energies Article Empirical Study on the Efficiency of an LPG-Supplied Range Extender for Electric Vehicles Jakub Lasocki * , Artur Kopczy ´nski , Paweł Krawczyk and Paweł Roszczyk Faculty of Automotive and Construction Machinery Engineering, Warsaw University of Technology, 02-524 Warsaw, Poland * Correspondence: [email protected] Received: 4 August 2019; Accepted: 11 September 2019; Published: 13 September 2019 Abstract: A range extender is an auxiliary power unit, usually consisting of an internal combustion engine and an electric generator, which is used to charge a battery of an electric vehicle in order to increase its range. This paper considers a range extender supplied with liquefied petroleum gas (LPG). The aim is to provide detailed data on thermal efficiency, brake specific fuel consumption (BSFC), and unit emission of carbon dioxide (CO2) in a broad spectrum of range extender operating conditions defined by rotational speed and torque. The experimental investigation has been conducted using a laboratory test stand equipped with an energy dissipation system of adjustable resistance. Measurement results, including fuel flow rate, were processed using custom algorithm for generating maps, i.e., two-dimensional dependencies of the considered parameters on the rotational speed and torque. The maps obtained for LPG supply were compared with those for gasoline supply. The results demonstrated feasibility of LPG-supplied range extender. Its BSFC and thermal efficiency were at a comparable level to those obtained for gasoline supply, but with less CO2 emission. The empirical data collected has been adopted in the simulation of extended-range electric vehicle in a driving cycle, showing the potential of utilizing the results of this study. Keywords: range extender; extended-range electric vehicle; range extended electric vehicle; thermal efficiency; brake specific fuel consumption; BSFC; carbon dioxide emission; CO2 emission; LPG 1. Introduction The number of electric vehicles (EVs) worldwide is growing rapidly and their future development prospects are excellent [1]. Vehicle electrification is the way forward for the automotive industry looking to address two main environmental challenges of road transportation: air pollution and depletion of crude oil reserves [2]. A major concern is air quality degradation in urban areas and its implications on human health [3]. The most effective solution that towns and cities can take to counteract this issue is the introduction of clean air (zero/low emission) zones. This promotes the use of battery-electric vehicles (BEVs) that do not produce exhaust emissions, unlike conventional vehicles with internal combustion engines (ICEs) [4]. Other advantages of BEVs, including high efficiency of the powertrain and recuperative braking [5] or good prospects for the introduction of automatic steering systems [6,7], make them become competitive, especially for intraurban, short-distance applications such as commercial fleets or commuter vehicles [8]. However, BEVs are unlikely to substitute for ICE-based vehicles in long-distance applications unless a major breakthrough in battery technology is achieved. Due to high cost and limited capacity of currently used batteries, to store energy to provide satisfactory driving range for BEVs is not always possible [1]. The importance of providing adequate driving range to BEVs users has led to the development of extended range electric vehicles (EREVs), which combine an electric powertrain with an ICE [9,10] (Figure1). Their principle of operation is same as that of a standard BEVs, with the only di fference Energies 2019, 12, 3528; doi:10.3390/en12183528 www.mdpi.com/journal/energies Energies 2019, 12, 3528 2 of 23 Energies 2019, 12, x FOR PEER REVIEW 2 of 25 being thatthat anan additionaladditional ICE ICE acts acts as as an an on-board on-board generator generator (range (range extender) extender) to recharge to recharge the batterythe battery [11]. [11].EREVs EREVs operate operate exclusively exclusively as BEVs as BEVs when when battery battery energy energy is available is available and and have have full full performance performance in inelectric electric mode mode (e.g., (e.g., top top speed, speed, acceleration acceleration capability) capability) [12 [12].]