Thermodynamic Energy and Exergy Analysis of Low-Temperature Combustion Strategies

Abstract

Increasing thermal and fuel efficiency in Internal Combustion Engines (ICEs) requires thorough investigations on the combustion process and its thermodynamics. The first law of thermodynamics expresses the balance of the energy, while the second law specifies the maximum achievable work. In this article, Low-Temperature Combustion (LTC) strategies, including Homogeneous Charge Compression Ignition (HCCI), Reactivity Controlled Compression Ignition (RCCI), Partially Premixed Combustion (PPC), and Direct Dual-Fuel Stratification (DDFS) were analyzed by the first and second law approaches, and they were compared with ideal-diesel cycle and Conventional Diesel Combustion (CDC). HCCI and RCCI had the highest exergy efficiency of 50.8% and 49.2%, respectively, compared to other cases, and exergy destruction in these cases was the lowest (25.3% and 27.5%, respectively). Although all mentioned LTC strategies met the EURO6 regulation for NOX and soot, the thermodynamic analysis confirmed that DDFS was superior in terms of emissions and engine performance. The DDFS strategy uses two direct injectors in the combustion chamber, and the injection parameters have influential effects on the energy and exergy distributions as well as emissions. These injection parameters in a DDFS engine with direct diesel and gasoline injectors are energy fractions for each fuel (E-d and E-g) and their injection timing (SOI2 and SOI3), spray angles (theta(d) and theta(g)), the injection pressures (P-d and P-g). Furthermore, the Exhaust Gas Recirculation (EGR) was also studied as another effective parameter. The suitable ranges for desired exergy efficiency and exergy destruction levels for the mentioned parameters are as follows: E-d and E-g should be in the range of 4-6% and 25-30% of the total energy input, respectively. -100 degrees to - 80 degrees After Top Dead Center (ATDC) is suitable for the diesel injection timing (SOI2), and -8 degrees to - 6 degrees is a preferred range for the gasoline injection timing (SOI3). The suitable spray angle for both injectors was found to be about 65 degrees, and the injection pressure about 1500-2000 bar showed better results for both injectors.