IC Engine - Jeff Hanna

8/8/2019 IC Engine - Jeff Hanna

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EARLY WORK ON FLUID MECHANICS IN THEINTERNAL COMBUSTION ENGINE

John L Lumley

Annual Review of Fluid Mechanics Vol. 33 pp. 319-338

Jeff HannaApril 26, 2006


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OVERVIEW

Stimulus for understanding effects ofturbulence

Engine knockingTurbulence plays a significant role

Ricardo Early 1900s

Understand turbulence and its effect on knock

Early fuel had very low octane

Either limit compression ratio or increase turbulence

Investigate overhead valve engines and flat-head engines

National Advisory Committee for Aeronautics Mid 1900s

Performed two research projects on a simulated cylinder of

an aircraft engine to measure internal turbulence

Obukhov 1970s

Significant research on swirling motions in ellipsoids

Found two types of instabilities


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STIMULUSMain reason to investigate turbulence effects: knock

Auto-ignition of gasoline-air mixture that occurs above a certain

temperature and pressure. Ifthe mixture ignites before it is

supposed to, the engine cannot function properly.

This auto-ignition reaction takes time, and must not be completed

before the spark induced flame reaches all ofthe gases

Turbulence increases the flame speed, thereby decreasing the

amount

oftime

tha

tthe end gases mus

twai

t.-Desire to induce turbulence

Tumble is a rotational motion about an axis perpendicularto that

ofthe cylinder


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RICARDO EARLY 1900s

Very low octane content rating prone to knocking

Keep compression ratio low sacrifices performance

Use turbulence to increase flame speed

Alter shape of combustion chamber

Through lots oftesting, Ricardo became convinced thatthe higher

efficiency of overhead valve engines (compared to flat-head valve)

was due to much greaterturbulence, shorter flame travel, and was

thus less prone to detonate


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RICARDO EARLY 1900s

Overhead Valve Engine Ricardos Flat-head Valve Engine

De K Dykes et al (1965)

Lee (1939)


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RICARDO EARLY 1900s

Ricardos Flat-head Valve Engine

1. Concentrate main volume of

chamber overthe valves,

leaving minimum clearance

bet

ween pist

on and cylinderhead

2. Chilled portion of charge

trapped in laminum so it

could not detonate (squish)

3. Shortened flame travel bymoving sparkplug to the

center ofthe chamber

12

3

De K Dykes et al (1965)


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RICARDO EARLY 1900s

Ricardo was able to obtain the same power output as an overhead

valve engine ofthe same dimensions.

Therefore, turbulence levels can be assumed to be comparable

In general, turbulence levels in an engine cylinder scale with the mean

piston speed.

An overhead valve engine with tumble reaches a RMSturbulent velocity

of scale mean piston speed

An engine without squish reaches RMSturbulent velocity of mean

piston speed

Estimating the RMSturbulent velocity for squish reveals mean piston

speed

Combine this with the residual to obtain a RMSturbulent velocity of

scale mean piston speed just like an overhead valve engine.


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NACA 1938

In 1938, the National Advisory Committee for Aeronautics

performed two experiments on a simulated aircraft engine cylinder.

Using a glass cylinder and high speed

camera, they were able to calculate

speeds of chopped goose down in anoverhead valve cylinder with 4 valves.

Determined RMSturbulent velocity to

be approximately 1.6 times the mean

piston speed, with small amounts of

tumble.

Lee (1939)


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NACA 1938

Using shrouds on the valves

placed in various positions as

shown here, NACA

determined thatthe RMSturbulence velocities

increased to about 2.6 times

the mean piston speed, with

much more tumble.

Lee (1939)


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NACA 1938

In their second experiment, they removed

the glass cylinder and instead put a glass

window in place ofthe exhaust valves.

Observed thatthe highestturbulence level

during early combustion was fromshrouding arrangements D, G and F,

which were expected to produce the

highest levels ofturbulence.

The conclusion from these experimentswas thatthe higher levels ofturbulence

were directly proportional to the gas

velocities flowing through the valves.Lee (1939)


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OBUKHOV 1970s

Analyses of dynamical behavior oftumbling motion in ellipsoids that

can be utilized for flow in an engine cylinder.

Obukhov et al considered an incompressible, inviscid fluid system

and found thatthe simplest non-trivial system is a triplet which can

be writt

en in canonical form shown here, and which arethe same asEulers equations for force-free motion of a rigid body.


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OBUKHOV 1970s

From the rigid body equations, we know there is a second integral

of motion which corresponds to angular momentum.In a fluid case, this corresponds to the sum ofthe squares of

circulations aboutthe principal sections

Spin of a rigid body aboutthe middle axis is unstable, while spin

around the othertwo axes is stable (various textbooks on

mechanics).

Related to fluid mechanics, rotation aboutthe middle axis of an

ellipsoid is unstable and will overturn.


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OBUKHOV 1970s

Obukhov et. al experimented with transparent spinning ellipsoids

(filled with water) to look atthe instabilities associated with theflow.

The ellipsoid was rotated for a long period oftime to ensure solid

body rotation, and then quickly stopped.

The flow satisfied the force-free motion of a rigid body

equations until the boundary layers became too large, which

took approximately 5 fluid revolutions.

If initial rotation was aboutthe short axis, the motion was

stable and continued.If rotation was aboutthe intermediate axis, it flipped over and

rotated aboutthe shorter axis within about one fluid revolution


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OBUKHOV 1970s

Overturning process forrotation of a fluid aboutthe

intermediate axis of an

ellipsoid (Obukhov 2000)

Obukhov (2000)


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OBUKHOV 1970s

Stability of rotation aboutthe

long axis can be demonstratedifthe long axis is less than 2x

the short axis.

Asthe long axis leng

threaches 2x the short axis, the

flow flips and forms two

vortices, parallel to the short

axis.

As the long axis is increased,

the motion becomes stable

again, and then unstable etc.

Obukhov (2000)


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APPLICATION TO AUTOMOBILE ENGINE

Ricardo proved that increasing the turbulence in the combustion

chamber increased flame speed, making engines more reliable.

NACA demonstrated that valve arrangements make it possible to

introduce tumble in a cylinder

We expect conservation of angular momentum to amplify the tumble

during the compression stroke, as the vortices get smaller.

Obukhov showed how rotational flow aboutthe intermediate axis was

unstable and would turnover.

2 Problems Ellipsoid is not a cylinder

Cylinder is symmetric


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Problems with Obukhov:

The rotation in a cylinder will be of higher order.

Truncating the system at least allows for a qualitative idea

of what is happening, even though it is not exact.

Because the the cylinder is symmetric, two ofthe axes will be

the same length. If rotation is aboutthe long axis, I1=I2,

corresponding to r=0 from the rigid body equations. This results

in a table situation, and no overturning would be present.


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Multi-vortex instability

Stabilities change as the piston moves up the cylinder.

Initially the long axes is the axis ofthe cylinder, but once the

piston moves half way up the cylinder, it becomes the smallest

axis, with two equal longer axes perpendicularto it.The tumble will break up into a number of smaller cortices

with axes at right angles to the axis of initial tumble.

As the piston moves more and more, the number of vortices

becomes greater and the individual vortices smaller in

diameter.

Gledzer & Ponomarev (1992) indicated that when the piston is

half-way up the cylinder, the tumble becomes unstable to half-size

vortices a

trigh

tangles

tothe original axis, fur

ther backing

this.


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QUESTIONS?

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