Abstract
Research and/or Engineering Questions/Objective
A combustion concept that has been successfully applied on large stationary engines and to some extent on heavy-duty engines is dual-fuel combustion, where a compression-ignited diesel pilot injection is used to ignite a homogeneous charge of compressed natural gas (CNG) and air. However, it does not exist at all for light-duty engines. In this paper, a comparison between different possible combustion modes is presented based on heat transfer, efficiency and emissions analysis of a multi-cylinder light duty engine operated at part load. The objective of this study is to identify and understand the potential of different combustion modes in order to reduce emissions and increase engine efficiency simultaneously.
Methodology
Previous research carried out by the authors described the role of dual-fuel combustion efficiency on overall engine efficiency at high diesel substitution ratio. At low loads, high diesel substitution ratios result in excessive total unburned hydrocarbon emissions due to bulk flame quenching, and consequently lower substitution ratios in combination with intake air heating, throttling and increased turbulence should be adopted (or even full diesel operation) in order to meet emission legislation targets. At those operating points where combustion efficiency is not a limiting factor, engine operation can be really flexible and different options can be implemented. Conventional dual-fuel (CDF) pilot-ignition and reactivity controlled compression ignition (RCCI) combustion have been evaluated and compared at different dilution levels (via exhaust gas recirculation (EGR) or excess air at stoichiometric or lean conditions) based on emissions, heat transfer and efficiency.
Results
The results presented in this paper show that CDF is a really robust combustion mode but efficiency and NOx emissions are not optimal. Diesel injection close to TDC promotes fuel rich areas and NOx is formed in near-stoichiometric regions surrounding the diesel jets. Efficiency is reduced due to the effect of fuel trapped in crevices and excessive combustion temperatures, resulting in high heat losses through combustion chamber walls. For these reasons, RCCI low temperature combustion mode allows a more efficient use of CNG at high substitution ratios than CDF while NOx emissions are sharply reduced without significant increase in methane emissions at part load.
Limitations of this study
The emission measurement equipment used is capable of measuring total unburned hydrocarbons instead of methane molecules and diesel-like hydrocarbons separately. The engine is not equipped with turbochargers and they are simulated via intake heating, intake pressure and backpressure independent control.
What does the paper offer that is new in the field including in comparison to other work by the authors?
There are not so many publications of analysis of combustion modes from a heat transfer point of view in dual-fuel engines. The authors have focused the previous experiments at low loads, while this paper presents results at mid-load, where combustion efficiency is not a limiting factor.
Conclusions
Higher engine efficiencies than conventional spark ignition natural gas engines can be reached when using dual-fuel CNG-diesel combustion. RCCI combustion showed great potential for simultaneous improvements in efficiency and NOx emissions, while keeping methane emissions below legislation limits.
Keywords : Natural gas, dual-fuel, heat transfer, efficiency, emissions.