internal commercial engine (ice)

8/21/2019 Internal Commercial Engine (ICE)

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Internal Combustion Engine (ICE)

Some Important terms:

TDC = Top dead centre

BDC = Bottom dead centre

GT = Gas Turbine

CR = Compression ratio

1.What are functions of connecting rods in ICE?

Ans: It is a link between the piston and crankshaft, whose main function is to transmit force from the

piston to the crankshaft. Moreover, it converts reciprocating motion of the piston into circular motion of

the crankshaft.

2.What are functions of piston rings in ICE?

Ans: There are two sets of rings mounted on the piston. These are circular rings and made of special

steel alloys which retain elastic properties even at high temperature. The function of the upper rings is

to provide air tight seal to prevent leakage of the burnt gases into the lower portion. The function of the

lower rings is to provide effective seal to prevent leakage of the lubricating oil into the engine cylinder. It

also reduces friction (less contact area) and so less wear and tear of cylinder wall.

3.What should be the length of the connecting rod?

Ans: The upper end of the connecting rod is fitted to the piston and the lower end to the crank. So the

length of the connecting rod should be such that it does not touch the cylinder when the crank position

is 90. So the longer connection is better. The radius of the cylinder should be more than 2r, where r is

crank radius.


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4.What are the functions of lubricating oil in ICE?

Ans: The functions of lubricating oil in ICE are given below:

Prevents metal to metal contact between moving parts there by makes the motion possible.

Reduces friction between load-bearing surfaces.

Damps mechanical shock in gears to other mounting park (bearing etc.).

Dissipates the heat generated from the moving parts due to friction and Limits the temperature

by carrying away heat from fluid friction and combustion of fuel i.e it works as a coolant.

Cleans the metal surfaces from sludge, carbon soots. Where this is working as detergent and

dispersant.

Makes the piston movement air tight (filled of oil).

5.Why inlet valve does not close at BDC in 4 stroke ICE?

Ans: In 4 stroke ICE when the inlet valve opens, charge is sucked into the cylinder and the piston

moved downwards from the top dead center (TDC) and it continues till the piston reaches its

bottom dead center (BDC). But the charge continues to come into the cylinder because the area of

valve is small and piston area is big. Moreover the piston speed is high. So inlet valve is kept open as

long as charge comes in the cylinder. When piston travels upward, inlet valve is closed because the

charge wants to go out through the inlet value after BDC. The angle after BDC of crank will depend

on piston speed and valve area. Lower piston speed and more inlet valve area makes less no of

degrees after BDC for valve closing.

6.Why exhaust valve does not close at TDC stroke ICE?

Ans: The exhaust valve is open as the piston moves from BDC to TDC. This movement of the piston

pushes out the products of combustion from the engine cylinder through the exhaust valve into the

atmosphere. Because the valve area is small compared to piston area and speed of piston is high.

The gases continue to go out, when the piston is at TDC and we allow the exhaust to go out even

when the piston goes down. Moreover the inlet valve opens before TDC and hence both the valves

are open at a time and the charge enters the cylinder and drives out the exhaust through the

exhaust valve. There by in the clearance volume fresh charge comes in replacing the exhaust gases

giving more power. This is scavenging. So to get rid of exhaust gases from cylinder, the exhaust valve

closes after TDC.


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7. Dia of piston = 10cm, crank radius = 6 cm, CR = 8, how much is clearance volume?

Ans: We know,

= 1 +

= 8-1 =9.25

= 1.34 104

8.Why there can be 3 valve in each cylinder of ICE?

Ans: In cylinder there are 3 valve because exhaust is by pushing of piston. But inlet is by suction.

There can be 3 valves, 2 inlets and one outlet to give more area for inlet valves to get more charge

(which is by suction) in the cylinder. To get more power from the engine.

9.Why we need to cool lubricating oil in diesel engine?

Ans: The oil is supplied to lubricate the rocker arm gear, operating the inlet and exhaust valves and

to the cranks and crankshaft drive piston and cylinder, bearings in diesel engine. During the process

of lubrication, it also cools the parts and the oil is heated. This hot oil is to be cooled to supply its for

lubrication. So oil gets heated by the parts of the engine and is cooled before sending it for

lubricating by pump and oil cooler.

10.Why CR of diesel engine is higher than petrol engine?

Ans: The ratio of total cylinder volume to the clearance volume is known as compression ratio.

Diesel fuel is injected under higher pressure into cylinders than petrol engines. The reason

being is that in diesel engines air is sucked and compressed during compression stroke. At the end of

the compression, oil is injected in the cylinder. The temperature of air in the cylinder should be such

that the oil will burn by itself. So temperature in the cylinder is more than SIT (Self Ignition

Temperature) of oil. To get this we must compress to a high pressure and it is possible because we

compress only airs. Thats why CR of diesel engine is more than petrol engine.


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11.What is scavenging in ICE? How it is clone in 4 stroke, 2 Stroke ICE?

Ans: The process of removing burnt gases, from the combustion chamber of the engine cylinder, is

known as scavenging.

In a four stroke cycle engine, the scavenging is very effective, as the piston during the exhaust

stroke, pushes out the brunt gases from the engine cylinder and when it is near TDC the inlet valve is

opened so the charge enters the cylinder and drives out the brunt gases. Inlet valve and exhaust

valve are kept open at the same time to get better scavenging.

In a two stroke engine, the scavenging is less effective as the exhaust port is open for a small

fraction of the crank revolution. Moreover, as the inlet and exhaust ports are opened

simultaneously during a part of the crank revolution. As the transfer, therefore fresh charge also

escapes out along with the burnt gases.

12.What is S.I.t? Why it is important for both diesel and petrol engine?

