Politecnico
di Milano
Experiences in aircraft hybrid
propulsion systems
Lorenzo Trainelli
Dipartimento di Scienze e Tecnologie Aerospaziali,
Politecnico di Milano, Italy
15 May, 2014
Experiences in aircraft hybrid propulsion
systems
Outline
•Introduction
•Electric aircraft, hybrid propulsion
•Project description
•Energy requirements
•Performance goals
•Power system design
•System sizing
•Component selection and arrangement
•Performance verification
•Concluding remarks
POLITECNICO di MILANO – Dept. of Aerospace Science & Technology
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Experiences in aircraft hybrid propulsion
systems
Outline - 1
•Introduction
•Electric aircraft, hybrid propulsion
•Project description
•Energy requirements
•Performance goals
•Power system design
•System sizing
•Component selection and arrangement
•Performance verification
•Concluding remarks
POLITECNICO di MILANO – Dept. of Aerospace Science & Technology
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Experiences in aircraft hybrid propulsion
systems
Introduction
A greener aviation
•
Modern aeronautics is highly concerned with reducing chemical and
acoustic emissions
•
Ambitious environmental targets set by the ACARE for 2020
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Experiences in aircraft hybrid propulsion
systems
Introduction
More electric aircraft (MEA)
A fundamental avenue towards the previous goals is electrification:
from various on-board systems to the propulsive system itself
•
MEA
The MEA concept advocates the utilization of electric power for nonpropulsive systems.
Replacing hydraulic, pneumatic, mechanical systems and sub-systems
with electric equivalents is becoming a dominant trend in the aviation
industry (B787, A350).
Advances in the areas of power electronics, fault-tolerant architecture,
electro-hydrostatic actuators, flight control systems, high density electric
motors, power generation and conversion systems are providing the
technology to improve efficiency and safety of aircraft systems operation.
Improvements in aircraft weight, fuel consumption, total life cycle costs,
maintainability and aircraft reliability are expected.
The ultimate goal is distributing only electrical power across the airframe.
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Experiences in aircraft hybrid propulsion
systems
Introduction
All-electric aircraft (AEA)
An ambitious challenge: removing the ICE (internal combustion engines)
from aircraft
•
AEA
In a AEA the propulsive system is based on electric motors (EMs), with
electric power coming from fuel cells, solar cells, ultracapacitors, or
batteries.
Pros
 No chemical pollution (during operation)
 Very low acoustic pollution
 High energy-conversion efficiency:
0.95
 Aeronautics: no power drop-off with altitude
Cons (batteries)
x Low energy density  Low endurance/range
x Long recharging time
POLITECNICO di MILANO – Dept. of Aerospace Science & Technology
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Experiences in aircraft hybrid propulsion
systems
Introduction
Advantages of hydrocarbon-fueled ICEs
The ubiquity and success of HC-fueled engines is the result of
1. Very high energy density (energy per unit weight) of the fuel
kWh
kg
2 orders of magnitude!
kWh
0.17
kg
2. Very high power density (power per unit weight) of the engine
12.5
Electric motors are reaching these values
Also, other factors play an important role
3. Liquid fuels are more easily distributed and handled than gaseous
(natural gas, hydrogen) or solid (coal) fuels
4. ICEs are made of almost inexpensive materials compared to other
solutions
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Experiences in aircraft hybrid propulsion
systems
Introduction
Hybrid propulsion concept
Get the best of HC-fueled ICEs and EMs by coupling them into an integrated
propulsive system
Ems vs ICEs
Electric motors are
•
highly efficient
•
provide exceptionally high power-to-weight ratios
•
provide adequate torque when running over a wide speed range
Internal combustion engines
•
run at their most efficient when turning at a constant speed
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Experiences in aircraft hybrid propulsion
systems
Introduction
Two basic hybrid architectures
Parallel
A mechanical clutch allows ICE
and EM to combine their powers
onto the propeller shaft
Series (or serial)
The EM is connected to the
propeller shaft, while the ICE acts
as a battery recharger
«Range-extended electric vehicles»
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Experiences in aircraft hybrid propulsion
systems
Introduction
Parallel hybrid systems
 Somewhat easier to design
 Less electric components
 Flexibility: power splitting
Ex.: high power ICE + small EM for TO
 Safety, especially «perceived» safety
Two-engine configuration, attractive for single engine aircraft
x Mechanical complexity: clutch and on-board arrangement
x Eco-advantages
ICE works at variable power settings
Higher noise emissions
x Architectural rigidity
ICE not easily replaced by other engines (ex. switch from piston to
gas turbine)
AEA target is farther
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Experiences in aircraft hybrid propulsion
systems
Introduction
Series hybrid systems
 Design more demanding
 Mechanical simplicity
 Safety, given high EM reliability
 High architectural flexibility
ICE is more easily replaced by other engines
AEA
target
is
much
closer
(ex.
