Latest Updates in Mechanical Engineering Interviews

• 1 HP=how much rpm | Latest Updates in Mechanical Engineering

We need Torque to find out.

Apply this formula:

T = HP x 5252
           rpm

T = torque (in lb-ft)
HP = horsepower
5252 = constant
rpm = revolutions per minute

• Which mechanism is used in automobile gearing system

Differential mechanism

• Why different types of sound are produced in different bikes though they say run on SI engine

Engine specifications are different in different manufactures like as bore diameter(cc), ignition timing. Also the exhaust passage take more responsible for sound.

• Why entropy decreases with the increase in temperature?

ds=dQ/T

Entropy is inversely proportional to the temperature so.as temp. increases,entropy decreases

• What type of metal is used in a propeller | Latest Updates in Mechanical Engineering

Aluminium,stainless steel,magnesium bronze, NIBRAL (nickel,brass,aluminium)

• Why involute curve used in gear | Latest Updates in Mechanical Engineering

1) it’s smooth to drive the gear.
2) in gears contact is only point, this type is curve take the more load than normal one

• How Trains take turns, though there is no any differential

Purpose of the differential is to enable vehicles to take turn, by allowing both the wheel to rotate with different speed. This is required when wheels, like in car, are connected with same axle. Interestingly, wheel in trains are not connected with the axle, they are independent and hence can take turns without ant differential. This is same as how the bullock cart takes turns, without any differential.

• In condensation of steam power plant, steam sent in shell side and cooling water on the tube side because

  1. To increase overall heat transfer coefficient of water side velocity can be increases if water is on the tube side
  2. Condenser can act as a storage unit for condensed steam
  3. Rate of condensation of steam is invariably smaller than the mass flow rate of cooling water
  4. It is easier to maintain vacuum on the shell side than on the tube side

Answer:

1. To increase overall heat transfer coefficient of water side velocity can be increases if water is on the tube side

Reason:

Specific volume of steam is larger so more volume is required in condenser

• Why freezer in the fridge is placed in top level? | Latest Updates in Mechanical Engineering

Because density of cold air is higher than warm air,hence it is placed at top. Cold air comes downward & cool the bottom part.

• What is more dangerous longitudinal stress or Hoop stress and why? | Latest Updates in Mechanical Engineering

Hoop stress is more dangerous than longitudinal stress, because the dangerous area coming under the hoop stress is circumferential,while in longitudinal,the axial area will come under dangerous area will will be less than circumferential area.So the hoop stress is more dangerous than longitudinal stress.

• In a spray washing system, if the water temperature is higher than the dry bulb temperature of entering air then the air is | Latest Updates in Mechanical Engineering

  1. Heated and dehumidified
  2. Heated and humidified
  3. Cooled and humidified
  4. Cooled and dehumidified

Answer:

Heated and humidified

• Why fine grained structure is harder than coarse grain structure? | Latest Updates in Mechanical Engineering

Answer:

The smaller the grain size, the more frequent is the pile up of dislocations. With decrease in grain size, the mean distance of a dislocation can travel decreases, and soon starts pile up of dislocations at grain boundaries. This leads to increase in yield strength of the material.

• A diesel engine is usually more efficient than a spark ignition engine because | Latest Updates in Mechanical Engineering

1. Diesel being a heavier hydrocarbon, release more heat per kg than gasoline
2. The air standard efficiency of diesel cycle is higher than the otto cycle, at a fixed compression ratio
3. The compression ratio of a diesel engine is higher than that of an SI engine
4. Self ignition temperature of diesel is higher than that of gasoline

Answer:

3. The compression ratio of a diesel engine is higher than that of an SI engine

• Consider air standard Otto and Diesel cycles, both having the same state of air at the start of compression. If the maximum pressure in both the cycles is the same, then compression ratio ‘r” and the efficiency ‘η’ are related by

