Vehicle Efficiency Pathways
How modern passenger cars are removing themselves from the environmental debate
John Bucknell
GM Powertrain
1
Abstract
Modern passenger cars must respond to market demand and regulation forces, delivering superior air
quality, utmost safety and ever-higher energy efficiency. This lecture will discuss efficiencies on
both the supply and demand pathways for improving energy efficiency in the context of emissions
and safety regulations. Well-to-wheel and pump-to-wheel efficiencies will also be covered in brief
to highlight the efficiency of Electric Vehicles
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Topics
 Transportation Efficiency
 State of the industry
 Supply-side Efficiency
 Powertrain Efficiency
 Driveline Efficiency
 Load-leveling
 Demand-side efficiency
 Aero, rolling-resistance, inertia
 Electric Vehicles & Fuel-Cell Vehicles
 Pump-to-wheels, well-to-wheels
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Market Economics
 Cost of ownership
 Market demand has illustrated that customers will purchase what they can afford. Technologies that
increase cost of ownership have great difficulty penetrating the market.
 Energy Costs
 Dual impact of increasing environmentalism and increasing energy costs have raised the visibility of vehicle
efficiency.
 Low energy cost of petroleum products has been the primary factor that has driven the market into a near
monoculture for it’s energy needs.
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Transportation Efficiency
 State of the industry
 Economies of scale drive allow manufacturers to compete on cost. Any technology that cannot make a
component at a minimum rate of one per minute requires additional sets of tooling, driving up investment
and increasing the number of sales to break even.
 Profit margins in the automotive industry are exceptionally small, as you’d expect with strong competition
for a very large revenue stream.
 State of the world has changed rapidly – developing new technologies that are sufficiently robust to be
used by every consumer can take a decade or more. The industry is responding to the need for greater
efficiency, vehicles on the market today are just the beginning.
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Regulation
 Tailpipe Emissions
 Air quality has been driven by the EPA and the California Air Resources Board. Details on how emittants are
formed and regulated follow.
 Passenger safety
 Customer awareness of impact performance on standardized tests has driven the industry to achieve a
minimum “Four star” rating in any test. The degree that of likelihood of injury to achieve the best rating has
decreased significantly over the last ten years. High strain-energy density materials, and large masses of
them have driven up body structure mass by about double in the same time frame.
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Emission and Fuel Economy Test Cycles
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Engine Fuel Balance
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Emissions Standards 1960 to 2008
History of Emission Control Standards
99.99%
Reduction
12
11
Typical 1960 Vehicle (pre-control)
10
9
HC St andard
8
7
6
5
4
3
2
0.5
1983 Federal Tier 0
1971 California Std.
1977 Federal Std.
1
0
0.4
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
HCStandard
NOx St andard
0.3
Federal Tier 1
0.2
0.1
NLEV
LEV2
ULEV2
0
0
SULEV2
0.1
0.2
0.3
0.4
0.5
NOx Standard
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Exhaust Aftertreatment
Catalysts have the capability of modifying the reaction rates of chemical processes (typically increasing reaction rates) without being
consumed while doing so.
The following chemical processes are of interest in automotive exhaust catalytic aftertreatment
•
•
•
•
HC + O2 CO2 + H2O
CO + O2 CO2
NO  N2 + O2
These reactions proceed toward equilibrium at very slow rates at prevailing exhaust temperatures - catalysts increase their
reaction rates to a degree that the exhaust aftertreatment becomes practical.
Conversion efficiency: (inlet concentration - outlet concentration)/inlet concentration
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Essential Components of a Catalytic Converter
Substrate
Mat
Can
Washcoat
Catalysts
Substrate: a ceramic honeycomb-like structure with thousands of parallel channels for applications of washcoat and catalysts
Mat: Provides thermal insulation and protects against mechanical shock and chassis vibration
Can: A metal package encasing the catalyzed substrate and mat
Washcoat: a coating that increases the surface area of the substrate for catalysis
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Source: Corning (2001)
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Catalysts for Exhaust Aftertreatment
The active catalytic material is typically a blend of platinum, palladium, rhodium and nickel.
Small amounts of these materials are distributed on a alumina (Al2O3) washcoat, which is specially processed to have very high
microscopic surface area. The high washcoat surface area helps to keep the catalytic material spread out to reduce the tendency to
agglomerate and thus loose surface area.
