 The frame of these engines was a large iron box casting that formed a single piece foundation for both the engine and boiler.Owing to the small height available with this pre-fabricated foundation, the semi-portable engine had no large flywheel, as was standard practice for stationary engines.The need for a more even power delivery, without the smoothing effect of a flywheel, encouraged the use of twin cylinder engines, even though a larger single cylinder would be cheaper and equally powerful.