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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APPLICATION TO AUTOMOBILE ENGINE
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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APPLICATION TO AUTOMOBILE ENGINE
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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