DPF regen in a slow moving bus requires TLC

By Christopher W. Ferrone

The emission standards that went into effect in 2007 have created further issues in an industry that has one of the lowest pollution emissions per passenger in the entire transportation industry. Nonetheless they are here to stay.

Now operators have only to focus their attention on how to deal with Diesel Particulate Filters (DPF) in a way that creates the least amount of disruption to safety and to business.

In general the DPF collects soot and ash produced by the engine and converts it to carbon dioxide gas. In this process of regeneration, or re-greening, the DPF heats to a temperature in the conversion and expels the gas out the tailpipe. At some point technicians may need to remove the filter and clean out the residual soot and ash using a special machine to regenerate — or regen for short — the DPF. The cost for this process is approximately $400 plus labor based on the operational profile, the climate and overall conditions during operation.

Here is the rub. A motorcoach operating at low average speed, such as a sightseeing bus or trolley may never achieve the mph required for automatic regeneration. As I have discovered in my fleet of sightseeing buses driving around Chicago, this becomes an inconvenience to the operator when the DPF does not regenerate automatically on public streets. But the DPF still needs periodic regeneration. This is why it is vital that operators of any slower moving vehicle understand the regen process, what triggers the DPF and how to regenerate it when normal operation does not trigger the process automatically.

This is not just a matter of a clogged DPF that pollutes more. Failure to regenerate automatically or manually can become a serious problem if the dash light illuminates and the condition goes uncorrected. The emission control monitor (ECM) will kick in and derate the engine power to where it travels only at very low speeds regardless of throttle position. This should force the operator to regenerate the DPF and not ignore the problem. Left alone this condition eventually renders the bus undriveable, stopped or slowed in a travel lane creating the potential for an accident.

Automatic and stationary regeneration
Automatic regen occurs when the vehicle is under a heavy-duty load cycle or above a predetermined road speed and rpm. In some cases the road speed threshold may only be 20 mph, but moving any slower the engine will not automatically regenerate.

During automatic regen the system takes over and conducts the process, which the operator does not detect with the possible exception of a slightly elevated noise from the turbocharger. This is due to the VGT feature on some engines that elevates exhaust temperatures to complete the process.

In buses and motorcoaches that normally travel at road speeds greater than the threshold of say 20 mph, the regen process occurs automatically without assistance from the operator. However for any vehicles that run at average speeds below this threshold, operators will need to conduct the regen process manually.

In the case of stationary regen, using the Chicago Sightseeing Co. fleet as an example, the average road speed for sightseeing buses is seven mph, according to the ECM. This is too low of a speed to ever trigger the automatic regen process, and calls for stationary or forced regeneration for this group of vehicles.

The stationary process is simple. OEMs have provided a dash switch that activates the regen process while the vehicle is stationary with the transmission in neutral and the parking brake applied. I have found in some cases the OEM has not activated the stationary regen capability within the ECM. In this case the technician will need to attach a laptop computer with the appropriate software and manually to change the stationary option from disable to enable. A simple process, but one that requires the correct equipment and a little know-how.

During the regen process the exhaust temperature can reach as high as 1,500 degrees F — well above the auto-ignition temperature of almost all fluids and other combustibles on the vehicle. Before initiating the process, park the vehicle away from people in a location where the heat will not damage the pavement and the surrounding area is not a fire hazard. Some OEMs have included an additional dash light that will illuminate if the exhaust temperature reaches a critical level. This indicates nothing more than the exhaust temperature and will not derate the engine power or stop the vehicle.

The most practical way to reduce the exhaust temperature is to begin driving. This is as simple as going around the block a few times at a slow rate of speed. This will wash cooler air over the DPF, creating a convective cooling effect and reducing the DPF temperature. In some cases the diagnostic software can initiate and perform the stationary regen process.

A forced duty cycle is a second stationary regen method. This method simply puts the vehicle of slow average speed into a duty cycle that exceeds the threshold required to trigger the regen process.

At Chicago Sightseeing we simply drive the vehicles out on the expressway periodically to force the DPF to regen. Typically, once the DPF light comes on, a 20-minute drive at 55 mph will force the regen process to convert the soot and ash and turn off the dash light. As with the stationary method, in some cases the vehicle will need to travel at a slow rate of speed until the DPF cools down and turns off the dash light.

Operators need to further understand there are different stages of DPF blockage. Some manufacturers cite as many as four stages of blockage. Once the system reaches stage four no amount of attempts, regardless of method, will clean the DPF and the dash light will remain illuminated. The only way to put the vehicle back into service at this point is to remove the DPF and have it cleaned in the appropriate cleaning machine.

For either regen action, the dash lights indicate the DPF needs cleaning. There are typically two dash lights for the DPF system, but the check engine light doubles as an indicator that the DPF system needs a regen.

Even if the DPF system senses the need but the condition is not severe, the DPF dash light will still illuminate.

When it hits severe levels and the system begins to derate the engine, the check engine light will illuminate simultaneously with the DPF light, indicating the DPF needs to regen immediately.

Christopher W. Ferrone is  president of Americoach Systems, Inc., Glenview, IL, an  engineering firm specializing in transportation technology, analysis and engineering safety.