Thermal efficiency is a dimensionless performance measure of a thermal device such as an internal combustion engine, a boiler, or a furnace, for example. The input to the device is heat, or the heat-content of a fuel that is consumed. The desired output is mechanical work or heat or possibly both. Because the input heat normally has a real financial cost of thermal efficiency is important. A Portable Emissions Measurement System (PEMS) is essentially a lightweight ‘laboratory’ that is used to test and/or assess mobile source emissions (i.e. cars, trucks, buses, construction equipment, generators, trains, cranes, etc.) for the purposes of compliance, regulation, or decision-making.