Great SAE Paper: The Aerodynamic Development of the Opel Calibra

[GasSavers_brick]

A few years ago I went to the SAE World Congress in Detroit, and on a whim picked up a book called "Vehicle Aerodynamics" (SAE PT-49) for cheap. I just found it and this thing is a gold mine! Probably the best paper in it is SAE 900317, "The Aerodynamic Development of the Opel Calibra" (Emmeimann, Berneburg, and Schulze of the Technical Development Centre of GM Europe). The paper goes through a detailed description of the development of the Calibra, which was was designed with aerodynamics in mind and has a CD of 0.26 (same as the Prius, yes?) I can't legally reproduce it in its entirety, but I thought I would share some tidbits that I've picked up after my first glance through.

Relevant to my current project, the undertray, is a series of wind tunnel tests that they performed with several aerodynamic accessories. The configurations tested were:
A: Control
B: No Wheels
C: B plus lower engine compartment cover
D: C plus partial underbody covers
E: D plus wheelhouse cover panels
F: No Wheels and wheelhouse cover panels
G: Partial Wheel simulation only (i.e. cut wheels off flush with the frame)
H: Smooth underbody, wheelhouse filled to body, and underbody contour (i.e. completely smooth in all respects.)

The configurations from best to worst were:
H (Delta CD = -0.098)
E (Delta CD = -.040)
D
F
C
G
B (Delta CD = -.020
A (Baseline)

So, obviously, the smoother the better. Also obvious, configuration H is completely impractical since our cars need wheels. If you compare configuration B (baseline without wheels) to E (add nearly every accessory imagineable) it looks like the best we can hope for with the aero mods tested is an improvement of 0.020, or 7.4% of a car with CD 0.27 (which is an OK assumption since very few of these mods made it to the production model).

Based on some crappy calcs I did a while ago, aero drag is responsible for roughly 75% of my total resistance at 60mph, so call it 65% to account for drivetrain losses on top of that. This is saying that the best fuel economy improvement that we can hope for with the undertray, skirts, etc. is 1-(1-(.074*.65))= .048 0r 4.8%. That translates to going from 40mpg to 42mpg at 60mph.

I'm a lousy engineer, but that sounds "sane" to me. Is that estimate roughly in line with what the experienced modifiers have seen?

[GasSavers_brick]

I should add that there was one modification that did make it into production after wind tunnel tests: a fuel tank spoiler that bridges the gap between the gas tank and the bottom of the rear bumper. Theirs is small (kind of like what I think I saw on Compaq888's Altima pictures) but reduced the CD by .003 all by itself. It goes to show you how important the rear of the vehicle really is.

[ZugyNA]

Yet the focus on this forum seems to be aero mods?

And a good bit of negativity about drivetrain related FE mods.

Where are the big gains coming from?

[MetroMPG]

Calibra!

[Mighty Mira]

Originally Posted by ZugyNA

Yet the focus on this forum seems to be aero mods?
And a good bit of negativity about drivetrain related FE mods.
Where are the big gains coming from?

I haven't seen any negativity towards drivetrain mods. Except for bs additives that don't do anything.

One thing to take into account is that aero mods determine engine load at highway speed. Thus it's better to get that optimized first rather than to optimize the drivetrain according to something that is going to change down the track.

The base variables that are going to improve a car's fuel economy are weight, drag coefficient, frontal (and side area for crosswinds) area, as well as tyre rolling resistance. These have a practical limit and will determine the requirements of the engine/drivetrain.

Thus for maximum fuel economy most people here rightly focus on those areas, as they are inexpensive, doing it the other way will likely result in having to optimize drivetrain/engine twice, and most people here have not optimized those variables yet.

[Mighty Mira]

Originally Posted by brick

So, obviously, the smoother the better. Also obvious, configuration H is completely impractical since our cars need wheels. If you compare configuration B (baseline without wheels) to E (add nearly every accessory imagineable) it looks like the best we can hope for with the aero mods tested is an improvement of 0.020, or 7.4% of a car with CD 0.27 (which is an OK assumption since very few of these mods made it to the production model).

That's a bit of a false dichotomy. The only option that approaches a full undertray is option H.

