A type of internal combustion engine named for its inventor, Rudolf Diesel (1858-1913). In the hundred-plus years of its history, the diesel engine has been used in a huge variety of applications and played a major role in the development of transportation and industry, in environmental and social ways as well as technical ones.

Technology in Brief

The diesel engine operates much like an Otto-cycle gasoline engine, except that it uses compression ignition: the incoming air is compressed until its temperature exceeds the flashpoint of the fuel, which is injected directly into the cylinder. In the absence of a spark plug, ignition timing is controlled by varying the time at which the fuel enters the combustion chamber. Along with the relatively low burn rate of diesel fuel, this also means that the engine does not need a throttle valve, and this reduced complexity gives the diesel engine its edge over other types in reliability and economy.

Because of these advantages, modern diesels are often found in large trucks, ships, railroad locomotives, and wherever continuous operation with minimal maintenance is paramount. But in Europe (indeed, almost everywhere outside the United States), diesel-powered passenger cars are commonplace, due to the lower cost of the fuel and the foolproof simplicity of the diesel mechanism. As technology has advanced in recent years, passenger-car diesels have begun to eclipse their gasoline counterparts in power, cleanliness and notably fuel economy, but their adoption in many markets has remained slow for other reasons (more on that later).


In the 1880s Rudolf Diesel was an engineer and business manager who had become fascinated with the problem of efficency – specifically, the poor performance of the steam engines which were then in wide use as industrial power sources (such as at the ice machine factory Diesel was managing in Paris). In Munich, he had begun studying the Carnot cycle and its theoretical description of an engine operating between two heat reservoirs, and by 1893 Diesel had in hand a patent for a compression-ignition engine of a "new, efficient" design. Initially he obtained financial backing from companies in Augsburg and Essen, who hoped that the new engine could burn coal dust (huge amounts of which were piling up in the Ruhr Valley as waste products of the coal-mining process). Though the coal dust avenue proved fruitless, Diesel successfully ran a 20-horsepower prototype, fueled by oil injected with pressurized air, in 1897. The inventor had always intended his engine to run on alternative fuels – if not on coal dust, then on oil-based fuels derived from plants – but economics dictated that petroleum soon became the fuel of choice.

In the early twentieth century, diesel engines began to supplant their steam counterparts in large stationary applications, and later aboard ships, helping to make Diesel himself a millionaire well before his death in 1913. Despite better fuel efficiency than gasoline engines, the early diesels were too heavy to use in cars or trucks, thanks to their higher compression ratio and the need for auxiliary air injection systems. In 1927 the Robert Bosch Company finally produced a mechanical diesel injection pump, which immediately helped proliferate automotive diesel engines and by the 1930s was being built under license by companies around the world. The first diesel-powered passenger car, a Mercedes-Benz 260D, came to market in 1936.

During World War II, diesel engines found their way into tanks, submarines and even airplanes (where the invention of the turbocharger vastly increased power and high-altitude performance), and after the war, diesel automobiles remained popular, due in part to the lower refining effort needed to produce their fuel. These engines were technically much like their pre-war ancestors, with indirect injection systems in which combustion occurs partially in a prechamber outside the cylinder – a technique which produces a smoother but less complete fuel burn. The next major advance came in the 1970s after the advent of electronic fuel injection systems, which allowed direct injection of fuel into swirl chambers in the cylinder head. Modern automobile engines built with such systems, including the Audi/Volkswagen TDI and the Ford Powerstroke, have achieved new levels of performance and economy. Still more efficiency can be squeezed from unit injector systems such as the Volkswagen Pumpe Düse (PD), which have individual high-pressure pumps for each fuel injector, operating at pressures as high as 30,000 PSI.

Environment and Other Factors

The diesel engine's traditional roles – in trucks, trains and the like, which are superficially large and dirty – have tended to cast it in an unfavorable light among environmentalist groups, some of whom have mounted aggressive and arguably short-sighted campaigns against the use of diesel in city buses and other urban vehicles. In spite of this, the diesel has the the highest thermal efficiency of any internal combustion engine, and new devices for emission control make it potentially one of the cleanest.

Diesel engines produce less greenhouse gases, such as carbon dioxide, in proportion to their lower fuel consumption. Higher operating temperatures also lead to increased emissions of oxides of nitrogen (a probable factor in acid rain), but the most visible byproduct of diesel combustion is soot, which manifests as a light gray to brown smoky exhaust discharge. The adverse health effects on humans who breathe this kind of particulate matter have become a growing concern in every industrialized country. Unless the engine is poorly maintained, and is burning its fuel incompletely, any particulates in the exhaust are most likely due to fuel quality problems. Unfortunately, diesel fuel in America exhibits on average the lowest cetane rating, highest sulfur content and lowest overall quality anywhere on Earth including Third World countries, and while European engineers have developed catalysts to reduce oxide of nitrogen, and soot traps which can address the issue of particulates, the huge amounts of sulfur in American fuel (as high as 500 parts per million) would quickly overwhelm any such device. The EPA has recently begun pushing for Ultra Low Sulfur Diesel to be adopted by 2006, but any real change remains far in the future.

Diesel engines can also be used with alternative fuels, continuing in Rudolf Diesel's original vision for his invention. The most widespread is biodiesel, which is refined using alcohol from plant materials (most often soy-based, but it can even be made using waste oil from sources such as deep fryers in restaurants). This produces a renewable, biodegradable fuel that offers higher mileage, fewer harmful emissions, less dependence on foreign oil, and even potential benefits for economically strapped American farmers. Some municipal bus systems, such as the Washington Metropolitan Area Transit Authority, have begun pilot programs with this and other non-petroleum fuels, but again, politics and economies of scale have so far kept biodiesel from making a major impact.

Elsewhere in the world, diesel development continues ahead of ever-advancing environmental standards. In fact, the current champion among fuel-efficient production cars is Volkswagen's Lupo 3L TDI, at nearly 90 miles per gallon, and the company recently demonstrated a two-seat diesel prototype which was driven from Wolfsburg to Hamburg at an economy of 239 miles per gallon. In North America, it will most likely take a sea change in the economic climate before diesel-powered cars become commonplace, but the technical groundwork is now being laid, and if past history is any indication, revolutions are more common than they seem.

Sources: about.com, www.tdiclub.com, www.deutsches-museum.de, www.vwvortex.com