There has been considerable buzz in the fire service regarding aftertreatment devices on fire apparatus and issues associated with regeneration of the diesel particulate filters. Although it has been difficult to get specifics on these issues that would enable the engine manufacturers to effectively troubleshoot these problems, it is clear that there is a great deal of confusion and misunderstanding present for fire departments and fire apparatus operators regarding aftertreatment devices and how they operate.
What Is Regeneration?
During the past two decades, the Environmental Protection Agency (EPA) has worked with diesel engine manufacturers to reduce emissions, specifically focusing on NOx and particulate matter (PM)-unburned hydrocarbons from the fuel that pass through the engine into the exhaust stream. The standards implemented in 2007 (EPA07) included a 90 percent reduction in allowed PM. To achieve that level, heavy-duty engine manufacturers included a diesel particulate filter (DPF) as part of the certified engine installation starting in 2007. The DPF is a canister that replaces the typical muffler. In addition to providing the necessary reduction in exhaust noise, there is also a ceramic matrix in the canister that captures particulate matter in addition to a catalyst to aid in the regeneration process. There were a limited number of engines available from 2007 to 2009 that did not require a DPF.
Over time, as the PM accumulates in the filter, the backpressure increases. Eventually, it becomes necessary to clear out the accumulated PM to maintain an acceptable level of backpressure. Regeneration is the process of clearing out the PM from the filter. There are subtle differences from one manufacturer to another, but the basic process is to increase the exhaust temperature enough to burn off the accumulated PM. A typical regeneration process could take 20 to 45 minutes to clear out the DPF.
The majority of fire apparatus built with EPA07 certified heavy-duty engines were equipped with a DPF. All fire apparatus built with EPA10 or later certified engines are equipped with a DPF. Any apparatus with a DPF will require regeneration.
There are two main classifications for regeneration: passive and active. The difference between passive and active regeneration involves whether or not the engine controls actively participate in starting or maintaining the regeneration event. When a diesel engine operates at significant load and speed, there is a considerable amount of heat energy passed through to the exhaust system. If the temperature of the exhaust is high enough, some of the PM present in the DPF will burn off on its own. This is passive regeneration. Under optimum conditions for passive regeneration, the PM loading of the DPF will be reduced. The duty cycle of most fire apparatus does not provide optimum conditions for passive regeneration, so eventually an active regeneration will be necessary.
There are two types of active regeneration: automatic and stationary. Stationary regeneration is also referred to as parked regeneration, high idle regeneration, or nonemission regeneration.
When an active regeneration is required, the engine controls will look for operating conditions where this can be performed in the background-for example while driving down the road or while in pump mode at a fire scene. Regeneration during pump mode is standard on some engines and optional on others. During these conditions (with minimum engine speed, load, and so on), the engine controls will actively initiate regeneration by dosing fuel into the exhaust stream, which then reacts with the catalyst in the aftertreatment device, thereby raising the exhaust temperature and burning off the PM. This type of regeneration, which requires no driver intervention, is an automatic regeneration.
If the engine does not see the necessary conditions for automatic regeneration, the driver is given warning lights (first the DPF lamp, then the check-engine lamp, and finally the stop-engine lamp) to indicate that the DPF is being loaded and needs to be cleared. To initiate a stationary regeneration, the vehicle should be parked outside in a clear area so the exhaust temps do not create secondary problems. The operator then sets the park brake and triggers the regeneration by activating the DPF regen switch. At this point, the engine speed will increase; fuel will be dosed into the exhaust stream; and, during the next 20 to 45 minutes, the DPF will be cleared of the stored PM.
When to Regen
The series of lights involved with the DPF system give the driver adequate warning for when to perform regeneration. The first indication a driver will receive is a solid amber DPF lamp. If the engine is operated further without regeneration, the next phase of warning is a flashing amber DPF lamp, followed by the check-engine lamp, and finally the stop-engine lamp. These warning phases occur during the course of several hours and will vary slightly from one engine manufacturer to another. The engine controls will prevent an operator from performing regeneration unless the filter has been loaded to a minimum level. The reasoning behind this is based on the EPA’s overriding goal to reduce emissions. Allowing regeneration prior to reaching a minimum filter loading would be counterproductive to that goal. As a result, it is not feasible to include a stationary regen in a fire department’s daily apparatus checks and maintenance.
