Off-Road Diesels Ready for Next Tier

Tom Berg, Truck Editor | October 11, 2010

Black smoke from the exhaust stack? Sooner or later it'll be a memory, along with less-visible diesel pollutants, all of which are being regulated and engineered down to almost nothing. "Sooner" is here now for smaller off-road diesels, whose exhaust gases are already somewhat cleansed by virtue of the federal EPA's Tier standards, which began in 1995 and are being phased in over 20 years. "Later" looms for some larger engines—the next deadline comes this January—but it'll be decades before smokey old iron wears out and is retired or exiled to other parts of the world.

What it means to buyers of diesel-powered off-road equipment is higher purchase and maintenance costs, and new supplies and procedures to get accustomed to. All diesels produced after January 2011 will have to use ultra-low-sulfur fuel and low-ash motor oil. Both are now readily available because on-road truck diesels are already using them. Tier limits for off-road diesels generally follow EPA's regulations for on-highway truck engines by three to four years. This means that lessons learned from extensive testing of truck-engine components and millions of miles of customer experience with them can be applied to off-road diesels.

Since the early to mid-1990s, EPA's on- and off-road limits have aimed to lower particulate matter (PM), commonly called soot, the black stuff we see blowing from stacks (though many particulates are so small we can't see them), and nitrogen oxides (NOx). Scientists say we breathe particulates into our lungs where they injure delicate tissue, especially in children. NOx is invisible, but it causes the characteristic diesel odor and helps form smog in the atmosphere.

These and other pollutants—carbon monoxide (CO) and unburned hydrocarbons (HC)—are being drastically squeezed out of diesel exhaust. Compared to many current engines, 2011-legal large diesels will emit 90 percent less PM and 45 to 50 percent less NOx. By 2015, those pollutants will drop to near-zero levels.

EPA's off-road limits began with Tier 1, then moved to Tiers 2 and 3, where most engines are now. A major step comes this January, when Tier 4-Interim standards take effect for engines of 175 to 750 horsepower. In January 2012, many engines below 175 horsepower will have to meet the Tier 4-I limits. The final step, called Tier 4-Final, takes effect for 50- to 75-horsepower engines in 2012, and then affects larger diesels in '13, '14 and '15. The scorecard on the facing page illustrates those Tiers.

EPA's Tier 4-Interim limits are also used in Canada and are almost identical with Europe's Stage IIIB standards that are also used elsewhere in the world. This allows manufacturers to develop engines for many markets, something they've not been able to do in the past. Some people think that the U.S. EPA's on-road truck-engine limits correspond to certain Tier limits, but they don't. Although pollutant numbers are similar, the truck limits took effect several years before comparable Tier limits. For example, EPA's 2010 limits for on-road diesels won't be initially required for off-road engines until 2014.

How it's done

Methods that truck-diesel manufacturers started using as far back as the early to mid-1990s began appearing later with off-road diesels. Notable are:

  • Advanced electronic controls, high-pressure fuel systems, and enhanced combustion designs allow precise fuel measuring and timing, multiple-event fuel injection with each power stroke, for ever-cleaner burning of fuel. Some truck diesels had electronic controls in the early to mid-1990s. Rail or injection pressures were steadily increased, and on many engines they now exceed 30,000 psi. Comparable systems have been applied to off-road engines, and more models will soon have them.
  • Variable-geometry turbochargers compress inlet air and alter its flow by movable vanes or nozzles, all run by the engine's electronic controls. These are sophisticated and costly to repair or replace, although the latest turbos are supposedly more durable and long-lasting.
  • Exhaust-gas recirculation (EGR) sends quantities of exhaust into the cylinders to displace some oxygen-rich air. This cools combustion and reduces the formation of NOx (which occurs in high temperatures). EGR has been applied to automotive gasoline engines since the 1970s, but it is a bigger deal for diesels because more of it goes into cylinders. Internal EGR, which opens an exhaust valve during the intake stroke to suck in some spent gas, was an initial method; next came external EGR, which pipes exhaust gas to the cylinders in varying amounts, all regulated by a special valve that's run by the engine's electronic control module. Simple EGR is now used on many off-road diesels, and higher amounts of exhaust gas will soon be used to meet Tier 4-I limits for NOx. Most makers will also go to cooled EGR. The cooler—usually a water-to-gas exchanger—pulls heat from the exhaust gas to assist the combustion-cooling process; heat is carried away in engine coolant, and the extra Btu's are dissipated by larger radiators.
  • Diesel particulate filters (DPFs) catch particulate matter as it comes down the exhaust pipe and temporarily store it in a honeycomb structure. Heat from hard running burns away the soot, which is called "passive regeneration." If the engine loafs a lot, the soot must be burned out by "active regeneration," where fuel is injected upstream of the DPF and either ignited by a spark plug or heated by a catalyst before it enters the DPF.

