Urba Centurion Update - (Kubota diesel, 100+ MPG in mixed driving - Fall 2012 on)

[sheepdog 44]

In my opinion, it's two things. 1, is BSFC as Sendler already mentioned. Which is ideal for the centurion; being at the optimal engine loading in an underpowered diesel at a very aero efficient speed. When a motorcycle can be overpowered in many instances.

2, The ratio of aerodynamic resistance to vehicle mass. The problem is a motorcycle has a great deal of aerodynamic resistance for it mass. Meaning it doesn't coast as far in "pulse and glide." Adding weight may achieve a longer coast while improving the BSFC of the pulse, netting overall better mpg. Improving aerodynamics would be the ideal solution since adding weight is only a half remedy.

Another part is momentum. Your speed multiplied by the mass of your vehicle. Trading Kinetic energy (speed) for potential energy (elevation) is a very efficient way to climb and descend hills. You want to use as little power ascending, and then convert your elevation back to speed at the bottom. The less mass you have, the more you rely on speed. You would need to drive faster to have enough momentum to carry you to the top of the hill. And the faster you drive the more air resistance there is!------There is an ideal weight of course, given how aerodynamic your vehicle is. It's all a trade off. At the GGP many cars were to heavy to corner turn one at an ideal speed and thus couldn't reach the bottom with enough speed to use most of that momentum to carry them to the top of the hill climb. The centurion's light weight was ideal for the course. Once you have momentum, if you can convert it to elevation and back again (without scrubbing speed, or going too fast) it is efficient. Real world exceptions would be maintaining freeway speed up an extended hill climbs where the descent is to steep of an angle to coast efficiently down.

Which brings me to another aspect. The ratio of mass to aerodynamic resistance determines how shallow a negative grade you can coast down (in gear or in neutral). With a high drag low mass motorcycle, you need a relatively steeper grade to maintain a certain speed than you would if you added more mass to it. Adding mass to make up for the drag means you can maintain a higher continuous speed on a shallower negative grade while using no power from the engine. On the GGP track adding weight may mean you can engine off coast from the top of the back straight all the way down the shallow descent culminating at the bottom of turn one! If you didn't have the mass to maintain speed, you might have to restart the engine to get another push to keep going. It would be incorrect to say more weight is always better. But as long as you get the right ratio for the given course or route it should help. These are just the unique problem of motorcycles that aerodynamics would be the better solution to. If you can't improve aerodynamics, only adding mass, or lowering your speed will suffice.

Originally Posted by changzuki

There is likely a limit as to how heavy you could make the bike but with different sets of real world numbers maybe we could see what the cut-off point is and possibly come up with a forumla where "X" added weight can yield "Y" gain in mpg. Also, if the extra weight is general in nature then this may be the easiest and most cost effective way to raise mpg. Would you do this in the name of science and ecomodder camaraderie'? What say you?

~CrazyJerry

This is the formula you should concern yourself with: i = ((krMs) + (kaAsv2d)) / (gMs)
You can figure out the exact negative grade that will maintain the constant speed of your vehicle while coasting. This would be the ideal negative grade to look for in a route so you could EOC in Neutral without gaining excessive speed, or slowly loosing speed. If you know the gradient of the GGP track, you can calculate the ideal amount of mass to add to your motorcycle!

where

P = power required (in watts)
kr= rolling resistance coefficient
M = mass of bike + rider
s = speed of the bike on the road
ka= wind resistance coefficient
A = the frontal area of the bike and rider
v = speed of the bike through the air (i.e. bike speed + headwind or – tailwind)
d = air density
g = gravitational constant
i = gradient (an approximation²)

Just for fun: this is the formula used to determine the power required to climb a grade (there are other ways to write it). (I really do think this stuff IS fun!)
http://ecomodder.com/forum/showthrea...tor-27824.html
Power = (coefficient of rolling resistance) + (aerodynamic resistance) + (road grade x mass x speed, ie the amount you fight gravity!) *likewise how much energy gravity can give you.

