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Engine heat transfer - PowerPoint PPT Presentation


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Engine heat transfer. Dr. Primal Fernando primal@eng.fsu.edu Ph: (081) 2393608. Internal combustion engines use heat to convert the energy of fuel to power. Not all of the fuel energy is converted to power. Excess heat must be removed from the engine.

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slide1

Engine heat transfer

Dr. Primal Fernando

primal@eng.fsu.edu

Ph: (081) 2393608

introduction
Internal combustion engines use heat to convert the energy of fuel to power.

Not all of the fuel energy is converted to power.

Excess heat must be removed from the engine.

In engines, heat is moved to the atmosphere by fluids--water and air.

If excess heat is not removed, engine components fail due to excessive temperature.

Engine temperature is not consistent throughout the cycle.

Heat moves from areas of high temperature to areas of low temperature.

Introduction
slide3
When fuel is oxidized (burned) heat is produced.
  • Only approximately 30% of the energy released is converted into useful work.
  • The remaining (70%) must be removed from the engine to prevent the parts from melting.
additional heat is also generated by friction between the moving parts
Additional heat is also generated by friction between the moving parts.
  • This heat must also be removed.
heat transfer
Heat transfer
  • Peak burned gas temperature ≈ 2500 K
  • Maximum metal temperature for the inside of the combustion chamber is much lower values due to
    • Cracking on materials (cast iron - 400°C, aluminum alloys - 400°C
    • Prevent deterioration of lubrication oil (keep below - 180°C)
    • Spark plugs and valves must be kept cool to avoid knock and pre-ignition problems
  • Should maintain the combustion temperature: high heat transfer reduce the engine efficiency
  • Effects for emissions
  • Heat transfer to inlet manifold reduces the air flow
cooling system
Cooling System
  • An automotive cooling system must perform several functions
    • 1. Remove excess from the engine
    • 2. Maintain a consist engine temperature
    • 3. Help a cold engine warm-up quickly
    • 4. Provide a means of warming the passenger compartment
slide7

Cooling system operation

  • Engine heat is transferred . . .
    • through walls of the combustion chambers
    • through the walls of cylinders
  • Coolant flows . . .
    • to upper radiator hose
    • through radiator
    • to water pump
    • through engine water jackets
    • through thermostat
    • back to radiator
slide8

Cooling system operation

  • Fans increase air flow through radiator
    • Hydraulic fan clutches
    • Hydraulic fans consume 6 to 8 HP
    • Electric fans
  • Coolant (ethylene glycol)
    • 50/50 mixture increases boiling point to 227°F (≈108°C)
    • Automotive cooling systems operate around 180-212 degree F (≈82 - 100°C)
    • pressurizing system to 15 PSI increases to 265°F (≈ 1 bar, 130°C)
  • Coolant (propylene glycol)
    • Less protection at the same temperatures
    • Less toxic
cooling terms
Thermal Conductivity

Ability of a material to conduct and transfer heat

Thermal expansion

Expansion of a material when it is heated.

Thermal growth

Increase in size caused by heating.

When cooled does not return to normal size.

Thermal distortion

Asymmetrical or nonlinear thermal expansion.

Three means of heat transfer:

Conduction

Convection

Radiation

Cooling Terms
heat movement
Heat Movement
  • Conduction
    • Movement of heat through materials ; Fourier’s Law:
  • Convection
    • Movement of heat by fluids; Newton's Law of cooling
  • Radiation
    • Heat movement by transfer from one body to another.

Stefan-Boltzmann constant

two cooling systems
Two Cooling Systems
  • Small engines use two cooling systems;
    • Air
    • Liquid
  • Both systems have two common features.
    • Heat is transferred from the combustion chamber to the crankcase by the oil.
    • A large portion of the excess heat is removed with the exhaust gases.
  • The difference is in the medium used to move the heat from the engine to the atmosphere.
air cooled heat movement
In air cooled engines the excess heat in the combustion chamber moves through the cylinder walls by conduction.

The heat transfers from the engine parts to the air at the exterior surfaces and into the atmosphere by convection.

The air fins increase the surface area between the engine and the air--increasing heat transfer.

