Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy Comparison of flash boiling resistance of two injector designs and the consequences on downsized gasoline engine emissions Changzhao Jiang a , Matthew C. Parker a , Daniel Butcher a , Adrian Spencer a, ⁎ , Colin P. Garner a , Jerome Helie b a Loughborough University, UK b Continental Automotive SAS, Toulouse, France HIGHLIGHTS • Spray collapse has been imaged in-cylinder during PN and NOx emission measurements. • A link between macroscopic spray plume characteristics and PN emissions was observed. • Nozzle design can improve flash boiling resistance by minimising plume interaction. • Diffusion flame from fuel wetted surfaces was identified for main PN increases. • Improving fuel-air mixing reduced PN but increased NOx across all conditions tested. ARTICLE INFO Keywords: Flash boiling Fuel injector Spray collapse Optical diagnostics Endoscope imaging PN emissions ABSTRACT This paper presents a comparative study of two injectors designed for the same Gasoline Turbocharged Direct Injection engine, one featuring 5 holes and one with 6 holes. Hole diameter and circumferential spacing also differed between the two injectors in order to optimise targeting while maintaining flow rate and drop size distribution. By comparing the macroscopic spray characteristics of the two injectors, this study investigated possible design features which may better maintain a spray’s intended morphology under severe flash boiling conditions. The sprays of each injector were firstly investigated by imaging in a quiescent pressure vessel before also being imaged in an endoscopically accessed version of the target 3-cylinder downsized engine to understand the impact of the spray morphology on performance and emissions. Near field images from the pressure vessel indicated that the 5-hole injector could tolerate a greater superheated degree before experiencing spray collapse, maintain its intended morphology better and exhibited a wider plume and shorter penetration length than the 6- hole injector for a given condition. Endoscopic images from the engine indicated that the spray area of the 5-hole injector was always wider under a range of start of injection timings, leading to a better air-fuel mixture and the observation of less diffusive combustion. The PN (particulate) emissions of the 5-hole injector was also con- sistently lower than the 6-hole injector under different injection timings due to better mixing and less piston impingement, whilst also being less sensitive to changes of injection timing due to its ability to maintain its spray morphology. 1. Introduction Worldwide concern surrounding CO 2 and other harmful emissions from combustion engines continues to drive the research and devel- opment of more efficient and less polluting forms of ground transpor- tation. Major automotive market regions, such as Europe, North America and China, require CO 2 emissions to reduce annually by 3–6% by 2025 [1]. Increasing market penetration of electric vehicles are a part of this trend, but their relatively short range, charge times and the infrastructure required to produce a charging network are prohibitive to full adoption of electrification [2,3] in the short term. Combustion engines are therefore still going to be a major part of ground transport strategy for a considerable time, at least as part of hybrid powertrain solutions [4]. In such hybrid systems the engine is likely to be down- sized since, compared to a naturally aspirated engine, this strategy can alone reduce fuel consumption by around 18% under the New https://doi.org/10.1016/j.apenergy.2019.113735 Received 6 December 2018; Received in revised form 10 August 2019; Accepted 11 August 2019 ⁎ Corresponding author. E-mail address: A.Spencer@lboro.ac.uk (A. Spencer). Applied Energy 254 (2019) 113735 Available online 20 August 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved. T