Characterization and optical imaging of dual-fuel combustion

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Insinööritieteiden korkeakoulu | Master's thesis
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Date

2017-12-11

Department

Major/Subject

Elective Studies (KON)

Mcode

K901-W

Degree programme

Master’s Programme in Mechanical Engineering (MEC)

Language

en

Pages

60+10

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Abstract

As the regulations for engine-out emissions around the world are getting increasingly stringent, dual-fuel (DF) combustion involving natural gas or methane as primary fuel ignited by a very small diesel pilot can be an attractive alternative to diesel only combustion in heavy duty engines of both road as well as marine applications. Natural gas or methane have potential for reduction of carbon dioxide emissions because of their high hydrogen to carbon ratio compared to diesel fuel. Their highly premixed combustion also produces less NOx than diesel as the overall combustion temperature is lower. Furthermore, they have huge potential for exploitation at competitive prices in regions such as India, China and North America, thus helping to reduce dependency on conventional diesel and gasoline. In this thesis work, an attempt has been made to characterize the DF combustion of premixed methane and diesel pilot with the help of optical and thermodynamic measurements. DF combustion experiments were carried out in a single cylinder, optical research engine with electro hydraulic valve actuation system and custom made engine control program that enable research with great degree of parameter freedom. Crank angle resolved images of Natural Luminosity (NL) resulting from combustion were acquired using a high speed CMOS camera. NL signals showing the development of premixed flame fronts, cylinder pressure traces, heat release rate (HRR) profiles and ignition delays are considered as tools for characterizing the DF combustion in this thesis work. Influences of methane lambda (λCH4), of pilot fuel ratio (PR) and of charge air temperature on dual-fuel combustion were investigated separately using three different case studies. From optical and thermodynamic investigations, it was observed that the ignition delay and shape of HRR in DF combustion strongly depend upon lambda of the gaseous fuel. With methane substitution rate ranging from 82% to 88%, when λCH4 was varied from 1.1 to 1.9, it was observed that the periods of ignition delays were longer at both least as well as most lean case, with relatively shorter delays around the λCH4 1.5 region. The study of DF combustion as a function of PR revealed that the DF ignition delay decreased with increasing PR until a certain extent after which any increase in PR did not bring the ignition delay significantly forward. In the final case study, for both λCH4 1.1 and λCH4 1.8 cases, it was observed that higher intake temperatures shortened ignition delays, promoted second stage combustion and reduced cycle-to-cycle variations. However, at high intake temperatures, certain DF cycles also demonstrated excessive rates of pressure rise immediately after the onset of combustion. In summary, it is concluded that at high substitution rates (ca. 85%) and light load conditions as experimented in this thesis, DF combustion is dominated by the premixed flame front propagation mode rather than by the characteristics of diesel diffusion combustion.

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Supervisor

Larmi, Martti

Thesis advisor

Kaario, Ossi

Keywords

dual-fuel, optical, ignition, methane fuel, flame front

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https://urn.fi/URN:NBN:fi:aalto-201712187924

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[dipl] Insinööritieteiden korkeakoulu / ENG