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Wayne 962 Wayne 962 is offline
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Here's something I found on the Internet:

Quote:
In a four-stroke naturally aspirated engine, the theoretical maximum amount of air that each cylinder can ingest during the intake cycle is equal to the swept volume (displacement) of that cylinder (0.7854 x bore x bore x stroke). Since each cylinder has one intake stroke every two revolutions of the crankshaft, then the theoretical maximum volume of air it can ingest during each rotation of the crankshaft is equal to one-half its displacement. The actual amount of air the engine ingests compared to the theoretical maximum is called volumetric efficiency (VE).

There are many factors which determine the BMEP an engine can produce, but the fundamental determinant is the mass of air it can ingest into the cylinders, and there is a nearly-linear relationship between VE and BMEP. For contemporary naturally-aspirated, two-valve-per-cylinder, pushrod engine technology, a VE over 95% is excellent, and 100% is achievable, but quite difficult. Only the best of the best can exceed 110%, and that is by means of extremely specialized development of the complex system comprised of the intake passages, combustion chambers, exhaust passages and valve system components.

Generally, the RPM at peak VE coincides with the RPM at the torque peak. And generally, automotive engines rarely exceed 90% VE. There is a variety of good reasons for that performance, including the design requirements for automotive engines (good low-end torque, good throttle response, high mileage, low emissions, low noise, inexpensive production costs, restrictive form factors, etc.), as well as the allowable tolerances for components in high-volume production.

Knowing a claimed power and fuel-flow for a given engine, you can calculate an estimate of the VE required and use all these yardsticks for estimation of reasonableness.

Engine airflow at 100% VE, in sea-level-standard-day cubic feet per minute (scfm) is:

AIRFLOW (scfm) = DISPLACEMENT (ci) x RPM 3456

The air-to-fuel mass ratio at best power mixture is typically around 12.5 to 1. If you work through the arithmetic, you find that the airflow required at that mixture (12.5 to 1) is:

REQUIRED AIRFLOW (scfm) = FUEL FLOW (gph) x 2.72

So, if a 420 cubic inch engine produces 480 HP at 4400 RPM on a BSFC of .46, then the required airflow is 601 scfm and the VE is 107%. You already know that is a VERY big number.

Next Subject: Thermal Efficiency
I'm sure it's different for many different 911 engines.

-Wayne
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Old 12-29-2004, 02:45 PM
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