Exhaust-gas affects the period of ignition lag. If

Exhaust-gas recirculation

Concept

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Exhaust-Gas Recirculation (EGR) is a highly effective
internal engine measure to lower NOx emissions on diesel engines. A distinction
is made between:

·        
Internal
EGR, which is determined by valve timing and residual gas.

·        
External
EGR, which is routed to the combustion chamber through additional lines and a
control valve.

The NOx-reducing effect is mainly due to the following
causes:

·        
Reduction
in exhaust-gas mass flow.

·        
Drop
in the rate of combustion, and thus local peak temperatures due to an increase
in the inert-gas component in the combustion chamber.

·        
Reduction
in partial oxygen pressure or local excess-air factor.

The NOx-reduction is possible in three ways –

·        
Dilution
effect – It is the reduction in the inlet charge oxygen concentration (main
reason for NOx reduction).

·        
Thermal
effect – It is the increase in inlet charge heat capacity, and

·        
Chemical
effect – Modification in combustion process as a result of dissociation of CO2
and water vapour

Since high local temperatures (> 2,000 K) and a
sufficiently high partial oxygen pressure are required to form NOx, the
measures listed above result in a drastic reduction in the formation of NOx as
the EGR rate rises. Reducing the reactive components in the combustion chamber
also leads to a rise in black smoke, which limits the quantity of recirculated
exhaust gas. The quantity of recirculated exhaust gas also affects the period
of ignition lag. If EGR rates are sufficiently long in the lower part load
range, ignition lag is so great that the diffusive combustion component, that
is so typical of diesel engines, is strongly diminished, and combustion only
starts after a large percentage of the air and fuel has been mixed. This
partial homogenization is used in new or future (p) HCCI combustion processes
to achieve extremely low-NOx and low-particulate combustion in the low
part-load ranges.