Ans: S.I.T means Self Ignition Temperature.

It is important for both diesel and petrol engine because at this temperature combustion stars.

For the diesel engine S.I.T should be low so that when fuel enters the cylinder. It will burn

without accumulation but in petrol engine the S.I.T of fuel should be high so that it will not ignite by

itself. In diesel engine we want outer auto ignition but in petrol engine, we do not want auto

ignition. So S.I.T is important for both petrol (high S.I.T) and diesel engine (low S.I.T).

13.Why the thermal efficiency of 4 stroke petrol engine is more than 2 stroke petrol engine having

same CR?

Ans: In 4 stroke engine intake and exhaust pressures are well defined over a longer stroke and good

scavenging is there but in 2 stroke engine these processes are not well defined and takes only a part

of the stroke. Thats whythermal efficiency of 4 stroke petrol engine is more than 2 stroke petrol

engine.

14+15.Diffrence between 2 and 4 stroke petrol engine and difference between 4 stroke diesel and

petrol engine ?---------------Answer From Book (Page no 592)


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16.Draw valve timing diagram of 4 stroke cycle diesel and petrol engine.

Ans: The diagram is given below:

Fig: 4-stroke cycle petrol engine. Fig: 4-stroke cycle diesel engine.

17.Why we use a linear in the cylinder of ICE?

Ans: During operation piston and piston rings are wear out and so the cylinder also wears out. If we

put a linear inside the cylinder, the linear will wear out and can easily be replaced at less cost but

cylinder requires there. If not there cylinder, the engine block is to be replaced or over size ring and

piston is to be used. So we may use linear (which is another cylinder) in the cylinder.

18.Why the term CR is not used in gas turbine?

Ans: The ratio of total cylinder volume to the clearance volume is known as compression ratio.

= +

Clearance volume

There is no swept volume or clearance volume in gas turbine (unlike ICE). Reciprocating engine has

this term. Gas turbine is rotary engine. So compression ratio is not used in gas turbine.


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19.Why there is no cooling system similar to petrol and diesel engine, in gas turbine?

Ans: In gas turbine the air from compressor enters the combustion chamber in two ways. One air for

combustion enters at the center of the combustion chamber as primary air and the balance as

secondary air in the periphery of the combustion chamber (CC). This air flows along the walls of the

CC. There by absorbing heat from the burning flam and not allowing the heat to go to the walls of

the CC. There by the air become hot and mixes with the products of combustion and complete the

combustion. The mass flow rate is more than what is needed for combustion. This increased mass

flow rate increases the power of the gas turbine and there is no need to use any extra cooling for

combustion chamber like in petrol or diesel engine.

20.Draw P-V diagram for Otto Cycle, Diesel Cycle and Brayton (Joules) Cycle.

Ans:

Otto Cycle Diesel Cycle Brayton-Joules Cycle

21. At full load where AFR is highest + why? In petrol or diesel engine or gas turbine?

Ans: Air fuel ratio is highest in gas turbine. In a diesel too much fuel (a low AFR) means smoke and

engine heat. The AFR of diesel is medium (20 : 1). To increase the power AFR is used. In petrol AFR is

14 : 1 lowest and gas turbine AFR is (70 : 1) highest. To cool we use extra air which flows (for fluid

cooling) along the surface of the ce and hence AFR is highest in gas turbine.

22.Describe water cooling system of diesel engine and marine diesel engine.

Ans: The water is circulated through water jackets around each of the combustion chambers,

cylinder, valve seats, valve stems. The water is kept continuously in motion by a centrifugal water


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pump which is driven by a V-belt from the pulley on the engine crank shaft. After passing through

the engine jackets in the cylinder block and heads, the water is passed through radiator. In the

radiator, water is cooled by air drawn through the radiator by a fan. Fan and water pump are

mounted and driven on a common shaft. After passing through the radiator, the water (which is

now cooled) is drained and delivered to the water pump through a cylinder inlet passage. The

process is repeated again and again. By this process diesel engine and marine diesel engine areoperated. Sometimes there is a thermostat at radiator to increase efficiency of engine.

23.Diesel engines but not petrol engines are used in power plants, ships?

Ans: Because petrol is costlier than diesel oil.

In diesel engines cylinders are of very large diameter and its capacity is measured by HP

(hourse power) and

is high in diesel engine. Besides the compression ratio and

efficiency is very high. But in petrol engine compression ratio is low and if the ratio will high than

petrol engine compression will auto. A petrol engine has compression ratio from 6 to 10 and

thermal efficiency is upto about 26%. But in the diesel engine efficiency is upto 40% and

compression ratio from 15 to 25. So diesel engine are used in power plants but not petrol engine.

24.Engine RPM =1500, crane radius 5cm. Find average velocity of piston. Why it is called average

velocity?

Ans: Here,

Stroke length, L= 2 X 0.05 and N=1500

=.1 m

So; Average velocity, V = 2LN

= 2 X 0.1 X 1500

=300 m/minute

Answer: The average velocity is 300 m/minute

Since the velocity of piston varies, so, we have average speed. Velocity at TDE and BDC is

zero i.e stationary. So the velocity of piston is called average velocity.


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25.Which components has maximum percentage in diesel, petrol engine and gas turbine

exhaust at full load.?

Ans: N2 has maximum percentage in diesel. Petrol engine and gas turbine exhaust at full

load. When the fuel with air enters the cylinder we have 77% by weight of N2 in air and goes

to exhaust valve. Through this N2 comes out through the compressing the exhaust gas

contains maximum N2. The exhaust gas also contains C, H2 etc. Some N2 may form oxides of

N2 but N2 remains as N2. So that the percentage 77% remain highest and next is CO.

internal commercial engine (ice)

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