Stratified charge engine

The term "stratified charge" refers to the working fluids and fuel vapors entering the cylinder.

In a homogeneous charge system, the air/fuel ratio is kept very close to stoichiometric, meaning it contains the exact amount of air necessary for complete combustion of the fuel.

This gives stable combustion, but it places an upper limit on the engine's efficiency: any attempt to improve fuel economy by running a much leaner mixture (less fuel or more air) with a homogeneous charge results in slower combustion and a higher engine temperature; this impacts on power and emissions, notably increasing nitrogen oxides or NOx.

[1] This results in a stratified charge: one in which the air/fuel ratio is not homogeneous throughout the combustion-chamber, but varies in a controlled (and potentially quite complex) way across the volume of the cylinder.

Diesel engine operation on the other hand inhales and compresses air only by the motion of the piston moving to top dead centre.

As the fuel burns it expands exerting pressure on the piston, which in turn develops torque at the crankshaft.

By this process the piston and subsequently the crank experiences a greater torque, which is also exerted over a longer time interval than its petrol equivalent.

In this manner the engine fired on every power stroke and speed / output was controlled solely by the quantity of fuel injected.

Harry Ricardo first began working with the idea of a lean burn "stratified charge" engine in the early 1900s.

Hesselman engines used the ultra lean burn principle and injected the fuel in the end of the compression stroke and then ignited it with a spark plug, it was often started on gasoline and then switched over to run on diesel or kerosene.

The spark plug and CVCC inlet were isolated from the main cylinder by a perforated metal plate.

The Vespa ET2 scooter had a 50 cc two-stroke engine in which air was admitted through the transfer port and a rich fuel mixture was injected into the cylinder near the spark plug just before ignition.

On its downward stroke it compresses the rich mixture to about 70 psi at which time the rising pressure raises a spring loaded poppet valve off its seat and the charge is squirted into the cylinder.

FSI direct injection technology increases the torque and power of spark-ignition engines, makes them as much as 15 percent more economical and reduces exhaust emissions.