Cerium oxide is often added to this mix to mechanically stabilize the alumina microstructure against thermal degradation.
Typically there are 0.5-2 grams of catalytic material per liter of overall catalyst volume, and the overall catalyst volume is about 50 ~
80% of the engine displacement, depending on the application.
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Temperature Effects on Catalyst Capabilities
Catalyst efficiency at catalyst temperatures below 200oC is
extremely low.
Catalyst efficiency rapidly increases as its temperature rises
above 200oC and reaches its temperature plateau at about
400oC.
Light-off temperature: conversion efficiency reaches 50%
Current exhaust system design practice insures catalyst light-off
within ~ 20 seconds without special aids. Catalyst heating
devices in lowest emissions vehicles can achieve light-off in
under 10 seconds.
Source: Heywood (1988)
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Catalyst Efficiency with Air/Fuel Ratio
Steady improvements in fuelling
control, engine-out emissions
and catalyst technology has
made it possible to achieve
100% conversion rates of HC
and NOx after catalyst light-off.
Source: Heywood (1988)
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Emissions Summary
Fuel-burning engines create pollutants that are regulated which are ever-more stringent. Emissions-control technology
has evolved to the point where three-way catalysts are 100%
efficient in converting HC, NOx and CO – only if the feedgas
operates very close to stoichimetric air-fuel ratio.
Any lean-burning combustion process (Diesel or stratified
charge) which improves fuel consumption also prevents
catalytic NOx reduction by maintaining oxygen in the exhaust
stream. Several technologies are emerging which consume
fuel or reductant to purge Lean NOx Traps, at the cost of fuel
consumption or added complexity.
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Supply-Side Efficiency
Energy conversation pathway
 Powertrain Efficiency (Stratified Charge/HCCI, Downsizing/Boosting)
 Driveline Efficiency (Multi-speed Transmissions, CVTs)
 Load-leveling (stop-start, mild hybrid, series and parallel hybrids)
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Energy Distribution
in Passenger Car Engines
Source: SAE 2000-01-2902 (Ricardo)
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Modern Naturally Aspirated
Brake Thermal Efficiency Map
1
0.35
0.9
0.3
0.7
0.25
0.6
0.5
0.2
0.4
0.15
0.3
0.2
0.1
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.05
Fraction Maximum Engine Speed
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Fraction Thermal Efficiency
Fraction Maximum Torque
0.8
Compression vs Spark Ignition
Compression ignition achieves significantly higher compression ratios
than spark ignition – raising thermal efficiency
Spark ignition engines control load by throttling, introducing parasitic
losses at less than maximum load which reduces thermal efficiency
Smoke limits reduce power density of diesel engines to only about
80% of energy density of spark ignited of similar displacement. High
operating pressures require heavy construction which further lowers
power/weight ratio
Source: Heisler (1995)
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Powertrain Efficiency Pathways
- Engine Prior three slides show that the maximum fraction of fuel energy that reaches the brake is 30-40% of the fuel input energy, which is the
most that thermodynamics allows.
Spark ignition engines pay a loss to reduce load by throttling – which is effectively operating a vacuum pump. Several technologies
seek to reduce or eliminate pumping work:
•Exhaust Gas Recirculation (EGR) – load reduction by diluting incoming combustion air
•Variable valve timing (including cam phasers and variable lift/duration systems) – load reduction by reducing trapping efficiency and
adding residual (internal EGR)
•Stratified Charge with unthrottled operation – load control via fuel mass running lean
•Homogeneous Charge Compression Ignition (HCCI) – load control via fuel mass and residual preventing lean operation
•Downsizing/Boosting – Reduction in displacement of engine so use of lowest efficiencies is mostly avoided and then boosting to
enhance available load
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Powertrain Efficiency Pathways
- Driveline Knowing that an internal combustion engine is most efficient in a limited regime, the driveline can be optimized to enable engine
operation the least amount of time away from that regime.
•Multi-speed Transmissions – 6, 7, 8 speeds with ratio ranges from 5.0-6.0 give powertrain controller best option of matching engine
to current power demand
•Continuously Variable Transmissions – Same as multi-speed transmissions, but typically have high parasitic losses
•Load Leveling – Through use of onboard energy storage (electric or other), energy conversion can happen at most efficient point in
map. Hybrids achieve this through several different strategies – parallel, series or dual-mode are most-often discussed. Microhybridization also appearing due to low cost of implementation.
