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Treadmill Dyno
taylorp035:
To start with, no-heat added pressure at 10:1 C.R. is not air pressure x 10. It's much closer to an exponential equation using PV=nRT. With no heat, it should be 342 psi @9.5:1. With the fuel, it should be 441 psi. The area for the valve opening is .77 in^2, so that works out to 441*.77 = 340 lbf on the valve. With a richer air fuel ratio and depending on the spark timing, that pressure could be much higher. Hence the reason engines knock when you get the spark timing too early. For low speed and small diameter pistons with high compression ratios, advancing the timing from 5 deg BTDC to 10 deg could increase the pressure by 50-100% since the flame covers the whole piston area so quickly (relative to crank rotation).
We could skip the springs, but it would probably be harder to get a consistent force on the valve.
SparWeb:
Right. It's been a long time since I've used "adiabatic" in a sentence. :-[
Any risk to the engine if you do remove the springs?
Have you been adjusting the spark timing?
Now you're going to need a dynamometer!
taylorp035:
Made it back to see the mini OHC engine run, after it experiencing poor performance right before competition last year due to an improperly sized carburetor and leaking poppet valves. Now with all of that was fixed, a bunch of dyno runs were made and the results were pretty amazing. The specs on it are 53 cc's, 14:1 C.R., nickel-chrome bore, and dual spark plugs with a SOHC. Peak power recorded was 1.4 hp at 3,900 rpm. With no load, it was able to idle down to ~550 rpm, which is pretty amazing for an engine that small. At power levels above 1 hp, we were able to record efficiencies of 18-19%, with a peak of 19.7%, which is a record for us. Some more runs at the RPM's that it will see at competition need to be completed, but an average of possibly 11-13% seems to be where it's at right now. Last year's engine averaged about 5% for comparison. So far, we have not hooked up an O2 sensor for AFR measurements, so maybe there is a bit more efficiency to be found. Adding an exhaust will probably help too.
Here is a video of it with no load. Closer to the end of the video, we turn down the idle until it finally stalls out at ~500 rpm. We really liked the sound that it makes.
http://youtu.be/hU5pT8_dWGY
taylorp035:
The results from this year's competition was 1020 mpg and 5th place. The car was rolling very well based on our calculations, which meant we finally have validated Michelin's rolling resistance numbers and our drag coefficient. Unfortunately, the engine was running super rich, most likely due to the increased engine bay temperature when compared to our testing. Our calculations figure a 5% average engine efficiency, which is way lower than we were hoping for. Peak efficiency on the dyno maxed out at 21.7% before competition. Our estimated engine efficiency should of been 11-15% for the race, but the carburetor wasn't tuned very well for the conditions. Two blown tires halfway into the afternoon hampered the efforts to adjust the AFR. For the last run of the day, the team gambled on putting the best tires on the car and it unfortunately didn't yield a better result (the engine was probably even hotter by that time). Interestingly, it seemed like all of the other teams were having similar issues, as the best score was 1211 mpg... a very, very close set of scores by the top teams.
Penn State Behrend's car is the white one on the far right. 76.5 lbs weight, ready to run with fuel.
SparWeb:
A good result even though not close to last year's high point. Were there attempts to adjust A/F ratio by "shot in the dark" or did you stick to a gradual adjustment plan?
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