Estimating Emissions from Sources of Air Pollution

6.3 Estimating Emissions from Off-Road Mobile Sources

6.3.4 Determining Tailpipe Emission Rates
The emissions rates of off-road mobile sources vary depending on the equipment, mode of operation or duty cycle, condition of the equipment, and fuel used. The majority of off-road equipment is powered by diesel fueled internal combustion engines. However, many off-road sources can be powered by other fuel types, such as electric, gasoline (premixed or not), natural gas, or jet fuels. Obvious examples include jet powered airplanes, some ship and recreational boat engines, and lawn and garden equipment.

Many of the sources in this category operate similarly to on-road engines in that they are internal combustion engines operating on diesel or gasoline fuel. For these sources, the calculation of emissions is therefore similar to that discussed in Section 6.2. For these sources, the amount of emissions produced by the equipment depends upon the size of the engine, the engine design, the condition of the engine, and the power load placed on the engine. The estimation of emissions from off-road equipment is based upon these factors.

Off-road equipment is one of the last anthropogenic emissions sources to be controlled, and these controls started in the 1990s in the US and Europe. Therefore, some off-road sources will have after controls and emission standards that will affect the emission rate as well. As with all sources, the emissions of in-use vehicles and equipment may be much different than the original certified emissions.

The emissions rates for off-road equipment are usually measured in the laboratory on a simulated load cycle for a new engine. Then, this zero mile emission rate is adjusted for deterioration and in use effects. In-use effects can include fuel effects, temperature or other effects, duty cycle effects (how the equipment is operated), tampering, or other factors that affect engine performance in the real world. The in-use effects can be incorporated into the emission rate in two ways: either by adding an additional emission term or multiplying the original emissions by a factor. Either way, the true emission factor that should be used in developing the emissions inventory should be the one that takes into consideration all in-use effects. However, since the in-use effects may differ widely from region to region, caution must be used in applying in-use factors to areas that it was not originally designed for.

The use of duty cycles are developed for various operations and are used to better estimate the real-world operation and emissions profile of off-road equipment. The accurate accounting of duty cycles is important because the emission rate varies considerably depending on whether the engine is operating at an idle or transient, or full load. A standardized cycle is usually developed for different equipment types based on observation of actual use and used to develop the zero mile rate emission factor. The load factor is used to correct the base emission rates to a level appropriate to the load.

There are three main sources of data for off-road emission factors in the United States. Both the EPA and ARB’s latest off-road emissions inventory models use these three data sources for their official inventory planning and development.

1. The oldest compilation of emission rates for off-road equipment is presented in Section II of the AP-42 Volume II. These data were developed from testing and other work performed in the 1970’s and are now outdated. In the 1990’s the EPA worked to develop updated emission factors, resulting in the Non-road Engine and Vehicle Study or NEVES The NEVES report for construction, industrial, agricultural, logging, lawn and garden, recreational vehicles, ground airport equipment, and marine sources was published in 1991. The NEVES study not only improved emission factors but also collected activity information from these categories. At the time of the NEVES study, off-road sources were still uncontrolled and therefore all emission rates reported in this study are uncontrolled emission factors. The emission rates for 80 types of equipment are reported in grams/bhp-hr. This study reports two different emission factors – one that reflects new or certified pieces of equipment, and one that reflects the deterioration, temperature and fuel effects, malfunctioning and malmaintance of in-use operation. The NEVES study can be downloaded at Paper copies of the NEVES study can be ordered at, Publication #PB-92-126960.

2. Since the NEVES study, the EPA has done many studies to improve upon the emissions in the NEVES study. A list of reports can be found at

3. The CARB has conducted extensive testing and compiled a database of off-road emission factors from diesel and gasoline powered equipment, and recreational marine. The database and methodology for these emission rates can be found at

Outside the US, the European Union develops its off-road inventory and uses primarily US-EPA based emission factors. The inventory uses slightly simplified version of the emission factors. The methodology and emission factors used in Europe can be found at

Even with the data resources described above, the emissions profiles and understanding are still in the beginning stages as compared to other stationary or on-road mobile sources. Therefore, new information is continually being developed. Most recently, in-use emission rates from commercial marine vessels are being measured by the University of California, Riverside (Welch, 2007, As of October 2007, the ARB is funding the University of Missouri-Rolla and Aerodyne Research to understand and measure the emissions and speciation of emissions from jet aircraft.