EPA and CARB legislation mandates that as of 2018, all off road diesel generators in the 15-47kw category must adhere to clean diesel emissions. This has resulted in several technologies coming forward to help clean up the emission from diesel engines.  

Emissions

The EPA regulations aimed to drastically reduce two primary exhaust pollutants. Particulate matter, or PM, is mostly unburned hydrocarbons like soot that previously shot out the exhaust stack unimpeded, and nitrates of oxygen or NOx, which is a primary ingredient of smog.

In normal operating conditions, the exhaust temps are hot enough to incinerate most of the trapped PM in a DOC. But idling, cold starts and light load factors can accelerate PM accumulation. When a DPF becomes full enough to affect backpressure, the engine’s ECM injects a stream of diesel fuel into the DPF, raising temperatures and burning off the accumulated PM. This is called regeneration.

On some systems regeneration happens automatically without the operator needing to do anything. On others, a warning light comes on to let the operator know that he needs to activate the regeneration system. Regeneration temporarily raises the exhaust temperatures, making it important not to engage in a regeneration cycle when you’re around combustible material.

Eventually ash, which won’t burn off, collects in the DPF, requiring it to be cleaned or exchanged. The EPA requires manufacturers to size DPFs on engines 175 horsepower and up to last at least 4,500 hours between cleanings. For engines below 175 horsepower, the required maintenance interval was set at 3,000 hours.

Most heavy equipment manufacturers today offer guaranteed intervals on DPF replacement or cleaning. To prevent excessive ash buildup and possibly voiding your warranty, use a low-ash oil. These are typically designated with an American Petroleum Institute’s CJ-4 label.

Be mindful that DPFs are so good at scrubbing soot from your exhaust that they can take away the visible smoke that used to tell you of impending engine problems. DPFs can also be compromised by over extended drain intervals, faulty fuel injectors or cooling system problems so it’s important to keep the entire engine well tuned and in top shape.

SCR

Tier 4 Final regulations called for a drastic cut in NOx levels. In most engines this was more than EGR alone could handle. The solution was to install a new exhaust after-treatment system called selective catalytic reduction, or SCR.

In an SCR system, the exhaust passes through a DPF or DPF/DOC combination first and is then doused with a mist of water and urea (a common chemical used in commercial fertilizers) in a catalytic chamber. The urea/water solution is commonly referred to as diesel exhaust fluid, or DEF. The exhaust and DEF in the presence of the catalyst turns the NOx into mostly water and nitrogen.

Since 2010, on-road trucks have used SCR to reduce NOx, so the technology is well proven. It does add another layer of complexity, but there are several advantages. For one, the use of SCR means the engine can meet emissions standards with a less aggressive use of EGR – so these engines run cooler.

The disadvantages of SCR are that you have to keep DEF in stock and refuel the DEF tanks on your equipment. If you run out of DEF, the engines are programmed to derate, eventually to the point where you can no longer operate. Running out of DEF is just as disruptive to your operations as running out of diesel.

DEF consumption ranges from 3 to 6 percent of your diesel fuel consumption. But since DEF is priced at or lower than diesel fuel and since it actually helps improve fuel economy, most view the pricing issue as a wash.