IC Engines, Refrigeration and Air Conditioning Notes for SSC JE Mechanical

IC engines, refrigeration and air conditioning (RAC) are the main parts of the thermal engineering section of the SSC JE Mechanical paper, and 8 to 10 questions come from them every year. The questions are about the Otto and Diesel cycles, the engine terms, the knocking, the octane and cetane numbers, the vapour compression cycle, the refrigerants and the psychrometric terms. These IC engines, refrigeration and air conditioning notes for SSC JE Mechanical give you the formulas and the facts in simple tables, followed by 20 practice questions. Read them with the Thermodynamics Notes for SSC JE Mechanical, Theory of Machines Notes for SSC JE Mechanical and Strength of Materials Notes for SSC JE Mechanical notes.

Classification of IC Engines

• In an internal combustion (IC) engine, the fuel burns inside the cylinder; in an external combustion engine (the steam engine) the fuel burns outside.
• The compression ratio of the SI engine is 6 to 10 (up to 12 in the modern ones) and that of the CI engine is 14 to 22.
• The CI engine has a higher thermal efficiency, is heavier and gives more torque than the SI engine.

Four-Stroke and Two-Stroke Engines

• The four-stroke cycle: the intake stroke (the inlet valve open) draws the air-fuel mixture (or the air in the CI engine), the compression stroke compresses it, the power stroke gives the work after the ignition and the exhaust stroke removes the burnt gases.
• Scavenging is the process of removing the burnt gases by the fresh charge in a two-stroke engine.
• The usual firing order of the four-cylinder engine is 1-3-4-2 or 1-2-4-3, and of the six-cylinder in-line engine 1-5-3-6-2-4.
• The valve timing: the inlet valve opens before the TDC and closes after the BDC; the exhaust valve opens before the BDC and closes after the TDC (the valve overlap).

Air-Standard Cycles: Otto, Diesel and Dual

• For the same compression ratio and the same heat input, the efficiency order is: Otto more than Dual more than Diesel.
• For the same maximum pressure and temperature, the order is: Diesel more than Dual more than Otto.
• The efficiency of the Otto cycle increases with the compression ratio; it is limited by the knocking in the SI engine. The mean effective pressure (mep) is the constant pressure that gives the same work: mep = the net work / the swept volume.
• The cut-off ratio rho = V3 / V2 (the Diesel cycle); the swept volume = (pi / 4) D squared L.

Performance of IC Engines (Important Formulas)

• The compression ratio = (swept volume + clearance volume) / clearance volume.
• The power of an engine increases with the supercharging or turbo-charging (the air is delivered at a higher pressure by a compressor driven by the exhaust gas or the engine).

Engine Components and Systems

Combustion, Knocking, Fuels and Emissions

• The rich mixture gives more CO and HC; the lean mixture gives more NOx at the high temperature.
• The petrol has a high volatility and low self-ignition temperature; the diesel has a high cetane number, and the CI engine is more efficient.

Refrigeration: Basics and Vapour Compression System

• The domestic refrigerator uses R-600a (isobutane) or R-134a with a hermetic compressor and a capillary tube; the freezer works at about -18 degrees Celsius.
• Compressors: reciprocating, rotary, scroll and centrifugal (for the large capacities); the hermetic compressor has the motor and the compressor in a sealed casing.
• Condensers: air-cooled, water-cooled and evaporative; the filter-drier removes the moisture and the dirt.

Vapour Absorption System and Other Cycles

Refrigerants

• The ideal refrigerant has a low boiling point, a high latent heat, a high critical temperature, non-toxic, non-flammable, non-corrosive, a zero ODP and a low GWP.
• The refrigerants have a number: R followed by the digits; the ASHRAE number code; the R-7xx series are the inorganic refrigerants (R-717 ammonia, R-718 water, R-744 CO2).
• Montreal Protocol (1987) phases out the ozone-depleting substances, and the Kigali Amendment (2016) phases down the HFCs; India is a party to both.
• ODP (Ozone Depletion Potential) and GWP (Global Warming Potential) are the two environment measures.

