In order to understand the soot formation in diesel engine, a turbulent jet flame is used to simulate the combustion in the cylinder. The experimental study is performed to investigate the evolution of soot morphology for the maturation of nascent particle in a turbulent lifted jet flame fueled by n-heptane/toluene mixtures. An ultrasonic atomizer is used to evenly spread the fuel droplets to acquire single primary particles. Transmission electron microscopy (TEM) is used to study the morphology of the particle sampled from the flame at different heights. Small soot aggregates are acquired from all the samples. Particle maturation such as agglomeration is accelerated under a high temperature by comparing the particle morphology sampled on the centerline and the wings of the flame. The precursors of nascent particles are depicted as dark nucleation dispersed to semitransparent polycyclic aromatic hydrocarbons (PAHs). The nanostructure of nascent particles transforms from an amorphous state to an onion structure with the maturation of particles. Surface growth initially dominates the maturation of nascent particles in the direction of outside to inside for single particles. Agglomeration begins to emerge with the increased probability of collision at the end of maturation. Surface growth and agglomeration increase the mean particle diameter as the flame height increases. The oxidability of particles that undergo surface growth and agglomeration notably increases. The structure of nascent particles is inclined to be ordered and the mean particle diameter decreases with oxidation dominating the combustion reaction.
Spark-assisted compression ignition (SACI) is a potential way to improve thermal efficiency for gasoline engine with a relatively low compression ratio. The dual-injection system and spark strategy are considered to be an effective approach to control the combustion of SACI engine. Polyoxymethylene dimethyl ethers (PODE) is a potential fuel for carbon neutral with high oxygen content and unique molecule structure. In this study, the transition of combustion modes with different equivalence ratio and effects of direct injection (DI) ratio on SACI combustion and emissions fueled with PODE/gasoline under different loads were investigated. The results showed that SACI combustion could be achieved with the compression ratio of 13 and the brake thermal efficiencies (BTEs) at 2 bar, 3 bar and 4 bar under the dual-fuel SACI were increased by 49%, 29% and 27%, respectively, compared with the gasoline spark ignition mode. The increase in DI ratio first shortened the combustion duration and then prolonged. An appropriate DI ratio was shown to control the combustion process to achieve high efficiency combustion, at which Low THC, CO and PM emissions were achieved while the NOx emissions remained at a low level.
Homogeneous charge compression ignition (HCCI) has potential to improve thermal efficiency for gasoline engine due to the similar formation of mixture between HCCI and traditional spark ignition mode. High reactivity fuel combined with gasoline is used to achieve HCCI combustion with a relative low compression ratio. Polyoxymethylene dimethyl ethers (PODE) is a kind of low-carbon fuels with unique molecule structure, which is beneficial for carbon neutral. The intermediate products of PODE are different from those of long-chain alkane such as n-heptane and diesel. In this paper, the difference in spark-assisted HCCI (SA-HCCI) combustion combined with spark ignition and EGR between PODE/gasoline and n-heptane/gasoline under different operating conditions were investigated. The results illustrated that PODE/gasoline was more suitable for SA-HCCI combustion due to the positive relation between thermal efficiency and gasoline percentage, which can significantly improve the thermal efficiency while suppressing knock. Although the cetane number of PODE is greater than the n-heptane, the SA-HCCI combustion was more violent fueled with n-heptane/gasoline than that of PODE/gasoline. Low NOx and PM emissions are achieved for SA-HCCI combustion especially for PODE/gasoline. The advancing spark timing reduced the incomplete combustion and shortened the combustion duration, thus reducing the THC and CO emissions, especially fueled with PODE/gasoline. Although a low EGR rate can slightly improve the THC and CO emissions, violent combustion can be significantly improved. In general, high efficiency and low emissions could be achieved by PODE/gasoline and spark ignition combined with EGR. The brake thermal efficiency for SA-HCCI combustion fueled with PODE/gasoline increased by 30% compared with DISI mode under the same loads.
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