Definition and theory of volumetric efficiency.
Internal combustion engines are volumetric machines, therefore their first typical problem is to remove the burned gases, at the end of the power stroke, and to fill the working volume with fresh charge, at the beginning of each cycle.
To describe the gas exchange process we have to consider the two following masses of fresh air:
- the ideal mass m(t) that could fill the swept volume V, if inside the cylinder there were physical reference conditions in term of air density;
- the actual aspirated mass m(a), really present inside the cylinder, at the end of the intake process of the single cycle.
It is defined filling coefficient or volumetric efficiency the ratio of these two air masses:
V.E. = m(a)/m(t)
This parameter measures the effectiveness of the global intake process of a given engine.
Physical reference conditions.
The physical reference conditions, to define the air density may be taken as:
- either the atmospheric conditions aroud the engine, in which case V.E. measures the performance of the entire inlet system;
- or the conditions inside the inlet manifold, in which case V.E. measures the only performance of the inlet port and valve.
The physical conditions inside the inlet manifold are used only in supercharged engines, to distinguish what is due, in term of aspirated air mass, to the supercharging group and what is instead due to the engine inlet system.
Atmospheric conditions.
The atmospheric conditions are taken as reference, because they allow both an easier experimental determination of V.E. and a simpler use of it in prediction calculations.
In an engine test room: mass flow rate of aspirated air, crank-shaft rotational speed and ambient physical conditions are easily mesaured.
Then aspirated mass and air density are immediately calculated and from definition V.E. = m(a)/m(t) volumetric efficiency is determinated.
Typical maximum values of V.E. for naturally aspirated engines at full load are in the range of 90-98%.
Examples of volumetric efficiency maps.
First example is Honda Civic with ecu Keihin. Here you can see the values of V.E. (in %) by RPM and degrees of camshaft position sensor.
Max values corresponding to the middle RPM and high deg CAM.
Second example is Chevrolet Corvette with Delco engine control unit. In this case the volumetric efficiency V.E. is in function of RPM and air pressure (kPa).
Max values corresponding to high RPM and high air pressure.
Third example is V.E. for new Subaru with Hitachi ecu. As other examples % of volumetric efficiency is in function of RPM and kPa measured by MAP.
Max values starting from low-middle RPM and high air pressure.
Last example is a Kawasaki ATV with Mitsubishi engine control unit. Also in this case V.E. is in function of RPM and pressure.
Max values starting from low-middle RPM and high air pressure.
