As gasoline prices continue climb and recreational toys grow in size, truck buyers are increasingly drawn to the powerful, long-lasting, and relatively efficient diesel engines offered in heavy-duty, full-size pickups. Currently, only pickups with a gross vehicle weight rating of 8,500 pounds or higher offer both diesel and gas engines. These heavy-duty trucks are commonly referred to as 3/4- or one-ton pickups. Light-duty, or 1/2-ton, pickups have GVWRs from 6,100 to 8,200 pounds, and all are powered by gasoline engines. Jeep offers the lone exception to this full-size pickup diesel dominance with the '05 Liberty SUV, available for the first time with a trail-ready 2.8-liter/160-horse I-4 with 295 lb-ft of torque and EPA mileage rated at 22 city/27 highway.
As testament to the diesel engine's popularity, Ford reports its 6.0-liter Power Stroke diesel was installed in 63 percent of F-250/F-350 Super Duty pickups sold through the first nine months of 2004. GM's 6.6-liter Duramax diesel made up 44 percent of GMC Sierra HD 2500/3500 sales in the same time period (up significantly from just 12 percent in 2000) and 41 percent of the Chevy Silverado HD 2500/3500 sales (up from 33 percent in 2002). It appears that the most devoted diesel fans drive the Dodge Ram 2500/3500, where the 5.9-liter Cummins engine was sold in 80 percent of the heavy-duty lineup through September 2004 (up from 69 percent in 2002.)
Diesels don't have the same attraction in the U.S. passenger-car market, where they're just starting to gain showroom momentum following a high-profile but failed effort in the '80s. The 1973 and 1979 gas shortages prompted automakers to offer diesel engines, with their attractive fuel economy numbers, as a way to combat high gas prices. But diesels quickly developed a reputation for being noisy, dirty, smelly, difficult to start in cold weather, and sluggish to drive.
Haunted by the motors' unrefined past, American mainstream drivers have shunned diesels. Meanwhile, technology has improved in Europe, where some countries boast that diesel-powered cars make up more than 50 percent of 2004 new-vehicle sales. Currently only Mercedes-Benz and Volkswagen offer diesel-powered cars in the U.S., and those models accounted for fewer than 30,000 total sales through the first eight months of 2004, according to J.D. Power & Associates. Meanwhile, Detroit sold nearly a quarter-million diesel pickup units in the same time period and should clear 400,000 units by year's end.
The diesel engine's traditional advantage has been fuel economy. Since EPA estimates are not required on heavy-duty trucks, however, it is difficult to compare actual mileage between gasoline and diesel models over a certain time span. On average, a diesel engine offers 30 percent better mileage than its gasoline counterpart, but that figure can rise to as high as 50 percent under certain conditions such as constant low-speed driving. Since diesel fuel traditionally has been cheaper to refine than gasoline, it historically has been priced lower. This has meant the extra cost of purchasing a diesel-powered vehicle could be recouped in three or four years. However, the diesel-fuel price advantage is moot in today's volatile, unpredictable oil market, where diesel-fuel prices can be equal to or higher than gasoline in some areas. Also, diesel engines are now more expensive to purchase due to improved technology, increased demand straining supply, and generous cash and finance incentives offered on gas-powered trucks that widen the price gap. The fuel economy and longevity benefits remain true, reducing the overall cost of ownership, but diesels have lost some financial edge.
| The 17-year partnership between Chrysler Group and Cummins has produced the mightiest diesel in Ram history, a potent powerplant that delivers 610 lb-ft of torque at only 1,600 rpm.
So why are diesels so popular in heavy-duty pickups if they're not saving as much money? Torque.
Today's truck customers want pulling power for towing, and diesel offers that extra grunt. Horse trailers are now bigger, with some even featuring living quarters. Recreational travel trailers increase in size and weight with each new amenity. Race and show cars are being towed in bigger trailers to accommodate tools, supplies, and other needs. And everyone wants a bigger boat. With Interstate speed limits back up to 65 mph and higher, drivers towing these big trailers need more pulling power to enter on-ramps safely and maintain speed on steep mountain roads.
Gas vs. DieselGas engines may provide more horsepower but diesel engines inherently offer more torque for acceleration and pulling power. All of the Big Three's diesel engines are rated at near or more than 600 lb-ft of torque, while their biggest gas engine is rated at 455 lb-ft. This advantage is achieved through a number of dynamics that differentiate diesels from gasoline engines, beginning with the fuel. Diesel fuel is heavier and carries more carbon atoms than gasoline, therefore it has about 15 percent more energy density. Since engines convert heat energy from the fuel into mechanical energy, diesels offer more power and miles per gallon.
