Lean burn refers to the use of lean mixtures in an
internal combustion engineThe internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, which are produced by the combustion, directly applies force to a movable...
. The
air-fuel ratioAir-fuel ratio is the mass ratio of air to fuel present during combustion. When all the fuel is combined with all the free oxygen, typically within a vehicle's combustion chamber,...
s can be as high as 65:1, so the mixture has considerably less
fuelFuel is any material that is burned or altered to obtain energy and to heat or to move object. Fuel releases its energy either through a chemical reaction means, such as combustion, or nuclear means, such as nuclear fission or nuclear fusion...
in comparison to the
stoichiometricStoichiometry is the calculation of quantitative relationships of the reactants and products in a balanced chemical reaction...
combustion ratio (14.7 for petrol for example).
Principle
A lean burn mode is a way to reduce throttling losses. An engine in a typical vehicle is sized for providing the power desired for acceleration, but must operate well below that point in normal steady-speed operation. Ordinarily, the power is cut by partially closing a throttle. However, the extra work done in pumping air through the throttle reduces efficiency. If the fuel/air ratio is reduced, then lower power can be achieved with the throttle closer to fully open, and the efficiency during normal driving (below the maximum torque capability of the engine) can be higher.
The
engineThe internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, which are produced by the combustion, directly applies force to a movable...
s designed for lean burning can employ higher
compression ratioThe compression ratio of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber; from its largest capacity to its smallest capacity...
s and thus provide better performance,
efficient fuel useFuel efficiency, is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance...
and low
exhaustExhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal...
hydrocarbon emissions than those found in conventional
petrol engineA petrol engine is an internal combustion engine with spark-ignition, designed to run on petrol and similar volatile fuels....
s. Ultra lean mixtures with very high air-fuel ratios can only be achieved by
Direct InjectionIn internal combustion engines, gasoline direct injection is a latest variant of fuel injection employed in modern two- and four- stroke petrol engines...
engines.
The main drawback of lean burning is the large amount of
NOxNox may refer to:* Nox , the Roman translation of Nyx, the primordial goddess of the night in Greek mythology* Nox , a video game developed by Westwood Studios* Nox , a pop band from HungaryIn fiction...
being generated at relatively high air/fuel ratios (ie. greater than stoichiometric but less than 30:1), so a complex
catalytic converterA catalytic converter is a device used to reduce the toxicity of emissions from an internal combustion engine. First widely introduced on series-production automobiles in the U.S. market for the 1975 model year to comply with tightening EPA regulations on auto exhaust, catalytic converters are...
system is required unless ultra lean ratios are implemented. Lean burn engines do not work well with modern 3-way
catalytic converterA catalytic converter is a device used to reduce the toxicity of emissions from an internal combustion engine. First widely introduced on series-production automobiles in the U.S. market for the 1975 model year to comply with tightening EPA regulations on auto exhaust, catalytic converters are...
s, which require a balance of pollutants at the exhaust port in order to carry out both oxidation and reduction reactions, so most modern engines run at or near the stoichiometric point.
Chrysler Lean Burn computer
From 1976 through 1989,
ChryslerChrysler Group, LLC is an American automobile manufacturer headquartered in the Detroit suburb of Auburn Hills, Michigan. Chrysler was first organized as the Chrysler Corporation in 1925. From 1998 to 2007, Chrysler and its subsidiaries were part of the German based DaimlerChrysler AG...
equipped many vehicles with their
Electronic Lean Burn (ELB) system, which consisted of a spark control computer and various sensors and transducers. The computer adjusted spark timing based on manifold vacuum, engine speed, engine temperature, throttle position over time, and incoming air temperature. Engines equipped with ELB used fixed-timing distributors without the traditional vacuum and centrifugal timing advance mechanisms. The ELB computer also directly drove the ignition coil, eliminating the need for a separate ignition module.
