It is traditionally accepted within the Diesel engine engineering community that Bore-to-stroke (B/S) ratios in the range ∼0.85 to ∼0.95 provide the best thermodynamic optimization for light-duty engines, mostly due to the favorable surface-to-volume ratio in the central phase of combustion, which reduces heat rejection, and to the torque-oriented volumetric efficiency profile. As a consequence, most engines into production exhibit B/S in that range, with few B/S ∼1.00 exceptions mainly for packaging issues on some V engines, and, very interestingly, on the last-generation of small and mid-sized engines.
The analysis of the technical reasons behind this recent trend is performed in the present paper, by employing a 1D-CFD approach based on Design Of Experiment (DOE) methodology. A one-dimensional analysis was carried out using a detailed GT-Power model for a 1.6 liter light-duty Mid-sized Diesel Engine (MDE), characterized by best-in-class torque and power rating in its class. In addition to B/S ratio, the effects of compression ratio, boost pressure, exhaust restriction, peak cylinder pressure and exhaust temperature was studied, in order to grasp the mutual interrelations between these factors.
The results show that, contrarily to common engineering thinking, the “square design” actually enables excellent compromise between specific power rating and low-end torque, thanks to synergy between turbocharger matching and volumetric efficiency profile. Part load heat losses are also reduced on average, thanks to lower convection which more than compensates the slightly unfavorable surface-to-volume ratio. Finally, by lowering the piston mean speed, it also benefits reciprocating and rotating components stress and friction at high speed, resulting in further fuel consumption benefits.
A second paper dealing with 3D-CFD and experimental results will follow, highlighting the impact of B/S selection also on combustion performance (heat release profile, charge utilization, EGR tolerance, pollutant emissions,…), thus deepening the present analysis to the combustion system details.