We offer a general procedure to design and select a CHP unit based on an internal combustion engine. This work is devoted to provide a numerical model to estimate the amount of generated power and recovered heat, based on the engine performance features. We evaluated the mismatch of our numerical results and the available computational published values. We also evaluated our presented design CHP unit economically and technically. Finally, "Grey Systems Theory" is proposed as a multi decision making strategy to determine the optimal engine. Applying the numerical model on vehicles' engines provides the efficiency and power and heat production characteristics of the engine. It is observed that an optimum operation system must be chosen for each case, separately. Generally, using several smaller engines in parallel mode is preferred over a single large engine. This is due to two major enhancements achieved in the former scenario; longer operation period at full load operation and reliability level improvement. (C) 2016 Elsevier Ltd. All rights reserved.