5.6 Why do exhaust catalysts influence fuel composition?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
5.6 Why do exhaust catalysts influence fuel composition?
Modern adaptive learning engine management systems control the combustion
stoichiometry by monitoring various ambient and engine parameters, including
exhaust gas recirculation rates, the air flow sensor, and exhaust oxygen
sensor outputs. This closed loop system using the oxygen sensor can
compensate for changes in fuel content and air density. The oxygen sensor
is also known as the lambda sensor because the actual air-fuel mass ratio
divided by the stoichiometric air-fuel mass ratio is known as lambda or the
air-fuel equivalence ratio.
The preferred technique for describing mixture strength is the fuel-air
equivalence ratio ( phi ), which is the actual fuel-air mass ratio divided
by the stoichiometric fuel-air mass ratio, however most enthusiasts use
air-fuel ratio and lambda. Lambda is the inverse of the fuel-air equivalence
ratio. The oxygen sensor effectively measures lambda around the
stoichiometric mixture point. Typical stoichiometric air-fuel ratios are
[80]:-
6.4 methanol
9.0 ethanol
11.7 MTBE
12.1 ETBE, TAME
14.6 gasoline without oxygenates
The engine management system rapidly switches the stoichiometry between
slightly rich and slightly lean, except under wide open throttle conditions
- when the system runs open loop. The response of the oxygen sensor to
composition changes is about 3 ms, and closed loop switching is typically
1-3 times a second, going between 50mV ( lambda = 1.05 (Lean)) to 900mV
(lambda = 0.99 ( Rich)). The catalyst oxidises about 80% of the H2, CO,
and HCs, and reduces the NOx [76].
Typical reactions that occur in a modern 3-way catalyst are:-
2H2 + O2 -> 2H2O
2CO + O2 -> 2CO2
CxHy + (x + (y/4))O2 -> xCO2 + (y/2)H2O
2CO + 2NO -> N2 + 2CO2
CxHy + 2(x + (y/4))NO -> (x + (y/4))N2 + (y/2)H2O + xCO2
2H2 + 2NO -> N2 + 2H2O
CO + H20 -> CO2 + H2
CxHy + xH2O -> xCO + (x + (y/2))H2
The use of exhaust catalysts have resulted in reaction pathways that can
accidentally be responsible for increased pollution. An example is the
CARB-mandated reduction of fuel sulfur. A change from 450ppm to 50ppm, which
will reduce HC & CO emissions by 20%, was shown to increase formaldehyde by
45%, but testing in later model cars did not exhibit the same effect [32,58,
59]. This demonstrates that continuing changes to engine management systems
can also change the response to fuel composition changes.
The requirement that the exhaust catalysts must now endure for 10 years or
100,000 miles will also encourage automakers to push for lower levels of
elements that affect exhaust catalyst performance, such as sulfur and
phosphorus, in both the gasoline and lubricant. Modern catalysts are unable
to reduce the relatively high levels of NOx that are produced during lean
operation down to approved levels, thus preventing the application of
lean-burn engine technology. Recently Mazda has announced they have
developed a "lean burn" catalyst, which may enable automakers to move the
fuel combustion towards the lean side, and different gasoline properties may
be required to optimise the combustion and reduce pollution [81]. Mazda
claim that fuel efficiency is improved by 5-8%, while meeting all emission
regulations, and some Japanese manufacturers have evaluated lean-burn
catalysts in limited numbers of 1995 production models.
Catalysts also inhibit the selection of gasoline octane-improving and
cleanliness additives ( such as MMT and phosphorus-containing additives )
that may result in refractory compounds known to physically coat the
catalyst, reducing available catalyst and thus increasing pollution.
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