Gunning for high gas mileage

[DougPan]

I've been lurking in these forums for a while, and suppose it's time to finally dive right in. I'm in the super mileage vehicle team in our university, and the whole point of our club is to build a vehicle that gets the best possible gas mileage.

My club has spent a whole bunch of time with composites, and we can make some pretty nifty stuff (fully enclosed aerodynamic fairing, carbon fiber "skateboard" chassis). We've reduced the total weight of the car to ~40 kg and have an estimated drag coefficient of .1 .


However, in our rush to get the space age stuff done, we've neglected the most important part: the engine and drive train. We plan to use a Honda Gx25, and a double reduction gearing to drive a Nu Vinci hub. The gx25 has a custom made EFI setup and will be optimized (hopefully ) later.

The competition that we enter is requires that the "car" (it's really a motorized recumbent bicycle) travel an average of 15 mph for 9 miles. This means that we use a burn and coast strategy where we accelerate till around 30 and coast until we're almost stopped and then we accelerate again.

The major complication is that I have no idea how to properly setup a double reduction gearing. Many of the projects I've seen on the site use the station gearbox. However, in our current mounting, there is little room for the box.

More info on our EFI setup is available here (http://anuj.ch.googlepages.com/ucb_smv_efi)

Thanks for this wonderful resource!

PS I plan to motorize this old beach cruiser I have laying around the house. Any suggestions for a good frame type setup?

 

[skyl4rk]

The GX25 is a good choice. You will need to work out how to freewheel with the motor off, most kits cannot do this.

Pulse and glide is definitely the way to go.

I have heard the NuVinci has a lot of drag. If you really need a geared system, I would suggest using a chain drive to an internal gear hub. If you don't have a lot of money, find an old Sturmey Archer and rebuild it.

http://www.sheldonbrown.com/interna...com/freewheel-sprocket-axle-wbrake-p-297.html

If you don't use an internal gear hub, you might be able to use one of these freewheel kits and pull start (?) the motor every time you need to pulse.

http://www.bicycle-engines.com/freewheel-sprocket-axle-wbrake-p-297.html

If I were doing it cheaply, I would build a jackshaft similar to the Dax Titan and use a pull start motor with a freewheel hub. The trick is to get the right rpm range to pull you up to 30mph hopefully within the optimal torque curve rpm range for the engine.

http://www.thatsdax.com/

As for the frame motor setup for a cruiser, good luck, I am also looking and have been for years. The best option right now is the Grubee kit:

http://www.bicycle-engines.com/

 

[mabman]

Can you pedal?

Gas only. Answered my own question.

 

[skyl4rk]

http://www.honda-engines.com/engines/gx25.htm

So it looks like you will want to run the motor from about 5000 rpm to 7000 rpm then switch gears.

 

[mabman]

Because their engine is modified via the fuel injection and whatever else they have done to it, a dyno test would have to be re-run. Those graphs are for the stock motor.

I think the Nuvinci, in that it is an internal hub, with their new developer kit View attachment DevKit_Datasheet_V4Web.pdf that is due out any day would be an excellent choice for this project. The ability to program your gearing via software is about as exact as it can get. Contact the factory direct.

Any drag from the hub, and all hubs create drag, could be reduced greatly by having a freewheel between it and the other power sources. Something like this,

You know of course that the driver of that rig is going to want to turn the Ipod up to 10 and be wearing noise cancelling head gear as running an engine in that shell is going to be LOUD.

 

[DougPan]

Thanks for the replies, guys. It's been pretty helpful.

To skyl4rk:
Thanks for the many links, I'll take the time to slowly go through them and learn what I can. I am pretty new to a lot of bike terminology and that Sheldon Brown website is pretty fantastic.

To mabman:
The vehicle's motive force can only come from the engine. We currently have a set up where the engine will start reliably. However, the big problem, as you've stated, is to generate an accurate torque curve ( or even better, a brake specific fuel consumption curve). I've looked around this forum for a couple of ideas, and am trying to decide between two setups: an inertia dyno and one that drives a large electric motor. I think that the one with the electric motor would allow for longer tests thus letting us play with injection times and what not. But the inertial one seems much easier to make.

Secondly, thanks for the developer kit attachment. As a team, we were thinking of using either a linear actuator ($$$) or a tiny dc motor with an encoder that's been geared up (down?) so that it's able to pull the shifter. That way, we can link the motor rpm, and wheel rpm in order to have the nu vinci shift juuuuust right.

