A team at Japan’s New ACE Institute—an industry-funded research initiative founded to develop a new diesel combustion concept—has developed a new diffusion-combustion-based concept with multiple fuel injectors to overcome the trade-offs of thermal efficiency with energy loss and exhaust emissions typical of conventional diesel engines.
In a presentation at the 2016 SAE High Efficiency IC Engine Symposium, Noboru Uchida, general manager of research for New ACE, outlined the basic approach, which he said potentially offers a pathway to greater than 50% brake thermal efficiency without the use of waste heat recovery systems. A paper on their concept is also published in SAE International Journal of Engines.
The objective, said Uchida, was to find the optimum combustion strategy which overcomes the complex trade offs among emissions, brake thermal efficiency (BTE) and energy losses based on conventional diesel combustion with more degrees of freedom.
The proposed engine employs neither low temperature combustion nor homogeneous charge compression ignition combustion; it implements a kind of Sabathé cycle (limited pressure dual cycle in which heat addition occurs partly at constant volume and partly at constant pressure).
Uchida noted that the Sabathé cycle is superior in suppressing the in-cylinder pressure increase rate and in reducing losses by constant pressure combustion. The New ACE concept is to enable the consecutive heat release of premixed and shaped diffusion combustion areas within combustion chamber; the premixed combustion area and the diffusion combustion area are temporarily and spatially isolated.
|Piston crown design, injector orientation and spray directions. Okamoto and Uchida (2016). Click to enlarge.|
To achieve this, the cylinders are fitted with three injectors: one vertically mounted at the cylinder center as in a conventional direct injection diesel engine, and two additional injectors slant-mounted at the piston cavity circumference.
The sprays from the side injectors are directed along the swirl direction to prevent both spray interference and spray impingement on the cavity wall, while improving air utilization near the center of the cavity.
Fuel sprays from center and side injector might interfere with each other if fuel is injected at the same time or center injection timing is later than side injection timing. The team shaped the piston cavity to achieve a relatively large bore but deep bottom to avoid wall impingement by the side injectors, even at high compression ratios.
The desired heat release rate is obtained by independent control of injection timing and duration (fuel injection pressure was kept in constant) for each fuel injector. The center injector fires first, followed by the side injectors.
Experimental results showed reduced friction loss, heat loss and NOx emissions, while maintaining indicated thermal efficiency by suppressing the peak cylinder pressure, bulk average temperature, and spray flame impingement to the cavity wall.
Additionally, the New ACE researchers achieved a simultaneous reduction in smoke and NOx emissions was achieved without any deterioration in CO (carbon monoxide) and THC (total hydrocarbon) emissions, even compared with conventional diesel combustion.
Okamoto, T. and Uchida, N. (2016) “New Concept for Overcoming the Trade-Off between Thermal Efficiency, Each Loss and Exhaust Emissions in a Heavy Duty Diesel Engine,” SAE Int. J. Engines 9(2) doi: 10.4271/2016-01-0729