On the Road to Cleaner Air

Since the mid 1960s the automotive industry has focused its attention on reducing health-related emissions from vehicles. For gasoline vehicles, this has meant reductions in carbon monoxide (CO) and the gases that form photochemical smog: hydrocarbon (HC) and oxides of nitrogen (NOx). Since HC emissions can come from both the vehicle exhaust and the evaporation of fuel, both sources have been addressed.

Control of Exhaust Emissions
Since the pre-control days of the early 60s, exhaust HC and NOx emissions have been reduced by more than 99 percent (Figure 1). This remarkable achievement has required advancements in virtually all areas of automotive design, including basic engine design, air and fuel delivery systems and control of complex processes. But, the absolute star in this remarkable improvement has been the catalytic converter. Without catalytic exhaust after-treatment, industry could not have made these dramatic reductions. By chemically reacting the products of incomplete combustion, the catalytic converter allows today's vehicles to emit mostly nitrogen, water vapor and carbon dioxide (CO2).

Figure 1. California has led the country in emission standards to reduce levels of oxides of nitrogen (NOx) and hydrocarbons (HC) in all current emission standards by more than 90 percent.

Control of Evaporative Emissions
With the gains that have been made in reducing pollution from combustion, greater attention has been paid recently to evaporative HC emissions of gasoline. Both California and the U.S. Environmental Protection Agency (EPA) have reduced allowable evaporative emissions. In California, the lowest standard requires zero fuel evaporative emissions. The auto industry has responded with new designs and materials to eliminate even the smallest evaporation of fuel to the atmosphere.

By chemically reacting the products of incomplete combustion, the catalytic converter allows today's vehicles to emit mostly nitrogen, water vapor and carbon dioxide (CO2).

In California, the lowest exhaust standard (SULEV, or super ultra low emission vehicle) has been combined with the zero fuel evaporative standard and further strengthened by an emission control equipment warranty of 15 years/150,000 miles. These super clean vehicles are designated Partial Zero Emissions Vehicles (PZEV). At least 10 vehicles currently meet PZEV standards and more are on the way.

Decades of Success in Reducing Smog-Causing Emissions
Since 1970, HC (both exhaust and evaporative) and NOx has been reduced from 4,100 pounds per vehicle to less than 100 pounds for the average new vehicle and under 20 pounds per vehicle for the cleanest new vehicles (see Table). This has led the California Air Resources Board (CARB) to predict that urban smog will be eliminated over the next 10 years to 15 years, as the in-use fleet of cars and light-duty trucks (including most SUVs) is replaced by these newer vehicles.

Table. Average lifetime per-vehicle emissions of the total U.S. passenger car fleet have declined tremendously since 1970. (California, not United States, totals shown here.) (from Cackette)

Replacement of old-technology vehicles with the new also will continue to reduce the U.S. fleet emissions. Improved durability -- from 50,000 miles in 1975 to 120,000 miles in 2003 and later -- will help to keep the fleet cleaner nationwide.

Greenhouse Gases -- a New Challenge
As new car criteria emissions approach zero, what is the next frontier of emission reduction? In the passenger car and light-truck sectors, the probable answer is the area of greenhouse gases. There is massive scientific evidence linking CO2 emissions to the greenhouse effect and global climate change. Debate still rages over the magnitude and impact of greenhouse gases on climate change and the impact of human-made CO2. However, nearly all agree that limiting and ultimately reducing combustion-related CO2 is desirable and that it eventually may be directly regulated in the United States, as it is beginning to be in Europe.

Figure 2. Hybrid drivetrains fall into three main categories based on power architecture. Series hybrids run on electric motive power only. Parallel hybrids use both the engine and electric motor. Series/parallel (also called split parallel) can operate in electric-only mode, gasoline-only mode or a combination of the two.

