Marine I.C engines Cycles and Timing diagrams.

The IC engines or “Diesel Engines” as they are referred to, are named after Rudolf Diesel, who invented these engines in 1893.

The first working engine was built in 1897. It weighed 5 tonnes and produced 20 hp at 172 rpm with an efficiency of about 26%. (Modern low speed diesel engines can have thermal efficiency that exceeds 50%).

In 1912 the first ocean going ship to have installed diesel engines was “Selandia” which was powered by twin, 1010 BHP B&W, 8 cylinder, 4 stroke engines, which were direct coupled.

The worlds largest diesel engine is Wartsilla Sulzer RTA96-C common rail marine diesel of about 113,210 hp @ 102 rpm output.

Internal combustion engine, Diesel engine
Modern Diesel engines operate on Dual Combustion Cycle which is a combination of Constant Volume (Otto) and Constant Pressure (Diesel) cycle is shown in the sketch.
The area of the diagram represents the work done on the piston during one cycle.
Starting from point C, the air is compressed adiabatically to a point D. Fuel injection begins at D, and heat is added to the cycle partly at constant volume (shown by vertical line DP), and partly at constant pressure ( shown by horizontal line PE). At point E expansion begins. This proceeds adiabatically to point F when the heat is rejected to exhaust at constant volume.
The exhaust and suction strokes are shown by horizontal line at C, and this as no effect on cycle.

P-V Diagram of marine disel engine

This theoretical cycle deviates from practical cycle due to :
  • Unavoidable thermal, hydraulic and mechanical losses.
  • The manner in which, and the rate at which, heat is added to the compressed air is a complex function of fuel injection equipment.
  • The compression and expansion strokes are not truly adiabatic. Heat is lost to the cylinder walls to an extent which is influenced by coolant temperature and by design of the heat paths to the coolant.
  • Some of the useful work is expended to perform the induction and exhaust process. Greater losses are involved during exhaust as the unused energy is lost by compressed hot gases when the exhaust valve opens before the piston arrives.
  • Action arising out of reciprocating, rotating and rubbing components also contribute to losses.
  • Some energy is used to drive auxiliaries like lub oil pumps, jacket water pumps, etc.

Nickolaus Otto invented the 4 stroke cycle in 1862.In 1893 Rudolph Diesel invented the compression ignition engine, now named after him.

The four stroke cycle is so called because it takes four strokes of the piston to complete the process needed to convert the energy in the fuel to work. Because the engine is reciprocating, this means the piston has to move up and down the cylinder twice, and therefore the crankshaft must revolve twice.

The four stroke of the piston are known as induction stroke, compression stroke, power stroke and the exhaust stroke.

Induction Stroke : The crankshaft is rotating clockwise and the piston is moving down the cylinder. The inlet valve is open and the fresh charge of air is drawn inside the cylinder at a pressure existing in the intake manifold. The inlet valve closes at the end of the stroke.

Compression Stroke
: Both inlet and exhaust valves are closed and air is compressed by the piston as it moves up in the cylinder. Because energy is transferred into air the pressure and temperature of the air increases.

Power stroke : Just before piston is reaching the TDC ( top dead centre) fuel is injected into the cylinder by fuel injector. Fuel is ignited by the high temperature produced at the end of compression and the expanding gases forces the piston down the cylinder. The gases expand until at the end of the stroke when exhaust valve opens.

Exhaust Stroke : As the piston is approaching the BDC ( bottom dead centre) , the exhaust valve opens. As the piston moves up the cylinder the exhaust gases are expelled from the cylinder.

As the piston approached the TDC again the inlet valve starts to open and the cycle repeats itself.

Four stroke timing and cycle diagram

Timing Diagram : Position of crank at which each operation during the cycle is commenced and completed.

1-2       : Completion of aspiration. Air inlet valve closed. 145-155 BTDC
2-3       :  Compression.
3-4-5    : Fuel Injection. From 10-20 BTDC to 10-20 ATDC
5-6       : Expansion.
6-7-8    : Exhaust. Exhaust valve opens 120-150 ATDC
8-9-10  : Overlap. Air inlet valve opens 70-80 BTDC
10-1     : Aspiration. Exhaust valve closed. 50-60 ATDC
1-etc     : Aspiration continues for next cycle.

4 and 9 are TDC positions and 1 and 7 are BDC positions.


Two stroke cycle was invented by Sir Dugald Clerk in 1881.
The two stroke cycle is so called because it takes two strokes of the piston to complete the process needed to convert the energy in the fuel to work.

The working of two stroke cycle engine differs from that of four stroke cycle engine because of complete absence of two distinct strokes of induction and exhaust. A part of each of compression and expansion strokes in a two stroke engine is utilized for the process of induction and exhaust.

Induction and Compression Stroke : Air is admitted as the scavenge ports are uncovered by the piston. The cylinder is filled up with the charge of fresh air at the beginning of compression. The air is compressed by the piston moving upwards in the cylinder and pressure and temperature of air is increased.

Expansion and Exhaust : Fuel is injected towards end of compression and heat is added to the mass of air and fuel starts to burn as piston is passing over TDC. The expanding gases push the piston downwards.
 At the end of the expansion stroke the exhaust is opened first , the pressure drops below the scavenge manifold pressure and fresh air is admitted through scavenge ports. The cylinder is cleansed of residual products of combustion by incoming air and the cycle is repeated.

1-2    : Completion of scavenge. Scavenge ports close 130-150 BTDC
2-3     : Post scavenge . Exhaust closes 110-150 BTDC
3-4     : Compression
4-5-6    : Fuel injection. Begins at 10-20 BTDC and ends at 10-20 ATDC
6-7    : Expansion
7-8    : Exhaust blowdown. Exhaust opens 110-120 ATDC
8-1    : Scavenge. Scavenge ports open 130-150 ATDC
1-etc    : Scavenging then continues for next cycle.

Position 1 represents BDC and position 5 represents TDC


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