Why 2-stroke Engines are Used More commonly than 4-stroke on Ships?

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 a large ocean-going merchant vessel, a 2 stroke engine is more commonly used as the main engine and has a much better market.

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

2-stroke Engines


Some of the important reasons why 2 stroke engines are more popular than 4 stroke engines as the main propulsion engines 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 the 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 ratio as compared to 4 stroke engines.
  • 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 engines.
  • Less Maintenance: The maintenance requirement of the two-stroke engine is much lesser than 4 stroke engine.
  • Direction control: Direct starting and reversing are easier with two stroke engine.
  • No reduction attachments: As two stroke engines are low-speed engines, there is no requirement of reduction gear or speed reduction arrangement as required for high-speed four-stroke engines.

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 the running and maintenance cost of a 4 stroke engine.

In 2 stroke engine, the amount saved on high-grade fuel can compensate for all other disadvantages and also reduce the whole operating cost of a ship.

You may also like to read-What is Local or Emergency Manoeuvring on Ship?

Disclaimer: The authors’ views expressed in this article do not necessarily reflect the views of Marine Insight. Data and charts, if used, in the article have been sourced from available information and have not been authenticated by any statutory authority. The author and Marine Insight do not claim it to be accurate nor accept any responsibility for the same. The views constitute only the opinions and do not constitute any guidelines or recommendations on any course of action to be followed by the reader.

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About Author

An ardent sailor and a techie, Anish Wankhede has voyaged on a number of ships as a marine engineer officer. He loves multitasking, networking, and troubleshooting. He is the one behind the unique creativity and aesthetics at Marine Insight.


  1. please suggest a book which help me to gain knowledge about steam boiler and auxilary machinary use in marine ships.

  2. @ Santosh: Just type “Books” in our search box provided on right side of the website.

  3. Recently, I found your site and its very interesting & impressive. Could you please write on tug boats drydocking and maintenance etc.

    Thanks & Regards,

    Manzoor Ali

  4. Hi sir,
    i want make my future in shiping so plz give me some basic information about shiping.
    if is possible can u mail me basic of shiping.

  5. Thank you very much for the article. This will really help me with my final year project. I just have one query. You mentioned that ” two stroke engines are more reliable in operation compared to 4 stroke engine”, I have been going through a few articles on two stroke vs four stroke engines and they all say that 4 strokes are more reliable than 2 stroke engines. Can you please clarify why you stated that 2 strokes are more reliable when it comes to ships?

    Thank you in advance for your reply.

  6. Sir, I’m very interested with all of your share on your website, I’m the one of the student in maritime in transportation management that need a help for the workflow of engine and the engine working principle in terms to complete my task ..can you help me sir?

  7. dear sir,
    this web site is so marvelous and useful for persons like us..keep growing..all the best wishes

  8. I am very happy to come across such a wonderful site most especially for the mariners.Thanks for this wonderful site

  9. hello sir,
    i m a marine engineering student and recently found your website.i feel this is an amazing website for marine engineering student as well as for seafarers.
    thank you so much for creating such a excellent knowledge platform.

  10. Dear Anish, what would be the impact on the engine selection once new eco fuel/sulphur control begins?

  11. @ Milton: Sulphur control norms are already in place with several Existing Emission Control Areas and more to add on the lists, most of the engine manufacturer are providing new 2 stroke slow speed engine to comply with the situation. Duel fuel engine and auxiliaries to minimize the air pollution (scrubber tower etc.) are the game changers in the marine market.

  12. plz write something on reversing mechanism of main engines…
    i watched that video.. but its not helpful…. as i doesnt explain how it is actually happening…
    throw some light on what is happening in the distributor, operation of exhaust valve and firing order.
    Thankx and Regards
    Rohit F.

  13. @ Parisa: Hello regarding ur 2S vs 4S battel in, i would like to go with anish, as he says, but i didnt went through his article,but im sure u should get the answer.

    In my context i would like to tell u ” that its better to use 2S besides 4S, 2S has half the no of revolutions as compared to 4S, and the power develops in 2S is directly proportional to no of strokes. As Main Engine is concerned we are more into developing the power with minimum of the maintainance. and as u know wear and tear will be more if running hours are more at high speeds.
    Therefore with superlong strokes we can develop more power with less maintainance compared to 4S.

    I think it will help u.Tell me it not.


  14. Lose the silider head,the sump then luck 4 the con rod u which 2 change den saperat d con rod cap from d con rod den u tap from B.D.C to T.D.C wen it is out den u remove d safety key,gujon pin wich conect d piston & d con rod 2gedar dem replance with d now 1

  15. dear sir, im very much interested in marine engineering.i wanted to know how can i apply for it.also i want to know which courses should i go for before entering into matine stream.i think this is the right site that i have opted for…….

  16. Sir.can u suggest me a book for a book for comparative study of marine engine and their working process

  17. It was useful information.
    Can you enlighten a bit on,
    Advantages and disadvantages of v-type engines over conventional inline marine engines?
    thank you.

  18. Anish sir can you tell me why crosshead is used in 2stroke engine? why four stroke engines doesnt have one ? and which is more preferd engines 2s or 4s? in ships.

  19. So loved this site, thank you for sharing sir. I hope to read more useful and practical informations on your site

  20. Great work !tq u for providing such a useful information in a brief and easiest way.

  21. sandeep rakshit is correct. The thermal efficiency of four-stroke engine is more than two-stroke engine due to superior scavenging, no matter how many times it is repeated to the contrary. Two reasons you don’t see two-stroke engines in cars, trucks, motorcycles, and even lawn cutting and snow removal equipment anymore are efficiency and emissions. Engines are designed around the fuels they want to burn. The bunker oil we are talking about here is typically heavy, residual oil left over after gasoline, diesel and other light hydrocarbons are extracted from crude oil during the refining process. This otherwise largely unusable residual oil has greater energy potential than any of the refined fuels or crude, which makes it a bargain. Because of the fuel they burn, the inefficiencies of the two stroke, and the two-stroke cross head design, they are the most polluting engines since the coal burner. The fuel has a high sulfur content which results in sulfuric acid and eats metal. These engine/fuel combinations run 3,500 times dirtier per gallon than regular car diesel, when it comes to sulfur dioxide. Ships also emit a lot of nitrogen oxides, which acidifies soil and water. Today, ships change to cleaner fuel when getting close to Europe and North America in order to comply. The pressure is to make ships cleaner. Because of the amount of the fuel they use and the cost difference in fuels, it makes it cost effective for new engines to implement scrubbers and other containment technologies into the next generation engines.

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