Psychrometry and Air Conditioning

• The comfort conditions: about 22 to 25 degrees Celsius DBT and 50 to 60 percent RH.
• Air conditioning systems: the window, the split, the central (duct) and the packaged systems; the cooling tower cools the condenser water; the cooling load includes the sensible and the latent heat from the people, the lights, the equipment, the walls and the ventilation air.
• The psychrometric chart shows the properties of the moist air; the lines of the constant WBT are inclined and the lines of the constant specific humidity are horizontal.
• The summer air conditioning cools and dehumidifies the air; the winter air conditioning heats and humidifies the air.

Important Questions Explained

Question 1: Which cycle is the ideal cycle of a petrol engine?
Answer: The Otto cycle (heat addition at constant volume).

Question 2: Which cycle is the ideal cycle of a diesel engine?
Answer: The Diesel cycle (heat addition at constant pressure).

Question 3: What is the compression ratio of the CI engine?
Answer: About 14 to 22.

Question 4: What is 1 ton of refrigeration in kW?
Answer: About 3.517 kW (3.5 kW).

Question 5: What is the COP of a reversed Carnot refrigerator working between 250 K and 300 K?
Answer: COP = T2 / (T1 - T2) = 250 / (300 - 250) = 5.

Question 6: Which property decides the anti-knock quality of the petrol?
Answer: The octane number.

Question 7: Which refrigerant is a natural refrigerant used in the industrial plants?
Answer: Ammonia (R-717).

Quick Tips to Remember

• Remember the cycles in order: Otto (constant volume, SI), Diesel (constant pressure, CI) and Dual.
• Remember the efficiency order: for the same compression ratio Otto is the best; for the same maximum pressure Diesel is the best.
• Remember IP, BP and FP formulas: FP = IP - BP and the mechanical efficiency = BP / IP.
• Remember the octane number for the petrol and the cetane number for the diesel; the SI engine knock is the auto-ignition of the end gas.
• Remember 1 TR = 3.517 kW and the COP of the reversed Carnot cycle = T2 / (T1 - T2).
• Remember that the DBT is the highest and the DPT is the lowest of the three temperatures for the unsaturated air.

Common Mistakes to Avoid

• Writing the Diesel cycle heat addition as constant volume.
• Using N instead of N / 2 for the four-stroke engine in the IP formula.
• Confusing the octane number (petrol) with the cetane number (diesel).
• Writing the COP of the reversed Carnot cycle with T1 in the numerator.
• Thinking that the WBT can be more than the DBT.

Quick Revision Points

• Otto: constant volume; eta = 1 - 1 / r to the power (gamma - 1); Diesel: constant pressure.
• Four-stroke: 2 revolutions; two-stroke: 1 revolution.
• IP = pm L A n k / 60; BP = 2 pi N T / 60; FP = IP - BP.
• Octane: iso-octane 100, n-heptane 0; cetane: cetane 100, alpha-methyl naphthalene 0.
• 1 TR = 3.517 kW; COP (Carnot) = T2 / (T1 - T2).
• VCRS parts: compressor, condenser, expansion valve, evaporator; VARS uses heat.
• Refrigerants: R-12 banned; R-134a zero ODP; R-717 ammonia; R-600a isobutane.

