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This blog is expressly directed to readers who do not have strong training or backgrounds in science, with the intent of helping them grasp the underpinnings of this important issue. I'm going to present an ongoing series of posts that will develop various aspects of the science of global warming, its causes and possible methods for minimizing its advance and overcoming at least partially its detrimental effects.

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Tuesday, February 28, 2012

Efficiency and Decarbonization of Transportation

Summary.  Transportation policy plays an important role in meeting the overall objective of the IPCC to limit warming of the long-term global average temperature.  It is challenging to reduce CO2 emissions, or to decarbonize, the myriad sources involved in transporting people and goods, so that other solutions are being developed.  These include making vehicles powered by internal combustion engines more efficient, and migrating to the use of electric vehicles powered by electricity that has been generated using renewable technologies.

States, nations and regions are approaching the problem of reducing emissions from transportation in different ways.  Some use market mechanisms and others use taxation, to put a price on carbon or on vehicles that burn fossil fuels.  Others issue regulations with efficiency goals that reduce the emission of CO2.  The European Union is the only jurisdiction that has created a comprehensive transportation roadmap to reduce emissions and develop a trans-national integrated transportation system, all by 2050.
Lowering CO2 emissions by transport vehicles is an important aspect of overall global climate change policy.  Both government policy and private enterprise can play major roles in developing new technologies to accomplish this objective. 

Climate change policy adopted by the United Nations Framework Convention on Climate Change (UNFCCC), the organization of most nations of the world that sponsored the Kyoto Protocol and is seeking its extension, is based on a science-derived finding of the U. N.-sponsored Intergovernmental Panel on Climate Change (IPCC) in its 4th Assessment Report.  It seeks to limit the accumulated atmospheric concentration of CO2 (and other greenhouse gases expressed as CO2 equivalents) to 450 parts per million (ppm), which is estimated to constrain the long-term global average temperature increase above the temperature that prevailed before the start of the industrial revolution to 2ºC (3.6ºF). 
The pre-industrial atmospheric CO2 concentration was 280 ppm.  Presently the CO2 concentration is about 393 ppm, and the global average temperature increase to date is about 0.7 ºC (1.3ºF).  Both these numbers are growing as mankind uses more and more fossil fuels and emits more and more CO2 (and other greenhouse gases) into the atmosphere.  Since transportation accounts for about 25-30% of global CO2 emissions there is a strong motivation to make this sector more fuel efficient, when using fossil fuels, and to decarbonize  the movement of people and goods wherever possible (i. e., eliminate the release of atmospheric greenhouse gases).
Internal combustion engines are highly inefficient.  Personal passenger transport is powered by internal combustion engines (ICE) that are fueled mostly by gasoline, refined from crude oil.  Burning fossil fuels injects the greenhouse gas carbon dioxide into the atmosphere, in a one-way flow from the geological deposits containing the oil to the release of a car’s exhaust to the atmosphere.  Yet use of ICEs is highly inefficient in terms of converting the chemical energy contained in the oil into useful mechanical energy, namely, propelling a car along the road.  This is shown below.

Energy use and losses in driving an automobile powered by an internal combustion engine, for combined city/highway driving.  The useful energy is “Power to Wheels”, lower right.  Its percentage is slightly lower for all-city driving, and slightly higher for all-highway driving (see the website below).  Source: Energy Efficiency & Renewable Energy, U. S. Dept. of Energy;

It is remarkable that only 1/7 to 1/4 (depending on city to highway driving) of the energy contained in the fuel is used in moving the car.  It is even more surprising that “Engine Losses” include heat that is deliberately dissipated via the car’s radiator and exhaust, which constitutes about 56-64% of the energy in the fuel (depending on city to highway driving). 

The energy that propels the vehicle along the road must overcome the forces opposing forward motion, namely wind resistance, rolling resistance and braking (Power to Wheels, see graphic above).  These are susceptible of improvement.  Yet even if they were fully eliminated, which of course is not possible, there would still be the very high thermal losses (Engine Losses, see graphic above) that arise as long as the power source is an ICE. 

