fuel efficiency consideration article

[lvmci]

Hi All, Clarke Echols wrote an article in the BCM, about DD engines and different factors and efficiencies.He mentioned aerodynamic drag, but I have always thought that the older buses with the retro streamlined aerodynamic look, had a lower drag coefficient than modern day bread boxes pushing or cutting a hole thru the air. Recently it was mentioned roughened surfaces have better drag resistance, I was thinking rivets on the older style buses. So you guys, given same size, engines, axels, etc. does a old style streamlined bus compared to a new boxy style, get better mileage just because of the styling and is it of any consequential amount?.  Lvmci...

[Utahclaimjumper]

Compounding the problem is the difference in bus widths, older being 96 inches compared to the later 102s. That larger "wetted" area makes a large difference multiplied by the length.>>>Dan

[Jeremy]

It's an interesting question, and one I've wondered about before. My guess would be that the overall shape of an old Brill or similar is probably more aerodynamic than a modern bus, but that it is counteracted to an extent (but how large an extent I don't know) by the smoother body (flush glazing etc) of a modern bus. The surface roughness thing is about the boundary layer between the air and the skin and so works on a scale vastly smaller than the size of a rivet.

Aerodynamics is mostly a question of reducing turbulence - ie, keeping laminar flow across the body of the vehicle, and then separating it cleanly at the back. A teardrop shape for the rear is ideal (hence the old Brills etc), and there are systems (see truck trailer photo) which try fool the air into thinking that the square edges of modern vehicles are in fact teardrop-shaped. The other approach is to deliberately introduce a sharp-edged 'spoiler' (see Plaxton coach photo) to encourage the air to separate cleanly upwards, instead of swirling downwards from the square edge in the usual drag-inducing vortex. But how the end result compares to that old Brill I really don't know.





Jeremy

Edit: I should have said "largely a question of reducing turbulance". Frontal area is a big factor too.

[belfert]

A large factor in why the 50s and 60s buses get better mileage is simply the reduced frontal area and reduced weight.  I was parked next to an older bus at Bussin' 11 in Arcadia and my modern bus was at least a foot taller and maybe even 2 feet taller than the older bus.  I bet that older bus weighed between 10,000 and 15,000 pounds less than mine too.  My bus is also 6 inches wider.

[shelled]

aaaargh !

I am currently building a bus to test some aerodynamic and engine mods for fuel efficiency -- a 4104 and have spent decades (literally) before this learning aerodynamics and hydrodynamics in automobiles, buses, sailboats and sailplanes.

The ideal aerodynamic shape in fantasy is a teardrop with the largest cross section located about one third of the way from front measured in direction of fluid flow.  In practice, it has been known for a long time that this shape can be cheated by simply chopping the rear straight off -- sometimes known as a Kamm-back after the German engineer who developed it in the 1930's.

Boundary layer control is all about getting the passing airflow to "lay down" close to the surface.  The boundary layer is turbulent air between the surface and (hopefully) laminar flow which not only wastes energy in turbulence, but adds the weight and inertia of the air inside the boundary layer to the weight the engine has to move down the road.

Roughened skin is one type of boundary layer control, as are the dimples on a golf ball and the stitches on a baseball.  On the size scale of a bus, you would use what are known as "vortex generators" and you can see those on the wings of aircraft, usually about 25% of the way back from the leading edge.  Vortex generators induce small amounts of turbulence in the plane of the surface to delay seperation of laminar flow.

The front end shape of the PD4104 is probably 10% more efficient than a PD4106 based on discussions I had with an Aerovironment (developers of the prize winning human powered airplane) regarding their experiments with a Corvair van back in the 1970s as an example of how little visual difference there can be in aerodynamic efficiency.

I read the article which originally appeared on the Coach Conversion Central website with mixed horror and laughter.  The only good information in the article is that spending thousands of dollars to get tiny gains makes no economic sense.

e3

[Jeremy]

So, on the basis of that knowledge and experience, would you expect the modern bus or the old bus to be more aerodynamic? I haven't seen the original article which prompted this discussion incidentally, so I have no idea what it said.

Regarding the vortex generators, I've seen such devices sold as aftermarket accessories designed to be stuck right at the rear of truck and trailer bodies vehicles - given how very cheap they are to make, the fact that (to my knowledge) no truck or trailer manufacturers stick them there as standard makes me think that it's a mostly snake oil exercise. Using them further forward on an aircraft wing at the point where the flow normally separates certainly makes more sense, although it occurs to me that that point must surely move a long way backwards and forwards depending upon air speed, altitude and even air temperature.

Jeremy

PS. When I was a teenager I once spent ages modifying the rear of a boat to replace the sharp edges on the transom with a nice rounded curve, and did the same thing all the way along the chines - only later learning that sharp edges are always preferable on the transom, and at planing speeds at least they're preferable on the chines too.

[lvmci]

Would old paint roughened with sun exposure, a boundry layer control, contributing to a smother drag coefficient?  Yet another reason to not paint my bus! Ill put a sign on it, donot disturb the dirty paint, air drag experiment going on! Lvmci...

[Lin]

I have no legitimate experience in the field of aerodynamics, but that does not stop me from having an opinion.  I do not believe that anything you could do to the skin of your bus will make a worthwhile difference in fuel economy.  Airplanes and race cars--sure. Tanks and buses--no.  If you are looking to improve your fuel mileage, managing your foot is about the best thing you can do.

[Jeremy]

I do not believe that anything you could do to the skin of your bus will make a worthwhile difference in fuel economy. 

I agree that you almost certainly can't make a worthwhile improvement - you can sure make the aerodynamics worse though




Jeremy

[Ed Hackenbruch]

I believe that that photo is of the first BusN'USA  rally!

[shelled]

Jeremy, I totally agree with you that many, if not most, contrivances sold to change vehicle aerodynamics are, as you said, just snake oil.

Cliff, it makes a great story, but the boundary layer over most of a bus is at least 3 or 4 inches deep/thick, so your old paint is going to have to be (ahem) seriously rough. 

e3

[dukegrad98]

Good post, shelled.  I was recently directed to an "article" on turbochargers that was one of the most brain-damaged bits of writing in recent memory; it was so wrong-headed that I did not even expend the effort to explain it to the poster.  Your description of mixed horror and amusement rings home for sure.

Cheers, John