1. P M V Subbarao Professor Mechanical Engineering Department
Strategies to Achieve A Fast Cycle with High & Safe Peak Pressure in A SI Engine
P M V Subbarao
Professor
Mechanical Engineering Department
Fuel Economy Vs Pace Vs Safety?!?!

2. Mixture Burn Time vs Engine Speed
Recall that the turbulent burning velocity is proportional to the turbulent intensity and laminar burning velocity.
Turbulence level 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.
Needs less time to burn the entire mixture
Finally, the optimum combustion duration in terms of crank angle can be kept in the range of degrees.

3. Air Fuel Ratio Vs Flame speed
An equivalence ratio of  = 1.1 gives the highest burning velocity and high flame temperature (maximum PCV).
Best fuel economy is obtained for a equivalence F/A that is less than 1.0 (~0.89).

4. Engine Acceptability Tests : Mixture Burn Time vs Engine Speed

5. 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

6. Dangerous Accidents

7. 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.

8. 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.

9. Special Flame visualization Studies
Knock cycle
Normal cycle
Observation window
for photography
Spark plug
Intake valve
Exhaust valve

10. 20th Century Studies on 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.
P,T
time
end-gas
flame P

11. 20th Century Studies on 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.
shock
time
P,T

12. 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.

13. 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.

14. 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.

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

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

17. 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.
P,T T
Ignition x
End of combustion

18. Knock Mitigation Using Spark Advance
Spark advance set to 1% below MBT to avoid knock x
X crank angle corresponding
to borderline knock
% below MBT

19. Automotive Electronic Control Strategy for Sparking
Border line Knocking Line
Safe Automatic SPARK TIMING

20. Other Engine Design Parameters Causing the Knock
There are the two fundamental variables that determine the possibility of occurrence of knock in SI Engines.
The end-gas temperature.
The time available for the end gas before flame arrival.
Few other engine parameters that effect these two fundamental variables are:
Inlet pressure and temperature
Coolant temperature
This phenomenon is a fundamental issue of modern SI engine design methods.

21. Fuel : The Resource is the Culprit : Knock Scale
To provide a standard measure of a fuel’s ability to resist knock, a scale has been devised by which fuels are assigned an Octane Number ON.
The octane number determines whether or not a fuel will knock in a given engine under given operating conditions.
The higher the octane number, the higher the resistance to knock.
By definition, normal heptane (n-C7H16) has an octane value of zero and isooctane (C8H18) has a value of 100.
Blends of these two hydrocarbons define the knock resistance of intermediate octane numbers: e.g., a blend of 10% n-heptane and 90% isooctane has an octane number of 90.
A fuel’s octane number is determined by measuring what blend of these two hydrocarbons matches the test fuel’s knock resistance

22. Fuel Quality : An Important Reason for Knocking
Compression Fuel Quality Brake Thermal
Ratio Requirement Efficiency 5:
6: %
7: %
8: %
9: %
10: %
11: %
12: %

23. Knock Characteristics of Various Fuels
Formula Name Critical r RON MON
CH4 Methane
C3H8 Propane
CH4O Methanol
C2H6O Ethanol
C8H18 Isooctane
Blend of HCs Regular gasoline
n-C7H16 n-heptane
For fuels with antiknock quality better than octane, the octane number is:
where mT is milliliters of tetraethyl lead per U.S. gallon

24. Octane Number Requirement of a Vehicle
The actual octane requirement of a vehicle is called the Octane Number Requirement (ONR).
This is determined by using series of standard octane fuels that can be blends of iso-octane and normal heptane ( primary reference ), or commercial gasolines.
The vehicle is tested under a wide range of conditions and loads, using decreasing octane fuels from each series until trace knock is detected.
The conditions that require maximum octane are full-throttle acceleration from low starting speeds using the highest gear available.