ICE+generator+batteries to a fuel cell system)
Switch
from
 Eco-advantages
ICE working at constant max efficiency power setting
Lower noise emissions (possibility of «pure» electric operations)
x More electric components
x Triple energy conversion
x Single engine configuration («perceived» safety)
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Experiences in aircraft hybrid propulsion
systems
Introduction
Component
η
η
η η η η η η
Efficiency
Electric motor η
0.90
Converter η
0.90
Battery pack η
0.88
Charger η
0.90
Generator η
0.90
Conventional ICE η
≤ 0.25
A series architecture involves multiple energy conversions, but the ICE works
under constant load, at its peak efficiency
Break-even
Δη
i.e., for higher values of Δη
1
η η η η η
1 η
, the hybrid solution becomes more efficient
Under conservative assumptions, η
0.43 is required – not an
unreasonable demand for a modern, optimized ICE at peak setting
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Introduction
Experiences in aircraft hybrid propulsion
systems
Electric/Hybrid Manned Aircraft
Dimona E-Star
ENFICA - FC
eSpyder
SkySpark
2007
2009
2011
2013
PROJECT
PARTNERS
AIRPLANE MODEL
PROPULSION
ENFICA-FC
Politecnico di Torino
Israel Aircraft Industries
Jihlavan Aircraft
Jihlavan Rapid 200
Fuel-cell AEA
SkySpark
DigiSky
Politecnico di Torino
Alpi Aviation Pioneer 300
(Fuel-cell AEA)
Battery AEA
Dimona E-Star
Siemens
Diamond Aircraft
EADS
HK36 Super Dimona
Series hybrid
eSpyder
Yuneec International
Flightstar Spyder
Battery AEA
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Introduction
Experiences in aircraft hybrid propulsion
systems
DSTA-PoliMi project
“Long-EZ” hybrid conversion
Rutan Model 61 Long-EZ
Wing span [m]
7.90
Wing surface [m2]
7.62
Length [m]
5.12
Height [m]
2.40
Canard empennage span [m]
3.60
Canard empennage surface [m2]
1.19
Max take-off weight [kgp]
601
Empty weight [kgp]
340
Fuel tank capacity [l]
197
Power installed [hp|kW]
An experimental (home-built) airplane
115|86
Max cruising speed [kts|km/h]
160|298
• High performance
Cruise speed [kts|km/h]
125|232
• Uncertified – Lower development burdens
Range [km]
• Geometry and data available
• Possible future implementation
Service ceiling [ft]
3,200
27,000
Rate of climb [ft/min]
POLITECNICO di MILANO – Dept. of Aerospace Science & Technology
1,750
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Introduction
Experiences in aircraft hybrid propulsion
systems
DSTA-PoliMi project
Design a viable series hybrid propulsion system for the Long-EZ
that preserves the general performance and handling qualities of
the original airplane and…
Performance goals (FAI records)
1. Maximum airspeed at altitude of 8,000 ft, aim: 160 kts
(pure electric)
2. Climb up to 20,000 ft in less than 20 minutes
3. Maximum range with 80 l of fuel on board, aim: 2,000 km
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Experiences in aircraft hybrid propulsion
systems
Outline-2
•Introduction
•Motivation
•Project description
•Energy requirements
•Performance goals
•Power system design
•System sizing
•Component selection and arrangement
•Performance verification
•Concluding remarks
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Energy Requirements
Take-off performance items computed
as functions of installed electric power
Exp. data
TOW
520 kgp
82 kW
Ground run distance
(blue) and total critical
distance, i.e. aborted
take-off, ATO (red)
TO distances
900
ATO distance
ground run distance
800
700
65 kts
350 m
TO energy
0.75
600
L [m]
0.7
500
400
[kWh]
0.65
TO
300
0.6
E
Experiences in aircraft hybrid propulsion
systems
Take-off
200
50
55
60
65
70
75
P [kW]
80
85
90
95
100
Energy required to complete take-off
over a 50 ft screen height
0.55
0.5
0.45
50
55
60
65
70
75
P [kW]
80
85
90
95
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Energy Requirements
Energy required to reach the maximum airspeed at 8,000 ft altitude,
and to maintain this condition on a distance of 3 km and 15 km
Conditions
Power available (red)
and required (black)
at 8,000 ft, as functions
of true airspeed
performance diagram
140
120
100
Pure electric mode
Horizontal flight
Energy for max airspeed
11
Pa −− Pr [HP]
80
10
acceleration
3 km cruise
15 km cruise
60
9
40
8
E [kWh]
Experiences in aircraft hybrid propulsion
systems
Performance Goal 1: Max Airspeed
20
0
50
7
6
100
150
200
V [kts]
5
Energy required to reach (blue) and maintain
maximum airspeed for 3 km (red) and 15 km
(black) as functions of installed power
4
3
50
55
60
65
70
75
P [kW]
