Four-stroke engine

The invention belongs to the field of small engines, and particularly relates to a four-stroke engine capable of overturning by 360 degrees. The four-stroke engine comprises a cylinder body, a piston, a crankshaft assembly and a carburetor. The carburetor is used for providing mixed oil and gas for an air inlet channel, and the mixed oil and gas are connected to an air inlet valve assembly in a rocker arm chamber through the air inlet channel. The air inlet channel is provided with a branch communicated to a crankshaft chamber, and the branch is a mixed oil and gas lubricating channel. The lower portion of the crankshaft chamber is communicated with a one-way valve chamber, and the one-way valve chamber is communicated with a cam chamber. A one-way valve which is opened under positive pressure of the crankshaft chamber and closed under negative pressure of the crankshaft chamber is arranged in the one-way vale chamber. The small four-stroke overturning engine is used for providing power for portable tools, compared with a two-stroke overturning engine in the prior art, the advantages of being low in emission, low in oil consumption and small in noise are achieved, the four-stroke engine is particularly suitable for handheld portable tools, the working condition of an operator can be greatly improved, and oil consumption can be lowered by 30%-40%.