Volumetric Efficiency
- Thread starter weeds
- Start date
Volumetric efficiency is the volume of fuel/air mixture taken in by the enginedivided by the engine's displacement. Because air is compressible, the actual measurement is corrected to a standard set of conditions (29.92 in Hg and 60*F) so that different engines tested at different times can be compared on an equivalent basis.
Typically VE varies with RPM, increasing up to a point, holding steady within a narow range for a while and dropping off at higher RPMs. A VE curve necessarily looks very much like a torque curve for a well designed street engine, race engines are another matter. Short stroke engines and multi-valve engines tend to have higher VEs and at higher revs. Super/turbocharged engines have VEs in excess of 1.0 throughout most if not all of their operating range, many normally aspireated race engines exceed 1.0 in a narrow RPM range where the intake and exhaust tuning are maximized.
As for predicting VE from engine characteristics, there is some experimental data. Sir Harry Ricardo found that VE was maximized at a mean intake velocity of 140 FPS and remained acceptable within the range of 40-240 FPS. I've seen other data that showed VE optimized with a piston speed of 1600-2400 FPM and dropping off above that speed.
FWIW on the Ford 300, the 140 FPS intake velocity occurs at a hair over 2700 RPM and the 40-240 range is 775-4650 RPM. The 1600-2400 FPM piston speed occurs at 2400-3600 RPM. This means that from idle to max RPM the head is working as intended and the bottom end of the engine is maximized, as one would expect, in the fattest part of the torque curve.
Typically VE varies with RPM, increasing up to a point, holding steady within a narow range for a while and dropping off at higher RPMs. A VE curve necessarily looks very much like a torque curve for a well designed street engine, race engines are another matter. Short stroke engines and multi-valve engines tend to have higher VEs and at higher revs. Super/turbocharged engines have VEs in excess of 1.0 throughout most if not all of their operating range, many normally aspireated race engines exceed 1.0 in a narrow RPM range where the intake and exhaust tuning are maximized.
As for predicting VE from engine characteristics, there is some experimental data. Sir Harry Ricardo found that VE was maximized at a mean intake velocity of 140 FPS and remained acceptable within the range of 40-240 FPS. I've seen other data that showed VE optimized with a piston speed of 1600-2400 FPM and dropping off above that speed.
FWIW on the Ford 300, the 140 FPS intake velocity occurs at a hair over 2700 RPM and the 40-240 range is 775-4650 RPM. The 1600-2400 FPM piston speed occurs at 2400-3600 RPM. This means that from idle to max RPM the head is working as intended and the bottom end of the engine is maximized, as one would expect, in the fattest part of the torque curve.
A
Anonymous
Guest
To put it simply, mass airflow is what the IC engine needs. When a cylinder has the same pressure and density as the outside air, it is 100% filled. Anything less than 100% filled means that the engine cannot produce 100% of its theoretical power. When the engine is running at rpm, it takes time to "fill" each cylinder with air/fuel mixture. The valve timing events are such that (without turbo or supercharging), one cannot achieve 100% cylinder filling. Thus, the VE is less than 100% for real engines. The closer to 100%, the more the engine will produce power! Most modern IC engines are capable of 85% or higher VE. ... 