Although a diesel operates on a four-stroke cycle just like a gas engine, there are major differences in each stroke:
• Intake Stroke: Diesel engines have an open intake manifold, so a full charge of air can be drawn into the cylinders on every intake stroke. Gas engines have a throttle valve that regulates how much air-which is mixed with fuel in proportion to the air-is drawn into the cylinders. In its simplest terms, an engine is an air pump; more air moving through the engine results in more power.
| The key difference between gasoline and diesel engine designs is that gasoline powerplants intake a gas/air mixture, then ignite it with a spark. Diesel engines take in air, compress it, squirt fuel directly into the cylinder, and-kaboom! Combustion is triggered by the heat from the compressed air.
• Compression Stroke: Gas engines typically run a compression ratio of 8:1 up to 10:1. Diesel engines can run over 20:1. Higher compression facilitates efficient burning of fuel, hence more power and better economy. Gas engines cannot run high compression because the heat would cause the air-fuel mixture to self-ignite prematurely causing a power loss.
• Ignition and Power Stroke: Gas engines ignite the air-fuel mixture with a spark plug. Diesel engines inject a calibrated amount of fuel into the compressed air, where heat from the internal combustion chamber ignites the fuel. With a gas engine, the combustion is more controlled, but the burning process is limited. With a diesel, the instant combustion is more violent, lasts longer (depending on the length of the fuel-injection pulse), and literally pounds the cylinder walls and pistons, resulting in more noise. Diesel engines are also designed to have a longer crankshaft stroke than gas engines. The geometric principles are complicated, but a longer stroke combined with the longer burning results in considerably more torque.
• Exhaust Stroke: Both engines are similar in that the piston forces hot gasses into the exhaust manifold. But most diesel engines are equipped with turbochargers that use exhaust gasses to spin a turbine to pump large amounts of fresh air back into the intake manifold. Today's factory-installed turbochargers on diesels can provide 30 pounds per square inch of boost. Gas engines can hardly handle much more than 15 psi, unless they're modified.
Diesel Myths & TechnologyRecent 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.
Which to Buy?Gas engines have advantages that should be considered when choosing a vehicle. Gasoline doesn't smell as bad as diesel and is much easier to purchase, especially in urban areas. Gas engines generally have snappier throttle response for easier passing at highway and freeway speeds. For those living in cold-weather states, gas engines used to have a big advantage in starting. But recent advancements in heating elements for the fuel, engine block, and intake manifold on diesels have made starting much easier in freezing weather, narrowing the advantage of quicker-firing gasoline powerplants.
| For the first time in the United States, the 2005 Jeep Liberty is offered with a 2.8-liter I-4 turbodiesel, boasting the torque of a V-8 (295 lb-ft), the performance of a V-6, and the fuel economy of a four-cylinder engine, with 22 city/27 hwy EPA mpg.
Gas engines are perceived to pollute less because older diesels often belched black smoke, but the gap in actual tailpipe emissions is closing quickly. A new round of regulations went into effect for 2004, reducing diesel pollution by 90 percent from 20 years ago. In 2007, more regulations will reduce the particulate or soot content dramatically, which will virtually eliminate any smoke. And in 2006, low-sulfur diesel fuel will be mandated, removing most of the odor and allowing the use of catalytic converters. For those who want a diesel engine but truly scorn the diesel smell, they can always switch to biodiesel fuel. Users say the exhaust smells like French fries.
Another advantage beyond increased torque and mileage for diesels has surfaced in recent months: Resale trends show that vehicles with diesel engines tend to retain more value than those with gasoline engines. Of course, in the case of commercial vehicles, this benefit can be offset by a rough service life. Each example is different, but the investment in a diesel drivetrain can repay itself in several forms.
We continue to advise consumers that work or recreation requirements-not image-be the primary motivation when purchasing a heavy-duty truck. The same criteria are also important when choosing between gas and diesel engines. Fuel prices and new-vehicle incentives will be major factors in whether diesels can offer added value in addition to their ability to perform work assignments. But some truck owners simply want a diesel, regardless of need or economics, and they'll forgive any annoyances associated with the engine. According to J.D. Power and Associates, overall customer satisfaction is higher with diesel engines, even though owners report more problems. Should diesel penetration continue to increase in heavy-duty trucks, perhaps manufacturers will offer diesels in light-duty and even compact pickups. As diesels become lighter, quieter, and more efficient, there should be additional opportunities for all vehicles to benefit from this technology.
