The stratified charge engine_

The principle of the stratified charge engine is to deliver a
mixture that is sufficiently rich for combustion in the immediate
vicinity of the spark plug and in the remainder of the cylinder,
a very lean mixture that is so low in fuel that it could not
be used in a traditional engine. On an engine with stratified
charge, the delivered power is no longer controlled by the
quantity of admitted air, but by the quantity of petrol injected,
as with a diesel engine.

 HOW DOES IT WORK?

One approach consists in dividing the combustion chamber

so as to create a pre-combustion chamber where the spark

plug is located. The head of the piston is also modified. It contains

a spheroid cavity that imparts a swirling movement to

the air contained by the cylinder during compression. As a

result, during injection, the fuel is only sprayed in the vicinity

of the spark plug.

But other strategies are possible. For example, it is also possible

to exploit the shape of the admission circuit and use

artifices, like “swirl” or “tumble” stages that create turbulent

flows at their level.

All the subtlety of engine operation in stratified mode

occurs at level of injection. This comprises two principal

modes: a lean mode, which corresponds to operation at very

low engine load, therefore when there is less call on it, and a

“normal” mode, when it runs at full charge and delivers maximum

power.

In the first mode, injection takes place at the end of the compression

stroke. Because of the swirl effect that the piston

cavity creates, the fuel sprayed by the injector is confined

near the spark plug. As there is very high pressure in the

cylinder at this moment, the injector spray is also quite concentrated.

The “directivity” of the spray encourages even

greater concentration of the mixture. A very small quantity of

fuel is thus enough to obtain optimum mixture richness in the

zone close to the spark plug, whereas the remainder of the

cylinder contains only very lean mixture. The stratification of

air in the cylinder means that even with partial charge it is

also possible to obtain a core of mixture surrounded by layers

of air and residual gases which limit the transfer of heat to the

cylinder walls.This drop in temperature causes the quantity

of air in the cylinder to increase by reducing its dilation, delivering

the engine additional power. When idling, this process

makes it possible to reduce consumption by almost 40% compared

to a traditional engine. And this is not the only gain.

Functioning with stratified charge also makes it possible to

lower the temperature at which the fuel is sprayed. All this

leads to a reduction in fuel consumption which is of course

reflected by a reduction of engine exhaust emissions.