ELB was produced in both open-loop and closed-loop variants; the open-loop systems produced exhaust clean enough for many vehicle variants so equipped to pass 1976 and 1977 US Federal
emissions regulationsAutomobile emissions control is the study and practice of reducing the polluting emissions produced by automobiles.-Specific pollutants:Motor vehicles produce many different pollutants...
, and Canadian emissions regulations through 1980, without a
catalytic converterA catalytic converter is a device used to reduce the toxicity of emissions from an internal combustion engine. First widely introduced on series-production automobiles in the U.S. market for the 1975 model year to comply with tightening EPA regulations on auto exhaust, catalytic converters are...
. The closed-loop version of ELB used an
Oxygen sensorAn oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen in the gas or liquid being analyzed. It was developed by Robert Bosch GmbH during the late 1960s under supervision by Dr. Günter Bauman...
and a feedback
carburetorA carburetor or carburettor is a device that blends air and fuel for an internal combustion engine. It was invented by Karl Benz before 1885 and patented in 1886...
, and was phased into production as emissions regulations grew more stringent starting in 1981, but open-loop ELB was used as late as 1990 in markets with lax emissions regulations, on vehicles such as the Mexican Chrysler Spirit. The spark control and engine parameter sensing and transduction strategies introduced with ELB remained in use through 1995 on Chrysler vehicles equipped with throttle-body fuel injection.
Although Chrysler published extensive training and procedural manuals on ELB, it — like most early emission control systems — was complicated to troubleshoot without these manuals. Many Lean Burn computers have been replaced with a standalone electronic ignition module and centrifugal/vacuum advance distributor, a retrofit to maintain fuel economy and driveability.
Heavy-duty gas engines
Lean burn concepts are often used for the design of heavy-duty
natural gasNatural gas is a gas consisting primarily of methane. It is found associated with fossil fuels, in coal beds, as methane clathrates, and is created by methanogenic organisms in marshes, bogs, and landfills...
,
biogasBiogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Biogas originates from biogenic material and is a type of biofuel....
, and
liquefied petroleum gasLPG may stand for:* Liquefied petroleum gas* Laboratoire de Planetologie, Grenoble, France* Landwirtschaftliche Produktionsgenossenschaft * Llanfairpwll railway station, Wales; National Rail station code LPG.* LPG...
(LPG) fuelled engines. These engines can either be full-time lean burn, where the engine runs with a weak air-fuel mixture regardless of load and engine speed, or part-time lean burn (also known as "lean mix" or "mixed lean"), where the engine runs lean only during low load and at high engine speeds, reverting to a stoichiometric air-fuel mixture in other cases.
Heavy-duty lean burn gas engines admit as much as 75% more air than theoretically needed for complete combustion into the combustion chambers. The extremely weak air-fuel mixtures lead to higher combustion temperatures and therefore reduced forced induction possibilities (that would otherwise be limited by high exhaust gas temperatures), but generate higher theoretical efficiencies when compared to engines running on a stoichiometric air-fuel mixture.
Heavy duty gas engines may employ precombustion chambers in the cylinder head. A lean gas and air mixture is first highly compressed in the main chamber by the piston. A much richer, though much lesser volume gas/air mixture is introduced to the precombustion chamber and ignited by spark plug. The flame front spreads to the lean gas air mixture in the cylinder.
This two stage lean burn combustion produces low NOx and no particulate emissions. Thermal efficiency is better as higher compression ratios are achieved.
The Rolls-Royce Bergen K series marine gas engine is an example of a heavy duty gas engine using this solution.
Honda lean burn systems
One of the newest lean-burn technologies available in automobiles currently in production uses very precise control of fuel injection, a strong air-fuel swirl created in the combustion chamber, a new linear air-fuel sensor (LAF type O2 sensor) and a lean-burn NOx catalyst to further reduce the resulting NOx emissions that increase under "lean-burn" conditions and meet NOx emissions requirements.
