Sawtooth pattern to reduce fuel consumption of hybrid planes

Auteurs : H. Bourjot, S. Messe
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Sawtooth pattern to reduce fuel consumption of hybrid planes


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	    <date dateType="Created">Sun 1 Oct 2017</date>
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            <date dateType="Submitted">Sat 17 Feb 2018</date>
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Sawtooth pattern to reduce fuel consumption of hybrid planes H.Bourjot (1), S.Messe (2), 1 : SMA, SAFRAN Bat 34 Rond point René Ravaud Moissy Cramayel Cedex 77556 France, 2 : SMA, SAFRAN Bat 34 Rond point René Ravaud Moissy Cramayel Cedex 77556 France, Abstract In this paper, hybrid power unit for planes will be considered. We aim at demonstrating that hybrid aircraft could lead to fuel saving if we adapt our usage of such aircraft. We will present a plane numerical model and a hybrid power unit one. Both are designed for steady state operation point, and no transient effect will be considered. The numerical resolution of such models will be detailed. Furthermore this model will be used to determine fuel consumption of various flying patterns and we will conclude on the fact that by adapting pattern it is possible to reduce fuel consumption and maintain plane performance. Introduction The mainstream idea behind this work is: how can we use internal combustion engine (ICE) in a more fuel efficient way. The answer is known by every engineer, we need run the engine at its peak efficiency, which is close to full load. But by doing so, speed will increase as consumption because of drag force. In order to keep a constant speed it is possible to use an electrical power unit to store the extra amount of power, and not waste it in drag work. As it is impossible to store infinitely this energy, it has to be used in the most efficient way. Considering this, it seems logical to have a two phases flying pattern: - The first one will be a soft climb at full load on ICE in order to store some energy in batteries. - The second one will be a descent smoothed by the use of electrically harvested energy, while the ICE is shut down. This paper aims at proving the truth of this simple analysis. Model Fig. 1: Plane and hybrid power unit model and efficiency As a first estimation it has been chosen to use a quasi stationary approach to model the plane and its hybrid power unit. With this approach we are not able to consider any transient effect, but as we will see, the pattern is built with two steady points and transient effect could be neglected. Concerning power unit the engine consumption is modeled with a Willans line, see formula (i). Propeller and electrical power unit (battery, motor, inverter, harness...) are modeled with a constant efficiency (see figure 1 for details). (i) The plane is modeled with his polar curve and a mechanical equilibrium is considered to determine lift, drag force and get the required shaft power. The flying pattern (see figure 2) is a simple sawtooth. Fig. 2: Hybrid Sawtooth flying pattern Numerical Calculation Calculation was performed with a 4 seat civil airplane of 1.4 Ton and a 170 kW ICE. We assume a constant speed regarding ground of 140 kt. at a base altitude of 10 000 ft. Several climbing speed has been investigated and each time the matching descending speed was used to design the pattern. Regarding energy we get the following relation: Regarding speed we get the following relation: Therefore we get the following relation: This relation is used to build a pattern which is energetically balanced. The fuel consumption is calculated by using the following relation: Results Figure 3: fuel saving regarding climb rate of pattern Figure 3 presents the saving regarding climb rate of first phase. The base consumption is 43.4 L/h. We could reach a minimal fuel consumption of 38.2L/h, which represents a saving of more than 12%, but also requires high climb rate and will not make passenger comfortable, but is acceptable in case of UAVs A more feasible assumption is to consider an electrical drive of 70 kW. This solution leads to a gain of 9% regarding fuel consumption. For instance, a fuel gain of 10kg can be obtained on a 3-hour flight. On the opposite, to perform it, we need to add 10kg of batteries (2500Wh for a 7-minute cycle) and 30kg of accessories (harness, inverter, motor, heat exchanger). So with an overweight of 30kg, compare to 1 400kg for the plane, and adopting a dedicated flying pattern, we could reduce the fuel consumption by 9%. Conclusions A method to model a hybrid plane and the linked computational method has been presented. It has been shown that it is possible to reduce fuel consumption by 9% with an overweight of only 30kg. Of course the model is quite simple and some deeper investigations are needed to conclude on final gain, as prove in [1]. It’s also necessary to look at impacts for consumer, maintenance, certification… This result shows that to make planes ”greener”, it is not sufficient to adapt the power unit or to use hybridization, it is also necessary to rethink completely aeronautical application using all the levers hybrid technologies can offer. References 1 F. WINKE et al, Dynamic Simulation of Urban Hybrid electric Vehicles, MTZ, 2013, Vol.74, pp.56-62 2 J.B. HEYWOOD, Internal Combustion Engine Fundamentals, McGraw-Hill, 1988 Glossary  Climb angle  Descent angle E Energy stored during climbing phase h Altitude s Storage efficiency us Usage efficiency P1 Shaft power during phase 1 P2 Shaft power during phase 2 PICE,max Maximal mechanical power Ps Storage power Pus Usage power Qh,1 Fuel consumption during phase 1 sQh,2 Fuel consumption during phase 2 Qh Average fuel consumption QICE,max Maximal fuel consumption t1 Phase 1 duration t2 Phase 2 duration VG Speed regarding ground VZ,1 Climbing rate during phase 1 VZ,2 Climbing rate during phase 2 0% 5% 10% 15% 0 100 200 300 400 500 600 fuel saving Vz1, climb rate (ft/min) Unacceptable Battery weight