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

P M V SubbaraoProfessor

Mechanical Engineering Department

Fuel Economy Vs Pace Vs Safety?!?!

Main Requirement for User AcceptabilityMixture Burn Time vs Engine Speed

Mixture Burn Time vs Engine SpeedRecall 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 ~ NTherefore, at higher engine speeds the turbulent flame velocity is also higher and as a result need less time to burn the entire mixtureCombustion duration in crank angles (40-60 degrees) only increases a small amount with increasing engine speed.

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.

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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Recall the overall burn angle (90% burn) increases with engine speed, to accommodate this you need a larger spark advance.

Effect of Engine Speed on Spark Timing

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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

Symptoms of Normal Combustion in SI Engines

Cyclic Variation of Flame Volume

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.

Unexpected Engine Damage

Damage to the engine is caused by a combination of high temperature and high pressure.

Piston Piston crown

Cylinder head gasket Aluminum cylinder head

Dangerous Accidents

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.

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.

Observation windowfor photography

Spark plug

Intake valve

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Normal cycle

Knock cycle

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.

•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.

Engine Design Parameters Causing the Knock

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 don’t.

Knock limit as a function of CR and ON for moderate and high turbulence combustion chambers.

Effect of Initial Mixture Temperature on Available Combustion Time to Avoid 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.

Most Useful Engine Parameter to Control Knocking

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x End of combustion

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X crank angle corresponding to borderline knock

Spark advance set to 1% below MBT to avoid knock

1% below MBT

Knock Mitigation Using Spark Advance

Auto Sparking Strategy

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).

Effect of Fuel-air Dilution

Partial burn limit

Complete burns in all cycles

Partialburn regime

MBT spark timing

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