Side pressures and attached flow

[aerohead]

Originally Posted by JulianEdgar

I don't think you understand very well what is being described by throttle stop testing. We're talking changes in engine rpm of 40 or 50 rpm - basically inconsequential in terms of torque change. (And it's torque we're working with in this case, not power.)

But hey, don't take my word for it. Go and do some testing yourself, eg windows up / windows down and see if the measured drag changes matches what you'd expect in the two different configurations. Practical testing, not just theorising.

Torque and rpm is power.
You're not accounting for the actual net amount of power derived from the fuel charge. Sure the rpm can remain steady, but that says nothing about how efficiently, or not, the engine is converting the chemo-thermo energy into mechanical energy.
A course in internal combustion and air pollution was required in my course work. We had an engine test cell with a General Electric engine dynamometer at Texas Tech. BSFC is probably the most important thing to know about an engine.
I've already told you that I did coastdown testing to SAE protocols, and top speed at the Chrysler Proving Grounds. Top speed testing at Bonneville. And General Motors Aerodynamics Laboratory helped with the data reduction.
No theorizing necessary.

[JulianEdgar]

Originally Posted by aerohead

Torque and rpm is power.
You're not accounting for the actual net amount of power derived from the fuel charge. Sure the rpm can remain steady, but that says nothing about how efficiently, or not, the engine is converting the chemo-thermo energy into mechanical energy.
A course in internal combustion and air pollution was required in my course work. We had an engine test cell with a General Electric engine dynamometer at Texas Tech. BSFC is probably the most important thing to know about an engine.
I've already told you that I did coastdown testing to SAE protocols, and top speed at the Chrysler Proving Grounds. Top speed testing at Bonneville. And General Motors Aerodynamics Laboratory helped with the data reduction.
No theorizing necessary.

Ok, you definitely haven't read how throttle stop testing works.

The engine is at a constant throttle. RPM varies inconsequentially The air/fuel ratio is constant. Ignition timing varies inconsequentially.

But look, don't worry about it. The technique works brilliantly - if you can't get your head around it, that's OK.

[aerohead]

Originally Posted by JulianEdgar

Ok, you definitely haven't read how throttle stop testing works.

The engine is at a constant throttle. RPM varies inconsequentially The air/fuel ratio is constant. Ignition timing varies inconsequentially.

But look, don't worry about it. The technique works brilliantly - if you can't get your head around it, that's OK.

1) If you've altered the drag you've altered the road load horsepower requirement of the vehicle.
2) If the engine is not kept at a constant load, its BSFC will wander to a less efficient 'topographical region' of the engine's 'map.'
3 It doesn't matter if the throttle position, RPM, and stochiometric ratio are constant, you've altered the thermodynamic properties of the engine and they haven't been accounted for.

[JulianEdgar]

Originally Posted by aerohead

1) If you've altered the drag you've altered the road load horsepower requirement of the vehicle.
2) If the engine is not kept at a constant load, its BSFC will wander to a less efficient 'topographical region' of the engine's 'map.'
3 It doesn't matter if the throttle position, RPM, and stochiometric ratio are constant, you've altered the thermodynamic properties of the engine and they haven't been accounted for.

All right, it's obvious you don't know much about engines. For example, for a given engine and fuel, the stoichiometric ratio is a constant! (I think you mean air/fuel ratio.)

Don't worry about it. Maybe try doing some mapping of an engine from scratch, have your own chassis dyno, your own wideband air/fuel ratio meter, etc, etc, and then come back to me.

You know, experience and testing versus just theory (yet again!).

[JulianEdgar]

For anyone wondering....

In top gear, engine rpm varies little for speed changes. So for example, in the Insight, a few km/h difference in speed may be only 40-50 rpm.

In throttle-stop testing, the engine is at a constant throttle. The air/fuel ratio is constant. Ignition timing varies little - and in fact in the Insight with this variation in engine speed and manifold vacuum, is constant. Volumetric efficiency varies inconsequentially. Internal frictional losses in the engine vary inconsequentially.

As is then obvious, the torque output of the engine is very nearly constant over this very small change in rpm. This means the push backwards on the road by the wheels is very nearly constant.

The power output of the engine varies a little (very little, but a little) but that's why when we do the maths to calculate change in drag, we use the square rule (for force) not the cube rule (for power).

It's not like this was all just dreamed up in some random way: it was thought-through very carefully, discussed with some top experts for their thoughts, and then tested to see if a deliberate change in drag gave the expected test results. (Which it did.)

As with any car testing, you can do it really badly and get completely unreliable results (eg testing in low gear, on a peaky turbo engine at revs where it comes onto boost, on two days with completely different weather, and with different run-up speeds to the test section) but you can also do it and get excellent, repeatable results - and vastly better than normal coastdowns.

[aerohead]

Originally Posted by JulianEdgar

For anyone wondering....

In top gear, engine rpm varies little for speed changes. So for example, in the Insight, a few km/h difference in speed may be only 40-50 rpm.

In throttle-stop testing, the engine is at a constant throttle. The air/fuel ratio is constant. Ignition timing varies little - and in fact in the Insight with this variation in engine speed and manifold vacuum, is constant. Volumetric efficiency varies inconsequentially. Internal frictional losses in the engine vary inconsequentially.

As is then obvious, the torque output of the engine is very nearly constant over this very small change in rpm. This means the push backwards on the road by the wheels is very nearly constant.

The power output of the engine varies a little (very little, but a little) but that's why when we do the maths to calculate change in drag, we use the square rule (for force) not the cube rule (for power).

It's not like this was all just dreamed up in some random way: it was thought-through very carefully, discussed with some top experts for their thoughts, and then tested to see if a deliberate change in drag gave the expected test results. (Which it did.)

As with any car testing, you can do it really badly and get completely unreliable results (eg testing in low gear, on a peaky turbo engine at revs where it comes onto boost, on two days with completely different weather, and with different run-up speeds to the test section) but you can also do it and get excellent, repeatable results - and vastly better than normal coastdowns.

1) I located the videos back on page 5 of the forum.
2) I wanted to revisit them before responding.
3) The video portion plays, but there's no audio.
Any idea on how to proceed?