Strategies to Achieve A Fast Cycle with High

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Strategies to Achieve A Fast Cycle with High
Safe Peak Pressure in SI Engines

• P M V Subbarao
• Professor
• Mechanical Engineering Department

Fuel Economy Vs Pace Vs Safety?!?!
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Main Requirement for User Acceptability Mixture
Burn Time vs Engine Speed
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Mixture Burn Time vs Engine Speed
Recall the turbulent burning velocity is
proportional to the turbulent intensity St ut,
which increases with the piston speed . The
piston speed is directly proportional to the
engine speed, up N Therefore, at higher engine
speeds the turbulent flame velocity is also
higher and as a result need less time to burn the
entire mixture Combustion duration in crank
angles (40-60 degrees) only increases a small
amount with increasing engine speed.
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Timing to achieve Maximum Brake Torque Timing

• If start of combustion is too early work is done
  against piston and if too late then peak
  pressure is reduced.
• The optimum spark timing that gives the maximum
  brake torque, called MBT timing occurs when
  these two opposite factors cancel.

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Selection of Spark Timing

• Spark timing relative to TC affects the pressure
  development and thus the engine imep and power.
• Ignite the gas before TC to center the pressure
  pulse around TC.
• The overall burning angle is typically between 40
  to 60o, depending on engine speed.

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Effect of Engine Speed on Spark Timing
Recall the overall burn angle (90 burn)
increases with engine speed, to accommodate this
you need a larger spark advance.
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Effect of Throttle on Spark Timing

• At part-throttle the residual gas fraction
  increases, and since residual gas represent
  diluents it lowers the laminar burning velocity.
• Because of lower burning velocity overall burn
  angle increases so need to increase spark
  advance.
• At idle, the residual gas fraction is very high ?
  the burn time is very long
• Long overall burn angle requires more spark
  advance.
• In modern engines the ECU sets the spark advance
  based on engine data such as
• throttle position, intake manifold pressure and
  engine speed

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Symptoms of Normal Combustion in SI Engines
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Cyclic Variation of Flame Volume
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The Finale
An equivalence ratio of f 1.1 gives the highest
burning velocity (minimum heat loss) and flame
temperature (maximum PCV). Best fuel economy is
obtained for a equivalence F/A that is less than
1.0 (0.89). The overall burning angle is
achieved typically between 40 to 60o, depending
on engine speed by creating appropriate
turbulence.
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Unexpected Engine Damage
Damage to the engine is caused by a combination
of high temperature and high pressure.
Piston crown
Piston
Aluminum cylinder head
Cylinder head gasket
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Dangerous Accidents
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Description Knock and Surface Ignition

• There are two primary abnormal combustion
  phenomena knock and surface ignition.
• Knock is the engine sound that results from
  spontaneous ignition of the unburned fuel-air
  mixture ahead of the flame (the end gas).
• Surface ignition is the ignition of the fuel-air
  mixture by any hot surface, other than the spark
  discharge, prior to arrival of the flame.

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Signatures of Abnormal Combustion in SI Engine
Knock is the term used to describe a pinging
noise emitted from a SI engine undergoing
abnormal combustion. The noise is generated by
shock waves produced in the cylinder when
unburned gas autoignites.
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Knock cycle
Exhaust valve
Spark plug
Observation window for photography
Intake valve
Normal cycle
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Physics of Normal Combustion

• Under normal conditions the combustion is ignited
  by a spark at the spark plug.
• The flame kernel grows and propagates through the
  combustion chamber until it reaches the cylinder
  walls where it extinguishes.
• The flame front propagates with a speed much less
  then the speed of sound.
• Therefore the cylinder pressure can be considered
  nearly constant in the cylinder.
• The unburned gas in front of the flame is called
  the end gas.

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Physics of Knocking Combustion

• Knock is a phenomenon that occurs when high
  temperature and pressure causes the end gas to
  self ignite.
• This causes a very high local pressure and this
  generates pressure waves across the combustion
  chamber.
• These pressure waves excites the resonance modes
  of the cylinder.
• The frequency of the oscillations under knocking
  conditions depends on engine geometry, and is
  often in the range of 5 to 10 kHz.

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Reasons for Birth of Knock
The end-gas autoignites after a certain induction
time which is dictated by the chemical kinetics
of the fuel-air mixture. If the flame burns all
the fresh gas before auto-ignition in the end-gas
can occur then knock is avoided. Therefore knock
is a potential problem when the burn time is long.
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Engine Design Parameters Causing the Knock

• The end-gas temperature and the time available
  before flame arrival are the two fundamental
  variables that determine whether or not knock
  will occur.
• Engine parameters that effect these two
  fundamental variables are
• Compression ratio, spark advance, speed, inlet
  pressure and temperature, coolant temperature,
  fuel/air ratio.

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Important Engine Variables

• i) Compression ratio at high compression
  ratios, even before spark ignition, the fuel-air
  mixture is compressed to a high pressure and
  temperature which promotes autoignition.
• ii) Engine speed At low engine speeds the flame
  velocity is slow and thus the burn time is long,
  this results in more time for autoignition.
• However at high engine speeds there is less heat
  loss so the unburned gas temperature is higher
  which promotes autoignition.
• These are competing effects, some engines show an
  increase in propensity to knock at high speeds
  while others dont.

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Knock limit as a function of CR and ON for
moderate and high turbulence combustion chambers.
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Effect of Initial Mixture Temperature on
Available Combustion Time to Avoid Knocking
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Most Useful Engine Parameter to Control Knocking
Spark timing maximum compression from the
piston occurs at TC. Increasing the spark
advance makes the end of combustion crank angle
approach TC and thus get higher pressure and
temperature in the unburned gas just before
burnout.
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Knock Mitigation Using Spark Advance
Spark advance set to 1 below MBT to avoid knock
X crank angle corresponding to borderline
knock
1 below MBT
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Auto Sparking Strategy
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Effect of Fuel-air Dilution
Set spark timing for MBT, leaner mixture needs
more spark advance since burn time longer. Along
MBT curve as you increase excess air reach
partial burn limit (not all cycles result in
complete burn) and then ignition limit (misfires
start to occur).
Ignition limit
Partial burn limit
Complete burns in all cycles
MBT spark timing
Partial
burn regime