Download THE 6.2 LITER DIESEL ENGINE

5A. General Emission Systems
E x h a u s t
G a s
R e c ir c u la t io n ,
EGR
EGR PRINCIPLE
W hen a vehicle operates fo r a substa ntia l part of its tim e at part load — w hich is the norm al co n d itio n in tow n
tra ffic — em issio ns can be reduced by the use of Exhaust Gas R ecirculation. M ixing som e of the exhaust gas
w ith the inlet air at part load reduces the concentratio n of oxygen and lessens the o p p o rtu n ity to produce NOx .
R eduction in NOx o f som e 40% can be obtained w ith o u t increasing HC, CO or sp e cific fuel consum ption.
The increase in the temperature and NOx form ation in the main chamber depend upon the am ount of incom pletely
burned com pounds discharged from the pre-chamber, the oxygen concentration, and the tim ing of the discharge.
EGR is designed to control (Oxides of Nitrogen) NOx. This is done by blending the fuel-air w ith exhaust gases to
reduce the peak temperatures and oxygen concentration. The lack of oxygen lowers the possibility of nitrogen
com bining w ith oxygen to form NOx.
The effect of increasing the EGR on the pollutants shows that as EGR is increased NOx decreases, while both HC
and CO increase. The gradual decrease in NOx continues as EGR increases but the characteristics of HC and CO
change significantly above a value of EGR which varies w ith speed and load. It is possible to establish an amount
of EGR as a function of speed and load. Above 90% fuel no EGR is used. Also since during normal driving in town,
very little tim e is spent above 75% maximum speed, the use of EGR at high speed would have little effect on the
pollutants.
Spent gas is run through an EGR valve to the manifold, and then is part of the air intake (Figure 5-9). This is
introduced into the com bustion chamber. It takes up some of the volume of the incoming charge of air. When
ignition takes place, the spent exhaust gases cannot partake in the com bustion process, since they have already
been used previously, so they add nothing. During the period of “ rapid” com bustion, the temperature increases
quite rapidly. The tem perature rise causes the gases to expand. The temperature in the chamber is much higher
than the exhaust gases. The spent gases now take part in the process. They cannot add to the process because of
lack of oxygen. So the spent or inert gas acts as a sponge and pulls heat into itself causing it to expand. As it
does it absorbs heat of com bustion and drops the temperature approximately 500 °F. Carbon monoxide is not a
significant em ission factor w ith diesels. Hydrocarbons are controlled by the injection nozzles, pump tim ing and
com bustion cham ber design. The EGR valve is used on the LH6 (C) engine only.
5-4