions. The equivalent reductions in vehicle mass and tire rolling-resistance coefficient required to produce the same fuel saving are also shown.

 redpoint5 01-13-2021 12:09 PM

What has any of that got to do with the throttle-stop method?

The excerpt just says that you have to consider all the variables affecting efficiency to arrive at an EPA number following that protocol.

'influence coefficient'

This is the crux of the matter and what anyone using the throttle-stop technique must appreciate.
It's my opinion that the technique fails to quantify the known unknowns, as spelled out by Sovran et al..
Hucho devoted quite a bit of his text to address this challenge. Woe to anyone who sails without this chart.

 JulianEdgar 01-13-2021 03:31 PM

Quote:
 Originally Posted by aerohead (Post 640579) This is the crux of the matter and what anyone using the throttle-stop technique must appreciate. It's my opinion that the technique fails to quantify the known unknowns, as spelled out by Sovran et al.. Hucho devoted quite a bit of his text to address this challenge. Woe to anyone who sails without this chart.
So you actually have no points to your argument except "the technique fails to quantify the known unknowns".

1. What are the known unknowns that invalidate the test procedure?

2. How do these match with the actual results achieved when a known drag change is made?

3. How have you found the approach in actual use - or are you just theorising again?

points

Quote:
 Originally Posted by JulianEdgar (Post 640589) So you actually have no points to your argument except "the technique fails to quantify the known unknowns". 1. What are the known unknowns that invalidate the test procedure? 2. How do these match with the actual results achieved when a known drag change is made? 3. How have you found the approach in actual use - or are you just theorising again?
Powertrain efficiency and BSFC are variable. BSFC dominates the scenario.
'The 'influence coefficient' is a sensitivity factor that relates a percentage change in independent variable to its corresponding percentage change dependent variable.'
'Changes in other vehicle factors may occur as a consequence of drag change, and these must be identified and considered.'
'A drag change causes the locus of the engine's operating points during the test to fall in a different region of the engine's performance map. This can alter the BSFC.'
' aerodynamic influence factor can easily deteriorate to only 60% of its potential value.'

 JulianEdgar 01-13-2021 04:28 PM

Quote:
 Originally Posted by aerohead (Post 640596) Powertrain efficiency and BSFC are variable. BSFC dominates the scenario. 'The 'influence coefficient' is a sensitivity factor that relates a percentage change in independent variable to its corresponding percentage change dependent variable.' 'Changes in other vehicle factors may occur as a consequence of drag change, and these must be identified and considered.' 'A drag change causes the locus of the engine's operating points during the test to fall in a different region of the engine's performance map. This can alter the BSFC.' ' aerodynamic influence factor can easily deteriorate to only 60% of its potential value.' Any load reduction
I don't think you understand the throttle stop testing technique (eg BSFC is quite irrelevant), and very clearly you've never actually used it to assess drag changes.

Never mind. To put it mildly, you've not shown yourself to be very open to new ideas, and that's OK.

(Not of course, that testing for changes in top speed to quantify drag changes is new!)

understand

Quote:
 Originally Posted by JulianEdgar (Post 640597) I don't think you understand the throttle stop testing technique (eg BSFC is quite irrelevant), and very clearly you've never actually used it to assess drag changes. Never mind. To put it mildly, you've not shown yourself to be very open to new ideas, and that's OK. (Not of course, that testing for changes in top speed to quantify drag changes is new!)
I believe that I do understand it, and that's why I'd never use it. You may believe that you're in command of knowledge you're not in possession of. A reflection of your comments.
When it comes to testing, there's nothing more important than BSFC. I don't believe you actually understand it. That's not a cut, just an observation.
Some SAE Papers supporting your method would be instructive.

 JulianEdgar 01-13-2021 04:44 PM

Quote:
 Originally Posted by aerohead (Post 640601) I believe that I do understand it, and that's why I'd never use it. You may believe that you're in command of knowledge you're not in possession of. A reflection of your comments. When it comes to testing, there's nothing more important than BSFC. I don't believe you actually understand it. That's not a cut, just an observation. Some SAE Papers supporting your method would be instructive.
Brake Specific Fuel Consumption is irrelevant to throttle stop testing. Fuel consumption, whether specific to power being produced or distance being travelled, has nothing to do with the technique. Fuel consumed is not measured when using the technique.

Before I published the throttle stop approach, I did of course consult some aerodynamic experts - three in this case.

The only contentious point is one you have completely ignored, and that is whether the calculated change in drag depends on the square of the difference in speed, or the cube of the difference in speed.

Changes in top speed as indicating changes in drag has been used for over 100 years.

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