Reducing losses and increasing efficiency are considered in many sources (see References).  Among the most significant is weight reduction.  The inertia of an object is directly related to its weight.  It takes energy to change its inertia, for example when accelerating a car from a stop.  A lighter car will need less energy for acceleration than a heavier one.  A lighter car, needing less energy, can then incorporate a smaller engine, thereby decreasing weight even more.   

In addition, lighter materials can be used fabricate the frame and body of the car.  These include new steel alloys, alternative metals such as magnesium, aluminum and titanium, and nonmetallic composites such as carbon-fiber materials and strong plastics.  The Canadian Automobile Association states that vehicle weight can be reduced by as much as 40%, and that each weight reduction of 10% improves fuel economy by 5 to 7%.   

Smaller ICEs, in addition to being lighter, are also more efficient in converting the energy in the fuel to the forward motion of the car.  The Canadian Automobile Association, citing data from Natural Resources Canada’s 2008 Fuel Consumption Guide, shows that there is a much larger percentage increase in fuel economy in a compact car than in an SUV or a pick-up truck by making the engine smaller.  This factor is in addition to the considerable fuel economy achieved just by driving a compact car as opposed to an SUV or a pick-up truck.   

Streamlining the body lines of a car reduces its aerodynamic drag resistance to forward motion, a second important factor in optimizing efficiency of automobile transport.  In addition to the improvement in body shape that is obvious to the observer, shielding wheel wells and the underbody of the car further would improve its aerodynamic flow properties.   

Rolling resistance to forward motion refers to the deformation of tires as they roll along the road.  The tire, a semi-rigid object, is circular when it bears no weight, but is flattened out where it contacts the road when the car’s weight rests on it.  Energy is dissipated in the tire when this happens, and of course this goes on continuously as the car rolls along the roadway.  New high-efficiency tires, which optimize tread and sidewall design as well as incorporate new materials that dissipate less energy on deformation decrease rolling resistance.  Thus the losses ascribed in the graphic above to rolling resistance, amounting to 5-6% of the input energy of the fuel, can be reduced in some cases by as much as 20%.   

Capturing waste heat.  As seen in the graphic above, a major portion of the energy provided by burning fuel in an ICE, perhaps 60% or more, is lost as heat.  Research and development of technologies in ICE-driven vehicles that capture some of this heat are at an early stage, even though this aspect of vehicle inefficiency potentially offers the greatest gains in optimizing fuel economy.  

Thermoelectric conversion of heat directly to electricity relies on use of semiconductors that generate electricity when placed between two objects whose temperatures differ.  Schock and coworkers reported on research sponsored by the Energy Efficiency Renewable Energy program of the U. S. Dept. of Energy in a workshop in January 2011.  They fabricated and tested two different thermoelectric semiconductor materials, generating 70W or more.  They estimate that the payback period for the extra cost of a 1kW system is about 1 year, and for a 5kW system about 3 years.  Other thermoelectric systems, using various high-temperature  semiconductors, are being tested by BMW, Ford and Chevrolet, according to a report from May 2011.

Thermomechanical energy.  In 2005 Joaquin G. Ruiz, an undergraduate at Massachusetts Institute of Technology, proposed a way of capturing the heat generated in the catalytic converter in the exhaust train of an ICE-powered car to obtain more mechanical energy.  He estimated that overall thermal efficiency of fuel utilization (the numbers in the graphic above) could be improved by 7%, to be added to his estimate of 30% efficiency in current ICE fuel use.  In other words, his device would have a relative improvement in efficiency of more than 20%.  Honda is experimenting with a similar system that is reported to improve the thermal efficiency by 3.8% in a hybrid electric vehicle.

Cars powered by electricity, either partially or entirely, are expected to be far more efficient than full ICE-driven cars.  Electric cars were considered in an earlier post on this blog.  It discussed the all-electric Nissan LEAF, the two models of the all-electric Tesla Motors cars, the all-electric Mitsubishi iMiEV minicar, and the ICE-assisted electric Chevy Volt. 

