This paper outlines the principal of Lean Burn Combustion, 
and how it can improve engine efficiency.
This information is provided "as is" without any warranty, condition, or representation of any kind, either expressed or implied, including but not limited to, any warranty respecting non-infringement, and the implied warranties of conditions of merchantability and fitness for a particular purpose. In no event shall Franz Hofmann, or any directors, trustees, associates, or employees thereof be liable for any direct, indirect, special, incidental, consequential or other damages howsoever caused whether arising in contract, tort, or otherwise, arising out of or in connection with the use or performance of the information contained on this web site.  Access to and the use of this guide indicates the acceptance of these conditions.

Lean-burn combustion technology uses a concept of excess air. The main purpose of using excess air is to increase fuel consumption efficiency, but side benefits are:

  • Lowered combustion temperatures
  • Reduced NOx production
  • Reduced Carbon Monoxide

Lets review some combustion principals:

Ideally, during the combustion process, an engine ingests air, then supplies enough fuel to sufficiently burn the air in the cylinder.  Most engines consume a fuel, such as gasoline, diesel, natural gas, propane, or mixtures of these fuels.  Most of these fuels (with the exception of diesel) have an ideal air-to-fuel ratio, called the Stoichiometric Ratio.  This ratio describes the ideal thermochemical process, where all of the air and all of the fuel is consumed during the combustion cycle, leaving no residual exhaust by-products.

Also realize that all internal combustion engines control the amount of power by controlling the amount of air that is ingested in the engine, then supplying enough fuel to burn efficiently. (Diesel engines allow for an unthrottled operation, and control the amount power by varying the amount of fuel metered in the cylinders. Here are some ideal combustion ratios:

  • Gasoline 14.7 to 1
  • Natural Gas 16.8 to 1
  • Propane 15.5 to 1
(These ratios are based upon the weight of air to fuel.)

Also, there is another number; the amount of air by displacement;

  • Gasoline, about 3.5% (0.75 to 7%)
  • Propane, about 6.5% (2.1 to 9.6%)
  • Natural Gas, about 10% (4 to 13%
When a fuel is burned, its ideal Stoichiometric ratio is called the Lambda Ratio.  With all fuels, the Stoich Ratio at Lambda is identified by the number "λ".  The Lambda ratio is symbolized by the Greek character "Lambda" or the "λ" sign.  For example, gasoline with its air to fuel ratio of 14.7 to 1 is called "Lambda 1", or "λ=1".  If the air to fuel ratio is leaned by 10% to 16.17, the resulting Lambda would be "λ=1.1".   

Notice that diesel is absent from this chart. Diesel fuel has a very wide combustibility range. That is one of the factors that makes diesel fuel ideal for a combustion ignition engine (CI). As an overview of a CI engine, these engines operate unthrottled, meaning that the incoming air is NOT controlled, just the amount of fuel.

Now, why don't we just use diesel for lean burn? Simple, we are! At idle, a diesel engine may operate at air to fuel ratios in the 40 to 60 to 1 range! At full load, this ratio drops to 12 to 1 (and the accompanying smoke in the process, indicating unburned fuel).

Emissions

As a fuel is burned in an engine, various exhaust emissions are produced:
  • Hydrocarbons, HC's, from unburned fuel. HC's are formed from a rich fuel mixture (or a lean fuel mixture where there is excess air, leading to a lean misfire.
  • Carbon Monoxide, CO, from a rich fuel mixture, and never from a lean fuel mixture.
  • Carbon Dioxide, CO2, formed during any combustion process when oxygen and carbon are present in the primary combustion ingredients.
  • Oxygen, O2, from a lean fuel mixture.
  • Oxides of Nitrogen (Nitrogen Oxide) NOx. Nitrogen is present in the air we breath, and the engines air it consumes. Nitrogen displaces the air by approximately 75%. Nitrogen doesn't burn, but it can oxidize at temperatures over 2500 degrees F. NOx is a health hazard and one of the EPA's primary emission problems.
One method of controlling NOx is to reduce combustion temperatures. An EGR valve is the easiest device to use. It bleeds some inert exhaust gas into the incoming air stream, diluting the oxygen, and reducing combustion temperatures.

One other method of NOx reduction is to run a richer fuel mixture. By adding more fuel, the amount of air is displaced, reducing NOx. The leftover fuel is handled by the exhaust catalyst, converting the CO and HC into CO2.  With a liquid fuel engine, the addition of more fuel also lowers the combustion temperature by the condensing effect.  Here the fuel is evaporating and absorbing combustion heat.  With a vapor fuel, the reverse if true.  If the engine is running lean (over λ=1.2), the exhaust actually begins to cool down, thus reducing exhaust and combustion temperatures.  Now we understand the rational of "Lean Burn"!

