IC Engines, Refrigeration and Air conditioning:

SI & CI ENGINES

SI Engine: Spark Ignition engine; generally uses petrol/gasoline; ignition by spark plug.
CI Engine: Compression Ignition engine; generally uses diesel; fuel ignites due to high temperature of compressed air.
SI Engine: Air-fuel mixture is generally prepared before entering the cylinder; ignition by spark.
CI Engine: Only air is compressed first; fuel is injected near the end of compression.

Feature	SI Engine	CI Engine
Fuel	Petrol/Gasoline	Diesel
Ignition	Spark plug	Compression
Mixture	Air + fuel generally mixed before combustion	Fuel injected into compressed air
Compression Ratio	Lower	Higher
Speed	Generally higher	Generally lower
Efficiency	Generally lower	Generally higher

Four-Stroke Cycle

Suction/Intake: Fresh charge enters cylinder.
Compression: Charge/air is compressed.
Power/Expansion: Combustion produces power.
Exhaust: Burnt gases are expelled.

One complete four-stroke cycle = 2 crankshaft revolutions = 720°.

Two-Stroke Engine

One power stroke occurs in every crankshaft revolution.
Cycle is completed in 2 strokes = 360°.
Generally has higher power-to-weight ratio than a four-stroke engine.

ENGINE SYSTEMS & COMPONENTS

System/Component	Function
Cylinder	Working chamber for combustion
Piston	Converts gas pressure into reciprocating motion
Connecting Rod	Connects piston to crankshaft
Crankshaft	Converts reciprocating motion into rotary motion
Flywheel	Stores rotational energy and reduces speed fluctuation
Camshaft	Operates engine valves
Spark Plug	Produces spark in SI engine
Fuel Injector	Injects fuel into engine

Engine Systems

Fuel System: Supplies and meters fuel.
Air Intake System: Supplies air to the engine.
Cooling System: Removes excess heat.
Lubrication System: Reduces friction and wear.
Exhaust System: Carries combustion products away.
Starting System: Cranks the engine during starting.
Ignition System: Produces spark in SI engines.

IC ENGINE PERFORMANCE

Important Terms

Brake Power (BP): Power available at the engine crankshaft.
Indicated Power (IP): Power developed inside the engine cylinder.
Friction Power (FP): Power lost due to engine friction and other mechanical losses.
IP = BP + FP
Mechanical Efficiency: \( \eta_m = BP/IP \)

Power Formula

\[ BP = \frac{2\pi N T}{60} \]

N: Speed in rpm
T: Torque in N·m

Mean Effective Pressure

\[ MEP = \frac{\text{Work done per cycle}}{\text{Swept volume}} \]

IMEP: Indicated Mean Effective Pressure.
BMEP: Brake Mean Effective Pressure.

Thermal Efficiency

\[ \eta_{th} = \frac{\text{Power output}}{\text{Fuel energy supplied}} \]

Specific Fuel Consumption

\[ BSFC = \frac{\text{Fuel consumption}}{\text{Brake power}} \]

Lower BSFC → higher fuel economy.
Volumetric efficiency: Actual air inducted / Theoretical air capacity.

IC ENGINE TESTING

Dynamometer: Measures brake power/torque.
Willans line method: Used for estimating friction power in suitable operating conditions.
Morse test: Used to determine friction power in a multi-cylinder engine under specified test conditions.
Heat balance test: Accounts for distribution of fuel energy into useful power, cooling water, exhaust gases, radiation and other losses.

Important Performance Curves

BP vs speed
Torque vs speed
Fuel consumption vs speed
BSFC vs speed
Thermal efficiency vs load

FUELS FOR IC ENGINES

Petrol/Gasoline

Used mainly in SI engines.
Octane number indicates resistance to knocking in SI-engine fuel.
Higher octane number → greater anti-knock quality.

Diesel

Used mainly in CI engines.
Cetane number indicates ignition quality of diesel fuel.
Higher cetane number generally means shorter ignition delay.

Knocking

SI engine knock: Abnormal combustion involving auto-ignition of the unburnt end-gas.
CI engine knock: Associated with ignition delay and rapid combustion of accumulated fuel.

