m 1.iv diesel engines cycles

8/2/2019 m 1.IV Diesel Engines Cycles

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According to Operating Cycles.

(a) OTTO CYCLE (Constant Volume Combustion

Cycle).

It is the ideal air standard cycle for Petrol

engine, the gas engine and the high-speed oilengine. The engines based on this cycle havehigh thermal efficiency but noisiness resultsparticularly at higher power due to higher

pressures in the cylinders.



Otto cycle

T = temperature

S = Entropy

Constant volume cycle

1-2 isentropic compression

2-3 heat addition at constant

volume

3-4 isentropic expansion4-1 heat rejection at constant

volume

Air standard thermal efficiency

= 1-(1/r)

, r = comp. Ratio.



entropy (n) measure of the unavailabilityof a system's thermal energy forconversion into mechanical work. [Greek:

related to *en-2, trope transformation]



(b) DIESEL CYCLE (Constant Pressure CombustionCycle).

It is the ideal Air standard cycle for Diesel Engine,especially suitable for low speed Diesel Enginebut not for high speed Diesel Engine. (The

thermal efficiency is lower than Otto cycleengines but engines run smoothly due to lowerpressures in the cylinder.



(c) DUAL COMBUSTION CYCLE (Constant Pressureand Constant Volume Combustion Cycle).

Modern Diesel Engines do not operate purely on

constant pressure combustion cycle but somepart of combustion process takes place atconstant volume while the rest is completed atconstant pressure.

In general, this cycle resembles Constant volumecombustion Cycle more than constant pressurecombustion cycle. It is suitable for modernMedium and High Speed Diesel Engines. The

thermal efficiency is more than Diesel Cycle butless than Otto cycle. Also noise level is inbetween the two. This is a more practicalengine.



Dual combustion

Mixed cycle of otto and diesel cycle

Heat added partly at constant volume and partly at

constant pressure.hence having advantages of both cycles.



According to Strokes/Cycle. In an engine, the following events form a cycle:Filling the engine cylinder with fresh air.

Compressing the air so much that injected fuel ignitedreadily by coming in contact with hot air and burnsefficiently.

Combustion of fuel.Expansion of hot gases.

Emptying the products of combustion from the cylinder.Depending on how many strokes of piston are required in

completing this cycle, the engines can be divided intotwo classes:

1. Four Stroke Engine An engine, which needs 4 strokes of the piston (2 in and 2

out) to complete one cycle, is called Four-stroke engine.2. Two Stroke Engine An engine that needs only 2 strokes of the piston (1 in and

1 out) to complete one cycle is called Two-stroke engine.



According to Piston Connection.

Trunk Piston Type.



The piston is connected directly to the upper end

of the connecting rod. A horizontal pin (GudgeonPin) within piston is encircled by the upper endof the connecting rod. This construction is quite

common, especially in small and medium sizeengines.



(b) Cross Head Type.

The piston fastens to a vertical piston rodwhose lower end is attached to a ‘crosshead’, which slides up and down inguides. The crosshead carries a crosshead

pin, which is encircled by the upper end of the connecting rod. This more complicatedconstruction is common in double acting

engines and large slow speed single actingengines.



Comparison between Trunks Piston VersusCross Head Engine.

Most medium and small size engines use trunk pistons. Resulting side thrust causes the piston topress against the cylinder wall, first on one side,then on the other. At the top of stroke, when thegas pressure is greatest, side thrust is negligible

(due to small connecting rod angle). So most of wear takes place at the middle of stroke: makingpiston skirt increases thrust-bearing area, andhence reduces wear. In medium and small sizeengines, due to lower gas pressure, units’ side

pressure is so small that neither piston nor linerwears much.



In crosshead engines, crosshead takes the sidethrust, which will be high in large engines. So,crosshead engines have the following

advantages:Easier lubrication.Reduced liner wear.Uniformly distributed clearance around piston.

Simpler piston construction because the ‘Gudgeonpin’ and its bearing are eliminated.However these advantages of cross head engines

are offset by:

Greater complication. Added weight. Added height.Careful adjustments.



According to method of Charging.

(a) Natural aspirated Engine.

