The paper describes a numerical study, supported by experiments, on light aircraft 2-Stroke Direct Injected Diesel engines, typically rated up to 110 kW (corresponding to about 150 imperial HP). The engines must be as light as possible and they are to be directly coupled to the propeller, without reduction drive. The ensuing main design constraints are: i) in-cylinder peak pressure as low as possible (typically, no more than 120 bar); ii) maximum rotational speed limited to 2600 rpm. As far as exhaust emissions are concerned, piston aircraft engines remain unregulated but lack of visible smoke is a customer requirement, so that a value of 1 is assumed as maximum Smoke number.For the reasons clarified in the paper, only three cylinder in line engines are investigated. Reference is made to two types of scavenging and combustion systems, designed by the authors with the assistance of state-of-the-art CFD tools and described in detail in a parallel paper. The former is a uniflow system, featuring two exhaust valves per cylinder in the engine head, piston controlled inlet ports and a combustion bowl in the piston; the latter is a loop scavenged design, with both inlet and exhaust ports controlled by the piston, and a non conventional combustion chamber in the engine head (no bowl in the piston).All the calculations presented in the paper are performed by using GT-Power models calibrated by means of CFD-3D calculations (by KIVA-3V) and experiments. The experimental support is provided by two engines: the former is a modern commercial aircraft 2-Stroke Diesel engine, indirect injected, uniflow scavenged; the latter is a research engine based on an old marine unit, featuring loop scavenging and direct injection. The calibration of the GT-Power models is reviewed in the paper.