This stratified-charge approach to lean-burn combustion means that the air-fuel ratio isn't equal throughout the cylinder. Instead, precise control over
fuel injectionFuel injection is a system for mixing fuel with air in an internal combustion engine. It has become the primary fuel delivery system used in gasoline automotive engines, having almost completely replaced carburetors in the late 1980s....
and intake flow dynamics allows a greater concentration of fuel closer to the
spark plugA spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as, aerosol gasoline, Ethanol, and Liquefied petroleum gas by means of an electric spark...
tip (richer), which is required for successful ignition and flame spread for complete combustion. The remainder of the cylinders' intake charge is progressively leaner with an overall average air:fuel ratio falling into the lean-burn category of up to 22:1.
The older
Hondais a Japanese multinational corporation primarily known as a manufacturer of automobiles and motorcycles. Honda is the world's largest manufacturer of motorcycles as well as the world's largest manufacturer of internal combustion engines measured by volume. Honda surpassed Nissan in 2001 to become...
engines that used lean burn (not all did) accomplished this by having a parallel fuel and intake system that fed a pre-chamber the "ideal" ratio for initial combustion. This burning mixture was then opened to the main chamber where a much larger and leaner mix then ignited to provide sufficient power. During the time this design was in production this system (
CVCC, Compound Vortex Controlled CombustionCVCC is a trademark by the Honda Motor Company for a device to reduce automotive emissions called Compound Vortex Controlled Combustion. This technology allowed Honda's cars to meet United States emission standards in the 1970s without a catalytic converter...
) primarily allowed lower emissions without the need for a
catalytic converterA catalytic converter is a device used to reduce the toxicity of emissions from an internal combustion engine. First widely introduced on series-production automobiles in the U.S. market for the 1975 model year to comply with tightening EPA regulations on auto exhaust, catalytic converters are...
. These were
carburetedA carburetor or carburettor is a device that blends air and fuel for an internal combustion engine. It was invented by Karl Benz before 1885 and patented in 1886...
engines and the relative "imprecise" nature of such limited the MPG abilities of the concept that now under MPI (Multi-Port fuel Injection) allows for higher MPG too.
The newer
Hondais a Japanese multinational corporation primarily known as a manufacturer of automobiles and motorcycles. Honda is the world's largest manufacturer of motorcycles as well as the world's largest manufacturer of internal combustion engines measured by volume. Honda surpassed Nissan in 2001 to become...
stratified charge (lean burn engines) will operate on air-fuel ratios as high as 22:1. The amount of fuel drawn into the engine is much lower than a typical gasoline engine which operates at 14.7:1, the chemical stoichiometric ideal for complete combustion when averaging gasoline to be the petrochemical industries' accepted standard of C6H8.
This lean-burn ability by the necessity of the limits of physics, and the chemistry of combustion as it applies to a current gasoline engine must be limited to light load and lower RPM conditions. A "top" speed cut-off point is required since leaner gasoline fuel mixtures burn slower and for power to be produced combustion must be "complete" by the time the exhaust valve opens.
Applications
- 1992–95 Civic VX
The Honda Civic is a line of compact cars developed and manufactured by Honda. In North America, the Civic is the second-longest continuously-running nameplate from a Japanese manufacturer; only the Toyota Corolla, introduced in 1968, has been in production longer...
- 1996–2000 Civic Hx
The Honda Civic is a line of compact cars developed and manufactured by Honda. In North America, the Civic is the second-longest continuously-running nameplate from a Japanese manufacturer; only the Toyota Corolla, introduced in 1968, has been in production longer...
- 2001-05 Civic Hx
The Honda Civic is a line of compact cars developed and manufactured by Honda. In North America, the Civic is the second-longest continuously-running nameplate from a Japanese manufacturer; only the Toyota Corolla, introduced in 1968, has been in production longer...
