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 W friction ~ 10% (counted twice) W shaft t W b / m fqhv c brake W exhaust W loss W m = 10-35% f Q HV W W oil W coolant ambient 25-40% 20-45% 10-30% 5-15% 2-10%
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 Load 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 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)