Electronic Control Unit is an electronic system which controls different engine parameters for optimized performance. In SI engine, ECU controls the Injector opening timing and spark timing at different engine speed and load. Many ECU functionalities like Idle control, Cold start control, AFR control, Altitude correction etc. are explained in this paper which are helpful/main control strategies while designing an ECU. Also multiple fuel injection techniques are also discussed with their advantages and disadvantages. Multiple sensors are used for accurate fuel injection at different set points of engine operation. For achieving better accuracy of air and fuel ratio than carburetors, ECU estimates the opening time of fuel injector through different control algorithms. This paper includes the survey of different functional methodologies that are being used to reduce the pollutants.