Lean burn
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Lean burn is an internal combustion of lean air-fuel mixtures. It happens at very high air-fuel ratios (up to 65:1), so the mixture has considerably less amount of fuel in comparison to stoichiometric combustion ratio (14.6:1 for petrol).
The engines designed for lean burning can employ higher compression ratios and thus provide better performance, efficient fuel use and low exhaust emissions than those found in conventional petrol 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 the large amount of NOx being generated, so a complex catalytic converter system is required.
Chrysler Lean Burn Computer
From the late 1970s to mid 1980s, Chrysler equipped many of its North American production cars with a spark control computer which it called the Lean Burn Computer on the large sticker on the unit.
Mounted on the air filter housing of most rear wheel drive cars Chrysler produced during this time, it was responsible for adjusting spark timing based on manifold vacuum, engine speed, engine temperature and incoming air temperature; by doing this, Chrysler eliminated the traditional vacuum and centrifugal timing advance mechanisms used on distributors in order to provide more accurate spark timing. It also provided drive for the ignition coil directly, eliminating the separate ignition module.
Based on an early computer system, most Lean Burn Computers were an open-loop emissions control system with no provided diagnostic port or "Check Engine" warning light, were difficult to troubleshoot, and were greatly responsible for the poor reliability reputation which dogged Chrysler at the time.
Many Lean Burn Computers were replaced with the more reliable electronic ignition module and centrifugal/vacuum advance distributors used on earlier Chrysler vehicles, almost universally to improvements in fuel economy and driveability.
Heavy-duty gas engines
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 as much as 75% more air than theoretically needed for complete combustion into the combustion chambers. The extremely weak air-fuel mixtures lead to lower combustion temperatures and increased forced induction possibilities (that would otherwise be limited by high exhaust gas temperatures), leading to higher theoretical efficiencies when compared to engines running on a stoichiometric air-fuel mixture.
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