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Internal Combustion Engines

Lecture-24

Ujjwal K Saha, Ph.D.Department of Mechanical Engineering

Indian Institute of Technology Guwahati

Prepared underQIP-CD Cell Project

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Break-up of Energy

The energy released in the combustion chamber of an internal combustion engine is dissipated in three different ways.

About 35 % of the fuel energy is converted to useful crankshaft work, and about 30 % energy is expelled with the exhaust. This leaves about one-third of the total energy that must be transmitted from the enclosed cylinder through the cylinder walls and head to the surrounding atmosphere.

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Combustion Chamber Temperature

The temperature in the combustion chamber of an engine goes up to 2700 K, and the materials used in the engine cannot withstand this.

Further, this high temperature destroys the lubricating properties of the oil film on the cylinder walls. At the same time, thermal stresses will be developed thereby distorting the cylinders, head and piston.

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Energy Distribution

f fP m Q=

bthf f c

bpm Q

ηη

=

Power generated = exhaust loss accbp Q Q P+ + +

where = energy lost to exhaust= energy lost to surroundings by heat transfer= power required to drive engine accessories

exhaustQ

lossQaccP

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Temperature Distribution

Piston face300 C

Spark plug600 C

Exhaustvalve650 C

Intake valve250 C

Intakemanifold60 C

Cylinder wall185 C

Exhaustflow450 C

Pistonring220 C

Pistonskirt190 C

Oil 70 C

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Modes of Heat Transfer

In general, heat transfer by conduction takes place through the cylinder head, cylinder walls, and piston; through the piston rings to the cylinder wall; through the engine block and manifolds.

Heat transfer by forced convection occurs between the in-cylinder gases and cylinder head, valves, cylinder walls, and piston during the engine cycle.

Heat transfer by radiation occurs through the emission and absorption of electromagnetic waves. Radiative heat transfer occurs from the high temperature combustion gases and the flame region to the combustion chamber walls.

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Heat Transfer in Intake System

The walls of the intake manifold are hotter than the flowing gases, heating them by convection:

where, T = temperatureh = convection heat transfer coefficientA = inside surface area of intake manifold

( )wall gasQ hA T T= −

The manifold is hot, either by design on some engines or just as a result of its location close to other hot components in the engine compartment.

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Heating the Manifolds

Some are designed such that the flow passages of the runners come in close thermal contact with the hot exhaust manifold. Others use hot coolant flow through a surrounding water jacket. Electricity is used to heat some intake manifolds.

Some systems have special localized hot surfaces, called hot spots,in optimum locations, such as immediately after fuel addition or at a tee where maximum convection occurs.

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Heating the Manifolds

Engine systems using multipoint port injectors have less need for heating the intake manifold, relying on finer fuel droplets and higher temperature around the intake valve to assure necessary fuel evaporation. This results in higher volumetric efficiency for these engines.

Often, the fuel is sprayed directly onto the back of the intake valve face. This not only speeds evaporation, but cools the intake valve, which can reach cyclic temperatures up to 400ºC. Steady-state temperature of intake valves generally is in the 200º-300ºC range.

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Heat Transfer in Combustion Chambers

During combustion, peak gas temperature on the order of 3000 K occur within the cylinders, and effective heat transfer is needed to keep the cylinder walls from overheating.

Convection and conduction are the main heat transfer modes to remove energy from the combustion chamber and keep the cylinder walls from melting.

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Heat Transfer through a Cylinder Wall

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Heat transfer per unit surface area will be:

where, gas temperature in the combustion chambercoolant temperatureconvection heat transfer coefficient on the gas sideconvection heat transfer coefficient on the coolant sidethickness of the combustion chamber wall thermal conductivity of cylinder wall

Heat Transfer through a Cylinder Wall

( ) ( ) ( ) ( )/ / 1/ / 1/g c g cq Q A T T h x k h⎡ ⎤= = − + ∆ +⎣ ⎦

gT =

cT =gh =

ch =

x∆ =

k =

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Convection heat transfer on the inside surface of the cylinder is:

Convective Heat Transfer

( )/ g g wq Q A h T T= = −

Wall temperature should not exceed 180º-200ºC to assure thermal stability of the lubricating oil and structural strength of the wall.

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There are number of ways of identifying a Reynolds number to use for comparing flow characteristics and heat transfer in engines of different sizes, speeds, and geometries. Choosing the best characteristic length and velocity is sometimes difficult.