- 2002–05 Civic Hybrid
The Honda Civic is a line of compact cars developed and manufactured by Honda. In North America, the Civic is the second-longest continuously-running nameplate from a Japanese manufacturer; only the Toyota Corolla, introduced in 1968, has been in production longer...
- 2000–06 Insight
The Honda Insight is a hybrid electric vehicle manufactured by Honda and the first production vehicle to feature Honda's Integrated Motor Assist system. The first-generation Insight was produced from 1999 to 2006 as a three-door hatchback....
Manual transmission only
Mitsubishi Vertical Vortex (MVV)
In 1991,
Mitsubishiis the fifth largest automaker in Japan and the fifteenth largest in the world by global unit sales. It is part of the Mitsubishi keiretsu, formerly the biggest industrial group in Japan, and was formed in 1970 from the automotive division of Mitsubishi Heavy Industries.Throughout its history it...
developed and began producing the
MVV (Mitsubishi Vertical Vortex) lean burn system first used in Mitsubishi's 1.5 L
4G15The Mitsubishi Orion or 4G1 engine is series of straight-4 internal combustion engines introduced by Mitsubishi Motors in the 1970s, along with the Astron, Sirius, and Saturn. It was introduced in the Colt and Colt-derived models in 1978...
straight-4The inline-four engine or straight-four engine is a four-cylinder internal combustion engine with all four cylinders mounted in a straight line, or plane along the crankcase. The single bank of cylinders may be oriented in either a vertical or an inclined plane with all the pistons driving a...
engine. The lean-burn MVV engine can achieve complete combustion with an air-fuel ratio as high as 25:1, whereas conventional engines require 14.7:1. The result is 13% better fuel economy at over a conventional engine powered vehicle. This improved fuel economy means lower CO
2 emissions.
The heart of the Mitsubishi's MVV system is the linear air-fuel ratio exhaust gas oxygen sensor. Compared with standard oxygen sensors, which essentially are on-off switches set to a single air/fuel ratio, the lean oxygen sensor is more of a measurement device covering the air/fuel ratio range from about 15:1 to 26:1.
In order to speed up the otherwise slow combustion of lean mixtures the MVV engine uses two intake valves and one exhaust valve per cylinder. The separate specially shaped (twin intake port design) intake ports are the same size, but only one port receives fuel from an injector. This creates two vertical vortices of identical size, strength and rotational speed within the combustion chamber during the intake stroke: one vortex of air, the other of an air/fuel mixture. The two vortices also remain independent layers throughout most of the compression stroke.
Near the end of the compression stroke, the layers collapse into uniform minute turbulences, which effectively promote lean-burn characteristics. More importantly, ignition occurs in the initial stages of breakdown of the separate layers while substantial amounts of each layer still exist. Because the spark plug is located closer to the vortex consisting of air/fuel mixture, ignition arises in an area of the pentroof-design combustion chamber where fuel density is higher. The flame then spreads through the combustion chamber via the small turbulences. This provides stable combustion even at normal ignition-energy levels, thereby realizing lean burn.
The engine computer stores optimum air fuel ratios for all engine-operating conditions - from lean (for normal operation) to richest (for heavy acceleration) and all points in between. Full-range oxygen sensors (used for the first time) provide essential information that allows the computers to properly regulate fuel delivery.
Mitsubishi says air/fuel ratios of up to 25:1 are possible with its lean-system powerplant. The single-overhead-cam 1,468-cc 4-cylinder boasts a 10-20% gain in fuel economy (on the Japanese 10-mode urban cycle) in bench tests compared with its conventional MPI powerplant of the same displacement. The vertical vortex engine has an idle speed of 600 rpm and a compression ratio of 9.4:1 compared with respective figures of 700 rpm and 9.2:1 for the conventional version.
Diesel engines
All
diesel engineA diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression...
s can be considered to be lean burning with respect to the total volume, however the fuel and air is not well mixed before the combustion. Most of the combustion occurs in rich zones around small droplets of fuel. Locally rich combustion like this is a source of NOx and particles.