P = (krMs) + (kaAsv2d) + (giMs)

[Joggernot]

Yep, I like math. Do you have a book reference that discusses aerodynamics/formulas/etc?

[sendler]

Originally Posted by changzuki

[B]This may be a really cost effective way for folks to gain mpg's on the cheap.

Not suggesting that adding ballast to a commuter is a great way to improve daily fuel economy as an ecomod. Just stating some theory and examples of how adding mass to an over powered to weight vehicle can, counterintuitively, improve the result of effective PnG. You have to admit that adding 1300 pounds and still improving economy by 22% blows all weight weenie theories out of the water. Reducing weight and rotational mass makes for big improvements in PERFORMANCE in a race, but has little effect on economy of an over powered road vehicle if you don't have to touch the brakes as on a well known commute or cross country trip.
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I have already added weight to the CBR250R on all of my competitions. Carrying an unnecessary 40 pounds at my first Vetter challenge and 30 pounds extra at the GGP. I would have to strip the bike with no luggage, lighter exhaust, and lithium battery to perform your test.
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I often read comments by ecomodders that are worried that proposed aero mods will add weight to the vehicle. My point is that the weight of the mod, such as putting a full nose and tail on a motorcycle, is way down the ladder of importance compared to the economy that is gained from the improved aerodynamics.

[changzuki]

Originally Posted by sheepdog 44

In my opinion, it's two things. 1, is BSFC as Sendler already mentioned. Which is ideal for the centurion; being at the optimal engine loading in an underpowered diesel at a very aero efficient speed. When a motorcycle can be overpowered in many instances.

Where does the underpowered fit in? Was it the passing on the uphills part?
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My opinion is a bit different and I feel it is very "appropriately powered". RQ Riley did a wonderful job choosing the small Kubota for Centurion whose intended purpose was an urban commuter, not a drag car, and not a tow truck. And for the urban commuter application it's darn near perfect I'd say!
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Going one step further, it has roughly 17 horsepower per 1340lbs (weight on raceday)... The horsepower is just fine, it's not underpowered folks; it's way too heavy! A better way to look at it is: comparatively Centurion competed at a weight disadvantage when hp/wt is considered - and THAT is the drive behind lightening it up - it's a pig! Unlike sendler's experience with his motorcycle, everytime I lightened Centurion, the mpg increased. So for the GGP I ran with that and the numbers hit an all-time high - to include the trips to and from on the Interstate.
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Running with the adding-weight theory:
Sheepdog 44 - your Insight outporks Alfred State's by a longshot - but on the GGP track, do you really think you could at least "tie" with their best mpg? By this adding weight theory, surely you have a distinct weight advantage.
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Sendler - you've still missed the part of the passage "gap filled and boat-tailed" so to simply say that 1300 lbs is the sole reason for a stark improvement in mpg is lop-sided and ignores a couple of very important pieces. But let's run with it anyway... Bring the weight of your bike up to that of Centurion and at least tie my numbers on the Green Grand Prix track. You'll have every advantage of the weight you speak of for pulse-n-glide (which I did not do) and you have the advantage of the overpowered engine you mentioned.
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Also, for both Sheepdog 44 and sendler, the recent focus of the mpg discussion in this thread has been the Green Grand Prix, mostly because it mimics real world driving and a target mph, and not a specific scenario of say an exceptionally long downgrade to take advantage of coasting with the engine shut-off, etc, etc. But I see the conversation drifting to these very specific scenarios. So, yes, a point is proved by tipping the scale to those scenarios, but the real world is very different from those. Look at all the top mpg results from the past GGP (and elsewhere). I see a recurring theme: Aerodynamics, and, lightweight. Did they all miss the mark??
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~CrazyJerry

[changzuki]

Originally Posted by sendler

I would have to strip the bike with no luggage, lighter exhaust, and lithium battery to perform your test.

