Here in the US, the diesel engine is often associated with trucks and buses where its longevity and strong pulling power make it a better choice than gasoline. But diesel engines are also much more fuel efficient, which is increasing their popularity as car engines. Diesels account for more than 50% of car sales in some European countries, and 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 pressure and temperature, and then spray in diesel fuel (a less-refined and less-volatile form of petroleum). The hot compressed air ignites the diesel fuel, and the resulting combustion and expansion 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 the need for mileage-boosting devices such as auto shut-off systems and low rolling resistance tires.
Torque: Diesels produce lots of torque (pulling power) at low engine speeds. A four-cylinder diesel can produce as much torque as a six-cylinder gas engine, and at lower engine speeds. This strong mid-range torque gives diesel cars excellent passing power, though 0-60 acceleration often feels slower. Horsepower ratings for diesels tend to be lower, because horsepower is a function of engine speed and diesels have lower maximum (redline) speeds than gasoline engines. (Diesels are built from heavier components and therefore cannot spin as fast.)
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 go well past 300,000 miles with no major engine problems.
Alternative fuels: Unlike gasoline engines, diesels can run on renewable fuels such as biodiesel with no major modifications. Many manufacturers support operation on biodiesel mixes up to BD20 (20% biodiesel/80% petroleum-based diesel) without voiding the manufacturer's warranty; however many diesel owners run 100% biodiesel with no problems. Some diesel engines 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.
Simplicity: Gasoline engines require computer-controlled fuel and spark systems to control engine output. Diesels use a single master fuel pump, with no need for an engine electrical system.
Disadvantages of diesel engines
Noise: Unlike gasoline engines, which produce most of their noise from the easily-muffled exhaust system, a good deal of the diesel engine's 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 1/16th of its original volume (as opposed to gasoline engines, which compress their fuel-air mixtures to around 1/8th to 1/12th of its original volume). Therefore, they must be stronger than gasoline engines, which makes them heavier and more expensive to build.
Emissions: Though the diesel engine was invented well over a century ago, little attention was paid to emissions until recently. Most modern-day diesel cars rely on diesel exhaust fluid (DEF or "AdBlue"), a urea-based exhaust treatment that reduces the diesel engine's emissions. These cars have a DEF tank which must be refilled every 15,000 to 30,000 miles; per EPA regulations, if the tank runs dry, the engine cannot be started.
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.
Fuel availability: Diesel engines require dedicated diesel fuel -- they cannot run on gasoline -- and while it's true that not all gas stations offer diesel, the infrastructure is still excellent (remember, most over-the-road trucks and buses run on diesel). The diesel version of a given vehicle will usually have significantly longer range than its gasoline counterpart, since diesels share the same size fuel tank while getting significantly better mileage. So while diesel travellers might not find as many fuel stations, they can drive a lot farther between fill-ups.
Smoke: Diesel vehicles have a reputation of being smoky, smelly, and slow, a throwback to the low-tech diesels found in older vehicles. In truth, smoky diesel exhaust is usually a sign of a dirty air filter or an improperly-adjusted mechanical fuel injection system. Modern diesels are almost entirely smoke- and odor-free.
Poor cold-weather starting: Older diesels could be difficult to start in cold weather. These diesels used glow plugs, which had to cycle for 10 to 20 seconds before the engine could be cold-started. Newer diesels feature significantly faster pre-heating systems and will start almost immediately, even in very cold weather. However, at low temperatures diesel fuel can form clumps of wax-like material (paraffin) which can clog the fuel filters, so it is important to use winter-blended diesel fuel or additives when the weather turns cold.