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Load-Leveling
Engine Stop Start (ESS)
 Eliminates fuel consumed during deceleration and
idle
Fuel
On
Fuel
Off
Source: SAE 2001-01-0326 (GM)
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Load-Leveling
Mild Hybrid
 Regenerative Braking, Load-Leveling and Idle Stop
Source: SAE 2006-01-1502 (GM)
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Load-Leveling
Strong Hybrid
 Electric-only operation, Regenerative
Braking, Load-Leveling and Idle Stop
 Parallel, Series and Two-Mode e-CVTs
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Demand-Side Efficiency
 Not a true ‘efficiency’, however losses that are not minimized could be considered ‘in-efficient’
 Major Components
Inertia Loads (Kinetic Energy)
Aerodynamics
Rolling Resistance
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Demand-Side Efficiency
 Inertia Loads
 Vehicle mass requires proportional power to accelerate. Vehicle duty cycles with greater time spent accelerating will be
more sensitive to vehicle mass.
 Aerodynamics
 Pressure drag: The loss due to the difference in pressure on the front face versus the rear face of the vehicle. The dynamic
pressure (also called stagnation pressure) on the leading face is a measure of the kinetic energy of the displaced air.
 Friction drag: Losses due to viscosity effects are also substantial. Boundary layer theory says that particles immediately
next to a vehicle must be moving at vehicle speed as compared to at the free stream velocity. The shear force created by
the relative velocity of the fluid is proportional to vehicle speed and ‘wetted’ surface area moving through the fluid.
 The two speed-dependent components cause aerodynamic drag to increase primarily with the square of vehicle speed
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Demand-Side Efficiency
 Rolling-resistance
 Driveline: Seals, bearings, gears, CV and Cardon (universal) joints
 Any component using a viscous fluid to reduce contact stress for increased durability also suffers the
losses of viscous shear forces regardless of the load.
 Brakes
 Friction brakes work by rubbing two components together. Unfortunately due to the balancing of pad
retraction and response time, disc brakes will drag the pads against the rotors – a little or a lot
depending on the design. Drum brakes by their nature have very little hydraulic volume and thus can
retract far enough to not drag.
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Demand-Side Efficiency
 Rolling-resistance
 Tires
 Part of the suspension isolating the vehicle from surface irregularities. Tire is both a spring and a
damper, with greater spring rate and lesser damping force with lesser sidewall height. Spring rate is
proportional to inflation pressure. Greater isolation drives greater sidewall and lower pressure.
 Inflation pressure is same as tire contact pressure. Contact area is proportional to mass supported by
the tire. The greater the contact area, the more rubber has to deflect as it tracks across the surface.
Increased tire diameter decreases the degree of deflection. Rubber is not perfectly elastic, so some
energy is lost.
 The force to roll a tire is therefore proportional to the normal force and the volume of rubber deflected
per second which is proportional to rotational velocity.
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Measuring Vehicle Efficiency
 EPA and real world fuel economy (Efficiency) is impacted by the vehicle’s drag force. Drag is determined by
taking a vehicle to 70 mph and then shifting into neutral and measuring speed versus time and thus
deceleration rate. Knowing the mass of the vehicle, a drag force versus vehicle speed can be derived. This
drag force data is fitted to a 2nd-order polynomial whose coefficients are published by the EPA – called the
ABC coefficients.
 The chassis dyno where emissions and fuel economy data is taken has
 Rollers instead of pavement, with vehicle strapped down
 Only drive-wheels turning
 No aerodynamic loading
 The A,B,C coefficients determine the load which the dyno program must match over the course of the test
cycle
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Vehicle Drag Force Example
A = 28.73 lb
B = 0.7338 lb/mph
C = 0.01084 lb/mph^2
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Evidence of Vehicle Efficiency
 EPA data shows that there is no magic. Following slides show every vehicle for sale in 2008 Model Year in
the US.