Manufacturers of these electric cars emphasize their environmental advantage in having zero or minimal tailpipe emissions of CO2.  Electric motors such as used in electric cars are highly efficient, capable of converting more than 90% of the electrical energy into the mechanical energy of motion.  As pointed out in the earlier post, however, these cars actually have low or zero emissions only to the extent that the electricity used to charge the batteries itself is obtained from renewable or low-CO2 emitting generation sources.  Coal-fired electric generation is the least efficient, whereas modern natural gas-fired plants using combined cycle generation attain quite high efficiencies and much lower emissions of CO2.  By 2035, the U. S. National Academy of Engineering estimates that even for all-electric vehicles, the greenhouse gas emissions will remain at 30-50% as much as currently emitted by ICE-powered cars because electricity will still be  generated to a considerable extent from fossil fuels.  Optimally, use of renewable sources such as wind power, solar power, hydroelectric power and geothermal power will provide truly zero emission generation of electricity.

BMW electric drive-train cars, BMWi3 and BMWi8 (see this video), strive to achieve sustainability to optimize energy efficiency by radically new design.  The heavier weight of the large-capacity electric batteries is offset by replacing metal bodies with carbon fiber-reinforced plastic which is lighter than any metal used in car construction, yet is stronger in crash tests.  The video states that this is the first use of carbon fiber in production cars.
Hybrid-electric cars are powered in tandem by electric motors and ICEs; the cars are engineered so that the two energy sources share the burden of propelling the car.  The Toyota Prius and Honda’s Civic Hybrid and Insight are examples of hybrid-electric cars currently available.

California’s plan to decarbonize passenger vehicles.  In the U. S., California has the most advanced plan, affecting the most people, to reduce greenhouse gas emissions of all the states.  In an unofficial report detailing a path to achieving the state’s goal of reducing emissions by 80% by 2050, the California Council on Science and Technology (CCST) emphasizes the major role that will need to be played by decarbonizing the energy industry (see this post).  The report expects that personal transport will be achieved by electric vehicles, and that the electricity that powers these vehicles (and provides energy generally for the economy) will be generated largely by decarbonized sources.  Fossil fuels may continue providing the energy for electricity generation to the extent that the currently unproven technology of carbon capture and (geological) storage will be developed to industrial scale.  Otherwise renewable energy sources must be relied upon, in the view of the report.

U. S. government extends fuel efficiency standards.  In 2011 the administration of President Obama extended the Corporate Average Fuel Economy (CAFE) standard to 55.4 mpg for cars by 2025.  The previous CAFÉ standard issued by the Obama administration in 2009 raised the value to 35.5 mpg by 2016.  In addition the 2025 mandate covers new fuel efficiency standards on medium- and heavy-duty trucks.  It is expected to prevent emission of large amounts of CO2, save fuel costs to drivers, and reduce the need to import oil from foreign producers.  These savings, in the case of trucks, are expected to offset the extra cost of compliance with the standard, reaching payback within two years.

China and other developing countries will be responsible for a major increase in the number of passenger vehicles in use in coming decades, according to the International Energy Agency (IEA).  Its World Energy Outlook (WEO) 2010 (Executive Summary) analyzes present and projected world-wide production and consumption of energy over the period 2010-2035.  The New Policies Scenario of the WEO predicts changes in energy demand resulting from measures to be taken in response to the commitments made at the UNFCCC Copenhagen meeting of nations in 2009.  WEO judges that under this Scenario CO2 emissions continue to rise, by 21% over the level of 2008.

The growth in passenger vehicles in regions of the world, actual and projected under the New Policies Scenario, is shown below.

Actual growth in number of passenger vehicles (1980-2008) and projected growth (2020, 2035).  Other non-OECD (developing) countries includes India, for example.  Reproduced from World Energy Outlook 2010 © OECD/IEA.  The OECD has essentially similar membership as the IEA, plus 5 additional nations;

The growth for China reflects its pronounced economic growth over this period, resulting in a large shift of its population into a middle class that demands personal cars, among other amenities.  It is seen from the chart that other developing countries are likewise projected to experience large increases in the number of passenger cars.  

Current technology emphasizes powering passenger cars with fossil fuel-driven ICEs, leading to a large increase in greenhouse gas emissions worldwide from this source.  But the Edmunds Auto Observer reports that, as of 2009, China’s fleet-average fuel efficiency, including SUVs and minivans, was already 36.8 miles per gallon (mpg), and that the country has mandated an increase to 42.2-mpg by 2015.  A tax on vehicles based on their engine size provides a further economic incentive impelling Chinese purchasers toward smaller vehicles.   The current tax rates are shown in red bars in the graphic below.

