Exhaust emissions from internal combustion engines are one of the leading sources of fine particulate matter emissions in urban areas. Off-road engines account for a substantial portion of the total emissions, and are subject to increasingly strict legislation as well as voluntary emissions reduction programs. The benefits of various emissions reduction programs are typically quantified with simplified, short duration field tests. In an inquiry into the suitability of such tests, this paper examines experimental data collected by portable, on-board monitoring systems on truck, tractor, construction equipment, marine and locomotive diesel engines with rated power ranging from 130 to 1550 hp and displacement ranging from 4 to 163 liters. In engines operated extensively at low load, such as some locomotive engines, substantial amounts of solids and liquids accumulate inside of the engine and in the exhaust system. These deposits are then driven off during subsequent operation at high load. As a result, the emissions of particulate matter may be elevated for a rather long time, on the order of tens of minutes, which is longer than the duration of the individual modes of most field emissions tests. Therefore, the emissions of fine particles measured during short (units of minutes per mode) tests may be affected by the accumulated deposits. If this phenomenon is overlooked and not properly accounted for, the measurements may be less repeatable or comparable, and the particulate matter emissions based on such measurements may be overestimated. On the other hand, it is not clear that the problem can be remedied if the engine is diligently preconditioned prior to the measurement — in this case, the measured values do not account for “excess” emissions associated with extended low-load operation.

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