The vacuum is created when the piston movesaway from the combustion space draws in thefresh charge.

(b) Supercharged Engine.The charge is admitted into the cylinder at a

higher than atmospheric pressure. This highpressure is produced by a pump or blower orexhaust gas turbocharger.



According to Speed.

Slow Speed Engines: 100 to 150 r.p.m.

Medium Speed Engines: 300 to 1000 r.p.m.

High Speed Engines: More than 1000 r.p.m.



According to Bore/Stroke Ratio:a) Square Engine:If bore/stroke is about one, crankshaft web

dimensions become less compared to journal andcrankpin.

b) Over Square Engines (Short Stroke)If bore/stroke > 1, web dimensions (less height,

more thickness) are such that webs will be weak.So generally over square engines are not used.



(c) Long Stroke Engines.

Generally, engines have stroke/bore >1. This

gives crankshafts of good strength. Mostcommon ratio is stroke/bore = 2. 0: 2.2.

(d) Super-long Stroke Engines.

To have better propeller efficiency and bettercombustion even with lower grade fuels,lower r.p.m. engines with longer strokes are

gaining popularity. These engines havestroke/bore ratio = 3.



The Diesel Engines find the followingapplication on board merchant ships.

Main Propulsion.Electric Power generation.

Emergency Pumps (e.g. fire pump).

Life Boat.

Emergency Generator.

Emergency Air Compressor



REASONS FOR WIDE USE OF DIESELENGINES IN MARINE POWER PLANTS.

1. Small fuel consumption:

Diesel Engine is one of the most efficient heatengines. Hence it gives more power with lessfuel. It is an engine of high economy.

2. Cheap fuel:

Diesel engine uses fuel costing very less ascompared to other engines.

3. Economy at light loads:Diesel Engine is not only efficient when it is fully

loaded, but also when it is partly loaded.



4. Greater Safety:Diesel fuel is non-explosive and less flammable at

normal temperatures and pressures. It requires

special effort to make it start to burn. This featuremakes it very attractive in the marine trade,because it would be much safer carrying diesel oilon board ships.

Diesel exhaust gases are less poisonous than otherengines, because they contain less carbonmonoxide.

5. Ignition System is not required:Diesel engines do not require battery or magneto

running them.6. More power can be produced due to more

compression allowed.7. Diesel Engine is more robust and stronger.



8. Economy in small sizes: As great contrast to steam power plant, a small

diesel engine has nearly as good an economy as

a large one. This makes it possible to enlarge adiesel engine plant with additional units as theload grows. At all stages of growth, the efficiencyis high.

9. Sustained economy in service: Again in contrast to a steam power plant, dieselefficiency falls off very little during thousands of hours of use between overhauls.

10. Lightness and compactness:Diesel engine plants have less weight and space

per unit power. It is therefore well suited toportable and mobile installations.



11. Independence of water supply: A diesel engine requires very less water in

contrast to steam plants.12. Quick Starting.

A cold diesel engine can be started instantlyand made to carry its full load in few

minutes. It is therefore ideal for supplyingemergency power.13. Easily in Maneuvering:

A diesel engine can be made to run at full

power in either direction.14. Economy in Labour.No fire room force is needed.



15. Freedom from nuisance:There are no ashes to be disposed of, no noisy

and dusty coal handling and pulverisingequipment to maintain, no smoke, and noise canbe easily eliminated. Due to above mentionedreasons, Diesel engines are quite popular onboard ships.

These reasons can very well be regarded as theadvantages of Diesel Engines over other primemovers such as gasoline engines, gas turbines,steam engines, steam turbines and hydraulic

turbines.However, Diesel engines also have certaindisadvantages, which can be listed asfollowing:



1. Cost:Diesel engines, because of the higher pressures at

which, they work, require sturdier construction,

better materials and closer fits than gasolineengines. Therefore, they cost more to build.2. Weight:Because of sturdier construction, weight per power is

more than gasoline engines.3. Attendance: A diesel engine requires more attention than an

electric motor running on purchased current. It alsorequires more attention per unit of power producedthan a large steam turbine.

4. Fuel Cost:Oil used in Diesel engines is costlier than coal. Hence,

steam power plants using coal as fuel are cheaper in

operation.