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Inertial Loads
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Aerodynamics
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Rolling Resistance
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Downsizing
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Typical Mid-Size Vehicle Energy Distribution
Idle
0.90
Aero
0.21
Accessories
0.17
Rolling
0.34
8.29 units
Engine
1.25
D/L
1.00
Kinetic
Engine Losses
5.97
Driveline Losses
0.25
Braking
0.45
Urban Federal Test Procedure (FTP)
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FTP City – Mid-Size Sedan Simulation
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Simulation – Level of Hybridization
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Simulation – Hybrids with Downsizing
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Simulation – Advanced Powertrain Tech
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Electric Vehicles & Fuel-Cell Vehicles
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Well-to-Wheels and Tank-to-Wheels
USA Energy Consumption (%)
1960
1970
1980
1990
2000
Oil
44.1
43.5
43.6
39.8
38.5
Natural Gas
27.5
32.1
26.0
22.9
23.7
Coal
21.8
18.1
19.6
22.8
22.7
Nuclear Energy
.002
0.35
3.5
7.3
8.1
Hydro-,Geothermal,Solar, Wind,etc
6.6
6.0
7.2
7.4
6.9
USA Electricity Generation (%)
1990
2000
Coal
52.6
51.8
Petroleum
4.1
2.9
Natural Gas
12.5
15.7
Nuclear
19.1
19.9
Hydroelectric
9.7
7.2
Geothermal
0.5
0.4
Wood
1.0
1.0
Waste
0.4
0.6
Other Waste
0.076
.09
Wind
0.099
0.129
Solar
0.020
0.021
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 Any true discussion of energy diversity and it’s impact on GHG must discuss
the source of energy (ie the Well)
 Electric Vehicles will receive the bulk of their energy from coal-fired
generation for foreseeable future
 Coal-fired electrical generation was 35% thermally efficient in 2005 (EPA)
 Line-losses and battery/e-motor efficiency aren’t 0%
 Therefore from a GHG perspective -TNSTAAFL
43
Transportation Effects on GHG - Future
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By 2020,
1.1 billion vehicles (an increase of 300 million) will circle the earth 125 times.
Energy diversity is required in the future.
Reducing dependence on petroleum is imperative.
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“At GM, we believe tomorrow’s automobiles must be
flexible enough to accommodate many different energy
sources.”
“ And a key part of that flexibility
will be enabled by the development
of electrically driven cars.”
- Rick Wagoner
Chairman and CEO
General Motors Corporation
LA Auto Show 11/29/2006
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Hybrid, Electric & Fuel Cell Vehicles
Vehicle Type
Electric
Power
Onboard
Electric
Storage
Grid
Connected
Recharging?
Electric
- only
Driving
Mild HEV
low
low
no
no
Full HEV
med
low
no
limited
PHEV
med
med
yes
limited
E-REV
high
high
yes
Full
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Electrification
Introduction & Background – More definitions
48
Transportation Challenge –
Energy Diversity - Source Blending via Electrification
Oil
Petroleum Fuels
(Conventional)
Oil
Energy Carrier
Carrier
Energy
(NonConventional)
1st & 2nd Generation Biofuels
Liquid
Fuels
Syngas
CO, H2
Natural Gas
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Plug-In
Hybrid
CCGT
Shift
Reaction
(Solar, Wind, Hydro)
Nuclear
Fischer
Tropsch
Electricity
Renewables
Conventional ICE:
Gasoline/Diesel
Mild and Full
Hybrids
Biomass
Coal
Propulsion System
Propulsion
System
Electrolysis
Extended Range
EV
Battery Electric Vehicle
Thermochemical
Water-Splitting
Hydrogen
CO2
Sequestration
More Electrification
Conversion
Conversion
Battery Energy Storage
Energy Resource
Energy Resource
FC Electric Vehicle
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Transportation Challenge –
Energy Diversity - Source Blending via Electrification
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Advanced Technology and Sustainability…
GM Technology Strategy
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Chevrolet VOLT Concepts Illustrate
E-REV and FC Commonality
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Chevrolet VOLT E-REV Concept
•Global Compact Vehicle Based
•Electric Drive Motor
•120 kW peak power
•320 Nm peak torque
•Li Ion Battery Pack
•136 kW peak power
•16 kWh energy content
•Home plug in charging
•Generator
•53 kW peak power
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E-Flex Fuel Cell Variant
•Global Compact Vehicle Based
•Electric Drive Motor
•120 kW peak power
•320 Nm peak torque
•Fuel Cell Propulsion System
•Smaller Li Ion battery pack
•Hydrogen storage
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www.gm.com/corporate/careers/
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