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)