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.
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 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.
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.
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.
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.
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.
[8] Nissan QG engines are a lean-burn aluminum DOHC 4-valve design with variable valve timing and optional NEO Di direct injection.
The lean-burn MVV engine can achieve complete combustion with an air–fuel ratio as high as 25:1, this 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, which means lower CO2 emissions.
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.
[18][19] Near the end of the compression stroke, the layers collapse into uniform minute turbulences, which effectively promote lean-burn characteristics.
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.
[18][19] 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.