Heat Transfer in Engines Internal Combustion Engines
Energy Distribution
Removing heat is critical in keeping an engine and lubricant from thermal failure
Amount of energy available for use:
Brake thermal efficiency:
Energy distribution ~ 10%
(counted twice)
shaftW
HVQmW f
cfbt QmW HVbrake/
= 10-35%
25-40% 20-45% 10-30% 5-15% 2-10%
exhaustW lossW
oilW ambientWcoolantW
frictionW
Engine Temperature
Hot spots: Spark plug, exhaust valve and port, piston face
Exposed to high-temperature
combustion gases and
difficult to cool
Highest gas temperatures near
the spark plug during combustion
Engine Warm-up
As a cold engine heats up, thermal expansion occurs differently in all components
Engine bore limits piston thermal expansion
In cold weather, the start-up time to reach steady-state can be 20-30 minutes
A large percentage of automobile use is for this short trip
→ Air pollution
Airplane engines
better not to take off
before steady-state
Heat Transfer in Intake Systems
Convective heat in the intake system: The temperature
of intake gases increases to ~60°C
Thermal contact with the hot exhaust manifold, hot coolant
flow, electric heater
Vaporizing fuel → Homogeneous mixtures (esp. for carburetors,
throttle body injection)
Reducing volumetric efficiency: Reducing air density,
displacement of air due to fuel vapor
Higher chance of engine knock
Engines with multipoint port injectors needs less heating
for intake manifold
The aftercooler is used in some engines with super- or
turbochargers
Heat Transfer in Combustion Chambers
Three modes of heat transfer occur: Conduction,
convection, radiation
Convection on the inside surface of the cylinder is affected by
turbulence, swirl, etc.
Convection on the coolant side is fairly constant
Radiation accounts for ~10% of heat transfer in SI engines and
20-35% in CI engines (soot)
Heat transfer is cyclic
Minimum temperature at intake: Incoming gas may be hotter or
cooler than the cylinder walls → Heat transfer in either direction
Peak temperature of 3000 K during combustion → Cooling
needed
Slower heat transfer at the expansion and exhaust strokes due to
expansion cooling and heat losses
Heat Transfer in Combustion Chambers
Cooling problem at the piston face Cannot be cooled by the coolant
Splashing or spraying lubricating oil on the back surface of piston crown
Conduction through the connecting rod and piston rings
The cylinder wall temperature should be 180-200°C or lower to avoid the thermal breakdown of lubricating oil
Deposits on the wall create a thermal resistance and cause higher temperatures
Heat Transfer in Exhaust Systems
Heat losses from the exhaust system affect emissions
and turbocharging
Pseudo-steady-state exhaust temperatures
Pulsating cyclic flows
SI engines: 400-600°C with extremes of 300-900°C
CI engines: 200-500°C
Some automobile and large stationary engines have
exhaust valves with hollow stems containing sodium to
remove heat from the face
Engine Variables on Heat Transfer
Engine size: Larger engines are more efficient
Engine speed
Heat transfer increases with engine speed due to higher steady
-state temperature
Less time for combustion, self-ignition, knock but higher
temperature at high speeds
Load
SI engines: Throttle opens at heavy loads → Flow rate and heat
transfer increase
CI engines: More fuel is injected at heavy loads (constant flow
rate) → Temperature and heat transfer (including radiation due
to soot) increase but heat loss percentage is unchanged
Engine Variables on Heat Transfer
Spark timing: Spark set for maximum temperature
Fuel equivalence ratio: Greatest heat loss at stoichiometry
Evaporative cooling – Water injection
Inlet air temperature: Increasing temperature over the entire cycle
Coolant temperature: Increasing temperature of all the cooled components
Engine materials: Aluminum pistons operate 30-80°C cooler
Compression ratio: Cooler exhaust in CI engines
Knock: Potential to cause surface damage
Swirl and squish: Related to convection
Prob. 10-3
Air-cooled Engines
In many small and some medium-sized engines
Advantage: Light weight, low cost, no coolant system, no freeze-up, faster engine warm-up
Disadvantage: Less efficient (no control), noisier (no dampening water jacket)
Air has worse thermal properties than liquid
Air flow is directed with reflectors and ductwork
Finned heat-conducting metal surfaces
Cooling needs are different at different locations
Liquid-cooled Engines
The engine block is surrounded with a water jacket through which coolant liquid flows
Coolant: Water mixed with up to 70% ethylene glycol (C2H6O2, antifreeze) to prevent freezing and boiling, each causing engine failure and rust
Ethylene glycol has a freezing temperature of -11°C and boiling temperature of 197°C
Liquid-cooled Engines
Coolant system in automobiles: Fluid enters the water jacket, flows around cooling-needed locations, absorbs energy, and finally rejects enthalpy in the radiator (heat is exchanged with air sometimes with a fan), forming a closed-loop
Thermostat: Thermally activated go-no go valve to keep the coolant temperature from dropping below some minimum value
Coolant is pressurized to avoid boiling Localized boiling in small hot spots is desirable due to its better cooling, though
High-temperature coolant is used to heat the passenger compartments of automobiles
Other Issues in Heat Transfer
Oil as a coolant: The piston, camshaft, and connecting
rods can be cooled by spraying or splashing oil, e.g.
onto the back face of the piston crown for piston
cooling in the crankcase
Adiabatic engines: Not truly adiabatic but with reduced
heat loss from combustion chambers
Decreasing heat loss can increase brake power
Higher temperatures in engine components attributed to
advances in material technology
Small and light-weight because of the absence of cooling
systems
CI used because of a possible knock problem with SI
Other Issues in Heat Transfer
Some modern trends in engine cooling
Dual water jackets, dual-flow water jackets
Oil-only cooling systems
Safety features for cooling failure: No firing for some cylinders
intermittently for cooling
Thermal storage: “Thermal battery”
Waste heat is stored for the later use to preheat the intake
manifold, catalytic converter, lubricating oil, passenger
compartment, etc., or defrost car windows
The most common system to use a liquid-solid phase change in
a water-salt crystal mixture (freezing during storage and
melting during preheating)