Correction Factors for Power and Volumetric Efficiency

    2.12 Correction Factors for Power and Volumetric Efficiency

The pressure, humidity, and temperature of the ambient air inducted into an engine, at a given engine speed, affect the air mass flow rate and the power output.

Correction factors are used to adjust measured wide-open-
throttle power and volumetric efficiency values to standard
atmospheric conditions to provide a more accurate basis
for comparisons between engines.

Typical standard ambient Conditions used are:

The basis for the correction factor is the equation for one-
dimensional steady compressible flow through an orifice
or flow restriction of effective area A_£



In deriving this equation, it has been assumed that the fluid is an ideal gas with gas constant R and that the ratio of specific heats ( Cp/c_v = y ) is a constant; P₀ and T₀ are the total pressure and temperature upstream of the restriction and P is the pressure at the throat of the restriction.

If In the engine, P/P_o is assumed constant at wide-open throttle, then for a given intake system and engine, the mass flow rate of dry air m^._a varies as



For mixtures containing the proper amount of fuel to use all the air available and thus provide maximum power), the indicated power at full throttle p_i will be proportional to m^._a.The dry air flow rate. Thus if

                                                           P _i,s = C_F P _i,m

Where the subscripts s and m denote values at the standard and measured conditions, respectively, the correction factor C_F is given by



Where

p_s,d = standard dry-air absolute pressure
p_m = measured ambient-air absolute pressure
p_Vm = measured ambient-water vapor partial pressure
Tm = measured ambient temperature, k
Ts = standard ambient temperature, k

The rated brake power is corrected by using Eq ( D ) to correct the indicated power and making the assumption that friction power is unchanged. Thus

                                                                P _b,s= C_F P _i,m - P_f,m


Volumetric efficiency is proportional to m^._a / p_a, since p_a " air density " is proportional to P/T, the correction factor for volumetric efficiency, C'_F, is

Dry air pressure	Water vapour prssure	Temperature
736.6 mmHg	9.65 mmHg	29.4^O C
29.00 mmHg	0.38 mmHg	85 ^O F