Unforunately for the driver, we haven't taken her comfort too much into consideration yet. And the suggestion of noise canceling headphones is a good one. Another thing we were considering was an insulated enclosure for the engine. Since we pulse and glide, the heat generated when the engine is running will keep the engine warm and help avoid combustion in the transient regime. IE. when the motor is cold, it will not be as efficient as when the motor is hot.

Thanks for all the help guys. I really appreciate it. Is there any interest in the making of the aerodynamic fairing? If there is, I'll put it together. It's Christmas break, and I might as well do something useful.

Edit: I started studying the attached picture, and am not too sure what benefit there is in having the freewheel as opposed to a live axle would be. It seems that for a bicycle setup that has to juggle between pedaling and motor input, the freewheel is there so that the rider can rest his legs on the pedals as the motor revs up.

Edit 2: That said, I would really appreciate it if you had pictures of that set up from different angles. Other examples of that would be great too! Maybe I'm just not "getting" it.

 

[mabman]

I have some better pics around here somewhere of that setup and another currently under development and will try and dig them up later. But for our needs we want to be able to pedal, motor and coast independently while minimizing drive train drag especially for the pedaling and coasting part, yet also pedal and motor concurrently and as pedaling is not part of your parameters perhaps that setup would not be valid. The idea is to insulate each drive with a freewheeling mechanism, in this case simple bmx type freewheels through a jackshaft.

If you are solely dependent on the motor for your drive system and want to coast efficiently then you still need to reduce drive train drag to a minimum as you know, while keeping weight to a minimum also. As I said all internal hubs have drag, they also are heavy. The Nuvinci setup with the servo will give optimum shifting/rpm but maybe the weight and complexity is a detriment and you would be better off with a plain old derailleur type system and a well trained pilot? A small tach could be enlisted and optimum rpms/load determined easily on a small dyno to achieve this? As old as it is the old style chain/freewheel system is actually still the lightest and most effective drivetrain. It has less of a co-efficient of friction than the new belt drives actually. But the belt drives do not require maintainence of course.

Correct me if I'm wrong but the way I see it your greatest fuel expenditure will be from a dead stop? So the gearing you have to get off the line is pretty important in that it will do so effectively and that means a pretty low gear ratio to start. Because these motors all require gear reduction I think that the simple 5:1 box that is available through the scooter parts places like the one in the photo previously supplied would be a good place to start. It is relatively light in weight and compact. From there you can design your additional reduction necessary via sprockets probably? You just need a top gear that gets you to 30 which can be calculated easily using your wheel size and sprocket teeth front and final. I would say off the cuff that a 120" gear would do the job for 26" wheels and more for you as your wheels are 20"?

It just occurred to me that WWU here is probably in on this competition and I should see what they are up to. I talked to the head of the dept. there at one point and he wasn't at all interested in what we are up to trying to get MAB's recognized as LSV's.

 

[mabman]

And as long as your offering I will take one of these:


I like this design because it looks like it would really easy to insulate the engine from the cockpit via a simple insulated bulkhead.

And btw I thoroughly enjoyed your google pages, at least what I could understand and think that fuel injection system is bad. Now I want to make one!

As far as the competition goes I realize that there have to be parameters set and that 9 miles and 15 mph average is it. But I am more interested in the real world environment and i think that FI could help in that realm most of all that I saw on there. The current setups flirt with 200 mpg but as you load things down the mileage goes down also. Also for us it is a matter of how much you are willing to pedal which can be a big gas saver. My personal goal is 300 mpg with a bike capable of carrying 250 lbs. of cargo, while carrying said cargo. A fraction of 2800 certainly but I want to average 30 mph over 900 miles at that number.

 

[skyl4rk]

The NuVinci is a great system if you have power to burn. If you are in a mpg competition you cannot afford the internal losses due to friction. Note that you will be spending about two thirds (or more) of your time freewheeling with the motor off. Low drag during this period is critical go overall mileage per gallon.

http://hubstripping.wordpress.com/atc-nuvinci/

 

[ZnsaneRyder]

Since it's a small engine, how about using a centrifugal clutch with bicycle gears and shifter to get your MPG? Any type of CVT will consume more fuel. A clutch on the other hand, will fully lock at high RPM and you don't waste revs.

I'm getting 100+MPG with heavy 196cc engine and only one ratio, no shifting, so I know that with the right gearings, your 25cc should get 200MPG.