Truck builders have sounded a new alert regarding EPA '07 diesels: Routine regeneration of particulate filters in the exhausts might occasionally require drivers to stop during their runs to let the filters bake away accumulated ash. The so-called manual regenerations, or "regens," will be required only occasionally, and will probably never be needed in engines that work hard and generate enough heat to continually burn out ash that comes from motor oil. In most cases, the caution pertains to medium-duty diesels, not heavies.
Diesels that run cool, which could include many construction trucks, often run on city and suburban streets with a lot of stop-and-go operation and prolonged idling. Their exhausts don't get hot enough to burn out the ash during normal operations, so their electronic controls will order active regens.
These are done by injecting a little extra fuel into the cylinders or exhaust systems; the fuel causes a chemical reaction in a catalyst ahead of the diesel-particulate filters (DPF), and that reaction generates the heat needed to burn out the ash. (That's how it's done in most diesels; in Caterpillar diesels, fuel is injected ahead of the DPF and ignited by a spark plug, and the resulting heat burns the ash.)
Drivers won't notice an automatic active regen: The engine continues working normally. A small icon lights up on the instrument panel to indicate that the regen is occurring. When that happens, the inside of the DPF and the tail pipe can get hot, so the truck shouldn't be parked in tall grass (if it has a frame-hung exhaust pipe) or under trees (if it has a stack).
Some diesels run so cool even the automatic active regens won't be enough. When ash has accumulated in the DPF, the engine's electronic controls will illuminate a warning light — usually an amber followed by a red.
The driver will then push a button to initiate a manual active regen, but will have as many as 90 miles before he actually has to do so. Before doing so, the driver must park, set the parking brake, and prepare to wait about 30 minutes until the regen is completed. He must stop because, unlike an automatic active regeneration, a manual regen includes idling the engine faster and closing a valve upstream of the DPF to concentrate heat there. Under those conditions, the engine can't propel the truck or run a PTO, and the regen shouldn't be interrupted.
However, a manual regen should be rare, and drivers will have enough warning so they can usually finish the day's work. A manual regen can then be initiated when the truck returns for the night or before it leaves the next day.
Instructions about what do if a manual regen is required are included in truck manuals. Because few drivers read those books, they should be told about the possibility of a manual regeneration. In trucks without manual-regen switches, a check engine light might illuminate, and the vehicle should go to a dealer for DPF service.
Most OEMs will include warning lights and manual-regen switches in their medium-duty trucks because they want operators to have the option of handling the DPF regeneration. But not all will. Ford, for example, won't put the switches in any of its diesel-powered trucks, partly because engineers feel the need for manual regens will be extremely rare and also because they don't want to put customers in the position of causing damage to people or property. General Motors will include the switch in its midrange diesel-powered trucks, which are usually operated by professionals, but not in light-duty pickups, which are often owned by consumers or drivers. International is among most other builders who will include manual regen switches.
During any active regeneration, exhaust gas at the end of the tail pipe will be as hot as 1,200 degrees Fahrenheit — hot enough to set grass or leaves afire and burn flesh. That's why many OEMs have designed diffusers that mix the gas with outside air, cutting temperatures almost in half. The devices range from a simple pan-shaped pipe extender (Freightliner) to an extra cylinder with forward-facing vents that bring in outside air (GM and others).
Those who use diffusers put them on horizontal exhausts unless the trucks will regularly go under tree limbs, for example, municipal vehicles. However, Dodge says the long tailpipe on its pickups and conventional mediums will dissipate heat, while Hino says its diesels' exhaust is cool enough in the first place.
One other wrinkle involving diesels is in California. The state's Air Resources Board has placed strict limits on idling by engines that produce NOx above prescribed levels. Effective January 1, engines emitting more than what the state allows (which is less than federal limits) may not idle more than five minutes per hour. But at least two manufacturers — Isuzu and Cummins — say their diesels burn cleanly enough to be exempt from the anti-idling rules.
Isuzu 2008 models destined for California will have special "Certified Clean Idle" stickers that will allow drivers to idle engines longer than the five-minute limit.
Cummins says its auxiliary power units, used mostly by over-the-road truckers to heat and cool sleeper compartments, also qualify for the California exemption. California allowed engines in sleeper-cab tractors to idle without limit until January 2008 when the five-minute limit began applying to them, too. Ironically, the new limit applies only to trucks with EPA-'07 and newer diesels; older, dirtier engines are not affected.