Theory 3 - Mechanism Effectiveness

The mechanism effectiveness is the ratio of energy that is transferred by the mechanical parts from the working fluid to the buffer module and reciprocally from the buffer module to the working fluid.

When accounting for the mechanism effectiveness we can get an estimate of the mechanical efficiency of any engine over a complete cycle.

 Assuming the mechanism effectiveness is a constant value E, we get following equalities:

• The work effectively transferred from the working fluid to the buffer module is E*W₊
• The work transferred from the buffer module to the working fluid is W_- . Due to losses in the mechanism the buffer module has to provide a quantity of energy W_-' which is higher than W_-. Actually we have W_- = E*W_-'

Finally the work balance for the shaft is

W_shaft = E*W₊ - E*W_-' = ... = EW - (1/E - E)W_-

 We can deduce the overall mechanical efficiency of the engine using

 E_meca = W_shaft/W = ... = E - (1/E - E)W_-/W

where W is the indicated work of the thermodynamic cycle (surface of the closed loop).

Therefore, two parameters have a great impact on the overall efficiency:

1) the ratio W_-/W

2) the mechanism effectiveness E

Following graph shows the overall mechanical efficiency vs mechanism effectiveness E for  engines with different buffer pressures, resulting in different W_-/W values:

Note that for each W_-/W value there is a lower limit of mechanical effectiveness, below this limit the engine will not run. As seen in previous chapter W_- (and therefore W_-/W) is strongly dependent on the choice of buffer pressure.