151 Engine Service and Repair Technician Handbook Ignition Timing The spark plug ignites the fuel just before the piston reaches Top Dead Center (TDC) so that when gas expansion occurs, the piston is beginning its downward stroke. To obtain optimum power transfer from combustion, maximum combustion pressure should occur when the connecting rod reaches approximately 60є after TDC. As RPM increases, keeping maximum combustion pressure occurring at 60є after TDC can require ignition to occur sooner. This is referred to as timing advance (or ignition advance). Timing advance enables the engine to continuously operate at maximum power The ECM controls ignition timing based on:
• Engine speed • Air-fuel ratio • Engine load Technical Training 15