Page 257 - Understanding Automotive Electronics

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7 DIGITAL ENGINE CONTROL SYSTEM

Figure 7.10
Torque versus SA
for Typical Engine

FPO

advancing the spark relative to TDC increases the torque until a point is
reached at which best torque is produced. This spark advance is known as
mean best torque, or MBT.
When the spark is advanced too far, an abnormal combustion
phenomenon occurs that is known as knocking. Although the details of what
causes knocking are beyond the scope of this book, it is generally a result of a
portion of the air–fuel mixture autoigniting, as opposed to being normally
ignited by the advancing flame front that occurs in normal combustion
following spark ignition. Roughly speaking, the amplitude of knock is
proportional to the fraction of the total air and fuel mixture that autoignites.
It is characterized by an abnormally rapid rise in cylinder pressure during
combustion, followed by very rapid oscillations in cylinder pressure. The
frequency of these oscillations is specific to a given engine configuration and
is typically in the range of a few kilohertz. Figure 7.11 is a graph of a typical
cylinder pressure versus time under knocking conditions. A relatively low
level of knock is arguably beneficial to performance, although excessive knock
is unquestionably damaging to the engine and must be avoided.
One control strategy for spark advance under closed-loop control is to
advance the spark timing until the knock level becomes unacceptable. At this
point, the control system reduces the spark advance (retarded spark) until
acceptable levels of knock are achieved. Of course, a spark advance control
scheme based on limiting the levels of knocking requires a knock sensor such as
that explained in Chapter 6. This sensor responds to the acoustical energy in the
spectrum of the rapid cylinder pressure oscillations, as shown in Figure 7.11.
Figure 7.12 is a diagram of the instrumentation for measuring knock
intensity. Output voltage V of the knock sensor is proportional to the
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244 UNDERSTANDING AUTOMOTIVE ELECTRONICS