ENGINE EMISSIONS & CONTROL

Pollutant	Main Source/Reason	Control
CO	Incomplete combustion	Better air-fuel control, oxidation catalyst
HC	Unburnt fuel	Improved combustion, catalytic converter
NOₓ	High combustion temperature	EGR, NOₓ reduction systems
PM/Soot	Incomplete combustion, especially in diesel engines	Diesel particulate filter

EGR: Exhaust Gas Recirculation reduces peak combustion temperature and can reduce NOₓ formation.
Catalytic Converter: Reduces selected gaseous pollutants in exhaust.
DPF: Diesel Particulate Filter traps particulate matter.

VAPOUR COMPRESSION REFRIGERATION (VCR)

Basic VCR cycle: Evaporation → Compression → Condensation → Expansion.

Evaporator: Refrigerant absorbs heat from refrigerated space.
Compressor: Raises refrigerant pressure and temperature.
Condenser: Rejects heat to surroundings.
Expansion Device: Reduces pressure and temperature.

Four Basic Processes

Process	Approx. Process
1 → 2	Isentropic compression
2 → 3	Constant-pressure heat rejection
3 → 4	Throttling / isenthalpic expansion
4 → 1	Constant-pressure heat absorption

Coefficient of Performance

\[ COP_R = \frac{\text{Refrigerating effect}}{\text{Work input}} \]

\[ COP_R = \frac{Q_L}{W} \]

For ideal reversed Carnot refrigerator:

\[ COP = \frac{T_L}{T_H – T_L} \]

Temperatures must be in Kelvin.
Higher COP → less work required for a given refrigeration effect.

REFRIGERANTS

Refrigerant: Working fluid that absorbs heat at low temperature and rejects it at higher temperature.

Desirable Properties

Low boiling point
High latent heat of vaporization
Suitable operating pressures
Low toxicity
Low flammability
Chemically stable
Compatible with system materials and lubricant
Low environmental impact

Important Environmental Terms

ODP: Ozone Depletion Potential.
GWP: Global Warming Potential.
CFCs have high ozone-depletion potential and have been phased out under international controls.
Refrigerant selection considers safety, energy efficiency and environmental impact.

REFRIGERATION COMPONENTS

Compressor

Compresses low-pressure refrigerant vapour.
Raises pressure and temperature.
Types: Reciprocating, Rotary, Scroll, Screw, Centrifugal.

Condenser

Rejects heat from refrigerant to surroundings.
High-pressure refrigerant vapour becomes liquid.
Types: Air-cooled, Water-cooled, Evaporative.

Evaporator

Absorbs heat from the refrigerated space.
Low-pressure refrigerant evaporates.

Expansion Devices

Capillary tube
Thermostatic Expansion Valve (TXV)
Electronic Expansion Valve
Float valve

Throttling process: Approximately constant enthalpy.

VAPOUR ABSORPTION REFRIGERATION

Uses heat energy as the main driving input instead of primarily mechanical compressor work.
Common working pairs include NH₃–H₂O and H₂O–LiBr.
NH₃–H₂O: Ammonia is refrigerant; water is absorbent.
H₂O–LiBr: Water is refrigerant; lithium bromide is absorbent.
Main components: Absorber, Pump, Generator, Condenser, Expansion device and Evaporator.

VAPOUR JET REFRIGERATION

Also called steam jet refrigeration.
Uses a high-velocity steam jet to create low pressure and induce vapour from the evaporator.
Water is commonly used as the refrigerant.
Suitable where low-cost steam or waste heat is available.

THERMOELECTRIC REFRIGERATION

Based on the Peltier effect.
Electric current through a thermoelectric module causes one side to absorb heat and the other side to reject heat.
No mechanical compressor or refrigerant circulation is required.
Advantages: Compact, silent, low maintenance.
Applications: Small cooling systems, electronic cooling and portable refrigerators.

VORTEX TUBE REFRIGERATION

Uses the Ranque–Hilsch vortex effect.
Compressed gas enters tangentially and separates into hot and cold streams.
No moving parts.
Used for spot cooling and specialised applications.

PSYCHROMETRY

Psychrometry: Study of properties of moist air.