According to Operating Cycles.

(a) OTTO CYCLE (Constant Volume Combustion

Cycle).It is the ideal air standard cycle for Petrol engine,

the gas engine and the high-speed oil engine.The engines based on this cycle have highthermal efficiency but noisiness resultsparticularly at higher power due to higherpressures in the cylinders.



Otto cycle

T = temperature

S = Entropy

Constant volume cycle

1-2 isentropic compression

2-3 heat addition at constantvolume

3-4 isentropic expansion

4-1 heat rejection at constantvolume

Air standard thermal efficiency= 1-(1/r) , r = comp. Ratio.



(b) DIESEL CYCLE (Constant Pressure CombustionCycle).

It is the ideal Air standard cycle for Diesel Engine,especially suitable for low speed Diesel Enginebut not for high speed Diesel Engine. (The

thermal efficiency is lower than Otto cycleengines but engines run smoothly due to lowerpressures in the cylinder.

(c) DUAL COMBUSTION CYCLE (Constant Pressure



(c) DUAL COMBUSTION CYCLE (Constant Pressureand Constant Volume Combustion Cycle).

Modern Diesel Engines do not operate purely on

constant pressure combustion cycle but somepart of combustion process takes place atconstant volume while the rest is completed atconstant pressure.

In general, this cycle resembles Constant volumecombustion Cycle more than constant pressurecombustion cycle. It is suitable for modernMedium and High Speed Diesel Engines. Thethermal efficiency is more than Diesel Cycle butless than Otto cycle. Also noise level is inbetween the two. This is a more practicalengine.



Dual combustion

Mixed cycle of otto and diesel cycle

Heat added partly at constant volume and partly at

constant pressure.hence having advantages of both cycles.



According to Strokes/Cycle. In an engine, the following events form a cycle:Filling the engine cylinder with fresh air.

Compressing the air so much that injected fuel ignitedreadily by coming in contact with hot air and burnsefficiently.

Combustion of fuel.Expansion of hot gases.

Emptying the products of combustion from the cylinder.Depending on how many strokes of piston are required in

completing this cycle, the engines can be divided intotwo classes:

1. Four Stroke Engine

An engine, which needs 4 strokes of the piston (2 in and 2out) to complete one cycle, is called Four-stroke engine.

2. Two Stroke Engine An engine that needs only 2 strokes of the piston (1 in and

1 out) to complete one cycle is called Two-stroke engine.



According to Piston Connection.

Trunk Piston Type.



The piston is connected directly to the upper end

of the connecting rod. A horizontal pin (GudgeonPin) within piston is encircled by the upper endof the connecting rod. This construction is quitecommon, especially in small and medium sizeengines.



(b) Cross Head Type.

The piston fastens to a vertical piston rod

whose lower end is attached to a ‘crosshead’, which slides up and down inguides. The crosshead carries a crosshead

pin, which is encircled by the upper end of the connecting rod. This more complicatedconstruction is common in double acting

engines and large slow speed single actingengines.



Comparison between Trunks Piston VersusCross Head Engine.

Most medium and small size engines use trunk pistons. Resulting side thrust causes the piston topress against the cylinder wall, first on one side,then on the other. At the top of stroke, when thegas pressure is greatest, side thrust is negligible

(due to small connecting rod angle). So most of wear takes place at the middle of stroke: makingpiston skirt increases thrust-bearing area, andhence reduces wear. In medium and small sizeengines, due to lower gas pressure, units’ side

pressure is so small that neither piston nor linerwears much.



In crosshead engines, crosshead takes the sidethrust, which will be high in large engines. So,crosshead engines have the following advantages:

Easier lubrication.Reduced liner wear.Uniformly distributed clearance around piston.

Simpler piston construction because the ‘Gudgeonpin’ and its bearing are eliminated.However these advantages of cross head engines

are offset by:

Greater complication. Added weight. Added height.Careful adjustments.



According to method of Charging.

(a) Natural aspirated Engine.

The vacuum is created when the piston movesaway from the combustion space draws in thefresh charge.

