Diesel DoubtsDespite Their Efficiency, The Future Of Diesel Engines Is In DoubtEven with all of the current interest in fuel cells for automotive applications, most automotive engineers agree it's too soon to close the book on the internal combustion engine. The concept of these engines has been around for more than a hundred years and hasn't really changed that much.
Internal combustion engines operate by constantly repeating a cycle of sucking fuel and air into a chamber, compressing this mixture, igniting it and then pushing out the exhaust gasses. During the ignition phase, the chemical energy of the fuel is released, creating high pressure that forces the piston downward, producing power. In a gasoline engine, the air and fuel mixture is ignited by an electric discharge at the spark plug, and an automobile gasoline engine is only about 25-percent efficient. In other words, only one quart of every gallon of fuel actually goes into propelling the car forward-the rest is lost as waste heat and to friction and rolling resistance.
Contrast this with a diesel or compression ignition engine, where the air is so highly compressed that injecting diesel fuel into the combustion chamber causes automatic ignition. Diesels do a bit better, returning nearly 30-percent efficiency, but in general combustion engines are relatively inefficient at turning fuel energy into forward motion. For comparison, fuel cells-the current darlings of the auto industry-convert hydrogen into electricity at 35 to 40-percent efficiency.
How efficient can an engine really get? In 1824, French theorist Sadi Carnot described a theoretical cycle of compression and expansion that's quite similar to the four-cycle process of an internal combustion engine. Carnot's abstract model assumed no heat loss or friction and used an ideal gas as its working medium. In this most perfect of worlds, the Carnot cycle predicts a theoretical maximum efficiency of about 59 percent. In other words, fuel economy improvements can come from other areas such as improved aerodynamics and reduced rolling resistance, but even if engineers could build a perfect engine, it would never do better than twice the efficiency of today's diesel engines.
But the theoretical Carnot limit is still a worthy goal, and most engineers feel that the way to achieve it is with diesel power. In addition to the greater efficiency that comes from compression ignition, diesel fuel itself has a higher energy density than does gasoline. As 1 gallon of diesel fuel contains more energy than 1 gallon of gasoline, it will carry a vehicle just a bit farther as it burns.
Each of the vehicles created in the experimental Partnership for a New Generation of Vehicles (PNGV) program from the government and the U.S. automakers used a diesel powerplant to achieve a goal of 80 mpg. But not all of the concepts for high mileage are pie-in-the-sky. In Europe, Volkswagen has introduced its 3-liter Lupo (ec, 12/99). Three liters denotes not the engine displacement but the fuel burned during 100 km of driving. That's an equivalent of about 78 mpg. The engine in the Lupo is a three-cylinder 1.2-liter turbocharged and intercooled direct-injection diesel. The car itself is smaller than a VW Golf and the European VW Polo. Regular Lupos come with a choice of 1.1-liter or 1.4-liter gasoline engines or a 1.7-liter direct-injection four-cylinder diesel. With a diesel engine the regular Lupo burns 4.4 liters of diesel fuel for each 100 km (53 mpg).
VW engineers claim engine thermal efficiency between 40 and 45 percent for the three-cylinder diesel engine in the three-liter Lupo. This is an efficiency record for a production vehicle and comes amazingly close to the theoretical Carnot limit from 1824. Efficient combustion of fuel also reduces emissions, and the 3-liter Lupo meets emissions levels proposed in Europe for 2006. VW is looking at even more efficient powertrains. Ferdinand Piech, VW's chairman, was the man behind the super-frugal Lupo and said that VW currently has a research vehicle that will achieve an economy of 2 liters per 100 km, or about 118 mpg. Piech claims the car would cost more than $30,000 to build, as it uses a variety of lightweight aerospace materials.
Diesel cars have had their ups and downs in the marketplace. The world's first diesel-powered production car was the Mercedes-Benz 260D of 1936. Although diesel-engined 170 models were available after World War II, Mercedes-Benz didn't actually start selling diesels in the U.S. until 1960. Peugeot was another manufacturer that offered diesel engines in its passenger cars for many of the same reasons. These diesel-powered cars appealed at that time to people who wanted ultimate economy and longevity in a high-quality automobile.
Diesels got a real boost during the oil crises of the 1970s, as their inherently superior fuel economy gave them a significant advantage over similarly sized gasoline-engine cars. Pressure from the heavy trucking industry also kept diesel fuel prices artificially low, making diesel car ownership even more economical. Mercedes-Benz began offering a turbocharger on its diesel engines in 1978 that greatly improved performance, traditionally a weak point of diesel engines in passenger cars. In 1982, 78.9 percent of the 65,963 cars sold by Mercedes-Benz in the U.S. were diesel powered. In 1983, the company sold 54,044 diesel-powered cars, representing 73.3 percent of its sales in the U.S.