ISO 21006:2006 specifies how to make a declaration of the engine weight (mass) for internal combustion engines.For the purposes of this standard, the word weight is used as the direct equivalent of the word mass.ISO 21006:2006 concerns reciprocating internal combustion engines (spark-ignition or compression ignition engines) but excluding free piston engines, and rotary piston engines.NOTE These engines may be naturally aspirated or pressure-charged, either by a mechanical pressure-charger or a turbocharger.ISO 21006:2006 applies to engines for land, rail-traction and marine use, the propulsion of automotive vehicles, propelling (or powering) non-road mobile machinery, motorcycles, the propulsion of agricultural tractors and machines, the propulsion of earth-moving machinery as defined in ISO 6165, and the propulsion of recreational or other small marine craft of full length up to 24 m .In addition, ISO 21006:2006 may be applied to engines used to propel road-construction machines, industrial trucks and for other applications where no suitable International Standard for these engines exists.