Why Should I Worry About It?
If you do not clear out the DPF regularly, the backpressure can increase to a point that will effectively choke the engine exhaust and cause the engine or aftertreatment device to fail. To reach this point of filter loading, the operator has received and ignored several warning lights during several hours of operation. Excessive backpressure will cause increased fuel consumption and excessive thermal cycling of engine components, resulting in reduced component life. As backpressure increases, the engine must work harder to overcome the backpressure and will eventually stall. A worst-case scenario would be an uncontrolled regeneration where enough PM has built up and the backpressure has increased to the point where the heat energy from the exhaust is sufficient to cause the PM to light off on its own without the aid of the catalyst. This may result in a thermal failure of the aftertreatment device.
What If I Ignore DPF Warnings?
If you ignore the series of warnings, the department will usually need a certified engine repair facility to correct the fault codes and initiate the regeneration required to clear out the DPF. The distributor personnel will connect to the engine electronics with a computer to rectify the situation. The majority of heavy-duty diesel engines provided specifically for custom chassis fire apparatus will not derate or shut down because of DPF warnings. Consult your local engine distributor for clarification of the warning/shutdown strategy on the engine in your apparatus.
Using the Regen Inhibit Switch
The regen inhibit switch is required by the National Fire Protection Association (NFPA) for all custom chassis fire apparatus to prevent the engine controls from automatically initiating a regeneration event when the apparatus may be operating in a hazardous environment where elevated exhaust temperatures could create a safety concern. There are two types of regen inhibit switches: latched and momentary. A latched switch will stay engaged even after the ignition is cycled off/on. A momentary switch will reset to the default position whenever the ignition is cycled off/on. Check with your apparatus manufacturer to determine which type your apparatus features. Operators should not use the regen inhibit switch for normal operation. Reserve it for operating in hazardous environments. Excessive, unnecessary use of the regen inhibit switch may require more frequent stationary regeneration.
Should Apparatus Be Taken Out of Service?
It is not necessary to take apparatus out of service to perform a stationary regeneration. Operators can interrupt a stationary regeneration, if necessary, by applying the service brake or holding down the DPF regen switch. The engine speed will return to idle and the vehicle can be used as normal. If a regeneration is interrupted, the DPF lamp may stay illuminated to indicate that the regeneration has not been completed. Under these circumstances, the engine controls will look for the conditions to perform an automatic regeneration. Otherwise, the operator can resume the stationary regeneration at the next convenient opportunity.
Remember to Communicate
The most important aspect of these new systems for fire department personnel is communication. The engine and apparatus are communicating the aftertreatment device’s needs to the operator, who needs to understand what that communication really means to properly deal with these systems. It is critical that the lines of communication be clear and effective on how to interact with the apparatus and engine control systems. Engine manufacturers must clearly explain the systems and controls to the apparatus manufacturers. Apparatus manufacturers need to educate their dealers on how to properly operate these systems. And most importantly, dealers need to train each fire department on delivery of new apparatus.
When all else fails, ask questions. Your local engine distributor or dealer is your best source of information for questions regarding regeneration and engine operation.
The recent EPA Ruling (EPA-420- F-12-025) grants relief for emergency vehicles because of emission-related power loss or shutdown. The ruling does not specifically state what relief will be granted. It has been left to the engine manufacturers to propose to the EPA the specific relief for each engine or engine family.
BRIAN CHAPUT is a senior application engineer at Detroit Diesel, responsible for fire apparatus and motor coach applications. He has spent 16 of his 20-year career working with fire apparatus. He has worked with nearly a dozen different fire apparatus manufacturers in addition to supporting numerous dealer and distributor accounts and a variety of other applications.