Practice Questions With Answers

1. Which cycle is used in a petrol engine?
   Answer: Otto cycle (Constant volume).
2. Which cycle is used in a diesel engine?
   Answer: Diesel cycle (Constant pressure).
3. What is the compression ratio of the SI engine?
   Answer: 6 to 10 (Petrol).
4. In how many revolutions is a four-stroke cycle completed?
   Answer: 2 revolutions (720 degrees).
5. What is the formula of the efficiency of the Otto cycle?
   Answer: 1 - 1 / r to the power (gamma - 1) (Compression ratio r).
6. What is the formula of the friction power?
   Answer: FP = IP - BP (Engine).
7. What is the formula of the mechanical efficiency?
   Answer: BP / IP (Ratio).
8. What is the full form of MPFI?
   Answer: Multi-Point Fuel Injection (Replaced the carburetor).
9. Which number measures the anti-knock quality of petrol?
   Answer: Octane number (Iso-octane).
10. Which number measures the ignition quality of diesel?
    Answer: Cetane number (Higher is better).
11. What is the firing order of a four-cylinder engine?
    Answer: 1-3-4-2 (Or 1-2-4-3).
12. What is 1 ton of refrigeration in kW?
    Answer: 3.517 kW (About 3.5 kW).
13. What is the COP of a refrigerator?
    Answer: Refrigerating effect / work input (Performance).
14. Which valve reduces the pressure in the VCRS?
    Answer: Expansion valve (capillary tube) (Isenthalpic).
15. What are the four parts of a VCRS?
    Answer: Compressor, condenser, expansion valve and evaporator (Cycle).
16. What does a VARS use instead of the compressor work?
    Answer: Heat energy (Generator).
17. Which refrigerant pair is used in the VARS for the air conditioning?
    Answer: Lithium bromide-water (Water is the refrigerant).
18. Which refrigerant is isobutane?
    Answer: R-600a (Domestic refrigerator).
19. What is the temperature at which the vapour starts to condense called?
    Answer: Dew point temperature (Cooling at constant pressure).
20. Which process is done in a summer air conditioner?
    Answer: Cooling and dehumidification (Psychrometry).

How to Practise This Topic

1. Make a one-page sheet of the cycles, the efficiency formulas and the performance terms.
2. Learn the table of the refrigerants and the psychrometric terms by heart.
3. Solve two numerical problems each of the IP and BP and the COP.
4. Attempt the practice questions without looking at the answers.
5. Solve the SSC practice papers and the previous year papers to practise the JE pattern.

Related Notes and Practice

• Thermodynamics Notes for SSC JE Mechanical
• Theory of Machines Notes for SSC JE Mechanical
• Strength of Materials Notes for SSC JE Mechanical
• Mechanical Engineering (SSC JE) Notes
• Civil Engineering (SSC JE) Notes
• SSC practice papers

Frequently Asked Questions

What is an IC engine?

An internal combustion engine is a heat engine in which the fuel burns inside the cylinder and the pressure of the burnt gases moves the piston to give the work.

What is the difference between an SI and a CI engine?

A spark ignition (SI) engine uses petrol and a spark plug and works on the Otto cycle, while a compression ignition (CI) engine uses diesel, which burns by the heat of the compressed air, and works on the Diesel cycle.

What is the difference between a four-stroke and a two-stroke engine?

A four-stroke engine completes a cycle in two revolutions with valves, while a two-stroke engine completes it in one revolution with ports and gives a power stroke in every revolution.

What is knocking in an engine?

Knocking is the sudden and abnormal burning of a part of the fuel, which gives a metallic sound and reduces the power; it is controlled by the octane number in the SI engine and the cetane number in the CI engine.

What is the compression ratio?

The compression ratio is the ratio of the total cylinder volume to the clearance volume.

What is a ton of refrigeration?

One ton of refrigeration is the heat removed to freeze one ton of water at 0 degrees Celsius in 24 hours, which is about 3.517 kW.

What is the COP?

The coefficient of performance is the ratio of the refrigerating effect to the work input of a refrigerator.

How does a vapour compression refrigerator work?

The compressor raises the pressure of the refrigerant vapour, the condenser rejects the heat, the expansion valve reduces the pressure and the evaporator absorbs the heat from the cold space.

What is the difference between VCRS and VARS?

The vapour compression system uses the mechanical work of a compressor, while the vapour absorption system uses the heat energy and has a very small work input.

Which refrigerants are harmful to the ozone layer?

The CFCs like R-12 and the HCFCs like R-22 harm the ozone layer; they are being phased out under the Montreal Protocol.

How many questions come from IC engines and RAC in the SSC JE exam?

Usually 8 to 10 questions come from the IC engines, the refrigeration and the air conditioning in the SSC JE Mechanical paper.

How can I prepare IC engines and RAC for SSC JE?

Learn the cycles, the formulas, the refrigerants and the psychrometric terms in tables, solve the numerical problems and the previous year questions.