An otto-cycle engine is disclosed. The engine of the present disclosure consumes less work than a traditional engine for the following reasons: (1) the engine adopts constant volume exhaust and reduces the work consumed by forced exhaust and (2) in an intake stroke, the piston has a short stay at the top dead center and an intake valve has enough time to open to the maximum, thereby reducing negative pressure and reducing the work consumed by intake. By adopting otto-cycle technology, heat efficiency of the engine can be increased by more than 50%. And meanwhile, by adopting constant volume exhaust technology, power loss can be reduced, vibration of the engine can also be greatly reduced and an effect of a boxer engine is achieved.