One of the most common questions in Interviews and discussions; “What is the difference between two and four-stroke Marine Diesel Engine”? We all heard about it, we all learn about it. Yet at the time of necessity, we always forget one or two main points to say. So to brush up your knowledge and get you prepared for that next opportunity lets begin with its basics.
2 Stroke / 4 Stroke Engine = 2/4 + Stroke + Engine.
A four-stroke engine in a nutshell is a cylinder-piston assembly in which power is produced in just one of the four strokes. It’s a heavier complex design that is more fuel-efficient and produces less pollution.
Similarly, a two-stroke engine is a cylinder-piston assembly in which power is produced each alternating stroke. It is simpler by design, is lightweight, produces more torque, and is cheap; but is less efficient by design and causes pollution.
For your better understanding we will be explaining these differences in a point wise manner. But before we proceed let’s learn what an engine is and what are these strokes.
What is An Engine?
An Engine is a machine that converts the heat energy of fuel into mechanical energy. Fuel is burned in the combustion space which is then used to create force using a pistol, connecting rod, and crankshaft. It can be then classified on the basis of design, fuel, stroke, number, and position of the cylinder.
It can also be studied on the basis of its parts such as cylinder head, exhaust valves, inlet valves (2 strokes), scavenge space, liner, crankshaft, connecting rod, piston rod, piston, gudgeon pin and many more. With all such components working in order, it became the heart of the ship.
What is a stroke?
Even being used a lot many times in engineering (Mechanical + Marine). There are always some engineers who remain doubtful about what it is! A stroke in an I.C (Internal Combustion) Engine is the distance traveled by the piston from top to bottom dead center and vice versa.
In other words, They are the distance traveled by the piston in the 1800 revolution of the crank angle.
- Intake Stroke: This process begins with the piston moving from TDC ( Top dead center ) toward BDC ( Bottom dead center ). As the piston moves downwards the air within the cylinder expands and thus pressure reduces. Thus fresh air came in through the intake valve or scavenge space.
- Compression Stroke: Once the piston reaches BDC it starts to move back towards the TDC compressing the air within the cylinder. During this stroke, the intake and exhaust valves are shut.
- Combustion Stroke: This is where the power is created by the spontaneous combustion or by ignition using the spark plug as the piston reaches the TDC. IN the process both intake and exhaust valve are shut. The fuel is injected into the cylinder just before the piston reaches TDC by a process called atomization.
- Exhaust Stroke: The combustion produces energy and causes the gas to expand. This force the piston to move from TDC to BDC. Now when it starts to move the exhaust valve opens and exhaust gas release equalizing the pressure. The piston will move up the cylinder and begin the process again.
Difference Between Two and Four-stroke Marine Engines
A combustion cycle is the complete process of induction, compression, ignition, and exhaust. The difference between two and four-stroke engine arise with the time taken by the combustion process to complete. It depends on the number of times the piston moves up and down (stroke) during one cycle.
Four Stroke Engine
Two Stroke Engine
For one power stroke, it takes 2 revolutions of the crankshaft.
For one power stroke, it takes one revolution of the crankshaft.
Complicated design due to intake and exhaust valve mechanism.
Comparatively easier and simpler design.
It requires heavy and bigger flywheel due to uneven forces on the crankshaft.
It requires Small and light flywheel due to even forces on the crankshaft.
High Power to weight ratio.
Produce High torque and power.
Exhaust and inlet valves are mounted along with the fuel injectors on the cylinder head.
Exhaust valves may or may not be mounted along with the fuel injectors on the cylinder head.
Low initial cost.
High Initial Cost
High Maintenance and fuel cost.
Low maintenance and Fuel cost.
Lub-Oil Consumption is high.
Lub-oil consumption is low.
It requires less Space
These engines require more space.
Are generally medium and high-speed engines.
Are generally Slow-speed engines.
Low stroke to bore ratio ( 1.5 – 2.5:1 )
High stroke to bore ratio ( 4.2:1 )
Difficulty burning lower quality fuels.
Compatible with cheap/low-quality fuels.
Require Gear assembly.
Are generally direct Drive.
It contains unique Parts such as Rocker arm, Inlet valve, Gudgeon pin, and complex lubrication and cooling assembly.
It contains unique parts such as piston rod, scavenges space, scavenges ports with simple cooling, and lubrication assembly.
5 Reasons Why 2-Stroke Is Prefered Over 4-Stroke On Ships
There are many circumstances where using a 2 stroke engine proved to be more advantageous than using a 4 stroke counterpart. Although a four stroke engine offers some clear advantages such as more speed, less space, better fuel efficiency and less pollution by design; there are some vital advantages of using a 2 stroke counterpart.
- Fuel Selection: One of the reasons why we see large two-stroke engines on most ships. On a large ship, we use heavy fuel oil to reduce operational costs. But since only a two-stroke drive can burn low-grade heavy fuel oil but a four-stroke marine engine can not; we see less 4 strokes in ships than in automobiles.
- Large & Powerful: Using a two-stroke design allows for large cylinders and thus more power is produced per stroke. This can be seen in the form of high power to weight ratio ( for the same weight it produces more power ) and better thermal/mechanical efficiency.
- Vibration & Noise: Four-stroke drives are compact by design with more number of moving parts at fast speed. This leads to a good amount of vibration and heavy noise during its operation. Both 2 and 4 stroke engines are noisy but one is more compared to the other.
- Less Maintenance: Due to slow speed and less vibration 2 strokes design have an edge over 4 stroke when it comes to ease of maintenance. Further two-stroke engines require maintenance and the cost of doing it is also very low.
- Direct Control: A four-stroke drive has a very large operating speed. Thus reduction gears are used to connect them to the propeller shaft. But with 2 stroke design, we need not need either separate gears ( coupled directly to the propeller shaft ) and proved to be easier to start and reverse.
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