Is emission control disabling fire trucks?

Because of how fire vehicles are used, emission control technology can damage them; here's how to get around that problem

The subject of fire apparatus and ambulances shutting down because of Environmental Protection Agency required filters on their diesel engines becoming too dirty has garnered a lot of attention in recent months.

The prime culprit seems to be the diesel particulate filter (DPF), and it seems to be getting some assistance from folks who aren't as informed as they might be about emission-control standards and fire apparatus.

The EPA is responsible for administering the requirements of The Clean Air Act. Those requirements include regulating emission of pollutants that "endanger public health and welfare." Beginning in 1994, EPA started strengthening emission limits for diesel engine exhaust; similar decreases continued in 1998, 2004 and 2007, with the final goal being reached in 2010. 

Clearing the air

The fire service has been most affected by the mandated reductions in the amount of nitrogen oxide and solid particulates (soot) entering the atmosphere from diesel engines. NOx is a generic term for mono-nitrogen oxides NO and NO2 (nitric oxide and nitrogen dioxide). They are produced from the reaction of nitrogen and oxygen gases in the air during combustion.

The majority of original equipment manufacturers (OEMs) of diesel engines have met the challenges associated with reducing NOx emissions. In 2007, however, the EPA rolled out new requirements calling for a 90 percent reduction in the soot contained in diesel exhaust emissions. 

Finding the technological solution to meet this new level has posed a much great challenge for manufacturers.

The universal solution to date is a diesel particulate filter, a wall-flow substrate that is usually constructed using a porous ceramic media. As exhaust gases pass through the DPF, the ceramic media's surface area and interior pores trap soot particles which are then burned off during a regeneration event.

Here's where it gets interesting — keeping the DPF clean. Over-the-road trucks using diesel engines generate the required heat to burn off the soot in the DPF, that is, regenerate the filter, after about 20 minutes of continued operation under a load.

This is where the unique use of diesel engines in fire apparatus becomes important.

A breed of their own

We know that the life of a diesel engine in fire apparatus and ambulances consists of many short runs and idling for hours at a time. Such activity never gives the engine an opportunity to generate enough heat for a long enough period of time to completely oxidize the accumulated soot on the DPF. 

The OEMs overcame this problem in fire apparatus by adding heat to the exhaust gases before those gases reached the DPF. Most do this by adding a diesel oxidation catalyst upstream from the DPF.

When the vehicle's emissions system senses that the DPF is sufficiently full of soot, it dumps a charge of diesel fuel into the exhaust stream. The fuel passes through the oxidation catalyst, the fuel is oxidized, the exhaust is heated to a temperature that will burn off the soot in the DPF — and we have a process called active regeneration. The accumulated soot is burned off and life goes on.

Active regeneration can start because the vehicle's emission system sensed the need or it can be initiated manually at the direction of the vehicle operator.

While the catalyst can take care of the soot, which is mostly carbon, by ensuring it's burned off in the DPF, there will still be solid particles trapped in the DPF that cannot burn — otherwise known as ash. 

The bad news is that the only method for removing this ash is to have the DPF removed and professionally cleaned by an authorized service center. The good news is that such cleaning is typically only needed after several hundred thousands of miles of use. The DPF on most fire apparatus will likely never need to be cleaned.

Know the code

Warning lights on the dashboard of the vehicle are designed to keep the vehicle operator apprised of the level of soot in the DPF. The typical sequence begins with the DPF lamp coming on. This indicates that the DPF has become sufficiently loaded with soot and an intervention by the operator is necessary.

The required intervention needs to be either operate the apparatus at a sufficient load — by pumping or driving or conduct a manual regeneration.

If no action is taken following the initial illumination of the DPF lamp, the lamp will begin to blink. Continued inaction will cause the DPF lamp to flash, followed by the check engine lamp to light, which is then followed shortly by the stop engine lamp.

Here, the vehicle must be stopped and the engine shut off to avoid irreparable damage to the engine and its exhaust and emissions control systems. It is best to consult your operator manual for the exact sequence for your particular apparatus and engine.

The Fire Apparatus Manufacturers Association developed a white paper, entitled "Emergency Vehicle Emissions System Guidance Document — Custom Fire Apparatus," that covers just about everything you need to know about this subject.

FAMA says

FAMA recommends keeping the apparatus in service during manual regeneration. When the DPF lights up for the first time, take the next opportunity to do a parked regeneration following the manufacturer's process. This typically will take 20 to 40 minutes. 

The apparatus can respond to a call during the parked regeneration and operate as normal; after the call if the DPF hasn't already gone off — because the soot burned off during the call — initiate the process again. The system will pick up where it left off.

FAMA says to limit the use of the regeneration inhibit switch. This switch is meant to be used only if the apparatus is operating near a hazardous source of combustion.

Also, don't inhibit regeneration while pumping. Any time the DPF lamp is illuminated the emission control module is looking for minimum engine temperature and engine speed criteria to initiate active regeneration on its own.

If the PTO is engaged, the module will not affect the PTO speeds, but regeneration will take place if the required minimums are met. And update the emissions module programming regularly; always make sure you're running the latest version.

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