About lean burn engine

INTRODUCTION

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.
Ford CVH Lean-burn engine
In 1980 Ford introduced their CVH engine in the
Escort 3, as a replacement for the old fivebearing
pushrod OHV Kent engine. The engine's designation
derives from its design — Compound Valve angle,
Hemispherical combustion chamber.

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 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 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 that a complex catalytic converter system is required to reduce NOx emissions. Lean burn
engines do not work well with modern 3way
catalytic converter. which require a pollutant balance at the exhaust port
so they can carry out oxidation and reduction reactions—so most modern engines run at or near the stoichiometric point.

In an ideal, 100 % efficient internal combustion engine, the fuel would burn to give just carbon dioxide and water vapour. In practice, of course,  engines are far from efficient and the combustion process. also produces carbon monoxide, oxides of nitrogen and unburnt hydrocarbons, as well as carbon dioxide and water vapour.These byproducts of combustion are expelled as part of the car's exhaust gasses into the atmosphere where they cause pollution. In recent years, public concern about atmospheric pollution, and imminentEEC pollution control laws, has led to car manufacturers trying to find ways of reducing the level of these gasses in car exhausts. 

In 1980 Ford introduced their CVH engine in the Escort 3, as a replacement for the old fivebearing pushrod OHV Kent engine. The engine's designation derives from its design — Compound Valve angle, Hemispherical combustion chamber.Now the CVH has been reengineered to make it a true lean burn engine, capable of running on air fuel ratios of over 18:1. This meant a change in the cylinder head design to incorporate a lean burn kidney-shaped combustion chamber to ensure high mixture swirl and therefore more complete fuel burning.