Engine Downsizing Is a Proven Approach to Achieve Superior Fuel Efficiency
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Citation for published version: Hu, B, Turner, JWG, Akehurst, S, Brace, C & Copeland, C 2017, 'Observations on and potential trends for mechanically supercharging a downsized passenger car engine: a review', Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, vol. 231, no. 4, pp. 435-456. https://doi.org/10.1177/0954407016636971 DOI: 10.1177/0954407016636971 Publication date: 2017 Document Version Peer reviewed version Link to publication (C) 2016 Sage Publications. This is the author's accepted manuscript of an article published in: Hu, B, Turner, J, Akehurst, S, Brace, C & Copeland, C 2016, 'Observations on and potential trends for mechanically supercharging a downsized passenger car engine: a review' Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, pp. 1 - 22. Available via: http://dx.doi.org/10.1177/0954407016636971 University of Bath Alternative formats If you require this document in an alternative format, please contact: [email protected] General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Download date: 27. Sep. 2021 Observations on and potential trends for mechanically supercharging a downsized passenger car engine – A review Bo Hu, James W.G. Turner, Sam Akehurst, Chris Brace, Colin Copeland1 Abstract: Engine downsizing is a proven approach to achieve superior fuel efficiency. It is conventionally achieved by reducing the engine swept volume and employing some means of increasing specific output to achieve the desired installed engine power, usually in the form of an exhaust-driven turbocharger. However due to the perceptible time needed for the turbocharger system to generate the required boost pressure, turbocharged engines characteristic degraded driveability when compared to their naturally-aspirated counterparts. Mechanical supercharging refers to the technology that compresses the intake air using the energy taken directly from the engine crankshaft. It is anticipated that engine downsizing that is realized either solely by a supercharger or by a combination of supercharger and turbocharger would enhance a vehicle’s driveability without significantly compromising the fuel consumption at an engine level compared to the downsizing by turbocharging. The capability of the supercharger system to eliminate the large exhaust back pressure, reduce the pulsation interference and mitigate the surge issue of a turbocharged engine in a compound charging system will offset some of the fuel consumption penalty incurred in driving the supercharger. This, combined with an optimized down-speeding strategy, might further improve a downsized engine’s fuel efficiency performance whilst still 1 Faculty of Engineering and Design, University of Bath, Bath, United Kingdom Corresponding author: Bo Hu, Faculty of Engineering & Design, University of Bath, Claverton Down, Bath BA2 7AY, UK. Email: [email protected] 1 enhancing its driveability and performance at a vehicle level. This paper will first review the fundamentals and types of supercharger that are currently or have been used in passenger car engines. Next, the relationships between downsizing, driveability and down-speeding are introduced to identify the improved synergies between the engine and the boosting machine. Then, mass production and prototype downsized supercharged passenger car engines are briefly described, followed by a detailed review of the current state-of-the-art supercharging technologies that are in production as opposed to the approaches that are currently only being investigated at a research level. Finally, the trends for mechanically supercharging a passenger car engine are discussed, with the aim of identifying potential development directions for the future. Low-end torque enhancement, transient driveability improvement and low load parasitic loss reduction are the three main development directions for a supercharger system, among which the adoption of a CVT to decouple the supercharger speed from the engine speed, compressor isentropic and volumetric efficiency improvement and supercharger mechanism innovation seem to be the potential trends for mechanically supercharging a passenger car engine. Keywords: Mechanically, Supercharging, Downsizing, Passenger car, Engine 1 Introduction loss, improved heat transfer and better friction condition, thus shifting the engine operating points into Engine downsizing, which is the use of a smaller-swept- a more efficient area, are the major reasons for volume engine to provide the power of a larger one, is improved fuel efficiency in the frequently-used areas of widely accepted as one of the most viable solutions to low-load engine operation. The rated power and torque address the fuel economy and environment issues are conventionally recovered by means of facing passenger car engines [1-7]. Reduced pumping turbocharging and supercharging [8-9]. Turbocharged 2 engines are generally more fuel-efficient as they utilize needed. a proportion of otherwise-wasted exhaust gas energy. But most turbocharged engines are not ‘fun’ enough It might be worth noting that supercharging a due to the perceptible time needed for the passenger car engine can also address some other turbocharging system to generate the required boost inherent issues of a turbocharged alternative, among (so-called ‘turbo-lag’). Supercharged engines on the which the elimination of pulsation interference and the other hand do not suffer from this problem, because capability to reduce the high exhaust back pressure are the compressor of a supercharged engine is directly the two major aspects. Improvement of the driven by the engine crankshaft; however, they are not turbocharging system itself, such as modifying a as efficient as their turbocharged counterparts and conventional turbine to a twin-entry one [11,12] to increase engine friction directly. facilitate scavenging and improve the low-end torque particularly in four cylinder groups [13] and/or Although most of the downsized passenger car engines employing a variable geometry turbocharger (VGT) to in production are turbocharged and the driving achieve optimum efficiency in a wider range of speeds performance of turbocharged vehicles has been greatly and loads whilst improving the transient performance improved by the development of turbocharger system [14-15] and/or adopting divided exhaust period (DEP) itself (for example: reduction of turbocharger inertia) [16-27] or the so-called turbo-discharging concept [28- and proper charging system matching, with the 30] to reduce the backpressure to further improve a requirement of further downsizing to achieve superior turbocharged engine’s performance, can achieve some fuel efficiency [10] to satisfy ever-more strict fuel benefits but not as significantly when compared to a consumption regulations, supercharging technology supercharged counterpart. may have to be introduced to increase the low-end torque and improve the transient performance when Mechanically supercharging an engine was 3 commonplace in aero-engines [31-32], but the with the aim of identifying potential developments in development of this technology for a passenger car the future. only appears in recent years. This is especially the case for a compound-charging arrangement, in which two or Modeling methodologies that can capture the dynamic more charging devices are used to provide more system response of different boosting systems are detailed in a capability [33-36]. Considering the strong possibility separate section, as it is believed that only proper that the supercharging technology will be applied to modelling is able to assist in evaluating the engine’s fuel more passenger car engines in the near future to efficiency performance with different transient facilitate further downsizing and more aggressive behavior in a typical driving cycle. down-speeding, either in a Supercharger-Turbocharger 2 Fundamentals and types of supercharger or Turbocharger-Supercharger arrangement, a review of The term ‘supercharger’ in this paper refers to an air the current and concept approach to achieving pump that increases the pressure and thus density of supercharging is necessary. the charge air supplied to an internal combustion engine by directly connecting to an engine’s crankshaft In this paper, fundamentals and types of supercharger (and thus taking power from it) by means of a belt, gear will first be briefly introduced, following which the or chain [37]. paper will describe the relationship between downsizing, driveability and down-speeding for the In general, there are two main types of superchargers purpose of understanding the interactions between fuel defined according to the method of gas transfer: economy and vehicle performance. Some current mass Positive displacement and centrifugal compressors [37]. production and highly downsized prototype passenger car engine will then be discussed. Finally, conventional Positive displacement units offer a relatively constant and novel approaches to supercharging are compared, boost characteristic since they pump air at a fixed