The role of diesel in clean air policy is complex. A significant proportion of transportation-related pollution is the result of diesel-powered trucks (in California trucks account for 54 percent of diesel exhaust, followed by 19 percent for construction equipment, 10 percent for ships and boats, and 7 percent for farm equipment). On the other hand, diesel is more energy efficient than gasoline, primarily because of direct combustion via pressure; hence, a 20-30 percent higher fuel economy more than compensates for a 5-10 percent higher carbon content. As a result, total CO2 emissions would be lower with the equivalent diesel fleet. The downside is higher NOx and PM emissions, especially accentuated by diesel vehicles in high-density cities with narrower streets (creating health-damaging "chimney effects"). Nevertheless, diesel cars are making substantial progress in Europe because of their better fuel efficiency, lower fuel taxes on diesel than gasoline, and a greater concern with CO2 than with air pollutants. Also, there has been much less research on cleaning up diesel engines (because of their much smaller market share, at least of passenger cars), but there are significant possibilities. These include cleaner diesel fuel,5 particulate filters, lean-burn catalytic converters that work with diesels, and improved fuel-injection systems (UN IPCC, 1996).
There is an alternative that pollutes half as much, natural gas. A few businesses such as Ace Hardware and United Parcel Services use natural gas vehicles extensively. But they are more appropriate for local deliveries that involve a small enough daily VMT that the vehicles can be refueled at the home base overnight, given the sparse coverage of natural gas refueling stations. Their market penetration remains very modest; the Cummings Engine Co. stopped production of its liquefied natural gas truck engine in Spring 1999, after a short-term experiment of less than three years that resulted in a total of 4,000 sales compared with its 300,000 annual diesel sales (Cone, 1999a). About one-quarter of the trucking fleet makes local hauls, and could theoretically switch to natural gas, but the cost is prohibitive (about $30,000 more per vehicle; the differential with buses is even wider6). The natural gas engines are much more expensive and could increase trucking costs by up to 16 percent (if all the truck fleet were to be converted, it would amount to an annual cost of $2 billion). Fuel-cell heavy-duty commercial vehicles are in their embryonic stage; the few experimental vehicles in place (e.g. three buses in Chicago) cost 5-6 times as much as conventional vehicles. Another possibility is a new fuel (named Fischer-Tropsch), a sulfur-free liquid made from natural gas that can be burned in standard diesel engines (it compares with natural gas engines, approximately 50 percent of diesel emissions), but it costs about one-quarter more than diesel.
The problem with diesel engines is not only that they emit more NOx than gasoline or other fuels (thereby contributing to more smog in areas such as Los Angeles) but also, and more important, emit significant amount of soot particles (a truck emits the PM equivalent of 150 cars).7 It is generally recognized that PM is the most dangerous type of air pollutant from the key perspective of human health. A State of California panel estimated that 14,000 of California's population could die from the effects of diesel exhaust (although the link between exposure to diesel exhaust fumes and cancer risk remains somewhat controversial), not to mention lesser effects on asthma, hay fever and other allergy victims (Cone, 1999b). About 71 percent of the cancer risk from air pollution are because of diesel emissions. The risks vary by location, being much higher (by a factor of two) close to freeways.
The diesel risks explain why the Southern California AQMD (Air Quality Management District) is implementing a new rule to compel public fleets to abandon diesel by imposing emissions standards equivalent to methanol emissions, and has recently advanced the compliance date by two years. In October 2000, AQMD passed a regulatory measure that would require 60 government agencies operating 6,900 heavy-duty trucks in the district to begin purchasing cleaner fuel vehicles in July 2002, a replacement program expected to be stretched out over 4-10 years. The compliance costs are estimated to cost up to $150 million over a 13-year period. However, this will hardly make a dent in the diesel problem, because the public truck fleets are only a tiny fraction of the region's 57,000 trucks based in the region, not to mention the one million or more interstate trucks that pass in and out of Southern California. In April 2001 AQMD also passed a measure mandating the purchase of natural gas rather than diesel school buses, and promised to try to find funding to help school districts. Without funds, soot traps will be required, capturing 85 percent of the emissions. This is the first program of its kind in the country. There are about 8,800 school buses in the region (2,600 in the Los Angeles United School District), and it is expected that 3,360 could be replaced by 2007. This year AQMD has allocated $27 million for new buses and $14 million for natural gas refueling stations. Natural gas buses are more costly. Full implementation would result in eliminating 96 tons of NOx and soot annually. 70 percent of California's school buses are diesel, many of them without pollution controls. Children on school buses consume 4 times more diesel fumes than car drivers or pedestrians and 9 times more than the typical resident. Improved diesel engines produce 25 percent less NOx and much less soot than the standard engine; natural gas engines are even better (with two-thirds of the NOx emissions of the improved diesel engines).
In spite of these problems, sophisticated electronics have made more recent diesel trucks much cleaner. Unfortunately, their impact is very slow to take effect because of the slow replacement of the diesel truck fleet (the typical truck is on the road for at least 20 years). Nevertheless, particulate emissions from diesels are anticipated to decline by 29 percent in California between 1995 and 2005 and NOx emissions by 15 percent (California Air Resources Board data) as the newer vehicles are cycled in (both diesel PM and NOx emissions peaked in 1990 at 48 and 13 percent higher than at the 1995 level). The 2002 standards for trucks and buses (e.g. approximately a halving of the NOx emissions) and the 2008 standards for tractors and construction equipment will accelerate the trend.
On Dec 21, 2000, the Federal government approved regulations to cut diesel fumes from trucks and buses by 95 percent. These vehicles are a main source of ozone and PM-10. The rules have to be introduced by 2006, eliminating 3 million tons of emissions (equivalent to getting 13 million trucks off the road). Added cost estimates range from 4 cents (EPA) to 15 cents (industry) per gallon. Surprisingly, the regulations are supported by the California Trucking Association because it levels the playing field between California and the rest of the U.S. Addressing the diesel problem has lagged, although it has picked up with the increasing realization of its dangers; it escaped for 25 or more years. As suggested above, the long life of diesel engines makes the transition difficult (the full effect may take 30 years). By June 2006, refiners must produce diesel fuel with no more than 15 parts of sulfur per million, a 97 percent reduction. Sulfur (although an air pollutant) is more of a threat to emission devices such particle traps and catalysts for diesel engines. As much as one-fifth of fuel can be high-sulfur until December 2009. New engine specifications require a 90 percent reduction in soot emissions and a 95 percent cut in NOx. 50 percent of engines must meet these standards in 2007, and the rest by 2010. The Chicago-based International Truck and Engine Co. already makes a "green diesel" engine that meets these standards. Estimated costs of the rules are $4.3 billion by 2030, and a cost per truck of$1,900; however, these costs are expected to be swamped by health benefits.
Recognizing the dangers of PM particles in diesel fuel, 14 States (including California) collaborated in November 2000 to introduce new rules in advance of EPA mandates. The proposals would slash emissions in California and New York alone by 27 tons a day. The incentive to act was stimulated by a loophole in the 1998 agreement between the EPA and the diesel motor producers allowing them to stop testing engines on-road in 2004 until new Federal rules kicked in by 2007. The action is symbolic, representing one of the first times that States have moved ahead of the Federal government. CARB predicted a cost increase of $700-800 on trucks costing between $52,000 and $108,000.