Learning
X
Combustion Cycles
Gas Engines Basic
Concepts & Principles
Stoichiometry
✕
Gas Engine Fundamentals As discussed in the Gaseous Fuel Impact on
Engine Performance, stoichiometric combustion
Diesel vs. Gas Engine takes place when the air-fuel ratio is in the correct
proportion so that there is no oxygen or fuel
Compression Ratios remaining when combustion is finished. Cat gas
engine fuel systems are designed to operate in
Combustion Cycles relation to the stoichiometric combustion. The below
table and graph show the various Air/Fuel Ratio
Stoichiometry definitions for Cat gas engines as it relates to their
excess exhaust oxygen.
Stoichiometry
Air/Fuel Ratio Excess Exhaust Oxygen
Conclusion Range (%)
Rich Burn Ratio 0 - 0.5%
Course Survey
Standard Ratio 1.5 - 2%
Lean Burn (LB) Ratio 6 - 9%
Ultra Lean Burn 8 - 12%
(ULB) Ratio
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Rich Burn Combustion
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Rich burn engines are designed to operate at or
near the stoichiometric air-fuel ratio. Rich burn
combustion and stoichiometric combustion mean the
same thing: complete consumption of both oxygen
and fuel in the cylinders.
A key limitation of rich burn engines is that they
produce relatively high levels of exhaust
emissions, notably nitrogen oxides – known as
NOx . That is because stoichiometric combustion
creates high temperatures in the cylinders, and high
temperatures promote the formation of NO x .
By adding a three-way catalyst to treat the exhaust,
rich burn engines can still meet very strict NO x and
other emissions regulations.
In rich burn engines, power output – the engine
rating – is limited by two key factors: exhaust
temperature, and detonation. If either limit is
exceeded, engine damage will occur. Either the
exhaust temperature limit or the detonation limit
will define the rating. Which one imposes the rating
limit depends on the nature of the fuel, the
compression ratio, and the engine timing.
In general, rich burn engines have lower power
ratings than lean burn engines as their power
output is limited mostly by their inherently higher
exhaust gas temperatures. As a result, rich burn
engines will typically have significant power
limitations (or derates) when they burn fuels with
low methane numbers.
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Lean Burn Combustion
As the name implies, lean burn engines operate
with leaner air-fuel ratios than rich burn engines.
This means combustion takes place with an excess
of air in the cylinders. This excess of air enables
low NO x formation which allows to meet emissions
requirements without using a catalyst to treat the
exhaust. Lower emissions are a major reason why
lean burn engines are used increasingly in states
and countries that have strict air-pollution
standards.
Lean burn engines also have higher power densities
than rich burn engines and are typically more fuel-
efficient. They can burn gaseous fuels with a wide
range of methane numbers while maintaining
significant resistance to detonation.
The main difference in operation between lean burn
engines and rich burn engines is that lean burn
engines require higher-energy ignition systems.
That is because it takes a more powerful spark to
ignite the lean fuel mixture in the lean burn engine
cylinders.
Finally, lean burn engine ratings are limited by
detonation, but also by air system capacity and
lean misfire, which is related to the capability of
the ignition system.
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LAMBDA
An expression that determines how rich or lean the
fuel mixture is, is called Lambda. Lambda is the
ratio of the actual air-fuel ratio versus the
stoichiometric air-fuel ratio. The leaner the air-fuel
ratio, the higher the Lambda value. Because rich
burn engines operate at the stoichiometric air-fuel
ratio, their Lambda value is at or near 1.0. Lean
burn engines operate at higher values of Lambda.
Nitrogen Oxides (NOx ) and Carbon Monoxide (CO)
emissions remain low at higher Lambda values, up
to the point at which the engine experiences lean
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misfire. An air-fuel ratio that produces a Lambda
value of 1.63 approaches the ideal for NO x
reduction. Also, emissions of un-burned
hydrocarbons, generally, decline with higher
Lambda values. In general, as the air-fuel ratio
becomes leaner – meaning the value of Lambda
increases – combustion temperature in the cylinders
decreases, air pollutant emissions decrease, and
fuel efficiency improves.
The degree of emissions reduction the engine can
achieve is limited by the amount of energy available
at the spark plug to ignite the fuel mixture. If the
mixture becomes too lean, there may not be
enough energy in the ignition system to start the
combustion process, meaning that the fuel will not
burn, and the engine would go into lean misfire.
In summary, as Lambda decreases – as the air-
fuel mix gets richer – the risk of detonation
increases. As Lambda increases – as the air-fuel
mixture gets leaner – so do the risks of misfire
and air system limitations.
Engine Model vs Combustion
Type
Now that you have learned about the types of
combustion Cat gas engines operate on, find below
a matrix that shows the current engine models
based on their stoichiometric combustion.
Combustion Type Engine Model
Rich Burn G3300 /
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G3300B
G3400
CG137
G3500A
G3500 TA
Lean Burn (Gas G3500B**
Compression) G3500J**
Lean Burn (Power G3400C
Generation) G3500C
G3500E
G3500H
Conclusion
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