Ph.D Thesis

Ph.D StudentThawho Andy
SubjectStudy of a Multicomponent Alcohol Reformate Mixing in a
Combustion Chamber of ICE
DepartmentDepartment of Energy
Supervisor ASSOCIATE PROF. Leonid Tartakovsky
Full Thesis textFull thesis text - English Version


Thermochemical Recuperation (TCR) is a promising waste heat recovery method enabling utilization of the engine waste heat together with onboard hydrogen production. Methanol, as a low carbon intensity primary fuel, can be reformed at relatively low temperatures to produce a hydrogen-rich gaseous reformate (up to 75%mol H2 and 25%mol CO2) with increased heating value and improved combustion properties.

This method enables a significant engine efficiency improvement and gaseous emissions reduction over a gasoline reference case. At the same time, an unexpectedly high particle emission level as compared to the gasoline-fed engine was discovered in this study despite the combustion of a hydrocarbon-free hydrogen-rich reformate. In the existing literature, this phenomenon has not been described yet.

A direct experimental comparison between the reformate port fuel injection (PFI) and the direct injection (DI) methods performed on the same engine showed an increase in particle formation when the latter is applied. Based on these results, several hypotheses were suggested and investigated attributing to excessive lubricant involvement in the combustion process. Besides, for both injection strategies, a similar relatively high thermal efficiency (above 45%) was observed. This is a joint result of waste heat recovery and hydrogen combustion benefits. The experimental data showed that the PFI method, due to hydrogen induction into the intake manifold, results in maximal power output loss and abnormal combustion. Contrary, the DI strategy enables achieving maximal engine power. The heat release rate with the reformate DI is significantly higher compared to PFI for the same excess air-ratio. The higher in-cylinder turbulence induced by the direct injection process, compared to the premixed charge PFI counterpart, results in faster fuel burning.

To understand and describe the mechanisms leading to excessive particle formation in hydrogen combustion, several fundamental studies, both experimental and numerical, of the transient reformate jet behavior were conducted. A CFD simulation was performed to provide information on the initial period of the multicomponent jet development in the combustion chamber from the injection event till the wall impingement. The flow field of a confined, underexpanded transient round jet was investigated also using high-speed Schlieren imaging and particle image velocimetry (PIV) measurements. Distributions of the mean flow and Reynolds stresses revealed two different stages in jet development. In stage I, before shock cell appearance, the jet was characterized by a leading, toroidal vortex inducing recirculatory motion which inhibited the growth of the trailing jet's shear layer instabilities and radial spreading. In stage II, the jet became underexpanded and the flow characteristics resembled those of a ``co-annular'' jet. These findings provided much insight on the far-field flow characteristics of a gaseous transient underexpanded jet and helped to improve the understanding of the gaseous fuel-air mixing in DI ICEs.

The accomplished study allowed us suggesting a description of the particle formation mechanism in hydrogen combustion. The peculiarities specific for the reformate DI that result in enhanced particle formation are jet-lubricated-wall interaction affected by injection duration, lubricant vapor entrainment into the gaseous jet, and shorter flame quenching distance of hydrogen compared to gasoline.