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Volumetric Efficiency (VE)

  
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Volumetric Efficiency (VE)

 
dayers dayers
New User | Posts: 1 | Joined: 02/09
Posted: 02/04/09
07:42 PM

What is the best VE of an old (pre 80's) Ford 302 with stock components can we expect? Can a naturally aspirated engine come close to 100% without being NASCAR? I know that exhaust/intake overlap helps but what else such as intake runner length, exhaust manifold/header, etc.? Finally, which provides better VE, supercharging or turbocharging? Actually finally, how do you physically determine VE? Thanks.  

Outsider II Outsider II
Guru | Posts: 1140 | Joined: 08/06
Posted: 02/05/09
01:12 PM

Volumetric Efficiency (VE) is going to vary with rpm.  The maximum VE should closely correspond with the peak of the torque curve (although frictional and thermal loads can affect this).

The rule of thumb for OEM two-valve OHV wedge engines in bone-stock form is 80% AVERAGE VE. However, given the power output of most stock emissions-era small blocks, VE is often less than 80%.

Averages aren't really very helpful in modeling engine performance.

If you know the horsepower output and base specific fuel consumption (BSFC) for a particular rpm, you can solve for VE of an Otto Cycle engine as follows:

 REQUIRED VE = ( 9411 x HP x BSFC ) / (DISPLACEMENT x RPM)

With a ram-tuned intake, proper cam timing, adequate port and valve sizes and a tuned exhaust, it is possible to achieve in excess of 100% VE naturally-aspirated over a limited rpm span. Mismatch any of these variables and VE will suffer.Ergo, increases in rpm and matching the airflow motion variables to the targeted rpm range are the primary (but not exclusive) means of increasing naturally aspirated engine performance.

About 115% is the maximum observed VE limit for fully developed DOHC engines.

Plenty of naturally-aspirated street/strip engines have a peak VE above 90%, although the average will be less.

Calculating VE is somewhat misleading for supercharged and turbocharged engines.  Basically, the "natural" VE of a forced induction engine is enhanced by aspirating denser air.  Modifications that improve the natural aspiration ability of the engine tend to be multiplied by the density factor.  But the power output of the   engine is adversly affected by the additonal load required to increase the air density.

For street superchargers, that additional load can be anywhere from 45 to 150 h.p. or more, depending on the efficiency of the supercharger/blower and the manifold pressure level. All of this additional load comes from horsepower that would otherwise be available to the drivetrain.

For most street turbo set-ups, that additional load is 25 to 45 h.p. However, the turbo's additonal load is lower because part of the energy necessary drive the compressor is reclaimed from wasted thermal energy.  Although the exhaust turbine of a turbo would reduce "natural" VE (which is reflected in the "additional load"), the engine more than makes up for this through the increased air density multiplier effect -- at least up to the point where the turbo chokes airflow through the engine.  It's a bit like giving up five or ten cents on the dollar to gain another dollar or more.

In summary, turbo systems tend to be more powerful than supercharger systems for a given level of intake air density produced by virtue of high compressor efficiency and reclamation of waste thermal energy.  

VE, of course, is substantially different at part-throttle because the engine is running at far less-than-atmospheric air densities.  Also, Atkinson Cycle engines induct at a lower VE because of late intake valve closure which reduces the effective stroke, but expand the combusted charge using a longer effective stroke, which tends to extract more useable thermal energy from a qiven quantity of fuel.  

Hope this helps.  
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A nobody who never was . . . .