Combustion characteristics of a charcoal slurry in a direct injection diesel engine and the impact on the injection system performance

The paper presents the research results pertaining to the renewable biomass charcoal–diesel slurries and their use as alternative fuels for combustion in diesel generating plants. The utilization of charcoal slurry fuel aims to reduce diesel oil consumption and would decrease fossil green house emissions into the atmosphere. The paper investigates the formulation, emulsification, sprays, combustion, injection system operation, and subsequent wear with charcoal–diesel slurries. In the research, cedar wood chips were used for the production of charcoal to be emulsified with diesel oil. The slurry’s viscosity of 27cP achieved the target (<100cP) and gave prospects of good spray atomization and while maintaining a high calorific value. Thermal analysis studies found that cedar wood will oxidize about 75% of its original mass by 450°C. Charcoal slurry displayed a high vaporization rate of 75% by wt. at 300°C. Engine investigations showed that the top combustion pressure at 1200rpm and 100% load (7.8 brake mean effective pressure (bmep)) was 79bar for diesel fuel and 78bar for the charcoal slurry fuel. From the injection and heat release history was found an ignition delay of 1.7ms for diesel that increased to 2.1ms for the slurry fuel. A higher net heat release for charcoal slurry was observed, up to 180J/crank angle degrees (CAD) compared with the diesel at 145J/CAD The maximum combustion temperature reached 2300K for diesel and 2330K for slurry. The heat fluxes for both fuels have similar values and trends during the entire cycle showing the good compatibility of charcoal slurry with a diesel type combustion and low soot radiation. The exhaust temperatures were about 40–50°C higher for charcoal slurry at 19° before top dead center (BTDC) injection timing. The engine’s bsfc increased as expected due to the lower heating value of the slurry fuel. The smoke Bosch no. was lower for the slurry fuel at any load, and is believed that the oxygen from the charcoal had a beneficial effect. The measured emissions of slurry fuel were better at 13° BTDC than those of diesel fuel with the original engine settings and the remaining 6–10% oxygen content in the charcoal is thought to have a paramount role in helping the diffusion type combustion and diminishing the particulate matter formation. As the load was increased, the amount of time it took to notice a decline in engine efficiency decreased. This was due to the injector sticking open which was seen by a sharp increase in the exhaust temperature. The internal flow into the injector had the tendency to form deposits on the injector’s seat that were critical to the functionality of the injector. In order to alleviate this problem, a reduced charcoal particle size together with a new injector design were produced resulting in stable engine efficiency at 50% load for a period of 90min without injector sticking. Even with improvements, the needle’s seat into the injector body showed an accelerated wear 4–8 times faster than that in normal operation with diesel fuel and this cannot be sustained for long operational cycles. The investigations have proven that the new charcoal–diesel slurry can produce adequate sprays and burn with very good results in a direct injection diesel engine. The critical aspect of operation is the internal flow into the injector with the tendency to form deposits and wear in the injector.