This presentation was prepared by Mechanical Engineering students during their Internal Combustion Course.

1. ENGINE HEAT
TRANSFER

2. 2014-ME-322
2014-ME-326
2014-ME-335
2014-ME-350
GROUP MEMBERS

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:
Fourier’s Equation
𝑄
𝐴
= −𝑘
𝑑𝑇
𝑑𝑥
The figure below shows conduction through piston cylinder wall
MODES OF HEAT
TRANSFER

6. Convection:
𝑄
𝐴
= ℎ(𝑇1 − 𝑇2)
The figure below shows convection through piston cylinder wall
MODES OF HEAT
TRANSFER

7. Radiation:
𝑄
𝐴
= ε𝜎𝑇4
The figure below shows radiation 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
• exposed cylinder area and
• bore, and
• 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.
𝑄
𝐴
= ℎ 𝑇𝑔𝑎𝑠 − 𝑇𝑐𝑜𝑜𝑙𝑎𝑛𝑡
The coefficient varies with location and piston position. The coefficient is
found from a Nusselt - Reynolds number correlation
𝑁𝑢𝑠𝑠𝑒𝑙𝑡 𝑁𝑜 = 𝛼 × (𝑅𝑒𝑦𝑛𝑜𝑙𝑑′ 𝑠 𝑁𝑜) 𝑚
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.
𝜕𝑇
𝜕𝑡
= 𝛼
𝜕2 𝑇
𝜕𝑥2 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:
𝑥 = (𝛼𝑡)
1
2
𝑡 =
1
𝜔
=
1
1000
×
𝑚𝑖𝑛
𝑟𝑒𝑣
×
60 𝑠𝑒𝑐
𝑚𝑖𝑛
×
𝑟𝑒𝑣
2𝜋 𝑟𝑎𝑑
𝑡 = 0.01 𝑠𝑒𝑐
𝑥 = (𝛼𝑡)
1
2 = 20 × 10−6× 10−12 = 0.5 𝑚𝑚
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