The Physics of Getting Around by Steven Anderson (Micromobility Conference 2017)

The Physics of Getting Around
Or, why urban transport is the
killer app for electric
powertrains
MicroMobility Conference, Copenhagen, September 6, 2017
1

• MIT-trained mechanical engineer
• Journalist
• Technical Editor Cycle World
• Editor-in-Chief: Cycle Magazine
• Platform Director and Project Leader
at Buell Motorcycle Company and Erik
Buell Racing
Who am I to tell you this?
2

Who am I to tell you this?
3

Electric motorcyclesfrom start-
up companies, early 2010s.
Brammo Empulse Zero DS
Mission Motorcycles
4

Electric motorcycles from start-
up companies, early 2010s.
• Acceleration: Acceptable to very
good.
• Range: Awful (maybe 70-90 km at
highway speeds).
• Prices: Very high.
• My reaction, “These are insane
business plans.“
5

Why electric motorcycle range is
awful
Batteries have very low energy-density compared to fossil fuels.
Energy density (kWh/kg)
highest-power lithium-ion
high-energy lithium-ion
gasoline with typical efficacy
gasoline
0.0 3.5 7.0 10.5 14.0
6

Why electric motorcycle range is awful
Batteries are getting better, but not very quickly.
Effective energy density
of 18650 lithium-ion cells
and gasoline versus time
EffectiveWatt-Hours
0
750
1500
2250
3000
Year
1990 1998 2005 2013 2020
w-hr/kg of gasoline w-hr/kg of lithium-ion cells projected
7

Batteries are getting cheaper fast …
price per kWh
$0
$250
$500
$750
$1,000
2000 2005 2010 2015 2020
price per kWh
8

Steady-state operation of a motor vehicle is
overcoming two-principal forces:
1.Aerodynamic drag
2.Rolling drag
9

Aerodynamic drag.
Aero Power = CdAV3
• A is frontal area
• Cd is drag coefficient
• CdA is effective frontal
area
• V is velocity
10

How CdA varies across vehicles
meters2
11

Rolling drag.
Rolling Power = CrrWV
• Crr is coefficient of rolling
resistance
• W is weight of vehicle and
occupants
• V is velocity
12

Total Power requirements, Brammo Empulse
0.0
7.5
15.0
22.5
30.0
0.0 42.5 85.0 127.5 170.0
TOTAL power Aerodynamic power, kW Rolling resistance, kW grade power, kW
13

Total Power requirements, Tesla M3
0.0
10.0
20.0
30.0
40.0
0.0 42.5 85.0 127.5 170.0
TOTAL power Aerodynamic power, kW Rolling resistance, kW grade power, kW
14

Note that the Brammo requires only one less kW
to cruise at 120 kph than the Tesla M3.
The Tesla M3 has much more room and capacity
for a large and heavy pack (about 53 kWh) than
the 9 kWh the Brammo carried. The expected
price of a car in the BMW 3-series class also gives
much more room to afford such a pack.
Conclusion: with current tech, a one-to-one
replacement for current broad-use motorcycles
with an EV version doesn’t make much sense. You
need 30-40 kWh to match ICE motorcycles.
15

What is good about e-drive?
Power to weight (kW/kg) is exceptional for e-motor systems.
0.0
2.5
5.0
7.5
10.0
Honda Civic Mazda Wankel Tesla Remy e-Motor F1 KERS motors Boeing 777
16

Efficiency of a PMAC motor can be very good almost everywhere on
its operating map.
17

Contrast that with an ICE (internal combustion engine) BSFC map
3.2 kWh/kg
18

Effects of the different efficiency maps
The ICE masks vehicle energy usage effects.
• Peak range at 70-80 kph cruise.
• Range doesn’t drop with V2.
• You can’t have acceleration and ideal fuel
economy.
In contrast, with EVs:
• Peak range at very low speed, 20-25 kph or
less
• Range follows V2.
• Very small trade-off between acceleration and
range.
19

Non-Steady State: Kinetic Energy
KE = 1/2mV2
• KE is kinetic energy
• m is mass
• V is velocity
20

Kinetic energy
Kinetic
energy, W-hr
21

Kinetic energy
Regeneration
Theoretically you can get the
kinetic energy back as you
slow down, but:
• Braking and wheel lock,
particularly on two-wheelers
and low-friction surfaces
• Cold or full battery cells
• Driveline design
22

Gravitational energy
PE = mgh
• PE is potential energy
• m is mass
• g is acceleration of gravity
• h is change of altitude, or
height.
23

Gravitational energy
24

Summary
• Electric drive is very efficient, and
the motors are light.
• Batteries are still heavy, but
cheaper.
• Mass is bad for range.
• Aerodynamic drag is far worse,
and quickly dominates as highway
speeds are approached.
• Regeneration can recover energy
lost in ICE vehicles.
25

So, what happens if you forget about
speed . . .
26

So, what happens if you forget about
speed . . .
• For pure urban use, imagine vehicles that have
restricted top speeds: 25 kph, 45 kph, or 80 kph
• The vehicles might weigh from 30 kg to a few
hundred kg.
• Much like a Tesla, or even a BMW i3, the
acceleration would be expected to be brisk.
• What does their energy use look like, and what
would their batteries have to be like?
27

L1e (50cc) class two-wheeler
28

Quickly removable and
rechargeable battery pack
29

Battery pack must weigh
less than 9kg – 4.5 kg better
30

Over 50km from 1 kWh on WMTC 1
test cycle
31

• 2kWh/100km
• Removable battery about
1kWh
• 35 kg vehicle weight
• 0-45 kph in less than 4 seconds
• Top speed limited by software
32

E-bicycle
33

• For standard pedelec, much
less than 1kWh/100km
• Removable batteries typically
from 300wh to 1kWh
• 25 to 40 kg vehicle weight
• Acceleration generally low,
limited by legal power
• Top speed limited by software
or power
E-bicycle
34

Lightweight neighborhood vehicle
35

Lightweight neighborhood vehicle
• These four vehicles vary tremendously.
• Planet Rider vehicle uses less power than e-bicycle –
better aero and lightweight.
• Schaeffler Bio-hybrid maybe a little more than e-
bicycle, but close.
• Renault and Murray roughly similar to each other
• About 3-4kWh/100km on low-speed urban cycle
• About 10kWh/100km at 100 kph – worse than
Tesla M3 because aero!
• About 7.5kWh/100km at 80 kph – reducing top
speed pays in range.
36

Conclusion
• Lightweight electric vehicles are the most promising
urban transport to come alone in years.
• One has already taken off: the e-bicycle.
• Others will follow.
• They can be characterized by lightweight, high-
efficiency, and the potential of relatively low cost.
• Ownership model changes will likely drive growth of
new vehicle types.
37

The Physics of Getting Around by Steven Anderson (Micromobility Conference 2017)

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