Volumetric Efficiency

2-10   Volumetric Efficiency

One of the most important processes that governs how much power and performance can be obtained from an engine is getting the maximum amount of air into the cylinder during each cycle.

More air means more fuel can be burned and more energy
can be converted to output power.

However,because of the short cycle time available and the
flow restrictions presented by the air cleaner, carburetor ( if
any ), intake manifold , and intake valve(s),less than this
ideal amount of air enters the cylinder.

The intake system" the air filter,carburetor,and throttle plate
(in a spark-ignition engine),intake manifold,intake port, intake
valve - restricts the amount of air which an engine of given
displacement can induct.

The parameter used to measure the effectiveness of an
engine's induction process is the volumetric efficiency n_v.

Volumetric efficicncy is only used with four-stroke cycle engines which have a distinct induction process. It is defined as the volume flow rate of air into the intake system divided by the rate at which volume is displaced by the piston :
                                                                 2m^._a
                                                 nv =       -------------------------
                                                             p_ai Vd N

where p _air is the inlet air density. An alternative equivalent definition for volumetric efficiency is
                                                                  m_a
                                                 nv =       -------------------------
                                                                 p_ai Vd

where ma, is the mass of air inducted into the cylinder per cycle.

The air density pair can be evaluated at atmospheric conditions; n_v is then the overall volumetric efficiency. Or it can be evaluated at inlet manifold conditions; , (hence measures the pumping; performance of the cylinder, inlet port, and valve alone. This discussion will cover unthrottled (wide-open throttle) engine operation only. It is the air flow under these conditions that constrains maximum engine power. Lesser air flows in SI engines are obtained by restricting the intake system flow area with the throttle valve.

Volumetric efficiency is affected by the following fuel, engine design, and engine operating variables:

1.Fuel type, fuel/air ratio, fraction of fuel vaporized in the intake system, and fuel heat of vaporization
2.Mixture temperature uninfluenced by heat transfer
3. Ratio of exhaust to inlet manifold pressures
4. Compression ratio
5. Engine speed
6. Intake and exhaust manifold end port design
7. Intake and exhaust valve geometry, size, lift, and timings