A Numerical Exploration of Engine Combustion Using Toluene Reference Fuel and Hydrogen Mixtures

Massimo Borghi,Alessio Barbato,Valentina Pessina

doi:10.1051/e3sconf/202131207003

Abstract

Hydrogen-fueled internal combustion engines (H2ICEs) are capable of operating over a wide range of equivalence ratios: from ultra-lean mode to stoichiometric conditions. However, they provide maximum thermal efficiency and minimum NOx emissions if operated lean. Although NOx is produced, H2ICEs generate little or no CO, CO2, SO2, HC, or PM emissions. The main limitation to pure hydrogen fueling is power density. To overcome such an issue, mixtures of gasoline and hydrogen can be exploited, with small modifications to the engine feeding system. Due to the peculiar characteristics of hydrogen (in terms of thermophysical properties, molecular weight and propagating flame characteristics) care must be adopted when trying to address combustion using computational fluid dynamics (CFD) tools. In this work, we simulate the combustion of mixtures of toluene reference fuel (TRF) and hydrogen under largely different ratios. To simplify the problem, liquid and gaseous injections are neglected, and a premixed mixture at the inlet of the CFD domain is imposed. Due to the different laminar flame speeds of the mixture components, mass-fraction weighted in-house correlations based on chemical kinetics simulations are adopted. Outcomes are compared with those obtained using standard correlations and mixing rules available in most commercial CFD packages.

Highlights

The European Green Deal [1] sets out the European Union’s (EU) path to climate neutrality by 2050, through the deep decarbonization of all sectors of the economy; anthropogenic greenhouse gas (GHG) emission reductions are set for 2030
Sarathy et al [9] highlighted that the addition of oxygenates led to a reduction of the massbased Lower Heating Value (LHV) of the fuel blend because fuel-oxygen atoms do not contribute to heat production; the reduction of the LHV is proportional to the mass percentage of oxygen
The wide flammability limits together with the low ignition energy required and the high flame speeds could lead to undesired combustion phenomena such as autoignition and backfiring. The latter is limited to port fuel injection (PFI) operation and can be avoided with DI operation, as adopted in the modern H2ICEs

Summary

Introduction

The European Green Deal [1] sets out the European Union’s (EU) path to climate neutrality by 2050, through the deep decarbonization of all sectors of the economy; anthropogenic greenhouse gas (GHG) emission reductions are set for 2030. The wide flammability limits together with the low ignition energy required and the high flame speeds could lead to undesired combustion phenomena such as autoignition and backfiring The latter is limited to port fuel injection (PFI) operation and can be avoided with DI operation, as adopted in the modern H2ICEs. The correlation between mixture quality and NOx – the only relevant emission component in H2ICEs - is well documented in the literature. At fuel-to-air equivalence ratios (φ) of less than 0.5 (λ> 2), the engine operates without generating NOx emissions; increasing φ beyond this threshold results in an increase of NOx emissions peaking around φ~0.75 and a slight decrease approaching stoichiometric mixtures It has been demonstrated [15] that the use of a multiple injection strategy in H2-DI ICEs is an effective measure to significantly reduce NOx emissions, up to 95%.

Numerical Setup

Physical Models

Laminar Flame Speed Correlation

Results