And actually, option H is more realistic than you think. With properly faired wheels, the contribution to drag from the wheels will be small. Consider the AR-5. It is an airplane that has fixed landing gear and faired wheels. The total drag on the airplane is equivalent to a flat plate of 0.88 square feet. It uses fixed landing gear because the contribution to drag with fixed gear is less than the induced drag caused by the extra weight of having retractable landing gear! The contribution to total drag from landing gear is only 8%! (i.e. it has the same drag as a business card).

By fairing the wheels with skirts and deflectors, we are approaching something similar. It was unfortunate that they did not test this option. However, they did test option H which showed a 37% improvement. While faired wheels will not give the full 37% improvement, both that, undertray and grille should get most of the way there. Bear in mind that this 37% improvement represents a Cd of 0.162, which is among the very best of any automobile ever, present only in concept cars.

The other thing to bear in mind is that the opel calibra already has a nice bodyshape for drag. Many of our cars don't, and so we can get realistically good gains from things that help our cars approach or better the shape of the calibra. Hence my obsession with boattails.

The other reason for the obsession with lowering steady state load (i.e. air and rolling resistance) is that it opens up the possibility of a long range electric vehicle. And that's a liberating idea in itself - no overhauls, no oil changes, totally minimal cost.

Based on some crappy calcs I did a while ago, aero drag is responsible for roughly 75% of my total resistance at 60mph, so call it 65% to account for drivetrain losses on top of that. This is saying that the best fuel economy improvement that we can hope for with the undertray, skirts, etc. is 1-(1-(.074*.65))= .048 0r 4.8%. That translates to going from 40mpg to 42mpg at 60mph.

IMO that is woefully low. Mainly because you understimate the improvements that can be made to the drag coefficient, but also because in lowering the load on an engine, you also unlock possibilities that are possible by reducing engine size/taller gearing/ lighter weight etc.

I'm a lousy engineer, but that sounds "sane" to me. Is that estimate roughly in line with what the experienced modifiers have seen?

I'm curious... when you say "I'm a lousy engineer", do you have a degree and are just not that good at it, or are you an amateur who is just ok with figures? (I'm a EE btw, FWIW)

BTW that AR-5 I mentioned was not designed by an engineer, just a smart guy who had a lifetime interest in aerodynamics.

[Mighty Mira]

Originally Posted by brick

So, obviously, the smoother the better. Also obvious, configuration H is completely impractical since our cars need wheels. If you compare configuration B (baseline without wheels) to E (add nearly every accessory imagineable) it looks like the best we can hope for with the aero mods tested is an improvement of 0.020, or 7.4% of a car with CD 0.27 (which is an OK assumption since very few of these mods made it to the production model).

That's a bit of a false dichotomy. The only option that approaches a full undertray is option H.

And actually, option H is more realistic than you think. With properly faired wheels, the contribution to drag from the wheels will be small. Consider the AR-5. It is an airplane that has fixed landing gear and faired wheels. The total drag on the airplane is equivalent to a flat plate of 0.88 square feet. It uses fixed landing gear because the contribution to drag with fixed gear is less than the induced drag caused by the extra weight of having retractable landing gear! The contribution to total drag from landing gear is only 8%! (i.e. it has the same drag as a business card).

By fairing the wheels with skirts and deflectors, we are approaching something similar. It was unfortunate that they did not test this option. However, they did test option H which showed a 37% improvement. While faired wheels will not give the full 37% improvement, both that, undertray and grille should get most of the way there. Bear in mind that this 37% improvement represents a Cd of 0.162, which is among the very best of any automobile ever, present only in concept cars.

The other thing to bear in mind is that the opel calibra already has a nice bodyshape for drag. Many of our cars don't, and so we can get realistically good gains from things that help our cars approach or better the shape of the calibra. Hence my obsession with boattails.

The other reason for the obsession with lowering steady state load (i.e. air and rolling resistance) is that it opens up the possibility of a long range electric vehicle. And that's a liberating idea in itself - no overhauls, no oil changes, totally minimal cost.

I think that the Opel team discounted doing wheel skirts because they thought that was stylistically unrealistic. But mark my words: with high gas prices, wheel skirts (both front and rear) will become fashionable, and everyone will want their cars like that. It is human nature to compete, to have something better than other people, to be different and have something over your neighbour.

