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Originally Posted by lkewin
I have seen some heavy formula's thrown around in other threads and know we have a few rocket scientists in our midst, so.. is there a way to calculate the amount of time you would need to shut off the engine of a NH to overcome the "penalty" to re-start....or is there not more gas used to start an ICE?
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OK, I'll venture in:
assuming a warmed up engine, stoichiometric burning and 13:1 mass air/fuel ratio at sea level (fuel @ .75g/cc and air @ .0012g/cc)...approx. 8000:1 air/fuel volume ratio.
A 2 liter engine at 700rpm idle speed injects ~.125cc of fuel every rotation (1 injection per cylinder every 2 rotations for 4-stroke engine, so 1 liter "active" per rotation)
ASSUME it takes about 2 sec to start the engine to idle, and ASSUME that the mix is enriched and nonstoichiometric (=smelly emissions at startup), so ASSUME it uses triple the fuel for those 2 seconds. 3 x .125cc x 25rotations = ~9.5cc of fuel to start the engine. Call it 10. That's 2 teaspoons.
Back at normal idle, the engine will use 2 teaspoons of fuel in 80 rotations, or 7 seconds.
There you go. Idle for more than 7 seconds and you are better off stopping and restarting. Call it a bullride- 8 sec. Or 10 for us cityslickers.