Vehicle excise tax in China based on engine size.  Source: Huiming Gong, The Energy Foundation;

It is seen that there is a strong tax incentive to purchase smaller cars having smaller engines, and that this incentive became more pronounced for the largest cars after 2008.  In addition to this vehicle excise tax, there is a fuel tax as well.  Conversely, according to The China New Energy Vehicles Program , pilot programs are deploying electric vehicles in as many as 25 Chinese cities, beginning with government vehicle fleets. Purchases of electric vehicles by the public will be subsidized and vehicle charging stations will be deployed.  RMB 100 billion (USD 15.9 billion) will be devoted to new energy vehicles in the next 10 years.  While some reductions in CO2 emissions occur as a result of China’s shift toward use of electric vehicles, it is not as great as it could be in view of the fact that a major portion of China’s electricity is generated from coal-fired power plants.  These emit about twice as much CO2 per kWh as do modern natural gas-fired generating plants. 

Europe’s integrated economy-wide transportation plan.  The European Commission (EC) has developed a plan, currently being implemented by the nations of the European Union (EU), to limit greenhouse gas emissions from all sources by 20% below the levels of 1990 by 2020, and by at least 80% by 2050 (see this earlier post).  As part of this program the EC has set forth its transportation program in a White Paper on Transport (see References).  Its objective is to achieve a single EU-wide transport area that closely integrates all modes of transportation and unifies modes of transportation across national boundaries.  The plan intends to reduce the EU’s dependence on oil for transport by 60% by 2050, while enhancing efficiency and the mobility of goods and people, and promoting economic development.  The White Paper recognizes that action must begin without delay, since an extended period of planning, building and implementing the system will be needed.  The following are among the plan’s features. 

There will be incentives in urban areas to limit personal car travel, and migrate to mass transit and even bicycling and walking.  Generally personal vehicles, clearly involved mainly in short trips, will be powered other than by fossil fuel-driven engines.  This will contribute to lowering the dependence on oil, and reducing emissions of greenhouse gases and other polluting combustion products. 

Intermediate-range movements will emphasize development of multimodal means for transport, with efficient terminals facilitating the interchange of passengers and goods between modes such as vehicle use and rail use.  The White Paper observes that use of more efficient vehicles and phasing in of renewable fuels by themselves will likely not be sufficient to attain the intended objectives.  It proposes that common transportation modalities  including trains (including high-speed rail), buses and airplanes be developed to supplant personal vehicle use, and that more than 50% of freight be moved by rail and waterborne shipping by 2050 rather than by road as is currently done. 

For long distance travel, beyond the boundaries of the EU, the White Paper proposes enhancing the efficiency of aircraft and optimizing air traffic flow by developing information technology-based traffic efficiencies.  These steps should increase fuel efficiency and optimize the flow of passengers and cargo.  It is likely that the volume of air transport of the EU will double by 2050.  


Transport, which includes passenger vehicles, heavy duty vehicles, rail, air and shipping, accounts for about 27% of all the energy consumed worldwide.  Virtually all the energy used in transport is derived from burning fossil fuels, releasing the product, CO2, a greenhouse gas, into the atmosphere.  Vehicles powered by ICEs, and the other transport modes mentioned, are distributed sources for CO2 emissions.  There is no obvious way to capture CO2 from them in a way that would prevent it from entering the atmosphere. 

The number of transport vehicles is expected to rise in coming decades, due both to rising populations, especially in the developing world, and to advances in economic wellbeing as the economies of developing countries expand.  In the absence of policies that would lower the extent of CO2 emissions from transport, this sector will contribute significantly to ever increasing annual rates of greenhouse gas emissions in coming decades.  

CO2, once emitted into the atmosphere, persists for at least a century and probably longer.  Thus each year’s incremental addition accumulates, increasing the atmospheric CO2 concentration.  One can think of adding CO2 to a bathtub through its faucet; the bathtub’s drain, however, is closed so no CO2 leaves.  Even if the faucet were turned off (i. e., reducing the annual rate of CO2 emissions to zero), the bathtub would still have its full accumulated level of CO2 in it.  This is why the IPCC has warned of the need to limit CO2 emissions.  Lowering greenhouse gas emissions will help keep the level in the CO2 bathtub as low as possible, but can not meaningfully reduce its level. 

