top 25 mechanical engineering interview questions & answers

1. Explain the reason that why big tyres are used in rear of vehicles.

1. Large tyres provide larger surface area touching the ground, thus providing the more pulling power. Basically tractors have larger rear tyres because tractors don’t have to operate at higher speeds, thus getting the same ground speed with larger tyres. Moreover gear ratios don’t have to be as high as b/c of the tyres.

2. Explain the reason that why the tyres are always black in colour. Is this phenomena related to the heat conduction?

Tyres colour is black due to the proportion of carbon mixed in it during the vulcanization of the rubber, to make the tyre shear resistant. Without vulcanization tyres won't be able to bear the friction heat and stress of the road. Thus carbon is responsible for giving tyres the prominent black colour. Adding of carbon prevents tyres from rapidly deteriorating because when ozone combines with the ultraviolet light from the sun. It attacks the polymer of the tyre.

3. What is BHP? How can you measure BHP?

3. BHP is ‘Brake Horsepower’. It reflects the powerfulness of the brakes of the vehicles. BHP stands for how much power will be required to make the engine stop working i.e. to stop the rotation of the engine.
BHP = 2 times pi times torque times revolutions; all this divided by 550.
Pi is 3.1416 and torque is in pounds-feet, and revolutions are revs per second

4. What is the difference between scavenging and supercharging ?

4. Scavenging is process of flushing out burnt gases from engine cylinder by introducing fresh air in the cylinder before exhaust stroke ends. Supercharging is the process of supplying higher mass of air by compressing the atmospheric air.

5. What happens if gasoline is used in a Diesel engine?Diesel engine will work ?

5. No, It will not work,as the Compression ratio of Petrol engine is 6 to 10 & that of Diesel engine is 15 to 22. Thus on such high compression, gasoline gets highly compressed & it may blast.

6. 1 hp how much watt?

6.746.2Watt

7. How to calculate bearing number to diameter of the inner and outer

7.Divide the shaft diameter size by 5, it will give last two digitof the bearing no. and according to type of load we have tochose the type of bearing and that will give prior no. ofthe bearing.

8. Definition of Octane number & Cetane number

8. Octane No. - Octane number is defined as the percentage, by volume, of iso octane in the mixture of iso octane and h-heptane. It is the measure of rating of SI engine.

Cetane No. - Cetane number is defined as the percentage, by volume, of n-cetane in the mixture of n-cetane and alpha methyl naphthalene. It is the measure of rating of CI engine.

9. Poisons ratio is higher in (rubber/steel/wood)

9. When a material is compressed in one direction, it usually tends to expand in the other two directions perpendicular tothe direction of compression. This phenomenon is called thePoisson effect. Poisson’s ratio is a measure of the Poisson effect.
For rubber = 0.5

For steel = 0.288

10. What is fuel ratio ?

10. Fuel ratio is the ratio of its % age of fixed carbon to volatile matter.

11. Explain the second law of thermodynamics.

11.The entropy of the universe increases over time and moves towards a maximum value.

Mcq types:

12.The reference fuels for knock rating of spark ignition engines would include

A iso-octane and alpha-methyl naphthalene B normal octane and aniline C iso-octane and normal hexane D normal heptane and iso-octane

Answer: Option D

13.The diesel engines are also known as __________ engines.

A compression ignition B spark ignition Answer: Option A 14.The thermal efficiency of a standard Otto cycle for a compression ratio of 5.5 will be A 25% B 50% C 70% D 100% Answer: Option B 15.The brake power of a diesel engine, keeping other parameters constant, can be increased by A decreasing the density of intake air B increasing the temperature of intake air C increasing the pressure of intake air D decreasing the pressure of intake air

Answer: Option C

16.Pre-ignition is caused by the spontantaneous combustion of the mixture before the end of the compression stroke, and is due to

A cylinder walls being too hot B overheated spark plug points C red hot carbon deposits on cylinder walls D any one of these

Answer: Option D

17.In diesel engines, the fuel is injected in the form of very fine spray, into the engine cylinder, which gets ignited due to high temperature of the compressed air.

A Agree B Disagree

Answer: Option A

18.The object of supercharging the engine is

A to reduce mass ofthe engine per brake power B to reduce space occupied by the engine C to increase the power output of an engine when greater power is required D all of the above

Answer: Option D

19.Nuclear reactors are used

A to produce heat for thermoelectric power B to produce fissionable material C to propel ships, submarines, aircrafts D all of these

Answer: Option D

20.A moderator generally used in nuclear power plants is

A graphite B heavy water C concrete D graphite and concrete

Answer: Option D

21.The predominent isotope of the naturally occuring element is

A U235 B U238 C Pu233 D Pu239

Answer: Option B

22.The condition that causes vapour locking in a brake system is

A overheating of the fluid due to frequent brake application B overcooling of the brakes during high speed driving C keeping the vehicle without use for an extended period D an excessively high engine speed on a downhill road

Answer: Option A

23.The portion of a crankshaft which rests on cylinder block is called main journal.

A Yes B No

Answer: Option A

24.The motion of the cam is transferred to the valves through

A pistons B rocker arms C camshaft pulley D valve stems

Answer: Option B

25.If the engine coolant leaks into the engine oil, then engine oil

A appears milky B becomes foamy C turns black D none of these

Answer: Option A

26.The function of an alternator in an automobile is to

A supply electric power B convert mechanical energy into electrical energy C continually recharge the battery D partly convert engine power into electric power

Answer: Option C