Presentation on theme: "ENGINE HEAT TRANSFER P M V Subbarao Professor Mechanical Engineering Department Loss of Heat is encouraged only to keep engine safe…. It’s a penalty on."— Presentation transcript:
ENGINE HEAT TRANSFER P M V Subbarao Professor Mechanical Engineering Department Loss of Heat is encouraged only to keep engine safe…. It’s a penalty on performance……
Engine Cooling & Car Radiator History Heat dissipation is probably one of the most important considerations in engine design. An internal combustion engine creates enough heat to destroy itself. Without an efficient cooling system, we would not have the vehicles we do today. The original radiators were simple networks of round copper or brass tubes that had water flowing through them by convection. By the 1920’s some auto manufacturers, like GM, had switched to oval tubes because they were slightly more efficient. Not long after that, as engines grew larger and hotter, companies began to add fans for a constant flow of air over the radiator cores. These more efficient cooling systems eventually added a pump to push the water through the cooling tubes. All in all, the car radiator is a simple and lasting technology that will likely be around as long as we use internal combustion engines.
Engine Cylinder Cooling Systems There are mainly two types of cooling systems : (a) Air cooled system, and (b) Water cooled system.
Air Cooled System Air cooled system is generally used in small engines say up to 15-20 kW and in aero plane engines. In this system fins or extended surfaces are provided on the cylinder walls, cylinder head, etc. Heat generated due to combustion in the engine cylinder will be conducted to the fins and when the air flows over the fins, heat will be dissipated to air. The amount of heat dissipated to air depends upon : (a) Amount of air flowing through the fins. (b) Fin surface area. (c) Thermal conductivity of metal used for fins
Radial Temperature Distribution The equation for the temperature excess becomes
Heat Dissipation Capacity of Cylinder with Radial fins The heat flow through a fin is given by the heat flow at the base of a fin and can be expressed as The total heat flow from a fin array is the sum of heat flow from the fin body and the heat flow from the base surface without fin and can be written as
The temperature difference between a fin base and the fluid ( B ) due to total heat flow rate at the fin base can be expressed as
Development of Compact Finned Cylinder The heat flow through the base is The ideal heat flow Fin Efficiency
Engine Warmup As a cold engine heats up to steady-state temperature, thermal expansion occurs in all components. The magnitude of this expansion will be different for each component, depending on its temperature and material. Engine bore limits the expansion of pistons. In cold weather, the startup time can be as high as 20—30 minutes. Some parts of the engine reach steady state much sooner and some do not. Fairly, normal conditions may be experienced within few minutes, but it can take as long as an hour to reach optimum fuel consumption rates. Engines are built to operate best at steady-state conditions. Full power and optimum fuel economy may not be realized until this condition is reached.
Thermostat The thermostat's main job is to allow the engine to heat up quickly, and then to keep the engine at a constant temperature. It does this by regulating the amount of water that goes through the radiator. At low temperatures, the outlet to the radiator is completely blocked -- all of the coolantis recirculated back through the engine. Once the temperature of the coolant rises to between 82 - 91 0 C, the thermostat starts to open, allowing fluid to flow through the radiator. By the time the coolant reaches 93 - 103 0 C), the thermostat is open all the way.
Ebullient cooling systems In conventional cooling systems the water pumped into the cylinder jacket undergoes a rise in temperature as it absorbs heat while moving up the cylinder jacket. This results in a non - uniform temperature profile along the cylinder wall which produces severe distortions. Two-phase ebullient cooling systems involve the natural circulation of jacket water at or near the saturation temperature. These systems utilize the latent heat of vaporization to extract heat at constant temperatures. This results in a uniform wall temperature and no thermal stresses. The circulation can also be achieved by natural convection, removing the need for a pump.
A higher operating temperature, along with adequate heat dissipation, also helps in achieving more efficient operation. The water/ steam that needs to be circulated is also a fraction of what would be used in conventional systems given the high latent heat of vaporizations.
Engine Heat Losses For many engines, the heat losses can be subdivided: General range of various heat losses are: Type of lossRangeRemarks Cooling10 – 30 % 5 – 15% Diesel engines on higher side OilAt low load higher losses Ambient2 – 10% Friction10%
S I Engine Temperatures Three of the hottest points are around the spark plug, the exhaust valve and port, and the face of the piston. Highest gas temperatures during combustion occur around the spark plug. This creates a critical heat transfer problem area. The exhaust valve and port operate hot because they are located in pseudo- steady flow of hot exhaust gases. The piston face is difficult to cool because its is separated form the water jacket or finned surface.
Heat Transfer in Intake Systems Carbureted Engine: MPI Engine:
Gas to Surface Heat Transfer Heat transfer to walls is cyclic. Gas temperature T g in the combustion chamber varies greatly over and engine cycle. Coolant temperature is fairly constant. Heat transfer from gas to walls occurs due to convection & radiation. Convection Heat transfer: Radiation heat transfer between cylinder gas and combustion chamber walls is
Conduction Through Cylinder Liner & Innerwall Gas − − − = ×ln(/) / (2××× )
Heat Transfer from Wall to Coolant Q: The total heat transferred from gas to walls. Q 1 : Heat carried off by the cooling water Q 2 : Heat transferred across the cylinder block to the ambient. = ℎ ××( -outer − coolant )
Effect of heat load on heat transfer coefficient at different inlet temperatures of cooling water
Effect of inlet temperature of cooling water on heat transfer coefficient at different heat loads
Baseload Capacity (kW) bsfc (kJ/kWh) Exhaust Flow ( kg/hr) Exhaust Temperat ure C Exhaust Power(MJ /hr) Cooling Power (MJ/hr) 10012,6606350571295.4348.15 30010,409286005041086.651192.15 80010,297549004871951.752584.75 3,00010,0142200003645211.74610.35 5,0009,2403040003707395.556625.4 Waste Thermal Power
Baseload Capacity (kW) bsfc (kJ/kWh) Exhaust Power (MJ/hr) Cooling Power (MJ/hr) Lube System (MJ/hr) Total Power (MJ/hr) 10012,660295.4348.150643.55 30010,4091086.651192.1502278.8 80010,2971951.752584.7504536.5 3,00010,0145211.74610.351287.111109.15 5,0009,2407395.556625.42046.716067.65 Waste Thermal Power