University of Birmingham Development of a series hybrid electric aircraft pushback vehicle

The work presented in this paper is a progression to previous research which developed an overcurrent-tolerant prediction model. This paper presents some of the modelling and development techniques used for the previous research, but more emphasis is placed on the requirements of the case study; whereby an aeroplane pushback tug is converted into a series Hybrid Electric Vehicle (HEV). An iterative design process enabled the traction motor, transmission, generator and battery pack parameters to be tailored for this vehicle’s unique duty cycle. A MATLAB/Simulink model was developed to simulate the existing internal combustion engine powertrain as well as the series HEV equivalent for comparative analysis and validation purposes. The HEV design was validated by comparing the simulation results to recorded real-world data collected from the existing vehicle (torque, speeds etc.). The HEV simulations provided greater fuel savings and reduced emissions over the daily duty cycle in comparison to the existing vehicle.


Introduction
Carbon Dioxide (CO 2 ) is the primary greenhouse gas affecting global climate change [1] [2] [3]. Nearly 18% of the global CO 2 emissions generated each year from the burning of fossil fuels come from the transportation sector [4] [5]. It has been estimated that around 29,000 deaths each year are attributed to exposure to fine Particulate Matter (PM) in the UK alone [6] [7]. Nitrous Oxides (NOx) in the presence of ultraviolet light can become converted into photo-Engineering chemical smog, extended exposure to this causes eye irritation and can impair respiratory functions [8] [9]. The environmental impact and health issues from the emission generated from Internal Combustion Engines (ICE) has thus motivated greater research into Hybrid Electric Vehicles (HEV) and pure Electric Vehicles (EV) in a wide range of transportation sectors [10] [11] [12] [13].
An aeroplane pushback vehicle is required to move aeroplanes away from airport terminals and occasionally tow them across an airfield to the hangar to receive routine maintenance. Conventional pushback vehicles use high capacity Internal Combustion Engines (ICE) with a number of transmission ratios which are rarely fully exercised. Between pushback operations, the vehicle might rest for long periods until it is needed again. The ICE remains idling whenever the vehicle is resting to avoid any ICE start-up difficulties (particularly during cold weather). This is because airports deal with a high volume of aeroplane arrivals and departures each day, so an aeroplane tug failing to turn on could cause dramatic delays. These long idle times significantly increase the total fuel consumption and output emissions over the working day. One major advantage of a HEV pushback vehicle is that the generator is able to be turned off during the rest periods as long as there is enough energy stored in the Energy Storage System (ESS). Ideally, a series HEV would operate in fully electric mode for a substantial portion of the duty cycle to minimise fuel costs and output emissions. While the design methodologies for various light and heavy commercial vehicles have been reported in the literature [14] [15] [16] none has addressed the unique operational duty cycles of the aeroplane pushback vehicle consisting of diverse operations including series of short duration peak power operations, medium power, longer duration operations and high speed, low power operations. This paper establishes a development strategy for a hybrid vehicle with this type of unique and unusual duty cycle.
The proposed aeroplane tug will have a series hybrid topology as shown in Figure 1, incorporating an electric motor for traction, an engine-generator (genset) for electric power generation, and a battery pack to store electrical energy. The ICE and the HEV models were created in MATLAB/Simulink using standard vehicle dynamic equations and a new torque control strategy was incorporated into the traction motor. Electrical power management techniques were investigated to ensure the battery pack does not overcharge or over-discharge as well as prolonging the life of the genset. Section 2 will discuss some of the methodology and design criteria for the aeroplane pushback HEV. Further information not contained within this paper can be found from the author's previous research [17]. Section 3 will analyse the results of the final pushback vehicle.

