Characterization of the Effect of Exhaust Back Pressure on Crank Angle-Resolved Exhaust Exergy in a Diesel Engine

To enable efficient exhaust waste energy recovery (WER), it is important to characterize the exergy available in engine exhaust flows. In a recent article (Mahabadipour et al., 2018, Appl. Energy, 216, pp. 31–44), the authors introduced a new methodology for quantifying crank angle-resolved exhaust exergy (including its thermal and mechanical components) for the two exhaust phases, viz., the “blowdown” phase and the “displacement” phase. The present work combines experimental measurements with GT-SUITE simulations to investigate the effect of exhaust back-pressure (P_b) on crank angle-resolved exhaust exergy in a single-cylinder research engine (SCRE). To this end, P_b values of 1, 1.4, and 1.8 bar are considered for conventional diesel combustion on the SCRE. Furthermore, the effect of boost pressure (P_in) between 1.2 and 2.4 bar on the thermal and mechanical components of exhaust exergy is reported at different P_b. The exergy available in the blowdown and the displacement phases of the exhaust process is also quantified. Regardless of P_in, with increasing P_b, the cumulative exergy percentage in the blowdown phase reduced uniformly. For example, at P_in = 1.5 bar and 1500 rpm engine speed, the cumulative exergy percentage in the blowdown phase decreased from 34% to 17% when P_b increased from 1 bar to 1.8 bar. The percentage of fuel exergy available as exhaust exergy was quantified. For instance, this normalized cumulative exergy in the exhaust increased from 10% to 21% when P_b increased from 1 bar to 1.8 bar at 1200 rpm. Finally, although the present work focused on exhaust exergy results for diesel combustion in the SCRE, the overall methodology can be easily adopted to study exhaust exergy flows in different engines and different combustion modes to enable efficient exhaust WER.