# IC Engine - 2

## 21 Questions MCQ Test Mock Test Series for SSC JE Mechanical Engineering | IC Engine - 2

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Attempt IC Engine - 2 | 21 questions in 12 minutes | Mock test for SSC preparation | Free important questions MCQ to study Mock Test Series for SSC JE Mechanical Engineering for SSC Exam | Download free PDF with solutions
QUESTION: 1

### The efficiency of Diesel cycle approaches to Otto cycle efficiency when:

Solution:

When cut off ratio (α) is equal to zero then the efficiency of Diesel cycle approaches to Otto cycle efficiency.

QUESTION: 2

Solution:

QUESTION: 3

### The compression ratio for petrol engines is:

Solution:

The normal range of compression ratio for diesel engine is 16 to 20 whereas for spark ignition engines it is 6 to 10. Due to higher compression ratios used in diesel engines the efficiency of a diesel engine is more than that of the gasoline engine.

QUESTION: 4

________ is a transmission dynamometer -

Solution:

A dynamometer is a device used for measuring the torque and brake power required to operate a driven machine. It has a device to measure the frictional resistance.

Following are the two types of dynamometers, used for measuring the brake power of an engine.

1. Absorption dynamometers: The entire energy or power produced by the dynamometer is absorbed by the friction resistances of the brake and is transformed into heat, during the process of measurement.

Example: Prony brake dynamometer, Rope brake dynamometer, Hydraulic dynamometer

2. Transmission dynamometers: The energy is not wasted in friction but is used for doing work. The energy or power produced by the engine is transmitted through the dynamometer to some other machines where the power developed is suitably measured.

Example: Epicyclic-train dynamometer, Belt transmission dynamometer, Torsion dynamometer.

QUESTION: 5

A fuel is a mixture of 60% methane and 30% carbon mono-oxide and 10% oxygen by volume. Calculate the stoichiometric oxygen needed.

Solution:

CH4 + 202 → CO2 + 2 H2O

∴ 1 m3 of methane requires 2 m3 of oxygen

∴ 0.6 m3 of methane requires 1.2 m3 of oxygen

2 CO + O2 → 2 CO2

1 m3 of CO requires 0.5 m3 of oxygen, so 0.3 m3 of CO requires 0.15 m3 of oxygen

∴ Total oxygen requires = 1.2 + 0.15 = 1.35 m3

Oxygen already present = 0.1 m3

∴ Stoichiometric oxygen needed = 1.25 m3

QUESTION: 6

An SI engine sometimes continues to run for a very small period even after the ignition is switched off. This phenomenon is called

Solution:

Dieseling is the phenomenon in which an SI engine sometimes continues to run for a very small period even after the ignition is switched off. Dieseling may take place due to following reasons:

• Engine overheating
• Too high spark plug heat range
• High engine idling speed
• Oil entry into the cylinder
• Increase in compression ratio due to carbon deposits
• Incorrect adjustment of idle fuel-air mixture
• Sticking of throttle
• Requirement of tune up of engine
QUESTION: 7

An SI engine sometimes continues to run for a very small period even after the ignition is switched off. This phenomenon is called

Solution:

Dieseling is the phenomenon in which an SI engine sometimes continues to run for a very small period even after the ignition is switched off. Dieseling may take place due to following reasons:

• Engine overheating
• Too high spark plug heat range
• High engine idling speed
• Oil entry into the cylinder
• Increase in compression ratio due to carbon deposits
• Incorrect adjustment of idle fuel-air mixture
• Sticking of throttle
• Requirement of tune up of engine
QUESTION: 8

For the same peak pressure and heat input

Solution:

Otto cycle ⇒ 1 - 2 - 3 - 4

Diesel cycle 1 - 2’ - 3’ - 4’

It is clear from T - s diagram that the heat rejected by the Diesel cycle (area below 4’ - 1) is less than the heat rejection by the Otto cycle (area below 4 - 1); hence the diesel cycle is more efficient than Otto cycle for the condition of same maximum pressure heat input. The dual cycle efficiency lies between the Otto and Diesel cycle.

