Page 179 - Understanding Automotive Electronics

2735 | CH 5 Page 166 Tuesday, March 10, 1998 11:10 AM

5 THE BASICS OF ELECTRONIC ENGINE CONTROL

Figure 5.13
Oxidizing Catalyst
Conversion
Efficiency versus
Temperature

FPO

The Three-Way Catalyst
The three-way catalyst Another catalytic converter configuration that is extremely important for
uses a specific chemical modern emission control systems is called the three-way catalyst (TWC). It uses
design to reduce all three a specific catalyst formulation containing platinum, palladium, and rhodium to
major emissions (HC, reduce NO and oxidize HC and CO all at the same time. It is called three-way
x
CO, and NO ) by because it simultaneously reduces the concentration of all three major
x
approximately 90%. undesirable exhaust gases by about 90% if used optimally.
The conversion efficiency of the TWC for the three exhaust gases depends
mostly on the air/fuel ratio. Unfortunately, the air/fuel ratio for which NO
x
conversion efficiency is highest corresponds to a very low conversion efficiency
for HC and CO and vice versa. However, as shown in Figure 5.14, there is a
very narrow range of air/fuel ratio (called the window) in which an acceptable
compromise exists between NO and HC/CO conversion efficiencies. The
x
conversion efficiencies within this window are sufficiently high to meet the very
stringent EPA requirements established so far.
Note that this window is only about 0.1 air/fuel ratio wide (±0.05 air/fuel
ratio) and is centered at stoichiometry. (Recall that stoichiometry is the air/fuel
ratio that would result in complete oxidation of all carbon and hydrogen in the
fuel if burning in the cylinder were perfect; for gasoline, stoichiometry
corresponds to an air/fuel ratio of 14.7.) This ratio and the concept of
stoichiometry is extremely important in an electronic fuel controller. In fact,
the primary function of most modern electronic fuel control systems is to
maintain average air/fuel ratio at stoichiometry. The operation of the three-way
catalytic converter is adversely affected by lead. Thus, in automobiles using any
catalyst, it is necessary to use lead-free fuel.

166 UNDERSTANDING AUTOMOTIVE ELECTRONICS