Relationships betwwen Performance Parameters

   2.14   Relation Between Performance Parameters

The importance of the parameters defined to engine performance become evident when power, torque,and mean effective pressure are expressed in terms of these parameters.

From the definitions of

* engine power
* mean effective pressure
* Fuel conversion efficiency
* fuel/air ratio
* volumetric efficiency.

The following relationships between engine
performanceparameters can be developed.
For power P:

                  n_fm_a N Q_HV (F/A)
       p = ---------------------------------------------     Eq (a)
                            n_R

For four-stroke cycle engines, volumetric efficiency can be introduced:

                                                 n_fn_v N V_dQ_HV p_a,i(F/A)
                                          p = ----------------------------------------------------------                        Eq (b)
                                                                      2

For torque T:
                                             n_fn_v V_dQ_HV p_a,i(F/A)
                                          p = ----------------------------------------------------------
                                                                 4 pi

For mean effective pressure :

                                               mep = n_fn_v Q_HV p_a,i(F/A)

The power per unit piston area,called the specific power, is a measure of the engine designer's success in using the available piston area regardless of cylinder size. From Eq ( b ) the specific power is :

                                                            n_fn_v N LQ_HV p_a,i(F/A)
                                          p/A_p =     ---------------------------------------------------------
                                                                            2

Mean piston speed ( S_p = 2 LN ) can be introduced to give

                                                             n_fn_v S_p Q_HV p_a,i(F/A)
                                          p/A_p =     ---------------------------------------------------------
                                                                           4

Specific power is thus proportional to the product of mean effective pressure and mean piston speed.

These relationships illustrate the direct importance to engine performance of :

1 - High fuel conversion efficiency
2 - High volumetric efficiency
3 - Increasing the output of a given displacement engine by increasing the inlet air density
4 - Maximum fuel/air ratio that can be usefully burned in the engine
5- High mean piston speed