different types of marine diesel engines( slow, medium, fast, 2 stroke, four stroke)

Slow Speed Engines :

Slow Speed engines are engines in the range of 50 – 300 rpm at rated power. These are mostly two stroke, crosshead engines with between 4 to 12 cylinders in line and turbocharged.

The stroke – bore ratio of low speed engines varies between 2.5:1 to 4.2:1. These engines are called Super long stroke engines, and are well suited to operate on low quality fuels and have proven to be economically advantageous.

These engines are more fuel efficient and lub oil consumption is less when compared to medium and high speed engines, though the initial cost is considerably higher.

Low speed engines are generally intended for direct drive, hence connected to the propeller directly.

Medium and High Speed Engines :

Medium Speed engines are in the range of 300 – 800 rpm whereas high speed engines are above 800 rpm, though there are no clear physical features that enable the distinction to be made. These are usually four stroke, trunk piston engines which have up to 10 cylinders inline or up to 18-24 cylinders in V configuration, and mostly turbocharged.

Stroke to bore ratios are not very high, normally up to 1.5:1, because of the forces created by high speeds. These engines are lighter, more compact, and lower in acquisition cost than low speed engines.

These engines have advantage of high power to weight ratio and used extensively in passenger / cruise ships due to less head room required.

Though medium speed engines have proved capable of running on fuels of poor quality, high speed engines are normally restricted to distillate fuels.

The maintenance required on these engines is more and these are more noisy as compared to slow speed engines.

Medium and high speed engines, because of their higher rpm, drive propellers through speed reduction gears.

2 STROKE AND 4 STROKE ENGINES

TWO STROKE ENGINES :

Advantages :

* Able to develop much higher Power at lower rotational speed.

* Cheaper quality fuels can be used

* Slow speed so less wear and tear of moving parts

* Turning movement is more uniform, lighter flywheel can be used.

* Cost of lubricants is less.

* Crosshead bearings capable of sustaining high loads.

* High stroke bore ratio of about 4.2:1

Disadvantages :

* Requires more space due to super long strokes.

* Expensive

FOUR STROKE ENGINES :

Advantages

* High Power to weight ratio.

* Requires less head room

* Working parts are fewer and less costly.

Disadvantages :

* Gudgeon pin Bearing assembly not suited for higher loads

* Cost of lubricant is more

* Medium speed or high speed, more wear and tear of moving parts.

* Power developed is less.

* Due to 1 power stroke is every 2 revolutions the turning movement is not

so uniform. Heavier flywheel is required.

* Difficulty in burning cheaper quality fuels.

* Maintenance required is more