Reynolds Number

One way of defining a Reynolds number for engines that correlates data fairly well is:

( ) ( )Re /a f p gm m B A µ⎡ ⎤= +⎣ ⎦where, air mass flow rate into the cylinder

fuel mass flow rate into the cylinderborearea of piston facedynamic viscosity of gas in cylinder

am =fm =

B =pA =

gµ =

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A Nusselt number for the inside of the combustion chamber can be defined using the Reynolds number:

Nusselt Number

where, C1 and C2 = constants

thermal conductivity of cylinder gasaverage convective heat transfer coefficient

( ) 2

1Nu / Re cg gh B k C= =

gk =gk =

The Nusselt number and convection heat transfer coefficient on the coolant side of the cylinder walls can be approximated by conventional methods of forced convection heat transfer.

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Radiation heat transfer between cylinder gas and combustion chamber walls is:

Radiation Heat Transfer

where, gas temperature wall temperatureStefan-Boltzmann constantemissivity of gasemissivity of wall view factor between gas and wall

( ) ( ) [ ] ( ){ }4 41 2/ / 1 / 1/ 1 /g w g g w wq Q A T T Fσ −

⎡ ⎤ ⎡ ⎤= = − −∈ ∈ + + −∈ ∈⎡ ⎤⎣ ⎦⎣ ⎦⎣ ⎦

gT =

wT =σ =

g∈ =

w∈ =

1 2F − =

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Radiation Heat Transfer

Even though gas temperatures are very high, radiation to the walls only amounts to about 10% of the total heat transfer in SI engines. This is due to the poor emitting properties of gases, which emit only at specific wavelength.

N2 and O2, which make up the majority of the gases before combustion, radiate very little, while the CO2 and H2O of the products do contribute more to radiation heat transfer.

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Radiation Heat Transfer

The solid carbon particles that are generated in the combustion products of a CI engine are good radiators at all wave lengths, and radiation heat transfer to the walls in these engines is in the range of 20-35% of the total.

A large percent of radiation heat transfer to the walls occurs early in the power stroke. At this point the combustion temperature is maximum, and with thermal radiation potential equal to T4, a very large heat flux is generated. This is also the time when there is a maximum amount of carbon soot in CI engines, which further increases radiative heat flow.

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Local heat flux variation experienced at one location in a single cylinder of a typical engine for three consecutive engine cycles.

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Local heat flux variation experienced at three different locations within the combustion chamber of a single cylinder during one cycle of a typical engine.

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Cooling of PistonCooling of PistonThe piston face (A) is one of the hottest surfaces in a combustion chamber. Cooling is mainly done by convection to the lubricating oil on the back side of the piston face, by conduction through the piston rings in contact with cylinder walls, and by conduction down the connecting rod to the oil reservoir. High conduction resistance occurs because of lubricated surfaces at cylinder walls (X) and the rod bearings (Y).

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Heat Transfer in Exhaust System

Pseudo-steady-state exhaust temperatures of SI engines are generally in the range of 400º-600ºC, with extremes of 300º-900ºC. Exhaust temperatures of CI engines are lower due to their greater expansion ratio and are generally in the range of 200º-500ºC.

Some large engines have exhaust valves with hollow stems containing sodium. These act as heat pipes and are very effective in removing heat from the face area of the valve.

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Effect of Variables on Heat Transfer1. Engine Size: If two geometrically similar engines of different size (displacement) are run at the same speed keeping all other variables (temperature, AF, fuel etc.) as close as possible, the larger engines will have a greater absolute heat loss but will be more thermal efficient.

2. Engine Speed: As engine speed is increased, gas flow velocity into and out of the engine goes up, with a resulting rise in turbulence and convective heat transfer coefficient. This increases heat transfer during intake and exhaust strokes and even early part of the compression strokes. During combustion and power strokes, gas velocities within the cylinders are fairly independent of engine speed.

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Effect of Variables on Heat Transfer3. Load: As the load on an engine is increased (going uphill, pulling a trailer), the throttle must be further opened to keep the keep the engine speed constant. This causes less pressure drop across the throttle and higher pressure and density in the intake system. Mass flow rate of air and fuel, therefore, goes up with load at a given engine speed. The percentage of heat loss goes down slightly as engine load increases.

CI engines run unthrottled, and total mass flow is almost independent of load. When speed or load is increased and more is needed, the amount of fuel injected is increased. This increases the total mass flow in the latter part of each cycle by about 5 %. Thus, convection heat transfer coefficient within the engine is fairly independent of load.

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Effect of Variables on Heat Transfer

4. Inlet Air Temperature: Increasing inlet air temperature to an engine results in a temperature increase over the entire cycle, with a resulting increase in heat loss. A 1000C increase in inlet temperature will give a 10-15 % increase in heat losses.

5. Swirl and Squish: Higher swirl and squish velocities result in a higher convection heat transfer coefficient within the cylinder. This results in better heat transfer to the walls.

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Technologists, Addison Wisley.3.3. Fergusan CR, Fergusan CR, andand Kirkpatrick ATKirkpatrick AT,, (2001), Internal Combustion Engines, John

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The MIT Press, Cambridge, Massachusetts.

References

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