1. r Diesel > r Otto
2. r Diesel < r Otto
3. η Otto > η Diesel
4. η Otto < η Diesel

Answer:

1. r Diesel > r Otto and 4. η Otto < η Diesel

• Three engines A, B and C operating on Carnot cycle use working substances as Argon, Oxygen and Air respectively. Which engine will have higher efficiency? | Latest Updates in Mechanical Engineering

1. Engine A
2. Engine B
3. Engine C
4. All Engines have same efficiency

Answer:

4. All Engines have same efficiency

Explanation:

We know that, η = 1 – (T2 / T1)

Since efficiency of the engine purely depends upon source and sink temperature and independent of the working substances. All the engines have same efficiency.

• Which one of the following cycles has the highest thermal efficiency for given maximum and minimum cycle temperatures?

1. Brayton cycle
2. Otto cycle
3. Diesel cycle
4. Stirling cycle

Answer:

4. Stirling cycle

Explanation:

Stirling cycles efficiency is equal to Carnot cycle

• For a heat engine operating on the carnot cycle, the work output is 1/4th of the heat transferred to the sink. The efficiency of the engine is

1. 20 %
2. 33.3 %
3. 40 %
4. 50 %

Answer:

1. 20 %

• For maximum specific output of a constant volume cycle ( Otto cycle) | Latest Updates in Mechanical Engineering

1. The working fluid should be air
2. The speed should be high
3. Suction temperature should be high
4. Temperature of the working fluid at the end of compression and expansion should be equal

Answer:

4. Temperature of the working fluid at the end of compression and expansion should be equal

• For the same maximum pressure and heat input, the most efficient cycle is | Latest Updates in Mechanical Engineering

1. Otto cycle
2. Diesel cycle
3. Brayton cycle
4. Dual combustion cycle

Answer:

2. Diesel cycle

• The order of values of thermal efficiency of Otto, Diesel and Dual cycle, when they have equal compression ratio and heat rejection, is given by

1. η Otto > η Diesel > η Dual
2. η Diesel > η Dual > η Otto
3. η Dual > η Diesel > η Otto
4. η Otto > η Dual > η Diesel

Answer:

4. η Otto > η Dual > η Diesel

• Knocking tendency in a S.I. engine reduces with increasing | Latest Updates in Mechanical Engineering

1. Compression ratio
2. Wall temperature
3. Supercharging
4. Engine Speed

Answer: 

4. Engine Speed

• The air fuel ratio for idling speed of an automobile petrol engine is closer to

1. 10:1
2. 15:1
3. 17:1
4. 21:1

Answer:

1. 10:1

• By higher octane number of SI fuel, it is means that the fuel has | Latest Updates in Mechanical Engineering

1. Higher heating value
2. Higher flash point
3. Lower volatility
4. Longer ignition delay

Answer:

4. Longer ignition delay

• The tendency of petrol to detonate in terms of octane number is determined by comparison of fuel with which of the following?

1. Iso-octane
2. Mixture of normal heptane and iso-octane
3. Alpha methyl naphthalene
4. Mixture of methane and ethane

Answer:

2. Mixture of normal heptane and iso-octane

• For minimizing knocking tendency in S.I. engine, where should the spark plug be located? |  Latest Updates in Mechanical Engineering

1. Near inlet valve
2. Away from both the valves
3. Near exhaust valve
4. Midway between inlet and exhaust valve

Answer:

2. Away from both the valves

• In spark ignition engine knocking can be reduced by | Latest Updates in Mechanical Engineering

1. Increasing the compression ratio
2. Increasing the cooling water temperature
3. Retarding the spark advance
4. Increasing the inlet air temperature

Answer:

3. Retarding the spark advance

• The volumetric efficiency of a well designed S.I. engine is in the range of

1. 40 – 50 %
2. 50 – 60 %
3. 60 – 70 %
4. 70 – 90 %

Answer:

4. 70 – 90 %