Four Stroke Vs. Two-Stroke Marine Engines

The question of two-stroke vs. four-stroke in marine engines has probably been around as long as petroleum products have powered boats. When faced with a choice of engines, each may have its strengths and weaknesses, but the choice between two- and four-stroke marine engines is more often one of intended use.

  1. Basic Differences in Marine Engines

    • Two-stroke marine engines differ from four-stroke marine engines in the way the air-fuel mixture is delivered to the combustion chamber, the way the engine’s lubrication is managed and how often the engine’s power stroke occurs. The two-stroke engine is almost always an outboard, while the four-stroke marine engine can be found in both outboard and inboard engines. The most glaring exception is the inboard, two-stroke diesel engine.

    Four-Stroke Marine Engine

    • The four-stroke marine engine has an intake stroke, where the fuel-air mix is drawn into the cylinder through a valve that’s synchronized with the crankshaft though a cam or a belt. The second stroke compresses the fuel-air mixture. At the point of maximum compression, the spark plug fires and pushes the piston down – the power stroke. As the piston rises again, it forces the exhaust gases from the cylinder though the exhaust valve. These systems are designed to continue to run in spite of movement in the marine environment, such as violent rocking.

    Two-Stroke Marine Engine

    • Two-stroke marine engines use an arrangement of the intake and exhaust ports to draw fuel in and push exhaust out as the piston moves up and down in the cylinder. The fuel enters the engine through an intake port that’s slightly lower than the exhaust port. The port is closed off as the piston begins to rise. The fuel-air mixture is compressed in this single upward stroke of the piston, combining the intake and compression strokes of the four-stroke engine. The burned fuel – the exhaust gases – is forced from the cylinder through the exhaust port as the piston is forced down, combining the power and exhaust strokes. Like the four-stroke engine, the two-stroke's design compensates for the rocking and pitching of a vessel.

    Lubrication

    • The lubrication system of the four-stroke marine engine is internal. The crankcase contains the oil and the oil is pumped through a dedicated lubrication system to return to the crankcase and begin its circuit again. In a two-stroke marine engine, the oil is mixed with the fuel. The two-stroke engine’s moving parts are lubricated as the fuel mixture moves through the cylinder and crankcase. The oil is burned with the fuel and must be replenished from a separate tank outside of the engine. This does not limit the two-stroke engine’s usefulness; although the vessel must carry the oil needed to replenish that consumed by the engine, you might burn 20 gallons of fuel while consuming only 1 gallon of oil.