Thermodynamic Modeling of Military Relevant Diesel Engines with 1-D Finite Element Piston Temperature Estimation

In military applications, diesel engines are required to achieve high power outputs and therefore must operate at high loads. This high load operation leads to high piston component temperatures and heat rejection rates limiting the packaged power density of the powertrain. To help predict and understand these constraints, as well as their effects on performance, a thermodynamic engine model coupled to a finite element heat conduction solver is proposed and validated in this work. The finite element solver is used to calculate crank angle resolved, spatially averaged piston temperatures from in-cylinder heat transfer calculations. The calculated piston temperatures refine the heat transfer predictions as well requiring iteration between the thermodynamic model and finite element solver. Both the thermodynamics and the piston temperature predictions are validated against experimental data obtained from a heavy-duty single cylinder research engine equipped with a wireless telemetry system and piston surface thermocouples to measure piston surface temperatures. The piston backside conditions are critical to the performance of the temperature solver, therefore the tuning of piston backside conditions to match experimental data is considered and assessed. The validated model is then used to analyze the performance of the heat transfer correlations developed by Woschni and Hohenberg. The piston temperatures predicted by each of the correlations are compared to those measured in the experiment both in terms of the piston temperature swing and its sensitivity to injection timing. Finally, the capabilities of the coupled model are demonstrated by analyzing the effects of engine geometry on engine performance relative to critical limitations for military engines.