Brake Specific Air Consumption
01-01-2007, 10:46 AM
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Brake Specific Air Consumption BSFC (fuel flow/HP) is a figure of merit for motor efficiency and
a fudge factor in injector sizing calcs. But in an airflow limited
engine it seems to me that there is something to be learned from
the BSAC value, across tune-points. Optimizing use of the limiting
quantity as it were.
Do people do this, like on a dyno where you can see HP and (by
the scanner) airflow values, looking at the HP/air, and does the
null for that, correspond in any way to the peak-settings HP for
the mechanicals at hand?
Similarly I wonder if there is anything in the torque/CylAirMass
ratio that might show where the "best" tune point might lie.
a fudge factor in injector sizing calcs. But in an airflow limited
engine it seems to me that there is something to be learned from
the BSAC value, across tune-points. Optimizing use of the limiting
quantity as it were.
Do people do this, like on a dyno where you can see HP and (by
the scanner) airflow values, looking at the HP/air, and does the
null for that, correspond in any way to the peak-settings HP for
the mechanicals at hand?
Similarly I wonder if there is anything in the torque/CylAirMass
ratio that might show where the "best" tune point might lie.
01-01-2007, 10:56 AM
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ok so i had to read the post a few times before i understood, and i still dont. Are you saying/asking about tuning an engine with a set amount of airflow on a dyno while minimizing or exceeding the airflow seen while driving on the road? So by doing so your limiting the max gains of the tune to the amount of airflow given during the tune? If im way off, i'll just shut up and sit in the corner with my dunce hat on.
01-01-2007, 07:46 PM
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I was thinking more about, how to tell when you have
gotten the most out of the available airflow. Not trying
to limit it but to make the best of those limits that are
there.
Wondering if maximized airflow is the same as maximized
power output and under what, some or all conditions.
Thinking that understanding this might enable people to
find best power by street tuning from raw airflow readings,
same car, more air, always good? Or should there be some
windage put to it, etc.
gotten the most out of the available airflow. Not trying
to limit it but to make the best of those limits that are
there.
Wondering if maximized airflow is the same as maximized
power output and under what, some or all conditions.
Thinking that understanding this might enable people to
find best power by street tuning from raw airflow readings,
same car, more air, always good? Or should there be some
windage put to it, etc.
01-01-2007, 08:27 PM
#6
It's pretty easy to find out the maximum amount of power per some unit of airflow. You can do it with just the heating value of the fuel used.
You can also find indicated (and brake) hp/torque using AFR, heating value, VE, combustion eff., RPM, mechanical eff, and a few others. If your interested I can dig the formula up out of some old notes and post it for you.
In a quasi-steady state cycle and ignoring time the best power would be made at the highest airflow and stoich AFR where adiabatic flame temp is highest... Not really the case in a real engine where VE, combustion eff. and flame travel speed dependant of AFR
You can also find indicated (and brake) hp/torque using AFR, heating value, VE, combustion eff., RPM, mechanical eff, and a few others. If your interested I can dig the formula up out of some old notes and post it for you.
In a quasi-steady state cycle and ignoring time the best power would be made at the highest airflow and stoich AFR where adiabatic flame temp is highest... Not really the case in a real engine where VE, combustion eff. and flame travel speed dependant of AFR
