Green by Design
Part 5: Tighter Tailpipe Limits

Today, most vehicles sold either meet or are cleaner than the "average" Federal Tier 2 bin 5 vehicle emission standard. This is a dramatic improvement from even a handful of years ago, when only a few models were capable of achieving such levels. How did automakers cut pollution in these clean vehicles? In part, simply by continuing to improve their skills in designing emission controls. Car and light truck pollution has been evolving from an art to a science as automotive engineers find ways to refine their emission reductions techniques and industry leaders invent new technologies to create ever-cleaner gasoline-powered cars and trucks.

Exhaust pollution originates inside an engine's cylinders, where the mixture of air and gasoline is rapidly burned. The heat from combustion creates a high pressure, pushing the piston to produce the mechanical energy that moves the car. If fuel were perfectly burned, the only byproducts would be water vapor and carbon dioxide (CO2). CO2 is non-toxic (after all, it's what we exhale), although it does cause global warming—but that's another matter. Combustion is never perfect, and as a result, harmful pollutants such as unburned hydrocarbons (HC), nitrogen oxides (NOx), and carbon monoxide (CO) are created.

Most modern cars run their exhaust through a catalytic converter to clean up the emissions before they exit the tailpipe. Catalytic converters can be thought of as washing machines that chemically convert exhaust gases into less harmful compounds. These devices are not fully effective, however, and improvements have been necessary for the industry to meet more stringent air quality standards. An advanced catalyst can better neutralize the pollutants as they pass through it. Automakers are designing engines to decrease the emissions leaving the cylinders ("engine out" emissions), reducing the catalyst's workload. Improved sensors and electronics keep all of these complex engine and emissions control components working together in a carefully calibrated way. Here are some of the techniques being employed to lower emissions:

Improved control of the air/fuel mixture. Conventional three-way catalysts work best with the ideal "stoichiometric" ratio of air to fuel. By better controlling the air/fuel ratio, automakers can minimize incomplete combustion and maximize the effectiveness of the catalytic converter. Careful mixture control, helped in part by updated engine designs such as variable valve timing, is one of the ways automakers make their full-size pickups, SUVs, and minivans achieve cleaner standards.

Faster heating of catalysts. Today's catalytic converters can eliminate the vast majority of CO and HC emissions before they exit the tailpipe. In order to work properly, catalysts must operate at high temperatures (over 600°F). Below such temperatures, they are largely ineffective, which means they perform poorly while a car is still warming up. Recent developments in catalyst technology allow automakers to use smaller, more efficient catalysts. These new catalysts can be placed closer to the engine, where they heat up more quickly, reducing the amount of time that exhaust pollutants escape effective clean-up.

Exhaust gas recirculation (EGR). NOx emissions are generated differently than HC and CO emissions. HC and CO are the result of incomplete combustion of the fuel. NOx, on the other hand, is a byproduct, produced when nitrogen and oxygen in the air react at high temperatures. The higher the temperature, the more NOx is formed, and so reducing peak combustion temperatures will reduce NOx formation. One way to accomplish this is by pulling some of the engine's exhaust gas back into the cylinder. Because the exhaust gas has already been burned, it will not burn again and therefore slows down the rate at which the flame spreads from the spark plug through the cylinder chamber. This slower burn reduces peak combustion temperature and NOx formation. Increased EGR is one of the techniques utilized on PZEV models to achieve such low emissions levels.

Improved on-board diagnostics (OBD) systems. On-board diagnostics monitor a vehicle's emissions controls and inform the driver when a malfunction occurs. These systems have become more sophisticated in recent years, from dashboard warning lights and logged information in the vehicle's computer (which can then be accessed by mechanics), to vehicles emailing their drivers and informing them of possible problems. Regardless of the level of sophistication, the goal of OBD systems is to diagnose a problem before it becomes hazardous. Today's cleanest vehicles include a host of sensors that are used not only for OBD, but also to monitor conditions in real-time and adjust vehicle operation accordingly.

The Cleanest Gasoline Cars Yet

Compared to the ULEV I certification, the PZEV standard slashes tailpipe pollutants by a factor of four or more, taking some of the techniques discussed above to a greater degree of refinement. A number of these refinements are found on Volvo's 2.4-liter, 5-cylinder S40 and S60 sedans, and V50 and V70 wagons. The outstanding emissions performance of these vehicles is owed largely to computerized emissions control during the start-up process, and to a modified fuel system designed to reduce evaporative emissions.

A significant portion of a vehicle's emissions occurs immediately after start-up, before the catalytic converter has a chance to warm up to its proper operating temperature. To address this problem, Volvo uses new software to precisely control the start-up sequence and minimize engine-out emissions during this period. Computerized control of the engine's intake and exhaust valves, also known as variable valve timing (VVT), is used to manage emissions during the vehicle's cold start routine. First, the opening of the intake valve is delayed, which improves the air/fuel mixture and enhances stability during combustion. Second, the combustion process is run very late, burning up fuel on the cylinder wall and reducing HC and NOx emissions. Thirdly, the exhaust valve opening is delayed, bringing the hydrocarbon-rich exhaust gas "residual" back into the cylinder, further reducing HC emissions.

Further techniques shorten the catalyst warm-up time. For example, during start-up, the car is run with excess air in the cylinders. The excess air, also referred to as a "lean mixture," heats the gas in the cylinders quickly, hastening the warm-up of the catalyst. Additionally, the PZEV-certified Volvos use not one, but two different catalytic converters to achieve their emissions levels. The first catalyst is placed next to the engine's exhaust manifold, where it can capture the engine's heat—again to shorten its warm-up time.

These vehicles also cut evaporative emissions to near-zero levels, enabling the vehicles to receive Partial Zero-Emission Vehicle (PZEV) credits from the California Air Resources Board. Evaporative emissions are gasoline vapors that escape the tank, fuel system, and engine before the fuel reaches the inside of the cylinders. Volvo's engineers developed an evaporative emissions control system that uses two different canisters to trap the emissions. One canister sits close to the fuel tank and contains three chambers filled with activated carbon. Downstream from the first canister sits another, called the Hydro Carbon Scrubber (HCS), which contains a honeycomb structure coated with activated carbon. The carbon in these canisters traps the evaporative hydrocarbon gases, preventing them from leaking out into the atmosphere. Once the car is running, the trapped emissions are sucked out of the canister and burned up in the engine.

Cleaner Trucks, Too

Cars aren't the only vehicles getting into the clean tailpipe game. Recently, some of the most popular minivans, SUVs, and pickup trucks have been designed or redesigned to meet SULEV II and PZEV emission standards. Typically, trucks meeting these standards are still limited in availability to the clean-car states. However, some trucks, such as the Tier 2 bin 4 Ford Freestar minivan and SULEV II/Tier 2 bin 3 Toyota Highlander Hybrid SUV, are now available nationwide. One key aspect of progress in truck emissions is in the automakers' introduction of new engine designs. Continued improvements in catalysts, as well as better mixture control, upgraded calibrations, and new hardware, work together to hold emissions to lower levels on these larger vehicles. Light trucks usually still receive lower Green Scores than most passenger cars, because of their poorer fuel economy and generally poorer tailpipe emissions levels. Certain standout trucks designed to meet low tailpipe emissions, however, can be found at or near the top of their respective vehicle classes.

Continue to Part 6: Hybrids—Today and Tomorrow