A second type of innovative subsystem is represented by the HEPS. The aim of this technology is to produce more efficient propulsive power combining the endothermic source (i.e. the Internal Combustion Engine – ICE) and the electric source (i.e. the Electric Motor – EM) [31]. This kind of hybrid-electric propulsion is already consolidated in the automotive and nautical fields [32], pushed by the main advantages of reduction of fuel consumption and air pollution. However, this innovation in aeronautics is still premature. The weak point of the hybrid propulsion is indeed represented by the low energy density – meant as the amount of energy per unit of mass – of the electric accumulators. It is worth noting that the energy density of the AVGAS is about 44 MJ/kg, while the energy density of a lithium-ion battery could reach values only up to 0.6-0.8 MJ/Kg [33]. This means that heavy batteries could be installed aboard the airplane, affecting range and endurance of the aircraft.
While many aircraft with an all-electric propulsion have been designed, only a few examples of hybrid-electric concepts are worth mentioning. In 2011 Siemens, Diamond Aircraft Industries, and EADS presented at the Paris Air Show the first aircraft propelled by a HEPS, with the aim of demonstrating the feasibility of the hybrid technology in aeronautics [34]. This aircraft, a motor-glider named Diamond DA36 E-Star (Figure 7), is characterized by a propeller powered by an electric motor of 70 kW, which is supplied by both electric energy storage and a small 30 kW Wankel ICE linked to an electric generator.
Design of innovative on-board systems 11 This type of propulsion architecture is named series hybrid. A schema of the series hybrid architecture is depicted in Figure 8. As shown, the propeller is driven only by an electric motor, which is supplied by an electric generator connected to a thermal engine operating at a higher efficiency point, assisted by electric accumulators [35]. Advantages and disadvantages of this kind of hybrid architecture are listed in Table 1 [35].
Figure 8: Schema of the series hybrid architecture.
Table 1: Pros and cons of the series hybrid architecture [35].
Pros:
1. The ICE operates with optimal conditions of torque and rotary speed, entailing maximum efficiency
2. The ICE operates at a nearly constant angular speed. Hence, it’s more reliable and it requires less maintenance
3. The “only-electric” mode is feasible
4. Batteries could be recharged during descent
Cons:
1. The electric motor is sized for the maximum power, with consequent weight increment 2. Reduction of the global efficiency due to the energy conversions
A second hybrid propulsion configuration is defined parallel architecture. The parallel hybrid is characterized by the mechanical coupling via a gearbox or a belt of the ICE with an electric moto-generator (see the schema in Figure 9). The thermal and electric machines both supply mechanical power to the propeller during the take-off and other flight mission phases in which extra-propulsive power is required. During other mission phases, such as cruise, the subsystem could operate in “traditional mode”. The propulsive power is generated by the thermal engine,
12 Motivations and objectives of the research which besides moves the electrical machine generating electrical secondary power. An additional operative mode of the parallel hybrid system is the “only-electric” one, which could be operated during the ground taxi.
Advantages and disadvantages of the parallel hybrid architecture are listed in Table 2 [36].
Figure 9: Schema of the parallel hybrid architecture.
Table 2: Pros and cons of the parallel hybrid architecture [36].
Pros:
1. Powerboost is supplied when peak power is required
2. The ICE is downsized, entailing weight and volume reductions
3. The safety level in case of ICE failure is augmented. The electric drive entails an increase of the gliding distance
4. Power is recovered during the descent, using the propeller as Ram Air Turbine (RAT). Batteries could be recharged
5. The “only-electric” mode is feasible, for instance during the ground taxi phase
Cons:
1. A mechanical clutch could be required to connect/disconnect the ICE
Several studies have been conducted focusing on this type of architecture, as will be described in subsection 1.2.1. Moreover, the potentialities of the parallel hybrid architecture have been proved through a test bench realized by Flight Design [37] (Figure 10).
Design of innovative on-board systems 13 The solution proposed by Flight Design is characterized by a fixed connection through a belt of a 115 hp (85.8 kW) Rotax 914 engine and a 30 kW Permanent Magnet electric motor-generator. During the take-off and climb phases, the EM could be employed as electric motor, supplied by a 130 V Lithium Iron Phosphate (LiFePO4) battery pack. This type of energy storage is characterized by a good energy density (over 90 Wh/kg), entailing a total mass of nearly 30 kg. The power flow among the components is managed by an electronic controller, in which a control law based on the throttle position is implemented. According to this control law, for throttle values greater than 90%, both the ICE and the EM provide propulsive power (“hybrid” mode). Otherwise, the electrical machine is moved by the thermal engine, generating electric power (“traditional” mode). If the throttle is set between 20% and 85%, part of the generated electric energy is used for battery charging.
Figure 10: Flight Design’s parallel hybrid bench (adapted from [www.wired.com]). In conclusion of the present Section, it is worth mentioning the first aircraft powered by a parallel hybrid powertrain: the single-seat ultralight Alatus, conceived and realized at the University of Cambridge in 2010 [38]. This hybrid demonstrator is characterized by a 2.8 kW four-stroke thermal engine mechanically connected to a 12 kW brushless electric motor, which is supplied by Lithium-Polymer (LiPo) batteries with a capacity of 2.3 kWh.
14 Motivations and objectives of the research