. This This is is possible possible owingowing toto EREV’sEREV’s electric propulsion systemsystem andand batterybattery sized sized in in such such a a way way that that the the ICE ICE is is not not required required for for vehicle vehicle operation operation as aslong long as batteryas battery energy energy is available is available [12]. [12]. It is It therefore is therefore preferable preferable to move to move the vehicle the vehicle with thewith use the of use an ofelectric an electric motor motor only, i.e.,only, in i.e., "zero in emission""zero emission" driving driving mode, mode, but at but the sameat the time,same thetime, user the is user not afraidis not afraidof running of running out of batteryout of battery energy, energy, and so calledand so “range called anxiety”“range anxiety” (the term (the was term introduced was introduced in the late in 1990sthe late [13 1990s]) is alleviated. [13]) is alleviated. REX EV FSS BAT ICE MC EM FG CCU G Figure 1. Schematic diagram of extended range electricelectric vehicle: REX—range extender, EV—electric vehicle, FSS—fuel supply system, ICE—internal combustion combustion engine, engine, G—generator, G—generator, BAT—battery, BAT—battery, CCU—central controlcontrol unit, unit, MC—motor MC—motor controller, controller, EM—electric EM—electric motor, motor, FG—final FG—final gear. gear. Solid Solid lines marklines mechanical connections and dashed lines mark electrical connections. mark mechanical connections and dashed lines mark electrical connections. Despite their practicality, EREVs are not an ideal solution because the use of an onboard ICE to Despite their practicality, EREVs are not an ideal solution because the use of an onboard ICE to ensure sufficient range of the vehicle results in emission of pollutants. Recently, it has been proposed ensure sufficient range of the vehicle results in emission of pollutants. Recently, it has been proposed to limit this emission by supplying range extenders with alternative fuels [9,11,14,15]. While there to limit this emission by supplying range extenders with alternative fuels [9,11,14,15]. While there are are many gaseous and liquid fuels to consider [16], the use of liquified petroleum gas (LPG) and its many gaseous and liquid fuels to consider [16], the use of liquified petroleum gas (LPG) and its renewable version, BioLPG, are considered as currently viable options [11,17,18]. They offer not only renewable version, BioLPG, are considered as currently viable options [11,17,18]. They offer not only significant possibility to limit emission of harmful substances, but also good cost effectiveness over significant possibility to limit emission of harmful substances, but also good cost effectiveness over existing range extension technologies utilizing gasoline and diesel engines [11,19]. However, due to existing range extension technologies utilizing gasoline and diesel engines [11,19]. However, due to the early phase of development of this solution, range extenders supplied with LPG have not yet been the early phase of development of this solution, range extenders supplied with LPG have not yet been sufficiently tested and there is a very small number of scientific publications devoted to this topic. sufficiently tested and there is a very small number of scientific publications devoted to this topic. Recently it has been proved that EREVs can be successfully implemented in commercial Recently it has been proved that EREVs can be successfully implemented in commercial applications. Several car manufacturers offer EREVs for sale, e.g., BMW i3-REX, Chevrolet VOLT/Opel applications. Several car manufacturers offer EREVs for sale, e.g., BMW i3-REX, Chevrolet Ampera, other companies supporting the automotive industry, e.g., AVL, KSPG, Lotus Engineering, VOLT/Opel Ampera, other companies supporting the automotive industry, e.g., AVL, KSPG, Lotus MAHLE, have presented their own range extender concepts as modules for use in vehicles [20]. Most of Engineering, MAHLE, have presented their own range extender concepts as modules for use in them make use of spark ignition ICEs. BMW converted small-displacement two-cylinder ICE used in vehicles [20]. Most of them make use of spark ignition ICEs. BMW converted small-displacement the BMW C650 GT scooter [21] to be offered as an option for the BMW i3, initially designed as a pure two-cylinder ICE used in the BMW C650 GT scooter [21] to be offered as an option for the BMW i3, BEV. General Motors has chosen a different way by designing the Voltec powertrain [22,23], formerly initially designed as a pure BEV. General Motors has chosen a different way by designing the Voltec known as E-Flex, which includes an all-new four-cylinder Ecotec ICE [24], exclusively for Chevrolet powertrain [22,23], formerly known as E-Flex, which includes an all-new four-cylinder Ecotec ICE Volt/Opel Ampera. Voltec architecture is similar to a plug-in hybrid (PHEV), but is capable of operation [24], exclusively for Chevrolet Volt/Opel Ampera. Voltec architecture is similar to a plug-in hybrid on battery power alone up to 85 km, depending on the driving style, ambient temperature conditions, (PHEV), but is capable of operation on battery power alone up to 85 km, depending on the driving and climate comfort