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Treadmill Dyno
taylorp035:
Adjusting the AFR is difficult, because during a burn, it varies widely, from super rich to super lean. This causes the engine not to want to start. Also, it's impossible to tune the AFR without the correct load. The carb has an AFR screw for the low end and the main needle for the high end. The carb needed the needle moved a bit, but once you do that, the AFR and idle screws need tuned a lot, making this a risky decision in the middle of a race day. You can only drive the car on the track, so you wouldn't get many chances to tune the engine if you messed it up. Also, the low end AFR screw is super sensitive, so it's hard to play with. Varying air temperature for the intake also makes things a challenge, as the car heats up once the lid is put on.... this year, the nose of the car got so hot under where the front window was, that it was getting soft. Some teams put aluminum foil under the driver's feet... I can't imagine that helps visibility out of the already sketchy windows.
SparWeb:
So... next year's race will be in Nome Alaska? ;)
taylorp035:
It's interesting, because every year, the weather conditions are different, from windy to calm and then from cold to hot and sunny to stormy. Yet most teams aren't prepared for varying situations. Being able to have your car run at it's best in any weather condition is important. In the 5 years I've been there, I bet the insides of the cars has varied from the coldest to the hottest by 100F+. Just goes to show, learning how to make the best supermileage car takes more than a year or two... it really takes several generations of students to pass on the knowledge of all of the lessons learned.
And another thing that is so great is that there is no perfect answer for a car and that there is always room for improvement. A perfect car in the perfect conditions could achieve 6-10,000 mpg... yet only two teams in SAE history have cracked the 3,000 mpg barrier and only a handful to break 2k mpg.
SparWeb:
Did you install any sensors to help set it - oxygen / air flow / throttle position, etc.?
Did the unique valve system perform well?
taylorp035:
No, No, No, and No. We had an external O2 sensor that we could clamp to the end of the exhaust pipe, but that's it. Throttle position was up to the driver. On our EFI test engines, we have MAF's, but not on the carbureted engines. I have seen teams who have attempted a servo controlled throttle to vary the throttle % during a burn... fairly complex to effectively do so.
The engine in the car this year was the 53 cc SOHC ~14-15:1 CR custom head with dual spark plugs. It ran very well and experienced no problems that we know of, just that the AFR was way too rich.
The rotary valve for the SAE car has not been tried again. It needs to be re-sealed and glued to be run again. The only reason to do this is for the satisfaction to see it run, since no one would risk running it at competition. My homemade rotary valve still needs some attention and probably a bit of tuning on the AFR front..... which is hard to do with no O2 sensor and a coil wrap pull start.
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