Advancement in Fuel Injection Technology as an Enabler for Improved Engine Efficiency and Reduced CO2, 03-R8185
Inclusive Dates: 10/01/10 – 04/01/12
Background — Modern commercial diesel engines are expected to meet extremely strict NOx and Particulate Matter (PM) regulations as well as upcoming fuel efficiency and CO2 emissions standards. For the last 10 to 15 years, the diesel industry has combated smoke production through the use of ever-increasing fuel injection pressure. Current heavy-duty engines now regularly utilize injection pressure in excess of 2,500 bar. Increased injection pressure results in significant increase in engine fuel-system parasitic loss as well as engine efficiency penalty. Therefore, there is increased interest in alternative technologies that allow the effective control of PM emissions while minimizing engine efficiency losses. One such technology, called air-assisted diesel combustion, is proposed and investigated in the current research. It employs high-velocity air jets that aid fuel-air mixing by increasing turbulence in the diesel flame envelope and reducing PM formation.
Approach — Diffusion-controlled combustion of diesel sprays with and without air-jet assistance has been investigated for different injection pressures in an optically accessible constant volume reactor. Experiments were designed to provide a qualitative and quantitative assessment of the proposed air-assisted operation and compare it with the performance of commercially available high-pressure injection systems as well as injection systems expected to be available in the near future (up to 3,000 bar injection pressure). First, the air-assisted operation has been investigated using a computational fluid dynamics (CFD) numerical solver, the results of which guided the experimental work. Second, diesel combustion has been qualitatively investigated using high-speed Schlieren photography. This approach provided enhanced understanding of the impact of the air-assisted operation on the air-fuel mixing process as well as the temporal and spatial distribution of the fuel-rich combustion responsible for the production of in-cylinder soot. Finally, quasi-quantitative experiments using the laser-based light extinction method (LEM) measured the impact of the air-assisted combustion on the time-based evolution of soot optical thickness KL at a single location within the diesel jet.
Accomplishments — For high-pressure fuel injection, it was found that the laser extinction predicted a reduction of average KL values by a factor of two as injection pressure was increased from 500 to 2,500 bar. The air-assist jets investigated, which had momentums equivalent to a 500-bar fuel injection, produced a swirling flow field, which improved the fuel-air mixing. The air-assist technology showed promise, and while further work needs to be done before it may be ready for commercialization, this internal research project showed an alternative path towards soot reduction in high-EGR combustion systems that minimizes fuel system effort and improves the energy efficiency of the engine system. The results of this work have been detailed in an upcoming SAE publication.