Transport vehicles powered by ICEs (or diesel) can reduce, but not eliminate, CO2 emissions by efficiency steps such as outlined here, significantly increasing fuel efficiency.  Distribution of more efficient vehicles among buyers is facilitated by measures such as China’s excise tax which becomes more severe as engine size increases; by a “fee-bate” regime whereby the purchase of small, efficient cars is subsidized by a tax imposed on the purchase of larger, inefficient vehicles; by fuel taxes; or by pricing CO2 emissions using a cap-and-trade market system.  Alternative measures are exemplified by regulations that increase the required fleet-average fuel efficiency, such as imposed administratively in the U. S. Nevertheless, as long as ICE-powered vehicles remain in service, CO2 emissions can be reduced to near zero only by substituting renewable biofuels for fossil fuels. 

The CCST elaborated an ambitious program for reducing emissions by moving toward zero-emissions electricity to power transport vehicles and the economy more generally.  The CCST plan envisions using carbon captureand sequestration (CCS) to the extent that fixed generating facilities retain the use of fossil fuels as the primary energy source.  Yet, at the present time, CCS remains an experimental technology under development; it is not clear yet that it will become feasible at the industrial scale needed to accommodate fossil fuel-derived electric power.  Additionally, the CCST report stresses the development of renewable energy sources including wind, solar and biomass. 

China’s auto excise taxes induce its car-buying public to purchase smaller, more fuel-efficient car models.  The U. S., on the other hand, has been unable for more than a decade to enact a national energy policy which would have included transportation goals for fuel efficiency.  Instead, the present Obama administration has acted twice to extend previous regulations governing average fuel efficiency for passenger cars, first in 2009, then again in 2011.  The latter standard, to be effective by 2025, is quite ambitious.  Other than that, however, there is no unified national energy policy in effect in the U. S.  The state of California has partially filled that void, by enacting overall emission reduction goals that mirror those of the European Union.  The unofficial CCST plan for complying with its state’s mandate places strong emphasis on electrifying the energy economy with zero emissions, including, for transport, a virtually complete transition to use of electric vehicles or renewable fuels. 

Among the nations of the world, it is only the trans-national European Union that is addressing its energy economy overall, and its transportation policy in particular, in a cohesive, comprehensive fashion.  The EU’s “Roadmap to a Single European Transport Area” (see References) details the many interconnected aspects of transport policy, formulated with the objective of contributing significantly to the EU’s overall Roadmap 2050 for reducing greenhouse gas emissions by 80% by that year.  The EU fashioned its Roadmap, extending beyond the expiration of the Kyoto Protocol in 2012, independently of the fruitless negotiations under the UNFCCC seeking to formulate global energy policies beyond 2012.  

The need for global policies to reduce greenhouse gas emissions is critical.  Given the significant role that transport plays in contributing to these emissions, reformulating transportation modalities to reach low- or zero-emissions is an important facet of overall energy policy.  We should strive to achieve such goals as quickly as possible.  From the many examples cited above, it is clear that there is a role to be played both by government policy, including monetary support for new technologies, and by private industry driven by motives to generate profits.   


“Primer on Automobile Fuel Efficiency and Emissions”, Canadian Automobile Association, June 2009;  

“Reinventing Fire: Bold Business Solutions for the New Energy Era”, Amory B. Lovins and the Rocky Mountain Institute, Chelsea Green Publishing, White River Junction, VT, 2011. 

“Real Prospects for Energy Efficiency in the United States”, U. S. National Academy of Engineering, a component of the National Academies, 2010;  A free summary may be obtained here . 

California’s Energy Future: The View to 2050”, Summary Report, California Council on Science and Technology, May 2011; 

“Roadmap to a Single European Transport Area — Towards a Competitive and Resource-Efficient Transport System” (COM (2011) 144 final, European Commission White Paper, 28 March 2011;  

“The China New Energy Vehicles Program: Challenges and Opportunities”, World Bank and PRTM Management Consultants, Inc., April 2011;

© 2012 Henry Auer


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