|Four Stroke Engine||Two Stroke Engine|
|1.||It has one power stroke for every two revolutions of the crankshaft.||It has one power stroke for each revolution of the crankshaft.|
|2.||Heavy flywheel is required and engine runs unbalanced because turning moment on the crankshaft is not even due to one power stroke for every two revolutions of the crankshaft.||Lighter flywheel is required and engine runs balanced because turning moment is more even due to one power stroke for each revolution of the crankshaft.|
|3.||Engine is heavy. Power to weight ratio is low||Engine is light. Power to weight ratio is high|
|4.||Engine design is complicated due to valve mechanism.||Engine design is simple due to absence of valve mechanism.|
|6.||Less mechanical efficiency due to more friction on many parts. More wear and tear. More maintenance is required||More mechanical efficiency due to less friction on a few parts. Less wear and tear. Less maintenance is required|
|7.||More output due to full fresh charge intake and full burnt gases exhaust.
mep is high
|Less output due to mixing of fresh charge with the hot burnt gases.
mep is low
|8.||Power developed is less||Power developed is more (almost double)|
|9.||Gear box [to reduce speed & reversing] is connected between engine and propeller shaft||Engine directly connected to propeller shaft. Engine is reversible|
|10.||Engine consists of inlet and exhaust valve.||Engine consists of inlet port and exhaust port/valve.|
|11.||More thermal efficiency.||Less thermal efficiency, as effective compression ratio is less|
|12.||Separate cylinder L.O is not required||Separate cylinder L.O is required|
|13.||High speed & Short Stroke – Costly fuel of better ignition quality should be used||Slow speed & Long Stroke – effective scavenging and cheap fuel of low ignition quality can be burnt|