Engine Heat Transfer

Shahnaz Sultana

Visiting Faculty

Mechanical Engineering Department

PESIT, Bangalore

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Loss of Heat is encouraged only to keep engine safe….It’s a penalty on performance……

Energy Distribution

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• Around 35% of the total chemical energy that enters an engine is converted to useful crankshaft work.

• About 30% of the fuel energy is carried away from the engine in the exhaust flow in the form of enthalpy and chemical energy.

• About one-third of the total energy is dissipated to the surroundings by some mode of heat transfer.

Energy Flows in Engines

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There are three overall paths for energy energy flow: shaftwork, coolant, and exhaust. They are approximately equal, each about 1/3 of the energy of the incoming fuel/air mixture

Importance of Engine Heat Transfer

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Satisfactory Heat Transfer is required for a number of important reasons:

Material temperature limits Lubricant performance limits Emissions and Knock

Combustion in IC Engines is not continious as in External EnginesTemperatures of certain critical areas need to be kept below material design limitsAluminium alloys begin to melt at temps greater than 775KMelting point of cast iron is about 1800K

Differing temperatures around cylinder bore will cause bore distortion leading to:

Increased blow-by Oil consumptionPiston wear

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Engine Heat Transfer

Gas Temperature Variation during a Cycle

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Heat Transfer in Engines

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Why is heat transfer in engines important ?

There is a need to keep the temperatures of two critical areas below

material design limits. These areas are the piston crown and the exhaust valve.

Emission levels and octane requirements are a function of engine temperature.

Heat Transfer in Engines

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How do we determine engine heat transfer ?

The calculation of engine heat transfer is difficult, due to the periodic air and fuel flow and the complex geometry of the engine. We rely primarily on experimental results. With recent advances in computational fluid dynamics, computation of engine heat transfer is becoming more possible.

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What are typical heat transfer rates in engines ?

The majority of engines produced are automotive six cylinder engines, with about a 4" (100 mm) piston diameter (bore) and 4" (100mm) piston stroke , producing about 100 hp (75 kW). Since the heat transfer to the coolant and the heat convected from the exhaust are about equal to the power produced, the heat transfer to the coolant and to the exhaust will also be about 75 kW.

For this typical automotive engine, the total cylinder volume or displacement is typically about 300 cubic inches (0.005 m3), and the total cylinder area is about 0.2 m3. Therefore the power density is about 75 kW/ 0.005 m3 or 15 MW/m3 of displacement. The heat transfer per unit cylinder area will be 75kW/0.2 m3 or 375 kW/m3.

Heat Transfer in Engines

Heat Transfer Mechanism

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The three heat transfer mechanisms are:ConductionConvectionRadiation

Conduction

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Conduction through Piston-Cylinder Wall

Conduction heat transfer is energy transport due to molecular motion

and interaction. Conduction heat transfer through solids is due to

molecular vibration. Fourier determined that Q/A, the heat transfer per

unit area (W/m2) is proportional to the temperature gradient dT/dx. The

constant of proportionality is called the material thermal conductivity k

Fouriers equation :

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The thermal conductivity k depends on the material, for example, the

various materials used in engines have the following thermal

conductivities (W/m K):

Thermal Conductives of Common Materials

Copper 400

Aluminum 240

Cast Iron 80

water 0.61

air 0.026

Conduction

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Conduction

The thermal conductivity also depends on the temperature of the material.

For a cast iron 0.012m (½") cylinder block at steady state

Convection

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Convection heat transfer is energy transport due to bulk fluid motion. Convection heat transfer through gases and liquids from a solid boundary results from the fluid motion along the surface.

Newton determined that the heat transfer/area, Q/A, is proportional to the fluid

solid temperature difference Ts-Tf. The temperature difference usually occurs

across a thin layer of fluid adjacent to the solid surface. This thin fluid layer is

called a boundary layer. The constant of proportionality is called the heat

transfer coefficient, h.

Newton's Equation:

Boundary Layer

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The heat transfer coefficient depends on the type of fluid and the fluid

velocity. The heat flux, depending on the area of interest, is the local or

area averaged. The various types of convective heat transfer are

usually categorized into the following areas :

Table II. Convective Heat Transfer Coefficients

Convection

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For a cylinder block with a forced convection h of 1000, surface temperature of 100C , and a coolant temperature of 80 C, the local heat transfer rate is :

Convection

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Convection

For Flow through pipes or over plates

Nusselt No. Reynolds No. Prandtl No.

Radiation

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Radiation through Piston Cylinder Wall

Radiation heat transfer is energy transport due to emission of electromagnetic waves or photons from a surface or volume. The radiation does not require a heat transfer medium, and can occur in a vacuum. The heat transfer by radiation is proportional to the fourth power of the absolute material temperature. The proportionality constant s is the Stefan-Boltzman constant equal to 5.67 x 10-8

W/m2K4. The radiation heat transfer also depends on the material properties represented by e, the emissivity of the material.

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For a surface with an emissivity of e = 0.8 and T = 373 K (100C), the

radiation heat transfer is

For moderate (less than 100 C) temperature differences, it should be noted

that the radiation and natural convection heat transfer are about the same.

Radiation

Overall Heat Transfer

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Piston Temperature Distribution

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Cylinder Wall Temp Distribution

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Heat Transfer

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Heat Transfer &Engine Energy Balance

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Control Volume surrounding Engine

Heat Transfer &Engine Energy Balance

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Heat Transfer &Engine Energy Balance

Energy Flow Diagram

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Boundary Layer Behavior

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Component Temperatures

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