Air Cooled Heat Movement
  • The heart of the system is the fins on the flywheel which pumps the air around the engine.
  • The air flow is directed by the air shrouds.
water cooled heat movement
Water Cooled Heat Movement
  • Water cooled engines transfer the excess heat from the combustion chamber through the cylinder walls by conduction.
  • Water flowing past the exterior cylinder walls absorbs the heat and transfers it to the radiator.
  • Air flowing through the radiator absorbs the heat and transfers it to the atmosphere.
  • The system relies on a water pump to circulate the water through the system and a fan to move air through the radiator.
schematic of temperature distribution and heat flow across the combustion chamber
Schematic of temperature distribution and heat flow across the combustion chamber

Overall heat transfer from combustion chamber

overall heat transfer from combustion chamber
Overall heat transfer from combustion chamber

Gas side heat transfer

Through the wall

Coolant side

For force convection, convective heat transfer coefficient can be calculated by Nusselt theory

heat transfer and engine energy balance conservation of energy
Heat transfer and engine energy balance – conservation of energy

Energy in

Energy out

engine

Energy out by Power (or call brake power)+ coolant + (oil + convection + radiation ) + exhaust

Energy in by fuel+ air

heat transfer and engine energy balance conservation of energy1
Heat transfer and engine energy balance – conservation of energy

Brake power

Enthalpy of burned and unburned gas mixture

Heat rejected to oil (if separately cooled) convection + radiation engine’s external surface.

heat transfer and engine energy balance conservation of energy2
Heat transfer and engine energy balance – conservation of energy

Enthalpy of burned and unburned gas mixture

For the studies it is convenient to divide exhaust enthalpy into sensible part + reference enthalpy

Enthalpy relative to reference

heat transfer and engine energy balance conservation of energy3
Heat transfer and engine energy balance – conservation of energy

This equation can be rearrange to

Exhaust enthalpy loss due to incomplete combustion

Note: LHV uses when exhaust has water vapor (HHV=LHV+ hfg)

working fluid constituents
Working fluid constituents

Ф– fuel/air equivalence ratio

heat transfer analysis
Heat transfer analysis
  • Overall time averaged
    • Adequate for some analysis
  • Instantaneous
    • Necessary for realistic cycle calculations

Average values of temperatures, heat transfer coefficients are calculated at each point in the cycle and using following equations heat transfer per cycle is obtained, q().

Gas side heat transfer

Through the wall

Coolant side

convective heat transfer coefficients i
Convective heat transfer coefficients -I
  • For force convection, Nusselt correlation
convective heat transfer coefficients ii
Convective heat transfer coefficients -II
  • For time averaged heat flux, Taylor and Toong
    • Correlated heat transfer data for 19 different engines
    • They defined average effective gas temperature, Tg,a over the engine cycle, which is the temperature of the wall that stabilize if no heat is removed from the out side (obtained by extrapolating plotted data).
    • Nu plotted against Re
    • Suggested power law of 0.75
convective heat transfer coefficients iii
Convective heat transfer coefficients -III
  • For instantaneous spatial average coefficients - Annad
    • a varies with intensity of charge motion and engine design
    • Gas properties are evaluated at the cylinder average charge temperature,
convective heat transfer coefficients iv
Convective heat transfer coefficients -IV
  • For instantaneous spatial average coefficients - Woschni
    • Assumed average gas velocity equal to piston speed

For engine with swirl

convective heat transfer coefficients v
Convective heat transfer coefficients -V
  • For instantaneous local coefficients – LeFeuvre et al. and Dent Sulaiman
    • For direct injection diesel engines with swirl

Heat flux with any radius with Pr = 0.73

slide28
Radiative heat transfer – Diesel engines are about 20-35% of total heat transfer, SI engines small compared to convective part
  • Two sources radiative heat transfer within the cylinder
    • Gases
    • Soot particles (about 5 times compared to gases)

Annand

Flynn et al. for instantaneous heat transfer

example
Example

If radiation in the combustion chamber is negligible and time-averaged overall heat transfer of the engine can be approximated as

Give an expression for hc,o

solution
Solution

If radiation in the combustion chamber is negligible and time-averaged overall heat transfer of the engine can be approximated as

Give an expression for hc,o

Gas side heat transfer

Through the wall

Coolant side

solution1
Solution

Gas side heat transfer