Diesel Myths & Technology
Recent advancements in fuel-injection technology combined with improved engine design have refined diesel operation and efficiency. Many of the enduring myths and annoyances are no longer applicable.
Modern diesel engines are equipped with four valves per cylinder to increase air flow by reducing interference. This configuration also allows engineers to position the fuel injector in the center of the combustion chamber for uniform fuel dispersion within the cylinder. Computers can manage the fuel injectors with such precision that multiple injections of different fuel volumes are made during each power stroke. By introducing a small dose of fuel, called a pilot injection, just a few ten-thousandths of a second before the main shot of fuel, the combustion is "softer" and noise is reduced considerably. A small, third aftershot of fuel can help reduce harmful emissions by lowering combustion temperature. Some engine manufacturers are even looking at fourth and fifth injections to shape the fuel curve and further smooth out the combustion.
The torque war continues, with the Duramax 6600 6.6L V-8 turbo-diesel boasting 605 lb-ft for 2005, providing grunt to heavy-duty Chevrolet and GMC pickups, Hummer H1, and medium-duty commercial trucks.
Key to programming such delicate fuel events is a common-rail fuel-injection system that supplies fuel at pressures from 2,000 psi up to more than 23,000 psi. Gas engines with electronic fuel injection run at 40 to 60 psi. Older diesels had separate fuel lines from the fuel pump to each injector. The pressure and injection timing were dependent on engine speed, which sometimes led to a fuel-delivery lag at high engine speeds. Today, a common-rail system links all the injectors to a feed line that is independent of engine speed, allowing the engine-management computer to control fuel exactly as programmed by the engineers.
A sophisticated fuel system and advanced turbocharger are just two reasons why diesels are more expensive than gas engines.
Because of the intense cylinder pressures, all diesel components are built to withstand more punishment, and the cooling system is designed to handle extreme temperatures. The high compression ratio requires an extremely strong starter and heavy-duty battery. Manufacturers also know that diesel truck owners drive their vehicles more-J.D. Power and Associates says diesel trucks are driven 22 to 39 percent more than their gas counterparts-and load them with more weight. Therefore, engine castings are thicker and moving parts such as the crankshaft, pistons, and valves are more robust. All this extra beef means diesels are heavier and more durable, so it's not uncommon for diesel engines to go 250,000 miles before a rebuild is even considered. On the downside, some maintenance procedures are more expensive. Diesels need about 12 quarts of oil, compared to five or six in gas engines, and diesel fuel filters and water separators need close attention. But gas engines have spark plugs and ignition components that need replacement.