(b) Supercharged Engine.The charge is admitted into the cylinder at a

higher than atmospheric pressure. This highpressure is produced by a pump or blower or

exhaust gas turbocharger.



According to Speed.

Slow Speed Engines: 100 to 150 r.p.m.

Medium Speed Engines: 300 to 1000 r.p.m.

High Speed Engines: More than 1000 r.p.m.



According to Bore/Stroke Ratio:a) Square Engine:If bore/stroke is about one, crankshaft web

dimensions become less compared to journal andcrankpin.

b) Over Square Engines (Short Stroke)If bore/stroke > 1, web dimensions (less height,

more thickness) are such that webs will be weak.So generally over square engines are not used.



(c) Long Stroke Engines.

Generally, engines have stroke/bore >1. This

gives crankshafts of good strength. Mostcommon ratio is stroke/bore = 2. 0: 2.2.

(d) Super-long Stroke Engines.

To have better propeller efficiency and bettercombustion even with lower grade fuels,lower r.p.m. engines with longer strokes are

gaining popularity. These engines havestroke/bore ratio = 3.



The Diesel Engines find the followingapplication on board merchant ships.

Main Propulsion.Electric Power generation.

Emergency Pumps (e.g. fire pump).

Life Boat.

Emergency Generator.

Emergency Air Compressor



REASONS FOR WIDE USE OF DIESELENGINES IN MARINE POWER PLANTS.

1. Small fuel consumption:

Diesel Engine is one of the most efficient heatengines. Hence it gives more power with lessfuel. It is an engine of high economy.

2. Cheap fuel:

Diesel engine uses fuel costing very less ascompared to other engines.3. Economy at light loads:Diesel Engine is not only efficient when it is fully

loaded, but also when it is partly loaded.



4. Greater Safety:Diesel fuel is non-explosive and less flammable at

normal temperatures and pressures. It requires

special effort to make it start to burn. This featuremakes it very attractive in the marine trade,because it would be much safer carrying diesel oilon board ships.

Diesel exhaust gases are less poisonous than otherengines, because they contain less carbonmonoxide.

5. Ignition System is not required:Diesel engines do not require battery or magneto

running them.6. More power can be produced due to more

compression allowed.7. Diesel Engine is more robust and stronger.



8. Economy in small sizes: As great contrast to steam power plant, a small

diesel engine has nearly as good an economy as

a large one. This makes it possible to enlarge adiesel engine plant with additional units as theload grows. At all stages of growth, the efficiencyis high.

9. Sustained economy in service: Again in contrast to a steam power plant, diesel

efficiency falls off very little during thousands of hours of use between overhauls.

10. Lightness and compactness:Diesel engine plants have less weight and space

per unit power. It is therefore well suited toportable and mobile installations.

11 I d d f t l



11. Independence of water supply: A diesel engine requires very less water in

contrast to steam plants.

12. Quick Starting. A cold diesel engine can be started instantly

and made to carry its full load in few

minutes. It is therefore ideal for supplyingemergency power.13. Easily in Maneuvering:

A diesel engine can be made to run at full

power in either direction.14. Economy in Labour.No fire room force is needed.



15. Freedom from nuisance:There are no ashes to be disposed of, no noisy

and dusty coal handling and pulverisingequipment to maintain, no smoke, and noise canbe easily eliminated. Due to above mentionedreasons, Diesel engines are quite popular onboard ships.

These reasons can very well be regarded as theadvantages of Diesel Engines over other primemovers such as gasoline engines, gas turbines,steam engines, steam turbines and hydraulic

turbines.However, Diesel engines also have certaindisadvantages, which can be listed asfollowing:

1 Cost:



1. Cost:Diesel engines, because of the higher pressures at

which, they work, require sturdier construction,

better materials and closer fits than gasolineengines. Therefore, they cost more to build.2. Weight:Because of sturdier construction, weight per power is

more than gasoline engines.3. Attendance:

A diesel engine requires more attention than anelectric motor running on purchased current. It alsorequires more attention per unit of power producedthan a large steam turbine.

4. Fuel Cost:Oil used in Diesel engines is costlier than coal. Hence,

steam power plants using coal as fuel are cheaper in

m 1.iv diesel engines cycles

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