The mass and overall dimensions of massively downsized engines for very high bmep (up to 35 bar) cannot be estimated by scaling of designs already available. Simulation methods coupling different levels of method profoundness, as 1-D methods, e.g., GT Suite/GT Power with in-house codes for engine mechanical efficiency assessment and preliminary design of boosting devices (a virtual compressor and a turbine), were used together with optimization codes based on genetic algorithms. Simultaneously, the impact of optimum cycle on cranktrain components dimensions (especially cylinder bore spacing), mass and inertia force loads were estimated since the results were systematically stored and analyzed in Design Assistance System DASY, developed by the authors for purposes of early-stage conceptual design.General thermodynamic cycles were defined by limiting parameters (bmep, burning duration, engine speed and turbocharger efficiency only). The unprejudiced assessment was based on variability of any other engine design feature. Holistic approach to all engine systems impacting brake efficiency and sensitivity analysis to yet unknown parameters occurred to be very robust tool with sometimes surprising results, giving impulse for the future engine research. The shortest combustion angle or unlimited peak pressure has not yielded the best results due to the same reasons. Too large stroke is disabled by mechanical efficiency, too small speed due to wall heat loss. Too efficient internal cycle leaves too small energy for a TC drive. Brake efficiency reached for passenger car engine is less than 50% if no waste heat regeneration is employed. The mass of cranktrain was estimated for limited fatigue safety factor and bearing loads.Comprehensive approach to iterative concept and configuration/parametric optimizations of thermal machines with internal combustion explained the limits achievable in the future. The procedures were described and stored in DASY environment. Coupling of the results to lightweight vehicles with properly scaled individual components and accessories of downsized/downspeeded engines is possible in this approach. Although not all methods for reaching the predicted optimum parameters have been already mastered, the study presents good base for finding new engine layouts and assessing their feasibility already in the concept phase.