Devices differ in design and design.
The engine is an integral component of any vehicle.
Of course, depending on the capacity and technical indicators the aggregates are divided. Moreover, there is a difference in manufacturers. Some designers, developing engines, first of all try to create an extraordinary engine that will not only fully cope with their work, but also be able to surprise as the appearance and technical parameters.
There are six of the most unusual engines that can not distinguish among similar units. So, as among the most interesting include: Circle cycle, Skyactive-X, LiquidPiston X Engine, Achates Power 2.7 L, Quasiturbine and X-3 Thruster. Each of these engines differ in their parameters. Moreover, unusual appearance only attracts additional attention.
Developing the engine manufacturers primarily focused on a specific model of machine for which it was created. This helped to give the units individual parameters, which allow to consider them unique.