Sensors on the DPF tell the engine controls when the filter is loading up with soot, and the controls will automatically trigger an active regeneration. Under some conditions a warning light will prompt the operator to punch a switch to initiate the "regen." Operators need to know about this. During an active regen, gases exiting the tailpipe can reach 900 to 1,200 F, so diffusers are fitted to dilute the gas with cool, fresh air. Even so, operators will have to keep the stack or tailpipe away from brush or grass to avoid igniting it. (Exhaust from gasoline engines often gets this hot and no one seems overly concerned about it, though grass fires have been touched off by hot mufflers and catalytic converters on cars and light trucks.)

The DPF also catches and stores ash that comes from motor oil that evades the piston rings. Ash stays in the honeycomb structure until the DPF is removed from the vehicle and blown out with compressed air or flushed with de-ionized water. This is why low-ash motor oil, called CI-4, is recommended for truck diesels built since January 2007, when DPFs were applied, and why it should be used in off-road engines soon to be fitted with DPFs. Some truck fleets have stayed with higher-ash CJ-4 motor oil with no ill effects.

The DPF also acts as a muffler and replaces it. But the DPF weighs far more, so special lifting tools are sometimes needed to safely remove it. Its honeycomb structure is ceramic coated with precious metals such as platinum, and it will break if the unit is dropped onto a hard surface. Completely removing a DPF is not only illegal, but it also renders the engine inoperative because its controls lose connections to the DPF's sensors.

Current low-sulfur fuel has too much sulfur (up to 500 parts per million) and will plug up the filter, which is why ultra-low-sulfur fuel (with no more than 15 ppm of sulfur) is required. DPF housings for trucks are fabricated of stainless steel so they don't corrode when exposed to road salts; more common steel might suffice for off-road equipment. The first off-road diesels will have DPFs this January. One more concern: Blown turbos or those with faulty seals send oil into DPFs, plugging and sometimes ruining them. It can cost several thousand dollars to replace one.

  • Selective catalytic reduction (SCR) injects a urea fluid into the exhaust, where ammonia forms to chemically break down NOx into nitrogen and water vapor. This allows tuning the engine for better performance and fuel efficiency, but it adds equipment to the exhaust system and a tank that must be replenished with "diesel exhaust fluid," a mixture of de-ionized water and urea. Most truck diesels built since January 2010 have SCR, and users are reporting higher fuel economy that should help pay for the more expensive equipment. SCR might be used by 2014 on some off-road diesels.
  • Advanced EGR deals with NOx in the diesel's cylinders, not in the exhaust, through higher doses of exhaust gas and enhanced combustion processes and controls. These are coupled with "credits" issued by EPA as a reward for production of cleaner-then-legal diesels in the past, and that allow slightly higher emissions. Only one truck-engine builder, Navistar International, uses Advanced EGR on 2010 engines. It claims lower weight and bulk, which is sometimes a real advantage with on-road trucks. (When its EPA credits run out, Navistar hopes to begin using on-board generation of urea and thus a nonfluid form of SCR.) John Deere Power Systems is evaluating what it calls Enhanced EGR.

What'll it cost?

Engine builders have each spent hundreds of millions of dollars developing the equipment and systems that make their diesels burn cleaner. They've been recouping these investments through steadily higher prices. Two big jumps for diesel-powered trucks came in 2007, with the advent of DPFs, and 2010, with SCR and A-EGR. Depending on weight class, the price of a diesel-powered truck increased by $3,500 to $10,000 each time.

Prices for off-road equipment have likewise risen, and this January, when DPFs appear on large engines, customers can expect similar per-machine hikes. Yet a $10,000 increase for a $500,000 wheel loader may be far less onerous than it's been for a $125,000 dump truck.

Expenses for everyday maintenance are more insidious and harder to quantify. Truck operators have reported niggling and frustrating problems with EGR'd diesels since 2002/04, and additional concerns with 2007-spec engines with DPFs (though the '07s seem better than the previous engines). It's too soon to make judgments about EPA-2010 diesels.

Crippled engines have interrupted deliveries and caused grief for owners. Fleet owners complain about additional tools and computer programs that they must buy to support the engines, and that dealers have been slow in obtaining support equipment and training for their technicians.

Reacting to those lessons, manufacturers have corrected faults and improved some components to increase reliability. Because off-road diesels will use technologies similar if not identical to their on-road counterparts, builders say they and their customers will benefit from experience with truck engines, and the path toward cleaner air should be less turbulent.

A final cautionary note: Old iron that's not retired and scrapped has traditionally been exported to the Third World, where buyers have less money to work with but whose mechanics have the "golden hands" to keep them running. This is becoming more difficult as electronics have become more pervasive (though computers are rapidly making people almost everywhere electron-savvy).

But there's a wrinkle: Engines with DPFs require ultra-low-sulfur diesel fuel to run properly, and if there's none available, the engines and the machines they power might not be usable. Then again, those resourceful mechanics might remove the DPFs and electronically fool the engines into running without them (that's already been done by some enthusiasts who own Dodge Ram pickups with Cummins Turbo Diesels, sometimes to visit ULSD-less Mexico). The resulting smoke will be something to watch.