Diesel engines are the workhorses for big trucks and construction equipment because of their efficiency, durability, and high torque (the force that produces wheel rotation and hauling power) at low speed. Modern diesel engines are quite different from the smoky and noisy engines of the 1970s and 1980s, with advances such as "electronic controls, high-pressure fuel injection, variable injection timing, improved combustion chamber design, and turbo-charging."19 Although they represent less than 1% of car and light truck sales in United States, diesels are becoming the car of choice in Europe, where gasoline prices are much higher, where fuel taxes favor diesel use, and where tailpipe emissions standards are less stringent.
Gasoline taxes in Europe and Japan are several times that of United States, and overall, their gasoline prices are two to three times that of ours. Gasoline in Japan, France, United Kingdom, and Germany costs between $3.60 and $4.60 per gallon.20 In France and Germany, diesel fuel costs about $1 per gallon less than gasoline. Diesels have some 40% of the market for cars in Europe, and by 2001 represented the majority of new cars sold in a great many European countries. They are 20% to 30% more fuel-efficient than gasoline.21
While diesels currently have higher emissions of particulates and oxides of nitrogen, they are steadily reducing their emissions. Many believe that with the large amounts of R&D funding currently aimed at diesels, they will be able to meet the same standards as gasoline engines in the near future, but probably at a price premium.
A new global warming concern about diesel emissions involves their high level of emissions of black carbon (BC) or small some particles smaller than one micron (PM1.0). Recent work by NASA's James Hansen and others suggest that black carbon is a potent greenhouse gas.22 One estimate even suggests that the black carbon emission from diesel engines may wipe out their global warming benefit compared to gasoline engines. As Princeton professor Bob Williams wrote in 2004, "Thus, ironically, the ongoing shift in Europe to diesel cars might lead to increased global warming even though it would help Europe meet its Kyoto obligations-because BC is not a greenhouse gas and is thus not covered by the
Kyoto protocol or the Framework Convention on Climate Change."23 Under Federal Tier I emissions standards, diesel cars may have no greenhouse gas benefit versus gasoline engines. Under the stricter Tier II emissions standards now being phased-in, the black carbon problem is largely solved for diesel cars, and their greenhouse gas benefit returns, but, as Williams notes, "this would be accomplished at a significant extra first cost," which could be $500 to $750-and this is on top of the significant price increment one must pay for diesels versus gasoline-powered cars. Today's diesels already cost "between one and nearly six thousand dollars more than 7 their gasoline counterparts."24
There is also be very legitimate concern that in the real world, emissions can be higher than mandated because cars and pollution control equipment often is operated differently than in tests and because some fraction of pollution-control equipment malfunctions.
The key to the success of diesels as a long-term greenhouse gas reduction strategy will be fuels with far lower net carbon dioxide emissions than current diesel fuels (possibly coupled with advanced particulate controls). This could include bio-diesels or, as will be discussed below, a synthetic diesel fuel made using a process that includes sequestration of carbon dioxide.