Generation gaps

An assembly line worker at Volvo’s New River Valley plant in Virginia installs one of Volvo’s unusual vertical diesel particulate filters into a Volvo VN. All 2007 certified engines lose some fuel efficiency when fuel is consumed to regenerate the DPF while running under lightly loaded conditions.

Heavy-duty engines went through more than a decade of improving fuel economy, increased performance, reduced emissions and increasing durability. In 2002, exhaust gas recirculation set a milestone.

Although dire predictions proved to be overstated, there were notable losses in fuel economy, as well as scattered failures of highly stressed EGR system parts.

Engine and truck makers proved resilient. They designed more durable EGR coolers and made dozens of other improvements to minimize the negative impacts. As the technology became more involved, costs rose.

Now after almost two years since the debut of the 2007 technology, enough real-world experience exists to assess the merits of the different engine types. Unless you’re planning to be an early adopter to the next round of engines, due in 2010, you’ll be driving 2007 or older technology for the foreseeable future. Each generation has its strengths and weaknesses.

Fuel economy
The general consensus is that fuel economy was reduced with the technologies developed to meet U.S. Environmental Protection Agency emmissions regulations that took effect in 2002 and 2007.

Data from the nation’s largest truckload fleet, Schneider National, showed an initial loss of fuel economy from using 2002 engines, according to a presentation to the Technology and Maintenance Council by Steve Duley, vice president of purchasing at Schneider. His figures average out fuel consumption for Caterpillar C15s and three other engine brands.

Duley’s numbers showed a gradual improvement in lost fuel economy: 4 percent in 2003, 3 percent in 2004, and 2 percent by the beginning of 2007. After the introduction of 2007 technology, with the advent of diesel particulate filters, fuel economy dropped 4 percent to 5 percent.

Jim Herman, fleet maintenance manager at Perdue Farms Inc., experienced a similar trend. “We lost 6 percent to 7 percent in 2002, but by 2006 we were getting good fuel efficiency,” he says. “It wasn’t as good as pre-2002, but it was pretty close.”

Measurements from engine and truck makers are similar, though not in every case. Ed Saxman, director of powertrains for Volvo Trucks of North America, claims, “Extensive data shows Volvo’s fuel economy has improved by 3 percent from EPA ’02 to EPA ’07 in similar operations.” These figures apparently apply to vehicles where little fuel is used to regenerate the DPF.

Detroit Diesel’s Don Kilfin, general manager national accounts, says, “It’s duty cycle and load sensitive, of course, but the initial fuel economy hit was 4 percent to 6 percent.” By 2006, he says the net loss had dropped to 2 percent to 3 percent.

The 2007 technology resulted in a further loss of 3 percent in fuel economy because of the fuel dosing to regenerate the DPF, Kilfin estimates. He says, however, that the late 2006 and 2007 data also reflect the introduction of ultra-low-sulfur diesel, which has lower BTUs, and thus requires more fuel to produce a comparable amount of energy.

Work was done to minimize the impact of EGR in succeeding years. Kilfin reports the engine makers provided a mount so the radiator could be fastened to the engine rather than the chassis. This change allowed the fan blades to fit more tightly into the shroud, and reduced the power required to operate the fan by 10 percent to 12 percent.

Engines were also fine-tuned by tweaking the “algorithms and fueling strategies,” as Kilfin puts it, and torque curves improved. Faster axles lowered cruise rpm, and multi-torque schemes were adopted to get drivers to stay in top gear longer.

Kenworth’s Chief Engineer Preston Reight explains that Kenworth not only offered more lightweight components, but was careful to place them to enhance weight distribution. He says Kenworth improved the aerodynamics on the T2000 and replaced the T600 with the T660, a more aerodynamic tractor. Laying out the engine compartment to improve air flow also increased flow through the radiator. This maintained the narrow radiator opening, which helps aerodynamics.

Maintenance and repair
OIL CHANGE INTERVALS. Drain intervals have remained the same even as Caterpillar’s technology has changed in recent years, says Mike Powers, product development manager.