Important Psychrometric Properties

Property	Meaning
Dry Bulb Temperature (DBT)	Temperature measured by ordinary thermometer
Wet Bulb Temperature (WBT)	Temperature measured by wet-wick thermometer
Dew Point Temperature (DPT)	Temperature at which condensation begins during cooling at constant moisture content
Relative Humidity	Ratio of actual water-vapour partial pressure to saturation pressure at same temperature
Specific Humidity	Mass of water vapour per unit mass of moist air
Humidity Ratio	Mass of water vapour per unit mass of dry air
Enthalpy	Total energy content per unit mass of moist air

DBT ≥ WBT ≥ DPT for unsaturated moist air.
At 100% relative humidity: DBT = WBT = DPT.

PSYCHROMETRIC PROCESSES

Sensible Heating: DBT increases; humidity ratio remains approximately constant.
Sensible Cooling: DBT decreases until dew point; humidity ratio remains constant before condensation.
Humidification: Moisture content increases.
Dehumidification: Moisture content decreases.
Cooling & Dehumidification: Temperature and moisture content both decrease.
Heating & Humidification: Temperature and moisture content both increase.
Adiabatic Mixing: Two air streams mix without external heat transfer.

COMFORT CHART & AIR CONDITIONING

Air Conditioning: Control of temperature, humidity, cleanliness and air motion to achieve desired indoor conditions.

Comfort Air Conditioning

Designed primarily for human comfort.
Important factors: Air temperature, humidity, air velocity, air purity and radiation conditions.

Industrial Air Conditioning

Designed mainly to maintain conditions required for a process/product.
Examples: Textile, pharmaceutical, electronics and precision manufacturing industries.

Comfort Chart

Shows combinations of temperature and humidity associated with acceptable human comfort.
Comfort depends on temperature, humidity, air velocity, clothing, activity and other environmental factors.

AIR-CONDITIONING LOAD CALCULATIONS

\[ \text{Total Cooling Load} = \text{Sensible Heat Load} + \text{Latent Heat Load} \]

Major Cooling Loads

Wall/Roof Load: Heat transfer through building surfaces.
Solar Load: Heat entering due to solar radiation.
Occupant Load: Sensible + latent heat released by people.
Lighting Load: Heat released by lights.
Equipment Load: Heat from electrical/mechanical equipment.
Ventilation Load: Heat and moisture introduced with outdoor air.
Infiltration Load: Heat and moisture entering through leakage.

Sensible Heat Factor

\[ SHF = \frac{\text{Sensible Heat}}{\text{Total Heat}} \]

\[ SHF = \frac{\text{Sensible Heat}}{\text{Sensible Heat} + \text{Latent Heat}} \]

HEAT PUMPS

Heat Pump: Device that transfers heat from a low-temperature source to a high-temperature sink using work input.
It operates on the same basic principle as a refrigerator.
COP of Heat Pump:

\[ COP_{HP} = \frac{\text{Heat delivered}}{\text{Work input}} \]

\[ COP_{HP} = COP_R + 1 \]

For ideal reversed Carnot heat pump:

\[ COP_{HP} = \frac{T_H}{T_H – T_L} \]

Temperatures are measured in Kelvin.

⚡ ONE-MINUTE REVISION

SI → Spark ignition → Petrol/Gasoline
CI → Compression ignition → Diesel
SI → Spark plug
CI → Fuel injection + compression ignition
Four-stroke cycle → 720° crank rotation
Two-stroke cycle → 360° crank rotation
IP = BP + FP
Mechanical efficiency = BP/IP
BP = 2πNT/60
Octane number → SI fuel anti-knock quality
Cetane number → CI fuel ignition quality
CO → Incomplete combustion
NOₓ → High combustion temperature
PM → Soot/particulate emissions
EGR → Reduces NOₓ formation
VCR → Compressor + Condenser + Expansion Device + Evaporator
Expansion valve → Throttling → h ≈ constant
Evaporator → Heat absorption
Condenser → Heat rejection
Compressor → Pressure & temperature increase
COP_R = Q_L/W
Vapour absorption → Heat-driven refrigeration
NH₃–H₂O → Ammonia refrigerant
H₂O–LiBr → Water refrigerant
Thermoelectric refrigeration → Peltier effect
Vortex tube → Hot + cold streams
Psychrometry → Moist-air properties
DBT ≥ WBT ≥ DPT
100% RH → DBT = WBT = DPT
SHF = Sensible Heat / Total Heat
Total Cooling Load = Sensible + Latent Load
Heat Pump → Transfers heat from low temperature to high temperature
COP_HP = COP_R + 1