Monday, April 30, 2012

ECU Fuel Injection Loop Calibration

In this post we will look at how a precision DC source or an SMU can be used to calibrate the fuel injection loop in a automobile or other engine powered equipment. The fuel injection flow to an engine is controlled by a solenoid that is adjusted either more open or more closed by a driver output from the engine control unit (ECU). Fuel injection control has always been an important part or a vehicle design, but recently the precision it is tested to has become even more critical because it is closely tied to a vehicle's fuel efficiency. Tight control over fuel injection means at any given moment the engine is getting the right mixture of fuel and air to ensure maximum efficiency.

Below is a basic circuit diagram of a fuel injection control circuit. The inductor at the top represents the fuel injector solenoid. The amount of current flowing through the solenoid determines how open or closed it is, which in turn controls the flow of fuel to the engine. The amount of current flowing through the solenoid is controlled by the MOSFET, which is pulse width modulated by a control circuit from the ECU. The rate of the PWM is determined using the control signal from the accelerator and the output of the current sense circuit. The test we will look at is used to calibrate the control loop to the output of the current sense circuit at various current levels.

There are two ways the calibration factors could be determined. The first is by isolating the circuit from  Vbatt and the solenoid and just forcing a known amount of current through the circuit using a current source. Below is a an example of how the test was done by an engineer at an automotive company using the Agilent N6762A Precision DC Source.

The DC source is used in constant current mode so it acts like a current source. Precision DC sources have high accuracy current sourcing capabilities which is ideal for this application. To perform the test iterate the output current value of the DC source through the full range of the solenoid, open to closed. Then use the measured values to build a look up table or create a curve fitted equation. Instead of a precision DC source a source measure unit (SMU) could also be used. An SMU will typically deliver slightly better current accuracy, but at a higher price. 

The second way to calibrate the fuel injection control is by operating the whole circuit in normal operation with Vbat and the solenoid or at least something that simulates Vbatt and the solenoid. You then use an SMU as a zero ohm shunt in series with the current path. To use an SMU as a zero ohm shunt you simply insert it into the circuit set it for 0V so its output is regulated at 0V. That way there is no voltage dropped across it so it doesn't effect the circuit. The circuit current from Vbatt will then flow through the SMU (it is sinking the current). The SMU would then accurately measure the current flowing through it. Using an SMU as a zero ohm shunt is much better then using a real current shunt, because a real shunt will add a non-zero amount of resistance to the current path. For this type of setup you want to make sure the SMU has a fast transient response spec to ensure proper regulation at 0 volts in the face of dynamic current changes.

In this post we looked at two ways to calibrate the fuel injection control loop in an automobile using either a precision DC source or an SMU. If you have any questions on this post please email me and if you have any insights to add just use the "Comments" section below.


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