The University of Calgary Schulich School of Engineering Mechanical Engineering Calgary, Canada

1
The University of CalgarySchulich School of
EngineeringMechanical EngineeringCalgary, Canada

• The Fuel Lean Operational Limits of Spark
  Ignition Engines
• Ghazi A. Karim and H . Li
• ICAT 08

2
Why Alternative Fuels?Why Lean Mixtures?Why
Operational Limit?How to Define the Operational
Mixture Limit of an Engine?How Does Hydrogen
Affect the Limits?

3
Lean Mixtures

• The operation of internal combustion engines on
  lean fuel mixtures has potentially many
  attractive features.
• It can provide in principle, higher fuel economy
  with reduced exhaust emissions and the tendency
  to knock..
• However, lean mixture operation is associated
  with significantly low burning and energy release
  rates. These can produce increased emissions of
  unburned hydrocarbons and partial oxidation
  products with low efficiency and increased cyclic
  variations leading to erratic combustion and
  inferior performance.

4
The Lean Operational Limitsin Spark Ignition
Engine

• There are distinct operational mixture limits
  beyond which acceptable engine performance cannot
  be maintained. These limits vary in value
  depending on numerous factors that would include
  the type of engine and fuel used and other
  operational and design parameters.

5
Summary

• A quantitative approach for determining the lean
  operational limits of S.I. engines is suggested.
  In this approach, the operational limit is
  defined in terms of the operating conditions
  associated with the first appearance of a cycle
  with a negative indicated power among a large
  number of cycles monitored when leaning the
  operating mixture gradually.
• The suggested quantitative approach was validated
  against traditional approaches through comparing
  the operational limit values derived.
• Based on this approach, the lean operational
  limits of S.I. engine operated on CH4, H2, CO,
  gasoline, iso-octane and some of their mixtures
  were experimentally determined.

6

• The Equivalence Ratio is the fuel to air mass
  ratio relative to the corresponding
  stoichiometric value ,
• E.R., F

is the apparent equivalence ratio of species i,
is the apparent equivalence ratio of species i,
7

is the apparent equivalence ratio of species i
in a fuel mixture,
is the apparent equivalence ratio of species i,
is the total equivalence ratio,
8
Some Methods for Establishing the Lean
Operational Limit in Spark Ignition Engine

• Sudden and Dramatic increase in Cyclic Variation
  in IMEP
• Sudden Drop on Averaged Indicated Power
  Production Efficiency
• Misfire Noise
• Sudden and Dramatic Drop in Exhaust Temperature
• First Occurrence of Negative Indicated Power
• (in a cycle among very many)

9
Schematic Diagram of Experimental Apparatus
10
Comparison of the Limits as Determined by the
Different Approaches
11
Simultaneous variations of CO emissions, relative
unburned CH4 emissions, COV of imep and misfire
frequency with equivalence ratio for methane
operation
12
Variation in the misfire frequency based on 250
consecutive cycles with equivalence ratio for
methane

13
Variation of the indicated work evaluated per 20
consecutive cycles with changes in equivalence
ratio for a binary mixture of methane and
hydrogen, (0.70CH40.30H2)

14
Variation of the exhaust temperature with changes
in equivalence ratio for CH4 and 70CH430H2
operation,

15
Variation of the indicated power production
efficiency with changes in equivalence ratio for
CH4 and 70CH430H2 operation, fully open
throttle, CR8.5, ST15 ?CA- BTDC, Tin22 ?C,
N900 rev/min, full throttle .

16
Reduction in the combustion duration with the
increased addition of hydrogen to methane for
stoichiometric mixture

17
Variation of the operational limit for iso-octane
and a gasoline with changes in compression ratio

18
Variations of the lean operation limited
equivalence ratio with changes in composition of
binary mixtures of CH4 with gasoline iso-octane,
open throttle

19
Variation of the combustion duration with
equivalence ratio for gasoline, (92 ON)
operation at a CR 6.5

20
Changes in the length of the combustion duration
with equivalence ratio methane, methane and
hydrogen and hydrogen as fuels

21
Variation of the lean operational limit with
volumetric efficiency due to throttling with
methane as the fuel

22
Conclusions

• The examination of a number of approaches for
  determining the effective operational mixture
  limits in spark ignition engines with different
  fuels showed them to display significant
  variations in the resulting values of the limits
  under the same operating conditions.
• The quantitative approach proposed for
  determining the lean operational limits in terms
  of the operating conditions associated with the
  first appearance of a cycle with a negative
  indicated power among a large number of cycles
  monitored, (gt250) when leaning the operating
  mixture gradually was used to show the limits.
• Other approaches tended to be associated with
  unacceptable operational instability and
  excessive emissions I
• The addition of hydrogen or methane to the liquid
  fuels extends the limits very significantly with
  hydrogen being much more effective than methane.
• Throttling tends to raise the values of the
  limit. With sufficiently excessive throttling the
  limit approaches the stoichiometric mixture.

23
Acknowledgement

• The contribution of Dr. A. Sohrabi to the testing
  made is gratefully acknowledged. The financial
  support of the Canadian Natural Science and
  Engineering Research Council (NSERC) and the
  University of Calgary is acknowledged.