IC Engines and Refrigeration MCQ With Answers for JKSSB Junior Engineer (Mechanical)

Internal combustion engines and refrigeration and air conditioning are favourite topics in the mechanical technical paper, with direct questions on strokes, cycles, efficiencies, refrigerants and psychrometrics. This page gives a revision list and then solved MCQs with explanations.

Quick revision list

• Engines: the four strokes are suction, compression, power and exhaust, and a two-stroke engine fires once every revolution
• Fuels: octane number measures knock resistance of petrol and cetane number measures ignition quality of diesel
• Efficiencies: mechanical efficiency is brake power divided by indicated power
• Refrigeration: the vapour compression system has a compressor, condenser, expansion valve and evaporator, and cooling happens in the evaporator
• Air conditioning: relative humidity, dew point and sensible cooling are read on the psychrometric chart

Worked numerical problems

• Brake power: an engine running at 3,000 rpm gives a torque of 100 N·m. The brake power is 2πNT ÷ 60 = 2π × 3,000 × 100 ÷ 60 = 31.4 kW
• Mechanical efficiency: if the brake power is 30 kW and the indicated power is 36 kW, the mechanical efficiency is 30 ÷ 36 = 83.3%
• Specific fuel consumption: an engine that uses 8 kg of fuel an hour while giving 30 kW has an SFC of 8 ÷ 30 = 0.267 kg/kWh
• Brake thermal efficiency: 8 kg/h of fuel is 0.00222 kg/s. With a calorific value of 42,000 kJ/kg, the heat input is 93.3 kW, so the efficiency is 30 ÷ 93.3 = 32.1%
• Compression ratio: a swept volume of 500 cc and a clearance volume of 50 cc give a ratio of (500 + 50) ÷ 50 = 11
• Swept volume: a bore of 80 mm and a stroke of 90 mm give π/4 × 80² × 90 = 452 cc
• Otto efficiency: with r = 9 and γ = 1.4, the efficiency is 1 − 9^(−0.4) = 58.5%
• Refrigeration: a refrigerating effect of 150 kJ/kg with a work input of 30 kJ/kg gives a COP of 5, and 5 tons of refrigeration is 5 × 3.517 = 17.6 kW

Petrol versus diesel engines

A petrol engine draws in an air–fuel mixture and ignites it with a spark. A diesel engine compresses air alone to a very high pressure and temperature and injects the fuel, which ignites by itself. Diesel engines use higher compression ratios, 14 to 22, so they are generally more efficient. Petrol engines are lighter and run at higher speed.

Lubrication and cooling

Lubrication reduces friction and wear, carries away heat and seals the piston rings. Common systems are splash, pressure and the mixed splash-and-pressure system. Cooling is by air, as in motorcycles, or by liquid, using a radiator, a water pump and a thermostat.

Refrigeration cycle summary

• Compressor: raises the pressure and temperature of the vapour
• Condenser: removes heat and turns the vapour into liquid
• Expansion valve: drops the pressure and temperature of the liquid
• Evaporator: the cold liquid absorbs heat from the space and turns into vapour

Psychrometric terms

For moist air, the dry-bulb temperature is read from an ordinary thermometer, the wet-bulb temperature from a thermometer with a wet wick, and the dew point is the temperature at which condensation starts. The dew point is never above the wet-bulb temperature, and the wet-bulb is never above the dry-bulb. At 100% relative humidity, all three are equal.

Questions candidates ask

What is the difference between indicated power and brake power? Indicated power is developed inside the cylinder, and brake power is what reaches the shaft, after friction losses.

Why do diesel engines have a higher compression ratio? Air alone is compressed, so the ratio can be high without knocking, and the heat of compression ignites the fuel.

What does one ton of refrigeration mean? It is a cooling rate of about 3.5 kW, equal to the heat needed to melt one ton of ice in 24 hours.

Which refrigerant is common in domestic refrigerators? R-134a, which replaced older ozone-depleting refrigerants, though newer ones are in use.

Practise

Solve the MCQs below, then read the exam guide. Next: theory of machines and machine design.