Unfortunately, the catalytic converter's "silver bullet" effect does not apply to CO2, which is the end product of the catalytic reaction. How then, will automakers reduce CO2? The answer is to simply burn less fuel. The rate of production of CO2 is tied directly to the amount of fuel consumed. There are two main pathways for manufacturers to reduce fuel consumption: to produce smaller, lighter vehicles or to make them operate more efficiently. The industry has reduced the weight of vehicles with the use of plastics and lighter metals, but consumer demand remains small for the smallest and lightest vehicles compared with heavier mid-to-large sedans and light trucks. Manufacturers can have a more effective impact on greenhouse emissions by improving drivetrain efficiency throughout the model fleet.

The conventional internal combustion gasoline engine is at best 35 percent efficient in select operating points. Over the range of typical city driving cycles, the overall efficiency is less than 20 percent for nearly all vehicles. Diesel engines run more efficiently than gasoline engines, but diesel emissions of criteria pollutants are problematic, especially in areas of particulate matter (PM) and NOx. Long-term replacement of internal combustion with fuel-cell power promises improved efficiency, but the required infrastructure for mass production, transportation and storage of the needed hydrogen fuel is a daunting challenge.

Improved Efficiency with Hybrid Drivetrains
Hybridization is another ready pathway to improved efficiency. Unlike fuel cells, it is available now; and unlike diesel, it does not require new technology to match PM and NOx emissions of most gasoline engines.

Hybrid vehicles are more efficient than conventional vehicles, with city efficiency in the range of 25 percent to 35 percent. The most advanced hybrid vehicle of today is the mid-size 2004 Toyota Prius, with EPA estimated mileage of 60 miles per gallon (mpg) city/51 mpg highway and a city efficiency greater than 35 percent. These seemingly "reversed" figures reflect the unique powertrain that's optimized for maximum efficiency in the kind of slow-and-go driving that most urban commuters experience.

Today's hybrid vehicles use two power sources -- gasoline and electricity. The inclusion of an electric motor in the drivetrain allows the power management computers and algorithms to load the electric motor at points in the driving cycle that would otherwise produce the most pollution from the engine alone. Starts and rapid acceleration are the most notable of these. In the Prius hybrid, the electric motor allows the control systems to reduce or eliminate the operation of the gasoline engine when it is prone to pollute the most, such as at cold starts and peak power.

Long-term replacement of internal combustion with fuel-cell power promises improved efficiency, but the required infrastructure for mass production, transportation and storage of the needed hydrogen fuel is a daunting challenge.

Honda also has hybrid gasoline/electric vehicles on the market. Ford, General Motors, DaimlerChrysler and Nissan have announced plans to bring hybrids on the market over the next two to three years.

In addition to the combination of an electric motor and a gasoline engine, a key to the hybrid system is the hybrid battery pack, which is a reservoir of power for motive operation of the automobile. The hybrid vehicle's gasoline engine drives a generator when the hybrid battery pack needs to be charged, so the vehicle never needs to be plugged in for recharging. Moreover, the battery pack allows the drivetrain to capture a portion of braking energy by having the motor act like a generator on deceleration. This charges the battery with some of the vehicle's kinetic energy, rather than losing it all as heat in the brake pads. All of these features contribute to improved miles per gallon and lower emissions.

A similar hybrid strategy using hydrogen and electric batteries also will be used in fuel-cell vehicles for greater efficiency and to allow use of a smaller fuel cell than would otherwise be needed. Thus, current developments in gasoline-powered hybrid vehicles can contribute directly to the zero emission fuel-cell hybrid vehicles of the future.

Automobile emissions of criteria pollutants have been cleaned up substantially over the past few decades and the end is beginning to appear in sight. As criteria emissions begin to reach ultra-low levels, the challenge of reducing greenhouse gas emissions will receive greater attention. Hybridization offers immediate gains in efficiency for the lowering of greenhouse gas emissions and opens an enabling pathway for the development of fuel-cell vehicles.

This article originally appeared in the October 2003 issue of Environmental Protection, Vol. 14, No. 9.

This article originally appeared in the 11/01/2003 issue of Environmental Protection.

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