Efficiency and raw emission benefits from hydrogen addition to methane in a Prechamber–Equipped engine

Patrik Soltic,Thomas Hilfiker

doi:10.1016/j.ijhydene.2020.06.123

Abstract

Providing high ignition energies to low reactivity (hard-to-ignite) fuels in premixed combustion modes is a challenging task. This is especially true if diluted combustion and high compression ratios are applied which are desirable for achieving high efficiency levels. One promising technology is to decouple the ignition process from the conditions in the main chamber using a prechamber, which is able to ignite the charge in the engine's main combustion chamber using turbulent jets. In this study, experimental results of a prechamber-equipped four-cylinder engine with 2 L displacement are discussed. The engine is designed to provide high efficiency using natural gas as a fuel. The prechamber is able to be fueled externally (called active prechamber mode), e.g. the air–fuel equivalence ratio λ can be decoupled from the λ in the main combustion chamber. Besides of pure methane operation, the engine is operated with the addition of 22.5 mol% (3.5 mass%, 8.0 energy%) hydrogen to methane. The results show that it is beneficial to use auxiliary fueling of the prechamber in pure methane operation mode in order to achieve highest possible efficiency at low NOx levels at λ of around 1.7. When adding hydrogen, λ of around 1.7 still provides highest efficiency but auxiliary fueling of the prechamber is not necessary any more to enable fast and robust combustion. Compared to the active prechamber operation with methane, the 3.5 mass% (8.0 energy%) addition of hydrogen is able to reduce the engine's fuel energy demand by 1.3–3.3% in relevant engine operating points, which leads to tailpipe CO2 reductions of 9.2–11%. Additionally, hydrogen addition is able to disproportionately lower the engine's unburned hydrocarbon (HC) emissions: The addition of 3.5 mass% hydrogen lowers HC mass emissions by impressive 30–40%. Simultaneously, at the lean conditions considered, CO and NOx emissions are lowered by around 20%.

Highlights

Alternatives to fossil liquid fuels for internal combustion engines of all kinds have the potential to lower greenhouse gas as well as pollutant emissions if they are economically feasible
The combustion characteristics are discussed for all three operating points E,F, K operated in active prechamber mode at an overall l of 1.7 whereas operating point F is operated in passive prechamber mode for HCH4
As hydrogen increases the laminar flame speed, a distinct shortening of the ignition- and early flame phase is observed by hydrogen addition

Summary

Introduction

Alternatives to fossil liquid fuels for internal combustion engines of all kinds have the potential to lower greenhouse gas as well as pollutant emissions if they are economically feasible. Burning methane produces around 55 g CO2 per MJ of LHV, which gives methane an energy-related CO2 advantage of around 25% This makes natural gas, with its major constituent methane, an attractive fuel for meeting CO2 regulations. In addition to the fossil sources, small-to-large scale bio-methane production technologies are established which make methane a feasible fuel with a long-term perspective. Hydrogen is the simplest chemical energy carrier It can be produced efficiently from water electrolysis, coal or biomass gasification, or natural gas reforming and a small number of hydrogen filling stations for vehicles already exists [6,7]. Large-scale storage of hydrogen is challenging, especially for seasonal balancing [8] so that synthetizing hydrocarbons from hydrogen and CO2 makes it easier and more cost-efficient to store and distribute [9]. The electrification of certain segments of the on-road mobility has a potential to lower CO2 emissions but only a meaningful combination of electrification and renewable fuels is able to maximize the CO2 benefits at reasonable costs [14e17]

Methods

Results

Conclusion