1. Engine heat transfer Dr. Primal Fernando primal@eng.fsu.edu
Ph: (081)

2. 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.

3. 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.

4. Additional heat is also generated by friction between the moving parts.
This heat must also be removed.

5. 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

6. 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

7. 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

8. 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 degree F (≈ °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

9. 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

10. 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

11. 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.

12. 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.
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.

13. 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.

14. Overall heat transfer from combustion chamber
Schematic of temperature distribution and heat flow across the combustion chamber

15. 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

16. 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

17. 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.

18. 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

19. 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)

20. Energy balance for automotive engines at maximum power

21. Working fluid constituents
Process
SI engine
CI-Engine
Intake
Air
Fuel (liquid + vapor)
Recycled exhaust-used to control Nox
Residual gas –within the cylinder
Compression
Expansion
Combustion products (Mixture of N2, H2O, CO2, CO, H2, O2, NO, OH, O, H)
Exhaust
Combustion products (Mainly N2, H2O, CO2, and either O2 ( Ф < 1) or CO and (Ф >1)
Combustion products (Mainly N2, H2O, CO2, and O2)
Ф – fuel/air equivalence ratio

22. 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

23. Convective heat transfer coefficients -I
For force convection, Nusselt correlation

24. 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

25. 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,

26. Convective heat transfer coefficients -IV
For instantaneous spatial average coefficients - Woschni
Assumed average gas velocity equal to piston speed
For engine with swirl

27. 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

28. 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

29. 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

30. 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

31. Solution
Gas side heat transfer