Emissions – How low can it go?

Automobiles and light trucks have been a favorite target of environmentalists for years, and with good reason. With urban air quality deteriorating in lock step with the rise in vehicle populations, the internal combustion engine has had to answer for much of the effects of smog. New “Tier Two” regulations, however, following the US Environmental Protection Agency, promise to bring light vehicle emissions down to the point where they simply won’t be a factor in most air quality issues. Getting there, however, will require overcoming significant technical challenges.

EGR, O2 and Catalysts

The historical heart of automotive emission controls were the combination of three-way catalysts, EGR, and air injection. When these technologies debuted in the 1970’s as add-ons to pre-emissions engine designs, the challenge was to overcome the need for very high combustion temperatures, needed to control hydrocarbons, while avoiding the byproduct of those temperatures: oxides of nitrogen, generically called “Nox”. The need to balance these conflicting requirements gave birth to EGR and ultimately to computer control, as “add-on” technologies required ever more complex and troublesome vacuum control for efficient operation.

Mike Ricciuto, manager, vehicle emissions, fuels and fuel economy for General Motors of Canada, has extensive engine design experience: “Now you need to reach a certain level of what we call “engine out emissions”. After treatment can only do so much. It all starts with combustion chambers, intake systems, rings, everything is designed to reduce emissions. When mapping out engine performance, today you only worry when the engine O2 sensors aren’t up and running. Before, we had more ‘closed loop” systems; you had to make sure that the engine was mapped very precisely. While you still have to be precise, as soon as the oxygen sensors are lit off, you know that fuelling will be very accurate. Today’s O2 sensors are very precise.”

Ricciuto states that some 90 percent of emissions are related to cold engine operation, which is the primary reason for the switch to electrically heated O2 sensors, which feed their signal back to the ECU much faster than exhaust heated units. Catalysts also need heat to light off, creating an interesting problem for vehicle engineers: How do you get the maximum heat into the converter as early as possible after engine start?

One answer is simple: move the converter as close as possible to the exhaust manifold. Another solution is to use a smaller converter upstream of the main unit to light off early and mop up cold start emissions, a process used by several current models including Corvette.

Other possible future routes may be traps that hold back a portion of exhaust during the first seconds of engine operation to give them time to react in the converter, heavily insulated exhaust plumbing to trap heat, or even electrically heated catalysts. For jurisdictions with inspection/maintenance programs, service technicians have learned that a re-test of a failed vehicle after thoroughly warming up the engine often lowers emissions to passing levels. Similarly, modified engines with headers or relocated converters may require considerable warm-up time to reach converter light-off temperature, a factor which has driven the performance aftermarket exhaust sector to concentrate on “cat-back” systems to avoid regulatory and emission warranty issues.

Secondary air injection is another I/M service factor. Look for belt-driven units with extensive plumbing to give way to electrically driven pumps operating under control of the ECU. Advantages include more convenient under hood packaging, better control and a longer service life since the pump doesn’t have to run constantly.

EGR is perhaps the weakest link in the system, mainly because of its operating environment. Exhaust gas recirculation involves channeling raw, hot exhaust through a relatively narrow constriction in the valve body, and regulating the flow with moving parts. Earlier designs using engine vacuum are susceptible to the usual suspects: vacuum line or intake manifold gasket leaks, ruptured diaphragms, stuck pintles and clogging.

More modern EGR technology uses multiple pintles; the state-of-the-art is stepper motor driven valves that allow tuned, variable flow. Tighter control is possible with the stepper motor design, and service is simplified by the ability of OBD to set a code in case of malfunction. The downside is the cost of replacement. In EGR, banging a clogged valve against the shop wall is no longer a service option. Where will EGR go? Ultimately, it may be possible to control valve timing or resonance in intake and exhaust systems to simply “leave” the correct amount of exhaust gas in the cylinder without an EGR valve, but that technology, if possible, would likely need pneumatic or solenoid-operated valves for precise cylinder-by-cylinder control.

And what about oxygen sensors? They’re a proven and robust technology that will likely appear similar to today’s units. Look for the number of sensors to increase, however, perhaps as units upstream and downstream of each converter in a multi-catalyst system to fine-tune engine parameters, especially during warm up as each converter lights off in sequence.