Kilfin reveals that Detroit Diesel’s Series 60’s process was improved for cylinder liner honing, monotherm pistons, oil control and fire rings, and combustion design during the past three years or so. Together with reduced sulfur in the fuel, the net effect was an increase in oil change intervals from 15,000 miles to 30,000.

Volvo’s Saxman says, “No change in oil change intervals occurred with EPA ’02 engines. For EPA ’07, oil change intervals increased from 25,000 to 35,000 miles in normal duty with VDS-4 oils,” which is CJ-4 oil that also meets Volvo’s requirements.

New engines such as the Mack MP and the Detroit Diesel DD15 offer longer change intervals than the engines they replaced, with the DD15 at 50,000 miles for on-highway trucks versus the Series 60’s 30,000 miles.

Perdue’s Herman says their entire fleet, using three engine brands, is on CJ-4 oil. Long-haul trucks have change intervals of 40,000 miles, up from 20,000 miles years ago, with the safety of the interval monitored by oil analysis.

All on-road diesels have reduced oil consumption to help decrease particulates and, ultimately, to minimize the impact of ash on the DPF. Duley says the latest engines he runs need only one quart in 7,000 miles.

Because of the ultra-high pressure injectors used today, fuel filters have a finer micron rating and need to be changed twice as often, Duley says.

COOLING SYSTEM. Initial EGR coolers were improved, but both Duley and Kilfin admit the high-temperature, acidic environment they operate in still means some maintenance issues for EGR system parts. Duley says variable geometry turbos are the most frequently replaced item at Schneider, followed by EGR valves, charge air coolers (which face increased pressure these days), sensors and controls, and exhaust manifolds and pipes. He reports, however, that technical support from all the engine makers has helped.

Kilfin says EGR coolers still fail sometimes, but it’s not a major repair. A good technician can replace one in two hours or less, and the part costs about $500.

Some 2007 and later engines also may need replacement of the coalescing filters used in the crankcase ventilation system, but this is a simple, inexpensive operation.

All the engine makers recommend paying more attention to coolant, but with extended life coolant, or the right regulated filter for adding SCAs, the bottom line is using the proper antifreeze.

DPF PROBLEMS. “Our 2007 engines have had a lot of issues, and have caused a lot of headaches. However, a lot depends on the application,” Herman says. “And, it’s mostly that we have to learn what we have to do, and not product quality.”

He has seen DPFs clog and fail to respond to attempts to initiate regeneration, necessitating a trip to the dealer for sensor repairs. However, drivers mostly just have to learn when to regenerate in those applications where the truck does not stay hot enough to burn off the soot in the DPF. While driver-initiated regeneration is rarely needed on over-the road trucks, it may be required weekly or more often in trucks that run locally.

Some DPFs are projected to go as far as 400,000 miles between cleanings. Powers says Caterpillar recommends cleaning at 250,000 miles or, “once during the typical life of the truck for the first owner.”

Durability
Schneider’s Duley has seen an increase in minor engine internal repairs such as head gasket replacement. Nevertheless, he said in his TMC presentation, “The base engine is reliable.”

Engine makers have improved durability since 2002. Detroit Diesel used only 14-liter block Series 60s to handle the increased EGR for 2007. The company also offers the stronger DD15.

Cummins, for its ISX, hardened the cylinder liners in 2002, and drilled the connecting rods to lubricate the piston pin under pressure, making it an even more durable design. Cummins also reduced the required air by redesigning the combustion chamber, thus eliminating the need to boost pressure to recycle more exhaust, says engineer Steve Charlton. Improvements in the EGR cooler for the ISX kept heat loads in line with previous engines for 2007, even though more exhaust was recirculated.

Other engine makers also have improved combustion chamber design. Volvo beefed up its D12 and increased displacement to 13 liters for 2007. Mack’s ASET engines were replaced with a stronger platform sharing many structural parts with the D13. Caterpillar ACERT engines have lower internal stresses than other engines, with no exhaust recirculation through the 2006 model. Even the 2007 models recycle less exhaust that can be run through the charge air cooler before it’s ingested, reducing both thermal and pressure stresses.