Wrong - I never said that. Please take the time to read my previous posting(s) that ask you to ADD WEIGHT to your bike. I'm in the camp of lightweight, but I'm encouraging you to add even more weight, increase your mpg, and give your theory some clout so that we may all benefit.
~CrazyJerry

[sendler]

Originally Posted by changzuki

[B]Running with the adding-weight theory:
Sheepdog 44 - your Insight outporks Alfred State's by a longshot - but on the GGP track, do you really think you could at least "tie" with their best mpg? By this adding weight theory, surely you have a distinct weight advantage.

C. Michael Lewis killed the best results that Alfred State ever achieved (low 70's) with his box stock Insight at 130 mpgUS on the track, coming back with 95 over the road with luggage, tool box, and food in the car. Bob must have felt that there was some fill up trickery involved (probably was for the first fill) so he just made up some numbers he liked and gave them a 62 so he didn't have to award them the $1000. But I talked to C. Michael while we were trying to pull his car out of the mud who told me what he really got.

[changzuki]

Originally Posted by sendler

C. Michael Lewis killed the best results that Alfred State ever achieved (low 70's) with his box stock Insight at 130 mpgUS on the track, coming back with 95 over the road with luggage, tool box, and food in the car. Bob must have felt that there was some fill up trickery involved (probably was for the first fill) so he just made up some numbers he liked and gave them a 62 so he didn't have to award them the $1000. But I talked to C. Michael while we were trying to pull his car out of the mud who told me what he really got.

Thank you for the response to my question Sheepdog 44, that was insightful.
~CrazyJerry

[changzuki]

Details - Exhibit #5: Rubbermaid

Aside from keeping Centurion as light as possible , other items were addressed along the way. One of them was the underside just behind the nose. Having previously tried two versions of an airdam, I ditched them both for three items from rubbermaid...


(Aerostang photo - thanks!)
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I'm not 100% certain this helped with cleaning up the underside, but, it did help in assisting radiator airflow. This Centurion is not using the stock Spitfire radiator, instead in uses the smaller Kubota radiator, which also has the added benefit of lighter weight .

There is very little frontal area on Centurion and there is also no traditional grille. There is however a small air inlet for cooling. You can probably spot it in this photo (courtesy of Aerostang at the 2014 Green Grand Prix- thanks!)


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So to further assist the very small air inlet (for the radiator) the license plate was angled to help direct more air into the opening:


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I don't have a real scientific way to test the relationship of airdams and such, but, Centurion left in its stock form, I was able to get a fair amount of hot air from the engine bay barreling into the cabin when the top of the console was removed. The faster the speed, the greater the flow from that uncapped console. By adding the rubbermaid trio, and the license plate angle (if that's worth anything), there is now very little air coming up through that same area, and with a bit more airflow from the angled license plate, the smaller (and lighter ) radiator cools the engine just fine.


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On deck: Taking a closer look at the lightweight ( ) Kubota D850....

~CrazyJerry

[UltArc]

I will need to get more interior (some) shots next year. Though going for lightweight, you won't want to drive with me in there lol.

It's interesting learning the why of why on the fine tuning and the vehicle itself. It would be nice if more(or any) manufacturers did this.

[aerohead]

I had this true-length drawing from Popular Machanics and did a 'Template' comparison.
The Centurion is a reverse-half-body.There is very little empirical wind tunnel data for such forms,but the best I can guestimate,is that with everything faired in,she might see Cd 0.13.
Typically,the reversed-streamline body has 60%,to 100% more drag than when it's boat tail is 'following' instead of 'leading' (according to Eiffel and Fachsenfeld).
As a half-body,in ground proximity,and with wheels,the penalty looks to be perhaps only 30%.
So where the Centurion 'backwards' might yield Cd 0.10 with a perfect cleanup,as she sits,Cd 0.13 would be nothing to sneeze at.And her frontal area is remarkable.