The invention relates to a modular internal combustion engine (1) comprising an engine housing (17, 117) with a crankshaft drive (40) having at least one reciprocating piston (4) in a cylinder (2), which acts on a crankshaft (6) by means of a connecting rod (5), and a variable-speed gear box (10) connected to the crankshaft (6) via a disengaging clutch (8), and a gearbox output train (12) which connects to at least one drive shaft (15) of a vehicle. In order to serve a large variety of vehicle types with a single low-cost engine design the proposal is put forward that the internal combustion engine (1) should have at least one main module (26) including the subassemblies engine housing (17), crankshaft drive (40), variable-speed gearbox (10), and gearbox output train (12), and at least one auxiliary module (27, 28, 29, 30, 31, 32) attachable to the main module (26), and that no mechanical connection be provided between the variable-speed gearbox (10) and the gearbox output train (12) within the main module (26) but that a rotational connection be effected by attaching a first auxiliary module (27) to the main module (26).