Page 174 - Understanding Automotive Electronics

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THE BASICS OF ELECTRONIC ENGINE CONTROL 5

BSNO = brake-specific NO concentration
x
x
r NO
= ---------- x
P b
where
r HC = HC rate of flow
r CO = CO rate of flow
r NO x = NO rate of flow
x
One specific air/fuel ratio is highly significant in electronic fuel control
systems, namely, the stoichiometric mixture. The stoichiometric (i.e., chemically
correct) mixture corresponds to an air and fuel combination such that if
combustion were perfect all of the hydrogen and carbon in the fuel would be
converted by the burning process to H O and CO . For gasoline the
2
2
stoichiometric mixture ratio is 14.7:1.
Stoichiometry is sufficiently important that the fuel and air mixture is
often represented by a ratio called the equivalence ratio, which is given the
specific designation λ. The equivalence ratio is defined as follows:

( air/fuel)
λ = ------------------------------------------------------
( air/fuel stoichiometry)

A relatively low air/fuel ratio, below 14.7 (corresponding to λ < 1), is called a
rich mixture; an air/fuel ratio above 14.7 (corresponding to λ > 1) is called a
lean mixture. Emission control is strongly affected by air/fuel ratio, or by λ.
Note from Figure 5.8 that torque (T) reaches a maximum in the air/fuel
ratio range of 12 to 14. The exact air/fuel ratio for which torque is maximum
depends on the engine configuration, engine speed, and ignition timing.
The air/fuel ratio has a Also note that the CO and unburned hydrocarbons tend to decrease sharply
significant effect on with increasing air/fuel ratios, as one might expect because there is relatively more
engine torque and emis- oxygen available for combustion with lean mixtures than with rich mixtures.
sions. Unfortunately for the purposes of controlling exhaust emissions, the NO
x
exhaust concentration increases with increasing air/fuel ratios. That is, there is
no air/fuel ratio that simultaneously minimizes all regulated exhaust gases.
Effect of Spark Timing on Performance
Spark timing also has a Spark advance is the time before top dead center (TDC) when the spark is
major effect on emis- initiated. It is usually expressed in number of degrees of crankshaft rotation
sions and engine perfor- relative to TDC. Figure 5.9 reveals the influence of spark timing on brake-
mance. Maximum engine specific exhaust emissions with constant speed and constant air/fuel ratio. Note
torque occurs at MBT. that both NO and HC generally increase with increased advance of spark
x
timing. BSFC and torque are also strongly influenced by timing. Figure 5.9
shows that maximum torque occurs at a particular advanced timing referred to
as minimum advance for best timing (MBT).

UNDERSTANDING AUTOMOTIVE ELECTRONICS 161