QUESTION: 9

With increase in compression ratio, flame speed

Solution:

Increased compression ratio reduces the clearance volume and therefore increases the density of the cylinder gases during burning. This increases the peak pressure and temperature and the total combustion duration is reduced. Thus engines having higher compression ratio have higher flame speed.

QUESTION: 10

Volumetric efficiency is a measure of

Solution:

Volumetric efficiency is defined as the ratio of actual volume flow rate of air into the system to the rate at which the volume is displaced by the system.

It indicates the breathing capacity of the system. It is to be noted that the utilization of the air is that determines the power output of the engine. Intake system must be designed in such a way that the engine must be able to take in as much air as possible.

QUESTION: 11

The three way catalytic converter cannot control the following emission

Solution:

Catalytic converters are called three way converters because it reduces the concentration of CO, HC and NOx in the exhaust (three concentrations).

QUESTION: 12

Octane number of natural gas is

Solution:

Octane number of natural gas is around 110 which makes it a very good SI engine fuel. Because of this high octane number the flame speed is higher and engine can operate with high compression ratio.

QUESTION: 13

A single - cylinder engine running at 1800 rpm develops a torque of 8 N - m. The indicated power of the engine is 1.8 kW. The loss due to friction power in the percentage is

Solution:

Brakepower=

Friction power = Indicated Power – Brake Power = 1.8 - 1.508 = 0.292

QUESTION: 14

In an SI engine very high compression ratio cannot be used because

Solution:

If a very high compression ratio is used, than the petrol will reach its self to the ignition temperature before the spark and the knocking will also occur. So compression ratio of petrol engine is less.

QUESTION: 15

Mean effective pressure at a given compression ratio is maximum when the air-fuel ratio is

Solution:

Mean effective pressure is the average pressure inside the cylinder of an internal combustion engine based on the calculated or measured power.

QUESTION: 16

In SI engines, maximum flame speed is obtained when the equivalence ratio is between

Solution:

The ratio of actual fuel air ratio to stoichiometric fuel air ratio is called equivalence ratio.

The highest flame velocities are obtained with somewhat richer mixture of equivalence ratio 1.1 to 1.2. When the mixture is made leaner or richer the flame speed decreases. Less thermal energy is released in case of lean mixture resulting in lower flame temperature. Very rich mixture leads to the incomplete combustion which results again in the release of less thermal energy.

QUESTION: 17

For CI engines, fuels most preferred are

Solution:

For CI engines, the normal paraffin’s are the best fuels and aromatics are the least desirable.

For SI engines the aromatics are the best fuel and the paraffins are least desirable.

The reason for this is normal paraffins exhibit the poorest antiknock quality when used in SI engines. But the antiknock quality improves with the increasing number of carbon atoms and the compactness of the molecular structure. The aromatics offer the best resistance to knocking in SI Engines.

QUESTION: 18

The intake charge in a diesel engine consist of

Solution:

In diesel engine intake is only air. The air is compressed and then the fuel is injected to the high pressure and temperature compressed air.

QUESTION: 19

A gas engine has a swept volume of 200 cm3 and clearance volume 20 cm3. The mechanical efficiency is 0.9 and the volumetric efficiency 0.85. The volume of the mixture taken in per stroke will be

Solution:

Volumetricefficiency=

∴ Actual intake volume = 200 × 0.85 = 170 cm3

QUESTION: 20

The adiabatic flame temperature of a reaction is controlled by

Solution:

For a combustion process that takes place adiabatically with no shaft work, the temperature of the products is referred to as the adiabatic flame temperature. This is the maximum temperature that can be achieved for given reactants. The maximum adiabatic flame temperature for a given fuel and oxidizer combination occurs with a stoichiometric mixture (correct proportions such that all fuel and all oxidizer are consumed). The amount of excess air can be tailored as part of the design to control the adiabatic flame temperature.

QUESTION: 21

Engine of different cylinder dimensions, power and speed are compared on the basis of

Solution:

For any particular engine, operating at a given speed and power output, there will be a specific mean effective pressure, mep. This mep is used as a basis to compare the performance.

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