Functional Description

0_Verbrennung_Uebersicht

1_Verbrennung1

2_Verbrennung2

3_Verbrennung3

Pos.

Designation

a

Valve

b

Spark plug

c

Piston

Normal Combustion

The graphics 1 show the desired type of combustion of the gas/air mixture in the combustion chamber. The ignition spark ignites the gas/air mixture. The flame front spreads out evenly in the combustion chamber with the specific laminar flame speed of the gas/air mixture. The cylinder pressure increases slightly during combustion.

Knocking Combustion

Knocking combustion arises if the gas/air mixture self-ignites before the actual flame front, but after the ignition 2. This system does not detect so-called early ignition.

The reason for this is an excessive increase in pressure and temperature of the as yet non-combusted mixture due to the pressure and temperature fronts preceding the normal flame front. The pressure and temperature fronts arising from the self-ignition, in turn, make further self-ignitions possible. High-frequency pressure waves arise in the combustion chamber, which are introduced into the engine structure via the walls of the combustion chamber and released as air-borne noise into the environment. The knocking becomes audible in this way 3.

Compared to normal combustion, significantly higher peak pressures arise, which may lead to major engine damage in addition to the higher thermal load.

Detonation Control System

The task of the DetCon detonation control system is to avoid engine damage from knocking combustion.

Vibration occurs in the engine compartment during the combustion process. These have a frequency which is characteristic for the engine type. The DetCon measures the vibratory energy within a narrow frequency range which is typical for the respective engine. The energy measured is proportional to the knocking level.

Measurement is only carried out within operating cycles in which combustion is possible. This increases the sensitivity of the measurement and minimizes its reaction to random noises. The operating cycles are determined according to application and the ignition controller used via an auxiliary synchronization output, an ignition sensor unit (ISU) or a camshaft sensor.

The following diagram and the explanations below it illustrate the basic control process of the system:

1_Funktionsbeschreibung

Term used in diagram

Explanation

Knocking Level

Example of the progression of knocking energy

IMMEDIATE STOP LIMIT

The maximum value at which the engine is stopped

IGNITION REDUCTION LIMIT

The maximum value upon which an ignition timing reduction is performed

ENGINE KNOCKING (binary output)

Signal on the binary output indicating knocking.

LOAD REDUCTION (binary output)

Signal on the binary output effecting load reduction.

TRIP (binary output)

Signal on the binary output indicating that the IMMEDIATE STOP LIMIT has been exceeded.

Timing Reduction (analog output)

Curve of the analog signal for timing reduction

MAX. LEVEL OF ANALOG OUTPUT

Maximum value of the timing reduction

Timing Reduction Gain

Speed of the timing reduction

Decrease Ramp

Speed of the timing reduction

Delay after load reduction

Delay time following a load reduction

The measured knocking energy (Knocking Level curve) is compared in every cycle with a preset maximum value (IGNITION REDUCTION LIMIT). If this maximum value is reached, the binary output ENGINE KNOCKING is activated. At the same time, the analog outputs change their values (Timing Reduction curve). The rate at which the value of the signal changes is specified by the setting Timing Reduction Gain. The analog signals are transmitted to the ignition controller, thus adjusting the ignition timing. If this causes the knocking energy to fall below the maximum value, the values at the analog outputs are also reduced. The rate of this reduction is adjusted according to the preset value Decrease Ramp.

If the ignition timing can no longer be corrected via the analog outputs and the engine is still knocking, the binary output for load reduction (LOAD REDUCTION) is activated. A master control (e.g. ALL-IN-ONE) can control load reduction via this output.

LOAD REDUCTION is deactivated again if the engine knocking stops. However, the analog outputs remain active for a further period which is set via the function Delay after load reduction. This period must be longer than required for reaching full load.

The third binary output, i.e. TRIP, is activated when the knocking exceeds the maximum value IMMEDIATE STOP LIMIT. This can be used as an emergency stop signal to force the engine to stop.