ZnsaneRyder said:
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.
We currently have the engine set up so that we have a centrifugal clutch and one way bearing so that we can accelerate accelerate the wheel, cut off the engine, and let the wheel coast without moving the chain. A video of our intended setup can be seen here (
http://www.youtube.com/watch?v=NXJhVOFls2I&eurl)
In the video, the wheel is running with minimal load and probably spinning way fast. Therefore we need some way to gearing the engine output down (up?) so that the chain speed that the wheel "sees" is in the vicinity of a bicycle's chain speed. That way, as mabman pointed out, we have flexibility in terms of which gearing type we use.
skyl4rk said:
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/
Hmmm. You're right. I was going off the specs provided by Nu Vinci in terms of drive train efficiency. According to "Bicycling Science" by David Wilson, chain and sprocket systems have efficiencies in the high 90's (95 -98%). I had thought about how heavy the wheel would be after sticking the nu vinci hub in, but did not realize that the car would also suffer a ~10% drive train efficiency drop.
I guess I'll be paying for the hub out of pocket and keeping it for myself then. No matter, the hub is cool enough to use for my own projects.
mabman said:
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.
.....
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.
The car starts from a dead stop, so the initial acceleration to top speed would consume the most fuel for that specific pulse and glide cycle. However, the track that we will race on is notorious for strong winds, so depending on the wind gods (lateral winds please!), there may be an entire section of the track where we're just burning gas just to maintain speed.
For an ideal case of a flat track, no wind, we still need to determine the actual drag coefficient of our fairing and actual wheel friction numbers. Based on that, we think our minimum speed of the line will be the point where the losses from the wheel are equal to the losses from wind drag. And the maximum speed will be the lowest possible speed to achieve the 15mph average. Could you elaborate on the 5:1 gear box reduction? I tried a simple google search for "scooter gearbox" and didn't get too many pertinent hits.
Regarding the EFI, the benefits of EFI increase the more time the engine is running, but right now, the guys who did the project can't tell me if it's more or less fuel efficient than a properly calibrated carb.
The big problem is that the oxygen sensor that we use is meant for use in cars so it needs ~6 seconds to warm up and after that we aren't even sure if there is enough exhausted gas to properly activate the sensor. Delphi makes a mini-switching 02 sensor, but I haven't had any luck in finding someone who actually sells it.
If you end up competing at the shell eco-marathon (31 days till end of registration!), maybe I'll see you there. It's a fun time, even though it's in the middle of the semester. Bring the bike too, it sounds pretty neat.