INTERNAL COMBUSTION ENGINE

(SKMV 3413)

Dr. Mohd Farid bin Muhamad Said

Room : Block P21, Level 1, Automotive

Development Centre (ADC)

Tel : 07-5535449

Email: mfarid@fkm.utm.my

Topic 11: Heat Transfer in

Engines

EFFECT OF ENGINE OPERATING

VARIABLES ON HEAT TRANSFER

Heat transfer within engines depends on so many different variables that it is difficult to correlate one engine with

another.

These variables include the:

air-fuel ratio,

speed,

load,

brake mean effective pressure,

spark timing,

compression ratio,

Materials

size.

EFFECT OF ENGINE OPERATING

VARIABLES ON HEAT TRANSFER

If two geometrically similar engines of different size (displacement) are run at the same speed, and all other

variables (temperature, AF, fuel, etc.) are kept as close to

the same as possible, the larger engine will have a greater

absolute heat loss but will be more thermal efficient.

If the temperatures and materials of both engines are the same, heat loss fluxes to the surroundings per unit area will

be about the same, but the absolute heat loss of the larger

engine will be greater due to larger surface areas.

Engine Size

EFFECT OF ENGINE OPERATING

VARIABLES ON HEAT TRANSFER

As speed increased, gas flow velocity into and out of the engine goes up increase turbulence and convection heat transfer coefficient.

Time of the cycle is less at higher speed, less heat transfer per cycle occurs. This gives the engine a higher thermal efficiency at

higher speed.

At higher speeds, more cycles per unit time occur, but each cycle lasts less time.

The net result is a slight rise in heat transfer loss with time (kW) from the engine.

This is partly due to the higher heat losses for part of the cycle, but is mostly due to the higher steady-state losses which the engine

establishes at higher speeds

Engine Speed

EFFECT OF ENGINE OPERATING

VARIABLES ON HEAT TRANSFER

All steady state temperatures within an engine goes up as engine speed increases.

Engine Speed

EFFECT OF ENGINE OPERATING

VARIABLES ON HEAT TRANSFER

Heat transfer to the engine coolant increases with higher speed:

To stay at the same steady-state temperature as engine speed is increased, more heat must be transferred to the surroundings from the

coolant in the automobile radiator heat exchanger.

At higher speed, there is also less time for heat transfer per cycle, which means the engine runs hotter, and a hotter engine has a greater knock

problem.

Engine Speed

= ( )

EFFECT OF ENGINE OPERATING

VARIABLES ON HEAT TRANSFER

As load increased, heat transfer within the engine also goes up by:

Engine temperatures increase with load.

At light loads less fuel is injected and burned, creating a cooler steady-state temperature. This decreases the corresponding heat transfer.

At heavy load more fuel is injected and burned, and the resulting steady-state temperature is higher. This causes a greater convective heat

transfer.

Engine Load

=

= convection heat transfer coefficient = surface area at any point = temperature difference at that point

EFFECT OF ENGINE OPERATING

VARIABLES ON HEAT TRANSFER

More power and higher temperatures are generated when the spark setting is set to give maximum pressure.

These higher peak temperatures will create a higher momentary heat loss, but this will occur over a shorter length of time.

With spark timing set either too early or too late, combustion efficiency and average temperatures will be lower.

These lower temperatures will give less peak heat loss, but the heat losses will last over a longer length of time and the overall

energy loss will be greater.

Spark Timing

EFFECT OF ENGINE OPERATING

VARIABLES ON HEAT TRANSFER

Other variables that effect on heat transfer include:

Fuel equivalence ratio

Evaporation cooling

Inlet air temperature

Coolant temperature

Engine materials

Compression ratio

Knock

Swirl and squish

Example 2