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Diesel Engine Fundamentals Assignment

1. With reference to 4-stroke turbo-charged diesel engine:(a) describe the cycle of events with suitable diagrams;(b) draw a fully labelled timing diagram.

1a) The cycles of events are induction, compression, expansion and exhaust.

In induction, the piston is moving downwards from TDC where the suction valve is alsoopened at the same time, allowing fresh charge of air to be drawn in to the cylinder.

Compression is when the piston is moving upwards from BDC where both suction anddischarge valves are closed and the swept volume of air is being compressed veryrapidly to TDC.

Expansion is when the piston is moving downwards from TDC due to the force of theexhausted gases expanding.

Exhaust is the period when the exhaust gases are driven off from the cylinder by theupstroke action of the piston.

Induction Compression Expansion Exhaust

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1b)

2. With reference to 4-stroke turbo-charged diesel engine,

(a) explain why the inlet valves do not open at TDC and close at BDC,(b) explain why the exhaust valves do not close at TDC and open at BDC.

2a) The inlet valve open slightly earlier than TDC to allow the fresh air to rush in removesome of the fine deposits on top of the piston; removing with the exhaust gas whilethe exhaust valve is still open.

The inlet valve closes slightly later than BDC, as air continues to enter due tomomentum allowing increased duration of for intake, thus increase amount of airtaken and reducing resistance to motion when piston changes direction after BDC.

2b) The exhaust valve slightly later than TDC to allow the removal of final traces ofexhaust gases and any fine deposits on top of the piston. It also increase valveoverlap period and capture the momentum of the exhaust gas.

It does not open at TDC but slightly earlier to allow the hot exhaust gases to bedischarged as the remaining energy in gases could not do significant work on thepiston. The gases also start to come out by its own energy (exhaust blow down) thusincreasing duration for exhaust.

Fuel Injection Ends

Fuel Injection Begins

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3. With reference to 2-stroke diesel engine:

(a) describe the cycle of events with suitable diagrams;(b) draw a fully labelled timing diagram.

3a) In the 2-stroke diesel engine, the cycles of events are compression, combustion,

expansion, exhausting (blow-down), scavenging, post scavenging.

In compression, fresh air is compressed from BDC to TDC; raising the temperatureof the air to approximately 550 oC.

In combustion, fuel oil is injected intro the hot air; igniting it.

In expansion, the piston is push downwards by the forces due to the expansion ofgases.

When the piston is almost reaching BDC, the exhaust valves open and blow-down ofexhaust gas takes place. Soon after the inlet ports are uncovered by the piston andfresh charge of medium pressure super charged air rushes into the cylinder.

Remaining exhaust gases are scavenged from the cylinder and at the same timefresh air fills the cylinder for the next cycle.

Compression Combustion Exhaust Scavenge

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3b)

4. State the differences between 2-stroke and 4-stroke engines?

1) Two stroke engine completes one cycle of rotation in only 2-strokes while afour-stroke engine needs four strokes to complete one cycle.

2) Two stroke engines are able to develop much higher power than four strokeengines.

3) 4-stroke engines have fewer working parts than 2-strokes engines.

4) In 4-stroke engines gas exchange takes place through cylinder head inletand exhaust valves.

whereas in 2-stroke engine it is carried out through scavenge ports andexhaust ports/valve.

5) 2-stroke engines require more head room than trunk piston type engines.

6) 2-stroke engines are costlier than 4-stroke engines.

7) Turbocharger is essential for 2-stroke engines to run but which is not thecase with 4-stroke engines.

Fuel Injection Begins

Fuel Injection Ends

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5. State the differences between crosshead engines & trunk type piston engines?

1) The transverse thrust due to the oscillation of the connecting rod is taken upby the crosshead assembly and its guide in the crosshead type of engines,whereas in the trunk-piston type of engines the transverse thrust is taken upby the piston skirt.

2) For crosshead engines, the upper part of the connecting rod is connected to acrosshead assembly consisting of crosshead block, pins and slippers.This crosshead assembly is in turn attached to the lower part of the piston rod.

Whereas in the trunk-piston type engines, the upper part of the connecting rodIs connected directly to the piston via the gudgeon bearing assembly.

3) Crosshead engines have a piston-rod and trunk-piston engines do not.

4) Crosshead engines have a diaphragm that separates the cylinder spacefrom the crankcase and trunk-piston engines do not.

5) There are fewer moving parts in trunk-piston engines comparisontoX-headengines.

6) They used different methods of liner lubrication. Crosshead type engineshave a separate lubrication system for the liner using a special gradecylinderoil.

The cylinder oil is injected into the liner by lubricator pumps through quillsarranged around the circumference of the liner.

Whereas in the most trunk-piston engines' liners are lubricated

by the splash lubrication method and the lubricating oil for the liner is thesame as that for the crankcase.

7) Crosshead engines are normally large 2-stroke slow speed engines,whereas the trunk type engines are usually 4-stroke medium or high speedengines.