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Burning of fuel with an excess of air in an internal combustion engine

Lean burn refers to the burning of fuel with an excess of air in an internal combustion engine. In lean burn engines the air–fuel ratio may be as lean as 65:1 (by mass). The air / fuel ratio needed to stoichiometrically combust gasoline, by contrast, is 14.64:1. The excess of air in a lean burn engine emits far less hydrocarbons. High air–fuel ratios can also be used to reduce losses caused by other engine power management systems such as throttling losses.

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 engines designed for lean burning can employ higher compression ratios and thus provide better performance, efficient fuel use and low exhaust hydrocarbon emissions than those found in conventional gasoline engines. Ultra lean mixtures with very high air–fuel ratios can only be achieved by direct injection engines. The main drawback of lean burning is that a complex catalytic converter system is required to reduce NOx emissions. Lean burn engines do not work well with modern 3 way catalytic converter—which require a pollutant balance at the exhaust port so they can carry out oxidation and reduction reactions—so most modern engines tend to cruise and coastdown at or near the stoichiometric point.

From 1976 through 1989, Chrysler 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 regulations, and Canadian emissions regulations through 1980, without a catalytic converter. The closed loop version of ELB used an oxygen sensor and a feedback carburetor, 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.

Lean burn concepts are often used for the design of heavy duty natural gas, biogas, and liquefied petroleum gas (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 twice as much air as theoretically needed for complete combustion into the combustion chambers. The extremely weak air–fuel mixtures lead to lower combustion temperatures and therefore lower NOx formation. While lean burn gas engines offer higher theoretical thermal efficiencies, transient response and performance may be compromised in certain situations. However, advances in fuel control and closed loop technology by companies like North American Repower have led to production of modern CARB certified lean burn heavy duty engines for use in commercial vehicle fleets. Lean burn gas engines are almost always turbocharged, resulting in high power and torque figures not achievable with stoichiometric engines due to high combustion temperatures. 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. Manufacturers of heavy duty lean burn gas engines include MTU, Cummins, Caterpillar, MWM, GE Jenbacher, MAN Diesel & Turbo, Wärtsilä, Mitsubishi Heavy Industries, Dresser Rand Guascor, Waukesha Engine and Rolls Royce Holdings.

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 is not equal throughout the cylinder. Instead, precise control over fuel injection and intake flow dynamics allows a greater concentration of fuel closer to the spark plug 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 Honda 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 Combustion) primarily allowed lower emissions without the need for a catalytic converter. These were carbureted 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 Honda stratified charge (lean burn engines) 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 the petrochemical industries' accepted standard of C8H18. 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.

In 1984, Toyota released the 4A ELU engine. This was the first engine in the world to use a lean burn combustion control system with a lean mixture sensor, called "TTC L" (Toyota Total Clean Lean Burn) by Toyota. Toyota also referred to an earlier lean burn system as "Turbulence Generating Pot" (TGP). TTC L was used in Japan on Toyota Carina T150 replacing the TTC V (Vortex) exhaust gas recirculation approach used earlier, Toyota Corolla E80, and Toyota Sprinter. The lean mixture sensor was provided in the exhaust system to detect air–fuel ratios leaner than the theoretical air–fuel ratio. The fuel injection volume was then accurately controlled by a computer using this detection signal to achieve lean air–fuel ratio feedback. For optimal combustion, the following items were applied: program independent injection that accurately changed the injection volume and timing for individual cylinders, platinum plugs for improving ignition performance with lean mixtures, and high performance igniters. The lean burn versions of the 1587cc 4A FE and 1762cc 7A FE 4 cylinder engines have 2 inlet and 2 exhaust valves per cylinder. Toyota uses a set of butterflies to restrict flow in every second inlet runner during lean burn operation. This creates a large amount of swirl in the combustion chamber. Injectors are mounted in the head, rather than conventionally in the intake manifold. Compression ratio 9.5:1. The 1998cc 3S FSE engine is a direct injection petrol lean burn engine. Compression ratio 10:1.

Toyota lean burn engine applications Unladen weight Fuel consumption, Japan 10 15 mode Fuel tank capacity Range Years Model Engine kg lbs L/100 km km/L mpg UK mpg US L gal UK gal US km mile Notes1984–88Carina T1504A ELU95021005.617.050416013.215.910566565spd manual1994–96Carina SG i SX i7A FE104022925.617.650416013.215.910566565spd manual1998–2000Corona Premio G3S FSE120026455.817.249416013.215.91034643Auto1996–97Caldina FZ CZ7A FE114025135.617.650416013.215.910566565spd manual1997 2000Avensis T2217A FE126027786013.215.91997–2002Caldina E7A FE120026455.617.650416013.215.910566565spd manual1997–2002Spacio7A FEAuto

Nissan QG engines are a lean burn aluminum DOHC 4 valve design with variable valve timing and optional NEO Di direct injection. The 1497cc QG15DE has a Compression ratio of 9.9:1 and 1769cc QG18DE 9.5:1.

All diesel engines 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 is a source of particulate matter (PM) emissions.