3. Modes of heat transfer
Piston heat transfer
Cylinder heat transfer
OUTLINE
4. There is a need to keep the temperatures of two critical regions below
material design limits: these regions are the piston crown and exhaust valve.
IMPORTANCE OF HEAT
TRANSFER
5. There are three modes if heat transfer: conduction, convection, radiation.
Conduction:
Fourierβs Equation
π
π΄
= βπ
ππ
ππ₯
The figure below shows conduction through piston cylinder wall
MODES OF HEAT
TRANSFER
8. The conduction and convection heat transfer in engines are processes that
occur in series and parallel with each other. A series path is convection
through the cylinder gas boundary layer, conduction across the cylinder wall,
and convection through the coolant liquid boundary layer; and a parallel path
is conduction through the cylinder wall and through the piston crown.
HEAT TRANSFER RESISTANCE
MODELING
9. There are two types of coolants used to remove the heat from the engine
block and head: air, and water. With air as a coolant, the heat is removed
through the use of fins attached to the cylinder wall. With water as a coolant,
the heat is removed through the use of fluid filled internal cooling passages.
The figure below shows three resistor network for piston cylinder wall.
HEAT TRANSFER TO
COOLANT
10. The figure below shows cooling system loop:
The water cooling system is usually a single loop where a water pump sends
coolant to the engine block, and then to the head. The coolant will then flow
to a radiator or heat exchanger and back to the pump.
HEAT TRANSFER TO
COOLANT
11. The piston crown conducts heat from the conduction gases to the cooling oil,
to the piston rings, and the piston skirt. Frictional heating is also a source of
heat.
The heat transfer coefficient depends on the
engine geometric parameters, such as the
β’ exposed cylinder area and
β’ bore, and
β’ piston speed..
PISTON HEAT TRANSFER
12. For the overall average heat transfer from the gas to the cylinder coolant,
convection type heat transfer equations are used.
π
π΄
= β ππππ β ππππππππ‘
The coefficient varies with location and piston position. The coefficient is
found from a Nusselt - Reynolds number correlation
ππ’π π πππ‘ ππ = πΌ Γ (π ππ¦ππππβ² π ππ) π
PISTON HEAT TRANSFER
13. The heat transfer from the hot combustion gases includes forced convection
through the hot gas boundary layer, conduction through the cylinder wall, and
forced convection (including boiling) into the fluid coolant in the head, engine
block, and piston. There is a small ( about 5 %) radiative component of heat
transfer from the gas to the cylinder walls.
ππ
ππ‘
= πΌ
π2 π
ππ₯2 unsteady conduction equation
CYLINDER HEAT
TRANSFER
14. The heat transfer process is periodic due to the piston motion. However, the
engine speed is usually high enough so that the temperature fluctuations only
penetrate about a millimeter into the cylinder wall.
The penetration depth of temperature fluctuations into the cylinder wall is
calculated as below:
π₯ = (πΌπ‘)
1
2
π‘ =
1
π
=
1
1000
Γ
πππ
πππ£
Γ
60 π ππ
πππ
Γ
πππ£
2π πππ
π‘ = 0.01 π ππ
π₯ = (πΌπ‘)
1
2 = 20 Γ 10β6Γ 10β12 = 0.5 ππ
CYLINDER HEAT
TRANSFER
15. The figure below is a representative graph of the cylinder heat flux as a
function of crank angle.
The heat flux begins rising when the combustion flame impacts the cylinder
wall, has a maximum at peak cylinder pressure when gas temperatures peak,
typically 20 degrees after Top Dead Center (TDC).
CYLINDER HEAT
FLUX