Methodology
Effective HEV design requires optimising the choice of mechanical and electrical components to meet the vehicle's target dynamic performance with the longest  driving range possible [18]. Numerous design iterations are often deliberated to ensure the mechanical and electrical parameters of the system allow the vehicle to achieve its target performance objectives. Part of the iterative process also requires compromising between other vehicle constraints such as the unloaded performance, cost, and component size. The pushback vehicle must be able to generate a large towing tractive force, but also have a relatively high top speed to travel quickly between pushback operations (~30 kph unloaded). The airport regulations and aeroplane manufacturers require that the tug must be able to generate a minimum tractive force for each aeroplane weight class the tug is designated to tow, an example of the tractive force requirements for a Boeing 737-7 is referenced [19]. Therefore a suitable combination of traction motor, transmission ratios and final drive ratios should be used to meet these regulations. As the mass of the conventional pushback vehicle is already very large, the additional mass of a battery pack which would normally be a constraint in the vehicle design is not of concern for this application.
The iterative design process for this vehicle was separated into two areas; finding appropriate mechanical components that will allow the vehicle to achieve its target longitudinal dynamics and the electrical storage/generation parameters that would allow the vehicle to fulfil its mission.  [20]. The membership functions for the FL speed controller are given in Table 1 and the rule base is given in Figure   2. This controller uses the longitudinal velocity error between the target velocity x V * and the instantaneous velocity x V at time t as the first input and its time derivative as the second input to generate a suitable pedal activation level.
An input range of ±5 kph in Figure 2 A controller output with a magnitude 1 indicates that the accelerator or brake pedal is fully activated. Positive speed controller outputs represent accelerator pedal activation levels and negative outputs represent brake pedal activation levels. An accelerator pedal activation level corresponds to the driver requesting a motor torque output to either accelerate the vehicle or maintain a steady cruising speed.
A vehicle model created in MATLAB/Simulink contains the basic forces acting upon the vehicle. The tractive force Traction F at the road-wheel interface (2), the rolling resistance Rolling F of the tyres (3), the aerodynamic drag Aero F (4) and the brake force Brake F (5) are all included in the model [16] [21] [22].

( )
The tractive force requires the output torque from the prime mover PM τ , gear Details of the parameters of the existing vehicle are shown in Table 2. It uses an IC engine with the torque-speed profile provided in Table 3, along with a final drive ratio and three gear ratios T N . Gear ratio 1 is used for the towing operation, gear ratio 2 is used for the maintenance runs, and gear ratio 3 is used for the unloaded solo runs. These gears cannot change while the vehicle is moving  and must be set before the operation begins. The ICE is accompanied by a fuel consumption map as shown in Figure 3 and a series of emissions maps shown in   (9). For a series-HEV or pure EV, the torque will be fully accommodated by the traction motor. For a parallel or power-spit HEV, the fraction of the command torque required to be supplied by the traction motor will take the role of the required output torque in (9).
An efficiency map can be used to find the motor's electrical power consumption at each point on the torque-speed curve using Equation (10). The tug also has a high auxiliary power demand from providing power to the aeroplane's air conditioning during pushback operations, warning lights, heating, and ventilation to the cabin. It is assumed that the auxiliary power would be constant throughout the duty cycle to indicate a worst case scenario. For the hybrid aeroplane tug to truly be comparable with the ICE counterpart, the battery pack SOE should be at the upper limit by the end of the duty cycle.
This is analogous to the ICE tug having the fuel tank filled at the end of the working day. Therefore, once the last pushback operation of the duty cycle is completed, the genset will turn on to begin charging the battery pack while the tug returns to the overnight storage area. The genset will remain on until the SOE reaches the upper limit where all auxiliary power can then be turned off.
Duty cycle data was recorded from the conventional ICE powered pushback vehicle (engine speed, output torque, longitudinal velocity, etc.) via the CAN bus.
This data was used to construct a target duty cycle for the HEV to follow. The pushback vehicle's duty cycle, obtained from several days' actual operation in the field, is broken down into four major areas as shown in Table 4 and explained as follows: • Low velocity pushback operations of heavy aircraft. This scenario occurs when the pushback vehicle is required to move aeroplanes away from the airport terminal. • Medium velocity towing operations of medium weight aircraft. This represents a maintenance run where the pushback vehicle tows an empty aeroplane for a longer duration across an airfield to receive routine maintenance.

Results
The final tug design parameters are given in Table 5 where a single large traction motor using a 2-speed transmission was used. The torque-speed curves for this motor and an efficiency map based on a 9-phase 245 kW IM is shown in Figure 6 [23]. A high gear ratio     The hybrid vehicle's dynamic profile is comparable to the target ICE counterpart, the battery SOE remains within safe working limits and the genset does not turn on/off rapidly throughout the duty cycle.
The blue shaded areas of Figure 8 show that the genset is providing full power to the DC-Link and that the traction motors are fully utilising this power during towing operations with the surplus power demand coming from the battery pack. The yellow shaded regions show that the genset provides full power to the DC-Link and the battery pack is being charged. The hybrid tug Engineering   Table 6. Assuming that the genset provided a similar reduction in engine emissions, the total volume of CO, NO x , PM, and HC's at the end of the duty cycle was also reduced.

Conclusion
This paper presented some of the design and modelling methods for a series hy-