Enclosed Pressure Analysis for Diesel-Fuel Combustion*

Diesel engines rely on precise mixing of fuel and air in an already hot cylinder which leads to autoignition and the combustion process. A near perfect combustion efficiency is critical for enabling the engine to get the most force possible out of the available fuel energy. However, diesel fuels are so complex that inefficiencies often arise during the reactions that lead to incomplete combustion and pollution. This study investigates the ignition behavior of diesel-like fuels in order to better understand their behavior and support effects to minimize inefficiencies. Studying the ignition process separate from the injection and engine piston motion requires specialized combustion vessel equipment. In this work, a system that was previously custom designed as an improvement over an existing commercial system was fully commissioned and used to complete preliminary experiments. The system supplies fuel via an air-multiplying pump and single-hole fuel injector that sprays into a pressurized inconel-based combustion chamber, where an induction heater surrounding the chamber heats the metal and thereby the air mixture to combustion-level temperatures. As fuel is injected precisely to achieve a desired global equivalence ratio, the auto ignition process is captured with an oscilloscope that reads the injector current and chamber pressure change against relative time. This allows for the fuels to be characterized at different pressures, temperatures, and equivalence ratios for the ignition delay behavior. Three fuels were considered: n-heptane, iso-octane, and a 80% n-heptane/20% butyl-acetate blend. A temperature sweep from 625 K to 850 K was performed for each fuel at an equivalence ratio of 1.0 and a chamber pressure of 5 bar. With these parameters, iso-octane was found to be the least reactive fuel by a wide margin. Additionally, the same temperature sweep was conducted for n-heptane itself and the blend at a chamber pressure of 10 bar. This experiment found that increasing the chamber pressure shortened the delay. With the n-heptane/butyl acetate blend, the ignition delay was slightly increased but not enough to deter butyl acetate from being used as additive. Moreover, butyl acetate could be considered as a useful renewable fuel for diesel engines.