80
85
90
POLITECNICO di MILANO – Dept. of Aerospace Science & Technology
95
100
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Energy Requirements
Performance Goal 2: Time to Climb
P = 50 kW
P = 100 kW
Energy required to reach 20,000 ft
altitude from sea level, aiming to a
climb time not exceeding 20 min
60
h [ft x 1000]
50
Conditions
Hybrid mode
Altitude vs. time to climb,
as a function of different
values of installed power
Minimum power
required airspeed
40
30
20
10
0
0
20
40
60
80
100
120
140
t [min]
Time to climb at 20,000 ft
energy to climb at 20,000 ft
24
19.5
22
19
20
18.5
E20000 [kWh]
[min]
20000
Left diagram: Time to
climb up to 20,000 ft
18
18
16
t
Experiences in aircraft hybrid propulsion
systems
climb diagram
70
Right diagram:
Energy required to
climb up to 20,000 ft
17.5
17
14
16.5
12
Both functions of
installed power
16
10
8
50
15.5
55
60
65
70
75
P [kW]
80
85
90
95
100
15
50
55
60
65
70
75
P [kW]
80
85
90
95
100
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Energy Requirements
Experiences in aircraft hybrid propulsion
systems
Performance Goal 3: Range
Range performance given a fixed value of the energy stored on
board, corresponding to the maximum amount of fuel usable
Energy management: “start and stop” strategy for the ICE
 cruise composed of alternating segments of “pure electric mode”
and “hybrid mode”
•
Pure electric mode (A)
ε
•
η
•
Hybrid mode (B)
•
•
,C
1
Batteries drive the electric motor from full
charge until a prescribed discharge level is
reached, and the ICE is kept off
Constant weight, power, airspeed
ICE power always higher than the power
required to drive the electric motor
 excess power exploited to recharge the
battery pack up to full charge ( )
Decreasing weight and power
Cruise-climb profile (constant airspeed)
,C
2
C
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Energy Requirements
Experiences in aircraft hybrid propulsion
systems
Performance Goal 3: Range
New endurance and range equations for the Hybrid mode
ε
ε
,C
,C
1
1
|
,C
,C
2
C
Power drop-off assumption:
ln
ρ
ρ
, if
0
, if
0
The equation that relates the battery rate of charge with the
portion of the excess ICE power output provides the final weight
η η
η
η η
|
,C
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Energy Requirements
Optimal endurance and range as functions
of installed electric power, based on the
previous analysis
Conditions
80 l fuel usable
60 kg battery pack
Endurance
Range
1100
2500
P = 50 kW
P = 100 kW
2400
1000
2300
900
R [km]
2200
ε [min]
Experiences in aircraft hybrid propulsion
systems
Performance Goal 3: Range
800
2100
700
2000
600
1900
P = 50 kW
P = 100 kW
500
0.4
0.6
0.8
1
1.2
C
L
Endurance
1.4
1.6
1.8
1800
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
C
L
Range
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Experiences in aircraft hybrid propulsion
systems
Outline-3
•Introduction
•Motivation
•Project description
•Energy requirements
•Efficiency
•Performance goals
•Power system design
•System sizing
•Component selection and arrangement
•Performance verification
•Concluding remarks
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Power System Design
Experiences in aircraft hybrid propulsion
systems
Electric Motor
Sizing from the previous analysis:
electric power output
= 50 ÷ 70 kW
Market survey: AC solution, synchronous
brushless direct drive
•
High efficiency & higher costs compared to
asynchronous designs
•
Direct drive avoids rpm reduction gear
Manufacurer
Model
Type
Max Power [kW]
Rotational Speed [rpm]
Size ΦxL [mm]
Voltage [V]
Current [A]
Weight [kg]
SICME Motori
“Va-lentino”
Brushless direct drive
67.5
2500
250x280
450
150
40
Yuneec International
Power Drive 60
Brushless direct drive
60
2400
280x209
190
220
30
Choice: “Va-lentino”
•
Higher values
of power and voltage
•
External rotor,
high number of poles
•
Used in the SkySpark
project
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Power System Design
Experiences in aircraft hybrid propulsion
systems
Battery Pack
Battery sizing crucial to power system sizing
•
Heavily impacting on all aspects of the design
•
Weight depends on required energy (from mission
requirements) and energy density (a characteristic
of battery technology)
Lithium-polimer batteries
•
Less prone to develop fire compared with Li-ion
batteries
•
No memory effects
•
Flexible, reliable, rugged
•
Higher costs + need management system (BMS)
30 different Li-Fe-Po battery cell types
produced by Kokam have been evalued
subjected to the mission power and
