Elsevier

Renewable Energy

Volume 148, April 2020, Pages 512-522
Renewable Energy

Effects of a wave-shaped piston bowl geometry on the performance of heavy duty Diesel engines fueled with alcohols and biodiesel blends

https://doi.org/10.1016/j.renene.2019.10.057Get rights and content

Highlights

  • Alcohol blends with Diesel-like cetane number offer coincident heat release curves.

  • Alcohols addition to fossil-free blends reduce soot emissions more than 60%.

  • The wave piston reduces the combustion duration for Diesel and oxygenated blends.

  • The wave piston increases thermal efficiency and reduces soot emissions.

Abstract

The effects of a new wave-shaped piston bowl design on combustion characteristics and engine out emissions were tested in a heavy duty Diesel engine fueled with conventional Diesel and fossil-free blends containing n-butanol, n-octanol, 2-ethylhexanol, hydrotreated vegetable oil, and rapeseed methyl ester. The compositions of the blends were chosen such that their cetane numbers matched that of fossil Diesel. Engine experiments were performed at four operating points from the European Stationary Cycle, with no modification of engine settings when switching between different fuels. A standard piston with omega geometry was tested using fossil Diesel and the fossil-free nBu30H (30% n-butanol and 70% hydrotreated vegetable oil by volume) blend, and the results obtained were compared to those achieved with the wave piston. In general, the fossil-free blends yielded significantly lower soot emissions than fossil Diesel but slightly higher NOx emissions. Relative to the standard piston, the wave piston accelerated the combustion of both Diesel and fossil-free blends, especially the diffusion combustion. The wave piston’s positive effects on thermal efficiency and soot emissions were more pronounced for conventional Diesel fuel than for oxygenated nBu30H.

Introduction

Replacing fossil fuels with renewable biofuels could greatly reduce vehicles’ well-to-wheel greenhouse gas (GHG) emissions and thereby help vehicle manufacturers comply with increasingly stringent regulations [1]. For instance, replacing fossil Diesel with n-butanol, isobutanol, n-octanol, or 2-ethylhexanol derived from non-food biomass could reduce well-to-tank equivalent carbon dioxide (CO2) emissions by 65%, 50%, 56%, and 62%, respectively [1,2]. Because of this potential to reduce lifecycle GHG emissions, there is great interest in using renewable longer-chain alcohols as alternative fuels for Diesel engines. Rakopoulos et al. [3] showed that replacing fossil Diesel with n-butanol/Diesel blends in a Diesel engine can reduce soot formation because such blends form overall ‘leaner’ fuel-air mixtures than conventional Diesel fuel. This reduction in soot formation is not accompanied by a significant increase in (NOx) emissions, offering a way to avoid the soot-NOx trade-off. Yao et al. [4] showed that using Diesel/n-butanol blends in conjunction with pilot- and post-injection strategies effectively reduced soot emissions from a heavy duty (HD) Diesel engine. However, the combined positive impact of using the blended fuels and the multiple-injection strategy was lower than expected based on their individual effects. Additionally, increasing the exhaust gas recirculation (EGR) rate amplified differences in heat release between butanol isomers, and therefore also amplified differences in emissions [5]. Higher percentages of n-butanol blends are able to achieve longer ignition delay and partially premixed combustion, reducing soot emissions further and showing the potential of high thermal efficiency [6,7]. Like other drop-in alcohols, pure n-octanol and its blends yielded a higher thermal efficiency and lower soot emissions than Diesel because of its oxygen content and higher latent heat of evaporation, which may reduce heat losses by decreasing the cylinder temperature [8,9].

The use of bio-based fuels such as hydrotreated vegetable oil (HVO) and rapeseed methyl ester (RME) in Diesel engines has been studied extensively. Replacing fossil Diesel with HVO in HD engines reduced NOx, soot, CO and HC emissions over a wide range of operating conditions [10,11]. However, under the same operating condition with same engine settings, the rate of heat release curves generated using HVO differed from those of fossil Diesel [12].

Piston bowl geometry strongly affects in-cylinder air motion because it influences the development of the complex turbulent flow field at the end of the compression stroke [13]. Simulations and experimental studies [14,15] have been done to prove that the diameter ratio of the piston bowl and cylinder bore influences the velocity field. Due to the different properties of biofuel compared with fossil Diesel, the optimization of injection timing and piston bowl geometry improves the thermal efficiency and reduces the emissions for the biofuel application [16]. A wave-shaped piston [17] was recently shown to improve late-cycle air mixing during diffusion combustion by efficiently guiding the near-wall jet flow back towards the chamber center. Increasing the level of turbulence in the reaction layer improves fuel-air mixing and promotes faster and more complete combustion, increasing thermal efficiency and thus reducing soot emissions. However, it is not yet known how the use of the wave piston affects emissions and thermal efficiency when using fossil-free fuels.

The aim of this study is therefore to evaluate the performance and emissions of HD engines with wave pistons when burning blends of various oxygenated fossil-free fuels, and to compare the wave piston’s effects to those of a conventional omega piston design. Combustion experiments were performed using fossil-free fuels including blends of n-butanol, n-octanol, 2-ethylhexanol, HVO, and RME, using typical production engine settings. To ensure that the ignition delay time was similar in all experiments, the CN values of the tested blends were adjusted to match that of fossil Diesel. Combustion experiments were also performed using fossil Diesel as a reference fuel. The combustion characteristics and engine out emissions achieved with the wave and conventional (omega) piston designs were compared using fossil Diesel and an n-butanol/HVO blend under the same engine settings as in the other combustion experiments.

Section snippets

Fuel properties

The fossil-free fuels tested in this work were blended by HVO, n-butanol, n-octanol, 2-ethylhexanol, and RME. Fossil Diesel was used as a reference fuel. Table 1 shows the properties of these fuels. Fossil Diesel satisfying the EN590 standard and containing no fatty acid methyl ester (FAME) was used. FAME blends used in commercial Diesel fuels are mainly synthesized from animal fats and some vegetable oils by esterification [18]. A commonly used FAME in the Swedish market is RME, which has very

Engine combustion characteristics when using the wave piston

Fig. 3 shows the apparent rate of heat release curves at A25, B50, C75, and B75 for the single cylinder HD engine. For all operating points, the rate of heat release profiles of the fossil-free blends closely resemble that of fossil Diesel. The variation between the fuels with respect to the timing of the start of combustion (SOC), i.e. the moment at which the heat release rate cross zero from negative, was less than 0.5 crank angle degrees (CAD) at each load point. This confirms that using

Conclusions

This paper investigated the effect of using a wave-shaped piston bowl on the performance and emissions of a heavy-duty Diesel engine (with factory-calibrated engine settings) fueled with conventional Diesel fuel and various fossil-free alcohol blends. The blends contained n-butanol, isobutanol, n-octanol, and 2-ethylhexanol, together with HVO and RME, and their cetane numbers were adjusted to match that of fossil Diesel.

When using the wave piston, the fossil-free blends and fossil Diesel

Acknowledgements

This work was performed as a part of the project -Butanol as an alternative fuel for Diesel engines, which is supported by the Swedish Energy Agency, Perstorp AB, Scania CV, Neste, Volvo GTT and Volvo Cars. The financial and technical support of these organizations is gratefully acknowledged. The authors would also like to thank Dr. Timothy Benham for his assistance and technical support during the experiments.

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