Smaller engines can improve efficiency via lower weight and less friction as long as the duty cycle allows for the lower power.
North American heavy-duty over-the-road trucking has been predominately served by 15 liter engines for the past few decades. Much development has been made by the engine manufactures for this engine size.
Recently, 11 to 13 liter engines have been developed by each of the engine manufacturers and their performance is being optimized. Smaller engines might also assist in improved aerodynamics and underhood air flow since they take up less space. Powertrain integration can improve the effectiveness of these smaller powerplants.
Duty Cycle Limitations
Smaller engines are not appropriate for all duty cycles because they provide less power.
Selling a truck with a smaller engine may be more challenging.