When a ship is being constructed in a shipyard, the most important machinery that is to be selected is the main propulsion machinery. Both 2 stroke and 4 stroke engines are widely available in the market but for large ocean going merchant vessel, a 2 stroke engine is more commonly used as main engine and has much better market.

Even with wide variety of advantages that 4 stroke engine offers like compact size of plant, much more RPM or speed etc, a 2 stroke engine outshines with few but vital advantages.

Some of the important reasons why 2 stroke engines are more popular than 4 stroke engines as main propulsion engine on ships

• Fuel Selection: The fuel prices have gone sky high and better grade fuel is adding higher costs to vessel operation. A two stroke engine can burn low grade fuel oil and hence reduce running cost of the ship.
• Efficiency: The thermal and engine efficiency of 2 stroke engine is much better than that of a 4 stroke engine.
• Power: Most of the 2 stroke engines are now large stroke engines that produce more power. Hence they have high power to weight ration as compare to 4 stroke engine.
• More Cargo: Ship can carry more weight and hence more cargo with 2 stroke engines because of high power to weight ratio.
• Reliability: Two stroke engines are more reliable in operation as compare to 4 stroke engine.
• Less Maintenance: The maintenance requirement of two stroke engine is much lesser than 4 stroke engine.
• Direction control: Direct starting and reversing is easier with two stroke engine.
• No reduction attachments: As two stroke engines are low speed engine, there are no requirement of reduction gear or speed reduction arrangement as required for high speed four stroke engine.

However, the ease-of-manoeuvring a two stroke engine is less than that of a four stroke engine and the initial cost of installation of a two stroke propulsion plant is also much higher than running and maintenance cost of a 4 stroke engine. In 2 stroke engine, the amount saved on high grade fuel can compensate all other disadvantages and also reduce the whole operating cost of a ship.