Invented in Germany in the 1890s by Dr. Rudolph Diesel, the diesel engine is known for its outstanding pulling power, longevity and fuel economy, hence its popularity in trucks, buses and boats. Over the last decade-and-a-half, small diesels are becoming more and more popular as car engines -- in fact, diesels account for more than 50% of car sales in some European countries. With the advent of low-sulfer "clean" diesel fuel in the United States, diesel cars are slowly edging their way back into the American market.
How diesels differ from gasoline engines
Gasoline engines are spark-ignition engines. Gasoline engines draw in a mixture of gasoline and air, compress it, then ignite it with an electric spark; the resulting explosion produces power. Diesel engines are compression-ignition engines. Diesels draw in a charge of air, compress it to increase its temperature and pressure, and then spray in diesel fuel. The diesel fuel burns, and the resulting combustion produces power. (See next page for a more detailed explanation.)
Advantages of diesel engines
Fuel economy: Diesel cars can easily approach the fuel economy of a hybrid without resorting to mileage-boosting devices such as auto shut-off systems and low rolling resistance tires.
Torque: Diesels produce large amounts of torque (pulling power) at low engine speeds; a small four-cylinder diesel can easily produce as much torque as a larger six-cylinder gasoline engine. This strong mid-range torque gives diesel cars excellent passing power. Horsepower ratings for diesels tend to be lower, because horsepower is a function of speed and diesels tend to have a lower redline (maximum operating speed) than gasoline engines. Diesels are built from heavier-duty components and therefore cannot spin as fast.
Older diesels produced the bulk of their power only at low engine speeds, which made for slow acceleration. Modern diesels are turbocharged, which allows them to produce power over a broader RPM range.
Longevity: Diesels are less prone to wear than gasoline engines. Heavy-duty truck engines will often run a million miles between overhauls, and diesel cars often run well past 300,000 miles with no major engine problems.
Alternative fuels: Unlike gasoline engines, diesels can run on non-petroleum-based fuel (biodiesel) with no major modifications. Many manufacturers support operation on biodiesel mixes up to 20% (20% biodiesel/80% petroleum-based diesel) without voiding the manufacturer's warranty; however many diesel owners run 100% biodiesel with no problems. Diesels can be converted to run on pure vegetable oil with only minor modifications.
Safety: Diesel fuel is less volatile than gasoline, and will only ignite under severe pressure and/or very high temperatures. That makes diesel fuel safer to handle, and reduces the chance of a fire or explosion should the fuel tank rupture in a crash.
Disadvantages of diesel engines
Noise: Unlike gasoline engines, which produce most of their noise from the easily-muffled exhaust system, most diesel noise comes from the engine itself. Diesel vehicles employ lots of sound insulation to soften the characteristic diesel clatter.
Expense and weight: Diesel engines employ much higher compression ratios than gasoline engines; they compress combustion air to about 1/25th of its original volume (as opposed to gasoline engines, which compress their fuel-air mixtures to around 1/8th to 1/10th of its original volume). Therefore, they must be stronger than gasoline engines, which makes them heavier and more expensive to build. Diesels also use high-pressure direct-fuel-injection systems, which require beefier (and more expensive) components than the indirect-injection systems used in gas engines.
Emissions: Though the diesel engine was invented well over a century ago, diesel emissions technology is still in its infancy. Several modern-day diesel cars rely on "AdBlue", a urea-based solution that provides ammonia to reduce the diesel engine's emissions; these cars have a tank which must be refilled every 10,000 miles or so.
Messy fuel: Diesel fuel is greasy, smelly, and can be difficult to wash off of hands or clothes. (Some diesel owners carry gloves to wear while refueling.) Diesel fuel doesn't evaporate as readily as gasoline, so the ground around diesel pumps is often slick, and tracking diesel fuel into the car can make the interior smell bad.
Smoke: Diesel vehicles have a reputation of being smoky, smelly, and slow, a throwback to the low-tech diesels found in many trucks. (In truth, smoky diesel exhaust is usually a sign of a dirty air filter or an improperly-adjusted mechanical fuel injection system.)
