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.

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. This heat must also be removed.

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 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

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

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

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 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. 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 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.

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

Energy balance for automotive engines at maximum power

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

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 For force convection, Nusselt correlation

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 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 For instantaneous spatial average coefficients - Woschni Assumed average gas velocity equal to piston speed For engine with swirl

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

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 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 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

Solution Gas side heat transfer