A new concept of actively controlled rate of diesel combustion for improving brake thermal efficiency of diesel engines: Part 1-verification of the concept

摘要

For further improvement in brake thermal efficiency of diesel engines, not only increase in indicated work but decrease in energy losses, for example, cooling loss and mechanical loss, are essential. Nevertheless, these demands are generally in trade-off with limited ability of combustion control by conventional diesel fuel injection equipment even though it can precisely control injection timing, duration, pressure, and frequency. To overcome the trade-off, the potential of heat release rate control with more degrees of freedom was first investigated by means of a zero-dimensional thermodynamic engine model. The result indicated that the optimum brake thermal efficiency is not achieved with constant-volume combustion (Otto) cycle, not with constant-pressure combustion (Diesel) cycle, but with Sabathe (limited-pressure or Seiliger) cycle. To experimentally confirm the numerical analysis result by a heavy-duty single-cylinder diesel engine, a new diffusion-combustion-based concept with the combination of multiple fuel injectors and soup plate type piston cavity has been developed. One injector was mounted vertically at the cylinder center like in a conventional direct-injection diesel engine, and two additional injectors were slant-mounted at the piston cavity circumference. The side injectors sprayed fuel downstream the swirl to prevent both spray interference and spray impingement on the cavity wall, while improving air utilization near the center of the cavity. The desired heat release rate profile was well achieved by independent control of injection timing and duration (fuel injection pressure was kept in constant) for each fuel injector. Results showed reduced friction loss, heat loss, and nitrogen oxides (NOx) emissions, while maintaining indicated thermal efficiency by suppressing the peak cylinder pressure, cylinder-averaged temperature, and spray flame impingement to the cavity wall. Additionally, a simultaneous reduction in smoke and NOx emissions was achieved, without any deterioration in carbon monoxide (CO) and total hydrocarbon emissions, even compared with conventional diesel combustion.