energy constraints
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Power System Design
Voltage sizing from electric motor power
•
Series of 122 cells, each with 3.7 V  450 V
Capacity sizing from technology constraints
•
Charging: maximum C-rate
•
Discharging: Peukert effect
Necessary power to battery recharge (8000 ft)
60
40
Energy requirements
•
Pure electric take-off
•
Hybrid take-off
•
Hybrid climb to 8,000 ft
20
ΔP [kW]
Experiences in aircraft hybrid propulsion
systems
Battery Pack
0
−20
−40
Type
Kokam #1
Kokam #2
12
31
345
780
129
147
∆ 3.69
3.69
C-rate
15
8
−60
−80
60
30 kW
40 kW
50 kW
60 kW
70 kW
39 Ah
36 Ah
25 Ah
80
100
120
140
160
180
200
V [kts]
2 cell types finally selected, in connection to the ICE selection
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Power System Design
Experiences in aircraft hybrid propulsion
systems
Internal Combustion Engine
Requirements
•
Airspeed range: 90 to 120 kts (usual cruise condition for the Long-EZ)
•
Adequate excess power in cruise (recharging power)
 ICE optimal power (max efficiency)
= 40 kW @ 8,000 ft
Manufacturer
Type
Cubic capacity [l]
Max power [kW]
Max power rpm
Max torque [Nm]
Max torque rpm
Optimum Power [kW]
Consumption [l/100 km]
Emissions [CO2 g/km]
Volkswagen
TDI
1.2
56
4200
180
2000
37
3
87
Smart
Gasoline
(turbocharged)
1.0
62
5250
120
3250
41
4.2
116
Type
Kokam #1
Kokam #2
12
31
Total pack
weight
45
95
ICE selection
Market analysis (automotive)
•
Smart engine chosen
•
Small size, high power output
•
Turbocharged (no power dropoff up to 8,000 ft)
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Power System Design
Experiences in aircraft hybrid propulsion
systems
Final Layout
Hybrid configuration
Item
[kg]
Sizing and lofting completed
including cooling system
Mass evaluation led to a
MTOW and CG travel
estimations fully compliant
with aircraft specifications
Original EOW
Native ICE LY-0235
Pilot
Oil & liquids
Fuel
ICE
Generator
AC/DC inverter
Battery pack
DC/AC inverter
Electric motor
12V battery
DC/DC converter
TOW
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-113
85
6
65
95
30
5
45
5
40
3
2
599
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Power System Design
Experiences in aircraft hybrid propulsion
systems
Performance Validation
Performance Goal 1: Max airspeed
•
160 kts achieved with Type 2 battery cells & fuel limited to 25 l
•
Flight program: 1) hybrid take-off and climb to 8,000 ft
2) hybrid loiter at 100 kts for 25 min for full recharge
3) acceleration dash to 160 kts (about 4 min)
4) maintain airspeed for 15 km
Performance Goal 2: Time to climb
•
17 min to 20 kft achieved with Type 2 battery cells & fuel limited to 25 l
•
Flight program: 1) hybrid take-off and climb to 8,000 ft
2) hybrid climb to the target altitude using full ICE power
Performance Goal 3: Range
•
2,000 km with 80 l fuel achieved with Type 1 battery cells
•
Flight program: 1) hybrid take-off and climb to 8,000 ft
2) stepped cruise-climb to 11,200 ft (a total of 23 alternating segments
of pure-electric mode and hybrid mode flight)
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Experiences in aircraft hybrid propulsion
systems
Outline-4
•Introduction
•Motivation
•Project description
•Energy requirements
•Efficiency
•Performance goals
•Power system design
•System sizing
•Component selection and arrangement
•Validation
•Concluding remarks
POLITECNICO di MILANO – Dept. of Aerospace Science & Technology
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Experiences in aircraft hybrid propulsion
systems
Concluding remarks
A first step towards a possible implementation of a novel hybrid
propulsion system
•
Starting from an existing GA airplane with favorable airframe
and aerodynamic characteristics
•
Aimed at breaking 3 performance records according to the FAI
regulations
The preliminary design process has been initiated
• Feasibility of the design based on the current technology
• Selection of COTS components, arrangement on-board and
preliminary verifications
• Performance verification shows promising results
2,000 km range: 80 l (hybrid) instead of 107 l (conventional) fuel
 25% fuel saving
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Experiences in aircraft hybrid propulsion
systems
Acknowledgements
Inspirator of the “Long-EZ” hybrid conversion
Gianni Zuliani, Technoline Engineering SA, Switzerland
Thesis students
Matteo Anselmi
Sara D’Andrea
Marco Ballarin
Andrea Alessio
Irene Gil
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