How Diesel Particulate Filters (DPFs) in modern engines make idling more costly than ever
You don’t have to be a diesel mechanic to know that extended idling is bad for business. Frequent or prolonged idling increases emissions, wastes fuel and puts unnecessary wear on your engine. That is why, due in large part to increasingly strict emissions standards, the trucking industry has seen a dramatic increase in idle reduction technologies. But what about those who can’t avoid idling? What is the answer for urban and vocational fleets—municipal buses and garbage trucks, etc.— that are frequent idlers due to the very nature of their work? That’s where things get a little more technical.
Aside from the obvious waste and wear associated with idling, one of the biggest disadvantages facing an idling vehicle has to do with its lowered exhaust gas temperature (EGT). To understand why this is important, let’s first look at the impact of higher EGTs in non-idling vehicles.
EGT in Non-Idling Vehicles
In a highway-driving truck, for example, the engine output required to sustain long periods of uninterrupted driving generates more heat in the combustion chamber, and, thus, yields a higher EGT. While extreme EGTs (1,400 degrees F or hotter) caused by excessive engine strain should always be avoided, the appropriate EGT achieved by the truck in this scenario (roughly 1,200 degrees F) will burn off much of the soot leftover from the combustion process before it reaches the Diesel Particulate Filter (DPF) downstream in the exhaust system. The DPF, which is standard on all modern diesel trucks, is designed to capture any remaining soot in the vehicle’s exhaust before it’s released into the air we breathe. Once the DPF fills up, the soot then needs to be combusted in order to clear, or “regenerate” the filter and maintain engine performance. This regeneration process (commonly known as “regen”) requires an EGT of 1,000-1,100 degrees F to combust the soot, which is then expelled through the exhaust pipe as nitrogen gas and carbon dioxide. For most highway-driving vehicles, the EGT required for regeneration is created naturally under normal operating conditions and a “passive” regen occurs automatically without any driver intervention.
EGT in Idling Vehicles
Now let’s look at what happens in idling vehicles. An urban delivery truck, for example, which travels at low speeds and idles frequently due to high traffic density, generates less heat in the combustion chamber, increasing the likelihood of soot and unburned particulate matter entering the exhaust system. Not only is more soot present in the exhaust, but because of the lower EGT, less of that soot is burned off before it reaches the DPF, ultimately necessitating more regens. And because the EGT isn’t naturally high enough to passively combust the soot, an “active” regeneration is the only way to clear the filter.
During an active regen, fuel is injected into the exhaust upstream of the DPF in order to raise the EGT high enough to burn off the soot in the filter. An active regen is triggered automatically by backpressure created by soot buildup in the DPF. The process can take anywhere from 20 to 60 minutes, consuming up to a half gallon of fuel, and can only occur while the truck is moving. Because the regen will stop if the driver stops the vehicle, a truck driving in a congested urban area can either get to a highway to initiate an active regen (or drive until the EGT is high enough for a passive regen), or perform a parked regen. A parked regen is essentially the same as an active regen while moving, but requires more fuel in the exhaust stream to achieve the necessary EGT. Whether the truck gets on a highway to regenerate the DPF, or performs a parked regen, both processes result in increased downtime and fuel spend.
In short, a high-idling vehicle not only wastes fuel in the act of idling, but also in the act of regenerating its DPF—an act that can require excess driving or pulling off the road for 30 minutes or more. If this is happening multiple times a day to several vehicles within a fleet, you can imagine how quickly it adds up. So how to minimize the cost of wasted fuel and time? The answer is not as complicated as you might think.
Since a regen is required when too much particulate matter builds up in the DPF, the key to minimizing the frequency of regens is in reducing particulate matter in the exhaust. Modern ultra-low-sulfur diesel (ULSD), while certainly cleaner than traditional diesel, is less thermally stable, which causes it to more readily break down into soot when introduced into today’s high-pressure fuel injection equipment. By treating ULSD with a thermal stabilizer, however, the fuel becomes less susceptible to thermal decomposition, reducing soot. To further minimize particulate matter in the exhaust, one should maintain a clean fuel system by using a diesel detergent, anti-foulant chemistry, and a corrosion inhibitor to prevent filter-plugging carbon deposits from forming and to keep injectors and other engine components in peak operating condition. Not only will a multifunctional treatment package reduce regens while idling, but it will restore horsepower and fuel economy while driving, increase service interval durations, reduce repair costs and associated downtime, and extend the life of your engine.
In conclusion, if you are requiring more than a few active regens per day, it’s time to stop being passive with your fuel. Start by working with reputable fuel and additive suppliers who have a firm understanding of the latest additive technology, appropriate treat rates, and diesel operability points. If you’re working with suppliers that pre-blend your diesel at the rack, make it a point to understand their operability expectations and declarations. And hold those suppliers accountable by monitoring your fuel—both in the field and through periodic laboratory screenings—to ensure that performance standards are achieved and that proper additive levels are used. Like a well-rounded additive package, a well-rounded oversight process will help maximize “up time” for your fleet no matter how long it collectively idles.