ENGINE HEAT TRANSFER

P M V SubbaraoProfessor

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

Finned Engine Cylinder

Geometrical Design of Finned Cylinder

Radial Conduction Equation

Define

TT fin

Radial conduction equation :

021

2

2

k

h

dr

d

rdr

d

The appropriate boundary conditions:

fin tip insulated and rr b bat

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

kh

m22

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 thefluid (B) due to total heat flow rate at the fin base canbe expressed as

Development of Compact Finned Cylinder

The heat flow through the base is

The ideal heat flow

Fin Efficiency

Liquid Cooling System

Liquid cycle In the system

Engine liquid passageways

Liquids for Engine Cooling

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.

Cold Startup of a SI engine.

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 - 910C, the thermostat starts to open, allowing fluid to flow through the radiator.•By the time the coolant reaches 93 - 1030C), the thermostat is open all the way.

Open & Closed Cooling Circuits

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:

ambientoilcoolantloss QQQQ

• General range of various heat losses are:

Type of loss Range Remarks

Cooling 10 – 30 %

5 – 15%

Diesel engines on higher side

Oil At low load higher losses

Ambient 2 – 10%

Friction 10%

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:

gaswall TThAQ

Thermal Analysis of Engine Cylinder

Gas

Heat Transfer in Combustion Chambers

cg

coolantgas

hkx

h

TT

A

Qq

11

Gas to Surface Heat Transfer

• Heat transfer to walls is cyclic.

• Gas temperature Tg 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

wallgasgcconv

conv TThA

Qq

w

w

g

g

wallgaswallgasgr

radrad

F

TTTTh

A

Qq

111

21

44

Cycle to Cycle Variation of Local Heat Flux:

Spatial Variation of Local Heat Flux:

Conduction Through Cylinder Liner & Innerwall

Gas

𝑇𝑤𝑎𝑙𝑙−𝑖𝑛𝑛𝑒𝑟−𝑇𝑤𝑎𝑙𝑙−𝑜𝑢𝑡𝑒𝑟 =𝑄×ln(𝐷𝑜/𝐷𝑖) / (2×𝜋×𝑘× )𝑙

Heat Transfer from Wall to Coolant

Q: The total heat transferred from gas to walls.Q1: Heat carried off by the cooling waterQ2: Heat transferred across the cylinder block to the ambient .

𝑄=ℎ×𝐴×(𝑇 -outer�𝑤𝑎𝑙 −𝑇coolant)

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

Cooling of Piston

Computed Temperature of A Piston

Heat Transfer in Exhaust System

Measurement of Engine Heat Transfer

Baseload Capacity (kW)

bsfc (kJ/kWh)

Exhaust Flow ( kg/hr)

Exhaust Temperat

ure C

Exhaust Power(MJ

/hr)

Cooling Power(MJ/hr)

100 12,660 6350 571 295.4 348.15300 10,409 28600 504 1086.65 1192.15800 10,297 54900 487 1951.75 2584.75

3,000 10,014 220000 364 5211.7 4610.355,000 9,240 304000 370 7395.55 6625.4

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)

100 12,660 295.4 348.15 0 643.55300 10,409 1086.65 1192.15 0 2278.8800 10,297 1951.75 2584.75 0 4536.5

3,000 10,014 5211.7 4610.35 1287.1 11109.155,000 9,240 7395.55 6625.4 2046.7 16067.65

Waste Thermal Power

Organic Substances must be selectedin accordance to the heat sourcetemperature level (Tcr < Tin source)