Howdy from Houston's 2009 Turbomachinery Symposium!

Randy Anderson, Principal Consultant, ACTT, as submitted to GMRC in 2008

The Y-Axis

The next part of the parabolic burning curve is identifying the unit of measure for the Y-axis of the graph. This section contains a series of graphs showing the following Y-axis elements:
•Heat generation by combustion
•Parabolic burning curve for various fuels
•Flame front velocity
•Peak firing pressure
•Misfires and detonations
•Emissions

Heat Generation by Various Fuels

Heat is generated by burning the air-fuel mixture.  The amount of heat generated is shown on the Y-axis.

Heat Generation on the Y-axis
UEL is the rich-limit, LEL is the lean-limit, and stoichiometric is where complete combustion occurs. Complete combustion releases more heat than incomplete combustion on either side of stoichiometric. Thus the parabola is introduced into the parabolic burning curve. The following figure shows the parabolic burning curve for methane.

Parabolic burning curve for methane
For methane, combustion is initiated at 5.64:1 or UEL and is extinguished at 18.96:1 or LEL The most heat is released at stoichiometric or 913 Btu.
The following is the parabolic burning-curve for ethane. Because the heat rate of ethane is higher than methane, 1,616 versus 913, the Y-axis was relabeled. UEL goes from 5.64:1 to 3.76:1 stoichiometric is 8.89:1 versus 9.52:1 and UEL is 16.15:1 as opposed to 16.15:1.

Randy Anderson, Principal Consultant, ACTT, as submitted to GMRC in 2008

Rich and Lean Sides of the Curve

Knowing the upper explosion limit, the lower explosion limits, and stoichiometry of a particular gas also allows rich and lean to be defined. A rich mixture goes from stoichiometric to UEL. A lean mixture goes from stoichiometric to LEL. Engines that run on the rich side of stoichiometric, such as rich-burn four-stroke cycle engines with non-selective catalytic reduction are called rich-burn engines. Engines that run on the lean side of stoichiometric, such as lean-burn four-stroke cycle engines with oxidation catalyst and most two-stroke cycle engines are called lean-burn engines.

Rich burn and lean burn sides of stoichiometric

Leaning and richening the mixture is the process of added or subtracting air or fuel. If the mixture is 6:1 and air is added, you are leaning the mixture. If the mixture is 16:1 and air is taken away, you are richening the mixture. The same can be done with fuel. If the mixture is 8:1and fuel is added, you are richening the mixture. If the mixture is 10:1 and fuel is taken away, then you are leaning the mixture.

Written by: Jim Rauh

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Randy Anderson, Principal Consultant, ACTT, as submitted to GMRC in 2008

The Sunday BBQ—UEL and LEL

Everyone knows the fire triangle shown in Figure 1. The three things necessary for combustion are oxygen, fuel and heat.


Fire triangle

Will any combination of oxygen, fuel, and heat initiate combustion?

Have you ever made preparations for a Sunday barbeque by dowsing the charcoal briquettes with lighter fluid only to have the match go out when it was tossed into the grill?  A match has always started the lighter fluid before, so what keep combustion from starting?  Answer, an incorrect air-fuel ratio. The mixture is too rich. Or change the scenario, your lighter fluid bottle is almost empty so only a small amount is sprayed on the charcoal. Again the match was tossed onto the bed of coals and again ignition did not occur. What happened? This time the mixture was too lean.

The richest mixture that will burn is called UEL or upper explosion limit and the leanest mixture that will burn is called LEL or lower explosion limit and it applies to any of the gases that compose “natural gas”.

So how hot is hot enough? The flammable (explosive) range is the range of a gas or vapor concentration that will burn or explode if an ignition source is introduced. Below is a list of common materials and their ignition temperatures.

Table 4: Combustion Temperatures of Various Materials

Randy Anderson, Principal Consultant, ACTT, as submitted to GMRC in 2008

One of the various first principles introduced in my training some 35 years ago, was the parabolic burning curve. It is a keystone to understanding peak firing pressure, different engines operations, emissions, engine balance, detonation, misfires, dead cylinders, flame front velocity, lower explosion limits and upper explosion limits. It is simple to explain, straightforward to understand, easy to remember and critical to mastering combustion characteristics in a reciprocating internal combustion engine.

There are three statements I would like to make about this paper:

• I have never been able to identify the original source of this information. Whoever you were, thank you!


• This is not intended to be a synopsis of trapped equivalence ratio, adiabatic flame temperatures or any other study of the basic laws of thermodynamics but rather a primer to help anyone understand the basic of air-fuel ratio and its affect on combustion and an instrument to help anyone remember this information forever.


• And most importantly, in 35 years it has never led me astray.