Engine cost
Duley estimates technology required to meet 2002 emissions regs carried a $4,500 premium over prior engines. He estimates the premium for 2007 technology at $13,000, which is the high end of what manufacturers were projecting in 2006.

Even with those costs, Duley forecasts no decrease in residual value due to emission controls. Larry Hess, a Western Star sales manager at Midway Truck Service in Bethel, Pa., agrees, saying the only exception would be the first trucks produced with 2002 emissions controls.

For owner-operators, the only certain tangible benefit of all this is that engines are quieter and run better. Some lubrication experts say the reduction in fuel sulfur may produce significantly longer cylinder liner life. So, it’s also possible that the combination of stronger engine structures, CJ-4 oils and ultra-low sulfur diesel will extend engine life significantly, thus reducing overhaul and life cycle costs.

Engine response
Kilfin says the variable geometry turbo’s sophisticated design, used on both 2002 and 2007 specification engines, promotes a flatter torque curve as well as much faster response. Detroit Diesel claims the DD15’s turbocharger responds even more quickly than typical variable geometry turbos because of its low mass.

Most diesels of today, including the DD15, feature split injection. This means a tiny pilot shot of fuel enters during compression to ignite fuel immediately during normal combustion, eliminating the characteristic diesel knock that results from ignition delay. Powers says the slower operating rpm of Caterpillar’s twin turbo setup, split injection and the DPF combine to reduce noise. Kenworth Chief Engineer Preston Reight says of the DPF, “There’s not much change in decibel readings, but the sound quality improves – it’s less harsh and offensive.”

Recirculating exhaust also helps warm a diesel engine on cold mornings, meaning less warmup idle time and faster heating of the cab.


Emissions reduction and fuel economy
Using exhaust gas recirculation to cut emissions hurts fuel economy. One reason is warmer intake air. When engines were outfitted with air-to-air charge air coolers, fuel efficiency improved because cool air wastes less energy during the compression stroke. Recirculating 20 percent or more of the exhaust, which is cooled to about 250 degrees F. to avoid condensation, raises the temperature of the intake air. That reduces efficiency.

The second reason is less efficient turbos. When a diesel with a standard turbo runs, the pressure in the intake manifold is higher than the backpressure in the exhaust. The intake air pushing down on the pistons during the intake stroke helps to push the crankshaft around and move the truck down the road.

When EGR came along, engineers knew they would have to upset this balance and raise the exhaust pressure till it was a few pounds per square inch higher than the intake pressure. This forced the exhaust through the jacket water cooler and into the intake air.

Most engine makers installed a variable geometry system in the exhaust side of the turbo to increase exhaust backpressure without increasing intake pressure. This meant the turbo could no longer help turn the engine over. However, modifications like advancing the injection timing became practical because the recirculated exhaust reduced nitrogen oxides, which minimized the loss of efficiency.

In the case of Caterpillar, something similar may have happened, though it’s hard to say because the company has kept a lid on its ACERT technology, introduced in 2002 as an EGR alternative. Jim Sayre, president of G.L. Sayre Peterbilt-International, says ACERT engines seem to ingest more air than earlier-model Caterpillars. There is also evidence that Caterpillar’s compounded, series turbocharging system and variable valve timing are used to create a slight pressure drop as the intake air enters the cylinder. The intake valve closes a little earlier than normal. This cools the intake charge, which has a favorable impact on both NOx and soot.

This may be how Caterpillar met the 2002 NOx standard without recirculating any exhaust. Caterpillar says ACERT engines take longer to break in than previous models.

“The engines were designed to maximize fuel economy when customers follow our operating recommendations,” says Mike Powers, product development manager. “When the engines are spec’ed right and driven right, the fuel economy between the various levels of EPA-certified engines is similar. The only difference is that, as emissions regulations have tightened, the sweet spot of the engine operating range for the best fuel economy has gotten smaller. Customers have to follow our recommendations closely for their application to maximize fuel economy.”

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