Unit 1: Internal Combustion Engines (types of engine ,2 stroke and 4 stroke engine (diffrences also), pv diagram, )

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IC Engine Working Principle is based on the 3 Fundamental Thermodynamic Theoretical Cycles.

Otto Cycle (heat add & reject at constant Vol)

Diesel Cycle (heat add at constant Pr & reject at constant Vol)

Dual Combustion Cycles (heat add partly at constant Vol & Pr & heat reject at constant Vol)

Actual engines do not operate strictly following these ideal cycles 

One Cycle consists of 4 operations:

(1) Suction / Induction / Scavenge

(2) Compression

(3) Power / Expansion / Working / Firing

(4) Exhaust

(i) 4 Stroke Cycle Engine

        One complete sequence of 4 OPERATIONS is completed in:

TWO REVOLUTIONS of the Crank Shaft

(or)

FOUR STROKES of the Piston

(ii) 2 Stroke Cycle Engine

      One complete sequence of 4 OPERATIONS is completed in:

ONE REVOLUTION of the Crank Shaft

(or)

TWO STROKES   of the Piston

Basic Marine Engineering BookPage No. 242   Fig 14.1  and    Page No. 243   Fig 14.2
 

 

PV Diagram



 

Difference between Four Stroke Engine &  two-stroke engine

  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