What Are the Barriers Against Brake Thermal Efficiency beyond 55% for HD Diesel Engines?

This study focused on the technology integration to aim beyond 60% indicated thermal efficiency (ITE) with a single-cylinder heavy-duty diesel engine as an alternative to achieve 55% brake thermal efficiency (BTE) with multiple-cylinder engines. Technology assessment was initially carried out by means of a simple chart of showing ITE and exhaust heat loss as functions of cooling loss and heat conversion efficiency into indicated work. The proposed compression ratio (28:1), excess air ratio and new ideal thermodynamic cycle were then determined by a simple cycle calculation. Except for peak cylinder pressure constraint for each engine, the technical barriers for further ITE improvement are mainly laid in cooling loss reduction, fuel-air mixture formation improvement, and heat release rate optimization under very high temperature and density conditions with very high compression ratio (smaller cavity volume). For the significant cooling loss reduction, a new concept of in-cylinder wall surface coating combined with intake chilling was experimentally investigated based on the hypothesis of the cooling loss mechanism which relates the heat transfer phenomena on the surface not only during combustion period but also during exhaust and intake strokes. For the mixture formation improvement, several numerical or experimental case studies were carried out to find the guideline for the chamber shape and fuel injection equipment design. For the heat release rate optimization, a new thermodynamic cycle was proposed. Although the engine has not been fully optimized yet, 59.5% ITE was experimentally achieved with the single cylinder engine by utilizing these technologies without any significant drawbacks.