The fuel issue

Clean engines need clean fuel to meet both manufacturers’ performance specifications and legislated emissions levels, but the definition of “clean” is far from universal. GM’s Mike Ricciuto relates, “We’re getting to converters with nine to twelve hundred cells per square inch of cross section. They’re almost microscopic. It increases surface area, and breaks the exhaust stream very finely, allowing more of it to react with the precious metals. That’s where fuels come into play. With a fine cell structure, impurities in fuels can clog them up quickly. Our biggest issue is sulfur in the fuel. We also have issues with manganese; we may have to live with manganese going forward, until we have catastrophic failures, but our sulfur rule going forward (30 parts-per-million, or PPM) is not bad in Canada. Today our Low Emission (LEV) vehicles could perform a lot better if we had fuel cleaner than the 400 to 450 PPM we have today.”

General Motors of Canada (as well as the Canadian Vehicle Manufacturer’s Association) favours Auto Maker’s Choice fuels, based on standards requested by vehicle manufacturers and set by the Worldwide Fuel Charter. Currently, Irving Oil is the sole Canadian supplier of Auto Maker’s choice fuels, which contain lower levels of several additives, including sulfur levels that are ahead of current legislation. According to Ricciuto, “The greater Vancouver regional district is the only region in Canada that regulates gasoline. They’ve shown that there is a positive benefit to regulating sulfur, detergency and other attributes to minimize the amount of emissions. Its disappointing that governments won’t enact legislation to make sure than we have cleaner fuel. I came from engine calibration work, and fuels can make a huge difference.”

For the companies that refine Canada’s fuel, however, the issue is less about parts-per-million as it is about politics. “The auto makers like to portray Canada as having a greatly different sulfur content than in the US. That’s false,” says Kerry Mattila, vice president of the Canadian Petroleum Products Institute, who adds, “60 to 70 percent of the gasoline sold in the US is conventional gasoline, and its sulfur content is no different than what it is in Canada. There’s more high sulfur gasoline sold in the US than in Canada. When you get out of RFG (reformulated gasoline, sold in “attainment” areas where smog is serious) zones and California, the standards are the same as they are here, 1000 PPM. When you take out the RFG and California zones, it averages somewhere over 300, and so does ours. All of this is an offshoot of the type of refineries they have.” Mattila states that by the standards of the U.S. Clean Air Act, there are no Canadian regions where the use of reformulated gasoline would be required: “We just don’t h
ave comparable smog levels”. Mattila reports that Canadian refineries are on the process of conversion to 30-PPM (sulfur) gasoline, with a nation-wide conversion ahead of U.S. refiners.

The other major fuel issue from an emissions perspective is MMT (methylcyclopentadienyl), the infamous octane enhancer that was the subject of major controversy and litigation by its manufacturer, the Ethyl Corporation. To Mike Ricciuto, MMT is bad news: “When something goes wrong, and the car runs roughly, no one goes to the oil company, they come to us. We have to change the converter design because it’s plugged with MMT. As a consumer that knows, I’d love to have a choice. I go to the gas station and knowingly have to pump MMT and sulfur into my vehicle.”

For their part, the CPPI stands behind the additive as a product with emission control benefits, according to Kerry Mattila: “They have taken a fight against MMT for over a decade, based in part on some problems that were linked to MMT back in the 1980’s, but which they were never able to prove were caused by MMT. The evidence put on the record from the Ethyl Corporation, the manufacturers of MMT, shows the opposite effect, that MMT acts as a scavenger in the catalyst and helps keep it clean. That’s why testing of long life catalysts in MMT cars shows that at the end of it’s life, the MMT car has lower emissions, because the catalyst is working more efficiency. It seems to be scavenging contaminants such as phosphorus. There’s huge controversy over it.”

Whatever the outcome of that controversy, the move toward zero emissions will have a significant impact on the way the new control technologies are serviced and maintained. How can we measure pollutants in vanishingly small quantities? Can automakers certify long-term system durability with the proposed fuels? Can OBD be developed that will set a code at the 1.5 times standard by inference from sensor data? Next month, SSGM will examine the issue of measuring emissions in ever-cleaner vehicles. SSGM