Based on some crappy calcs I did a while ago, aero drag is responsible for roughly 75% of my total resistance at 60mph, so call it 65% to account for drivetrain losses on top of that. This is saying that the best fuel economy improvement that we can hope for with the undertray, skirts, etc. is 1-(1-(.074*.65))= .048 0r 4.8%. That translates to going from 40mpg to 42mpg at 60mph.

IMO that is woefully low. Mainly because you understimate the improvements that can be made to the drag coefficient, but also because in lowering the load on an engine, you also unlock possibilities that are possible by reducing engine size/taller gearing/ lighter weight etc.

I'm a lousy engineer, but that sounds "sane" to me. Is that estimate roughly in line with what the experienced modifiers have seen?

I'm curious... when you say "I'm a lousy engineer", do you have a degree and are just not that good at it, or are you an amateur who is just ok with figures? (I'm a EE btw, FWIW)

BTW that AR-5 I mentioned was not designed by an engineer, just a smart guy who had a lifetime interest in aerodynamics. Once you see what is possible with airplanes (i.e. an AIRPLANE that gets 55mpg and does 200mph while doing it), you start to wonder what is possible with the car (that doesn't have to expend energy (other than RR, which is minimal) keeping itself aloft).

[GasSavers_brick]

Originally Posted by Mighty Mira

I'm curious... when you say "I'm a lousy engineer", do you have a degree and are just not that good at it, or are you an amateur who is just ok with figures? (I'm a EE btw, FWIW)

I'm mostly beating on myself. I'm actually a degreed ME, (did pretty well in school, passed the FE exam, blah blah blah), but I've been stuck at a not-so-exciting and only vaguely related desk job (pushing paper mostly) for the two years since I graduated. The only actual "engineering" I do these days is thinking about this kind of thing. Hence, this conversation is quite valuable to me!

Regarding your assessment of my quick-and-dirty analysis, I think you bring up a lot of excellent points. No doubt, 5% improvement in FE doesn't quite jibe with the 10-20% range that others tend to report. I think it was Basjoos who reported going from low 40s to low 60s (mpg) in his civic aerodynamics thread, which is unreal. My tendency is to take this kind of report with a big grain of salt (unless there's LOTS of data) but that might not be fair.

[The Toecutter]

I'm two semesters from an EE degee myself.

Interesting article. But you must keep in mind the Calibra is already relatively aero to begin with, and when conducting these tests, the overall body profile of the car remained unaltered. A tapered rear end among other things would greatly reduce drag yet more.

Cars like the GM Precept(.16 drag coefficient), Dodge Intrepid ESX2(.19 drag coefficient), Ford Probe V(.137 drag coefficient), Opel Eco Speedster(.20 drag coefficient), and Mercedes Bionic(.19 drag coefficient). For historical reference, low drag designes could have doubled and even tripled fuel economy half a century ago, with the 1957 Alfa Romeo BAT7(.19 drag coefficient), 1935 Tatra T77a(.21 drag coefficient), 1954 Fiat Turbina(.14 drag coefficient).

All have much lower drag coefficient than even the best of the modifications above listed would allow that Calibra to obtain. There is much altering to do to the basic body of the car itself. Subtle changes in the rear would not change exterior appearance, but could dramatically lower Cd, for instance.

[GasSavers_brick]

Originally Posted by The Toecutter

All have much lower drag coefficient than even the best of the modifications above listed would allow that Calibra to obtain. There is much altering to do to the basic body of the car itself. Subtle changes in the rear would not change exterior appearance, but could dramatically lower Cd, for instance.

This is all very valid. However, gassavers.org being what it is, I'm much more concerned with the modifications that we can make to our own vehicles. That 5% (even if it's actually 10% or 15%) isn't intended as an estimate of the absolute upper limit of auto design. It's intended as an estimate of what we can reasonably expect if we spend a weekend in the garage with some coroplast and a set of xacto knives. Now, I'm not saying that the subtler modifications are impossible for just that they would be much more difficult to get right. Unless I just misunderstand your point?

Anyway, I tried running through my calc except using the assumption that configuration H is the best approximation of a full belly pan. That brings the estimated total resistance reduction up to 9%, or equivalent to jumping from 40mpg to 43.6mpg at 60mph. Again, that's what one might expect from cutting and installing a nice, flat belly pan. The other optimizations (drivetrain, etc.) are valid, but a separate issue IMO.