Engine Efficiency

Conventional Otto cycle SI (Spark Ignited) engines have an efficiency ratio of approximately 29%.  That means that for every calorie of fuel consumed, only 29% actually reaches the flywheel.  The rest is lost during the combustion process, exhaust heat, internal friction, cooling system, cylinder block radiation, and so on.  A significant amount of heat energy is lost in the thermochemical process itself!  The single biggest robber of engine efficiency is the simple throttle!  As an example, in a large 7 liter engine, each cylinder is almost one liter in size.  With the throttle almost closed, (and with the engine intake manifold vacuum at 18" HG, or approximately 62% of absolute vacuum or zero psi absolute pressure) imagine trying to lift a plunger 4".  You would be lifting 208 lbs.!  (Assume a 4.25" bore, equaling 14.17 square inches, times 14.7 lbs/sq/in air pressure at sea level.)

Take this one step further, and multiply it times the number of cylinders, and you have some kind of idea how much power it takes to just spin an engine over without any combustion process taking place!

Diesel Engines

Now the marvel of a diesel engine.  No throttle!  That means that the pumping losses are non-existent.  You do have a higher compression, but that is a fraction of the pumping losses with a throttled engine.  This factor alone can raise the efficiency of a diesel engine to 37%, but now add in the BTU of a diesel gallon (145,000) versus a gasoline BTU (117,000) or Propane (91,500) and you can have an engine with a combustion efficiency approaching 40%.

The Lean Burn concept takes this practice one step further. By pumping in more air than is necessary, (see the chart), HC's, CO, and NOx are reduced by going past the "NOx Curve".

This chart readily shows how the NOx levels drop once the Lambda ratio crosses the 1.1 range and goes leaner.  Also note that the CO drops almost to nothing.  Hydrocarbons are cleaned up with an oxidizing catalyst.  What you have left is an engine that has very low NOx, low HC, low CO, and high levels of O2.

Easy?  Not on  your life!  Anyone who has operated an engine that is starving for fuel can realize that you lose power quickly once you run out of fuel.  So, how do we maintain the power levels with drastically reduced amounts of fuel?

We have to "help" the fuel and air into the engine.  The only way to do that is with forced induction.

Now for some technical data on lean burn engines.

  • You MUST use a gaseous fuel for lean burn (or vaporize gasoline);
  • You MUST use a turbocharger or supercharger.  If you attempt to operate a normally aspirated engine at elevated Lamda ratios, a severe loss of power will be noted, possibly accompanied by backfires.
Why can't we use a liquid fuel?  The fuel micro-droplets are too far apart to support linkage combustion (where one droplet helps to ignite its neighbor).  Even if the air mixture is compressed by using a supercharger or turbocharger, the fuel mixture now becomes a dense mass which has a difficult time supporting combustion.  There is research using ultrasonic atomizers to assist with this fuel problem, but we go one step further, we use a fuel that is already a gaseous fuel, Propane or Natural Gas.

Fuel control strategy

Even with supercharging, there is no (or almost no) boost at engine idle speed, so the fuel mixture must be at λ=1 (at idle).  As the power band is increased, the Lambda ratio may be increased.  Hard acceleration, or snap throttle acceleration might invite a spectacular engine backfire, so a fuel enrichment to λ=1 or to λ =.9 would be used, until the engine reaches no more throttle movement (degrees per second rotation below a set limit).  The real secret here is understanding the concept of boosting the intake pressure.  At stoichiometry, there is an ideal mixture of air to fuel, with the fuel mixture evenly dispersed in the combustion chamber.  As an engine operates in lean regions, the ratio of fuel to air decreases, reducing the relative amount of fuel in a cylinder.  Air and fuel under boost, with excess air, increases the amount of fuel in the cylinder, approaching the previous stoichiometric ratio, albeit at elevated air ratios.  This maintained fuel density allows for combustion.

Fuel injection is an ideal platform here, since there are electronic engine controllers available that would allow a tailorable fuel injector pulse-width algorithm, even while the engine is running.

The following parameters would be needed:

  • Fuel temperature;
  • Intake air temperature
  • Engine rpm
  • Exhaust Lambda ratio
  • Throttle position
  • Crankshaft position
  • Intake manifold pressure
This would be used to calculate the air mass density in preparation for developing the fuel injector tables.

Why can't a mechanical carburetor be used?

  • It can, if a method of leaning the fuel mixture on high boost is used.  One of the first laboratory engine experiments I ran used an Impco 200-A carburetor with a compensating pressure hose to the vaporizer.  This combination allowed a Cummins 8.3 liter 6 cylinder to produce 330 bhp at 2750 rpm to reach λ-1.45 with exhaust temps after the turbo of 870° F.  The exhaust did not require a catalyst to reach CARB standards (in 1995).  A catalyst would be required today.  I would NOT recommend a mechanical carburetor combination for an over the road Lean Burn engine.  You MUST have some means of electronic controls, either with fuel injection, or an electronically controlled mechanical system.

Summary:

Lean burn is a fascinating concept.  It will allow engine efficiency to almost reach diesel levels, with reasonably good performance.  Lean burn has been used for years in stationary application, where there is no ramp loading or throttle position changes that would affect the air to fuel ratio.

(This page is in the process of being updated.  This material was taken from a paper I wrote with the intention of incorporating it into a thesis.)

This paper is copyrighted by its author, Franz Hofmann.  No reproduction is allowed by any means without express written permission by its author or assignee.

Site uploaded Sept 12, 1999.
Page update Sept 25, 2004.

Paper date January 16, 1995.