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Internal Combustion Engines

Lecture-20

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

Indian Institute of Technology Guwahati

Prepared underQIP-CD Cell Project

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Fluid Motion in Combustion Chamber

Due to high velocities involved, all flows into, out of and within cylinders are turbulent. The exception to this are those flows in the corners and small crevices of the combustion chamber where the close proximity of the walls dampens out the turbulence.

As a result of turbulence, heat transfer, evaporation, mixing and combustion rates all increase. As the engine speed increases, flow rate increases with a corresponding increase in swirl, squish and turbulence. This increases the rate of fuel evaporation, mixing of fuel vapor and air, and combustion.

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Fluid Motion in Combustion Chamber

Turbulence in a cylinder is high during intake, and decreases as the flow rate slows near BDC. It increases again during compression, as swirl, squish and tumbleincreases near TDC.

The high turbulence near TDC when ignition occurs is very desirable for combustion. It breaks up and spreads the flame front many times faster. The air-fuel is consumed within a short time, and self-ignition and knock are avoided. The shape of the combustion chamber plays an important role in generating maximum turbulence and increasing the desired rapid combustion.

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Volumetric Efficiency

The inside surface of most intake manifolds are usually made smooth to maximize the volumetric efficiency.

However, in some engines where high power is not desirable, the inside surfaces of manifolds are roughened to promote higher turbulence levels to enhance evaporation and air-fuel mixing.

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Two-stroke Engines

Turbulence is detrimental in the scavenging process of two-stroke cycle engines. This is because, the incoming air mixes more with the exhaust gases, and a greater exhaust residual will remain within the cylinder.

Another negative result occurs during combustion when high turbulence enhances the convective heat transfer to the walls in the combustion chamber. This higher heat loss lowers the thermal efficiency of the engine.

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SwirlThe rotational motion of the fluid mass within

the cylinder is called swirl. Swirl greatly enhances the mixing of air and fuel to give a homogeneous mixture within a short time. It is also a main mechanism for very rapid spreading of flame front during the combustion process.

Swirl can be generated by constructing the intake system to give a tangential component to the intake flow as it enters the cylinder. This is done by shaping and contouringintake manifolds, valve ports and piston faces.

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Helical port

Tangential injectionSwirl motion

Contoured valve

Cylinder Swirl and its Generation

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Swirl is used to:

promote rapid combustion in SI engines

rapidly mix fuel and air in gasoline direct injection engines

rapidly mix fuel and air in CI engines

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Swirl TheorySwirl can be simply modelled as solid body rotation, i.e., cylinder of gas rotates at an angular velocity, ω.

Swirl Ratio: It is a dimensionless parameter to quantify the rotational motion within the cylinder, and is defined in two different ways in technical literature:

( )

( )

1

2

(1)

tan (2)t

p

angular speedSRengine speed N

uswirl gential speedSRaverage piston speed U

ω= = − −

= = − −

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Swirl Ratio

Remark: The average values of either the angular speed or tangential speed should be used in the above equations. Angular motion is very non-uniform within the cylinder, being a maximumaway from the walls and being much less near the walls due to viscous drag.

( )

( )

1

2

(1)

tan (2)t

p

angular speedSRengine speed N

uswirl gential speedSRaverage piston speed U

ω= = − −

= = − −

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The figure shows how swirl ratio changes through a cycle of the engine. During intake, it is high, decreasing after BDC in the compression stroke due to viscous drag with the cylinder walls.

Combustion expands the gases and increases swirl to another maximum part way into the power stroke. Expansion of the gases and viscous drag quickly reduce this again before blowdown occurs. Maximum swirl ratio as defined by equation (1) can be on the order of 5 to 10 for modern engine.

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Paddle-wheel Model

The mass moment of inertia (I) of cylinder gas is:

IωΓ =

where , m = mass of the gas, and B = bore = diameter of rotating mass

The volume within the cylinder is idealized to contain an imaginary paddle wheel that has no mass. As the wheel runs, gas between the blade turn with it with the result that all the gas rotate at one angular velocity.

The angular momentum (Γ) of the rotating gas is:

2 221 1

2 2 2 8B mBI mr m⎛ ⎞= = =⎜ ⎟

⎝ ⎠

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Many engines have a wedge shape cylinder head cavity or a bowl in the piston where the gas ends up at TDC.

Swirl

During the compression process as the piston approaches TDC more of the air enters the cavity and the air cylinder moment of inertia decreases and the angular velocity (and thus the swirl) increases.

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Squish

Squish is the radial flow occurring at the end of the compression stroke in which the compressed gases flow into the cavity in the piston or cylinder head.

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Tumble

As the piston reaches TDC the squish motion generates a secondary flow called tumble, where rotation occurs about a circumferential axis near the outer edge of the cavity or piston bowl.

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Squish and Tumble

Squish

Tumble

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Crevice Flow

Within the engine combustion chamber, there are tiny crevices which get filled with air, fuel and exhaust gas during the cycle. These crevices include:

clearance between the piston and the cylinder walls (80 % of total)

imperfect fit in the threads of spark plug or fuel injector (5 %)

gaps in the gasket between head and block (10-15 %)

un-rounded corners at the edges of combustion chamber and edges of valve faces.

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Crevice Flow

Although this crevice volume is of the order of 1-3 % of the total clearance volume, the flow into and out of it greatly affects the engine performance.

In an SI engine, air-fuel mixture is forced into these crevices, and some of the fuel ends up in engine exhaust thereby lowering thermal efficiency.

As fuel is added towards the end of the compression stroke in a CI engine, less fuel gets into the crevice volume.

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Piston Rings

Most pistons have two or more compression rings and atleast one oil ring. Compression rings seal the clearance gap between the piston and the cylinder walls. The oil ring scrape off most of the lubricating oil splashed on the cylinder wall, and return the oil to the crankcase.

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Piston Rings

Various designs of piston rings to minimize the leakage flow through the gap where the two ends meet.

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Summary

Efficient operation of an engine depends upon high turbulence in the air-fuel mixture, and the generated flows of swirl, squish and tumble. Swirl is the rotational motion generated in the cylinder during intake and compression, squish is the radial inward motion that occurs as the piston moves toward TDC, and tumble is created by squish motion and the shape of the clearance volume. All these motions enhance proper operation of the engine.

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Summary

Crevice flow is another flow motion that occurs during engine operation. Although crevice volume is only a small percent of the total combustion chamber volume, the flow into and out of it affects combustion and engine emissions. Some of the gas flow in the crevice between the piston and cylinder walls gets past the piston into the crankcase, where it raises the crankcase pressure and contaminates the lubricating oil.

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The MIT Press, Cambridge, Massachusetts.

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