Closed-loop ignition timing

The ignition system described in the foregoing section is an open-loop system. The major disadvantage of open loop control is that it cannot automatically compensate for mechanical changes in the system. Closed-loop control of ignition timing is desirable from the standpoint of improving engine performance and maintaining that performance in spite of system changes.

One scheme for closed-loop ignition timing is based on the improvement in performance that is achieved by advancing the ignition timing relative to TDC. For a given RPM and manifold pressure, the variation in torque with SA is as depicted in Fig. below. One can see that advancing the spark relative to TDC increases the torque until a point is reached at which best torque is produced. This SA 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. Fig. below 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 SA under closed-loop control is to advance the spark timing until the knock level becomes unacceptable. At this point, the control system reduces the SA (retarded spark) until acceptable levels of knock are achieved. Of course, an SA control scheme based on limiting the levels of knocking requires a knock sensor. This sensor responds to the acoustical energy in the spectrum of the rapid cylinder pressure oscillations, as shown in Fig.