Chemistry Reference
In-Depth Information
reduced via engine calibration, with the other pollutant being controlled using an
after-treatment device.
A relatively recent development has been the gasoline direct injection (GDI)
engine, in which fuel injection takes place in the cylinder. This allows better control
of the combustion process. Early (1990s) GDI engines operated using the lean-burn
principle over their complete range, in which proportionally more air is fed in the
cylinder [
13
]. Such engines benefited from better fuel consumption but the oxygen
abundance led to higher NO
x
emissions. Exhaust aftertreatment was therefore needed
to bring NO
x
emissions within the legislative limits [
14
]. Modern GDI engines can
switch between lean-burn (stratified) and stoichiometric combustion depending on
the operational mode to achieve low emissions and high fuel efficiency.
3.2.2 Exhaust Gas Recirculation
Exhaust gas recirculation (EGR) is a NO
x
-reduction technology which has been
used in both petrol and diesel engines for some years (since the mid-1990s in the
case of the latter). It works by redirecting a portion of the engine exhaust gas back
into the combustion chamber where it is mixed with the fresh fuel-air mixture. The
EGR gases act as a diluent, thereby lowering the peak flame temperature and hence
the rate of NO formation. Increasing the amount of gas recirculated reduces the rate
of NO formation. However, it also reduces the combustion rate, making stable
combustion more difficult to achieve [
15
].
In petrol engines the “internal” EGR concept is often implemented. This
involves adjusting the exhaust valve timing so that some of the combusted gas is
trapped in the cylinder [
16
]. This residual gas acts as a diluent for the next
combustion cycle, hence lowering the combustion temperature. Internal EGR can
only reduce NO
x
slightly, because no more than 5-10% of the exhaust gas can be
trapped without significantly affecting combustion.
In modern diesel engines the reduction in NO
x
emissions required by legislation
cannot normally be achieved using internal EGR alone. In this case an “external”
EGR loop is the preferred option, whereby some of the exhaust gas is fed back into
the cylinder by means of a pump, again reducing the combustion temperature. This
EGR configuration has the advantage of higher EGR rates (up to 40-50%) and the
possibility of adjusting the quantity of exhaust gas recirculated independently of the
valve timing. It also permits the introduction of a heat exchanger between the outlet
and inlet pipes (the so-called “intercooler”). This decreases the temperature of the
recirculated gas - and hence the combustion temperature - even further. On the
other hand, external EGR increases the complexity, size and cost of the system.
3.2.3 Three-Way Catalytic Converter
The TWC has been mandatory on all new petrol cars and vans sold in Europe since
the introduction of the Euro 1 emission standard in 1992 and has proved to be a very
robust technology for reducing emissions of NO
x
, carbon monoxide (CO) and
hydrocarbons (HC) (hence the name “three-way”).
