Austin Healey Electronic Fuel Injection - Page 2
- Category: PROJECTS
- Published: Sunday, 11 January 2009 19:25
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The Electronic Fuel Injection System: The EFI system is made up of two interlinked sub-systems, 1) the electronic elements, and 2) the fuel delivery components. These two sub-systems come together at the fuel injectors (see diagram). The system Ric has put together for the Austin Healey is a "closed loop" system, meaning that in steady state driving this system is constantly testing the sensors (such as O2 levels and atmospheric pressure) and working to adjust the injector firing duration to yield the most advantageous air/fuel burn rate (this is called the stoichiometric (ideal)) based on the conditions and the programmed fuel maps (i.e. delivery metrics). If, however, you are at other than steady state....such as full throttle application or start-up....the system will go into "open loop" and will follow the predefined fuel maps that are programmed into the electronic control unit (ECU, sometimes also called the electronic control module or ECM). This type of system is an infinitely variable, ever-changing fuel delivery curve instead of a single programmed set of values. In addition to the electronic elements, the EFI system is dependent on a high quality fuel delivery system, made up of a high pressure fuel pump, fuel filter, fuel pressure regulator, fuel rail, and fuel injectors. The Austin Healey electrical system needs to be converted from positive ground to negative ground to accommodate the EFI.
The Electronic Control Unit: The ECU is a microcomputer that is the "brains" of the fuel injection system. It controls the air/fuel mixture by evaluating the input from the sensors. Ric used a GM ECU which is readily available at junk yards and was produced in the late 80's and early 90's. This ECU is both simplistic by modern automobile standards and fairly advanced compared to general aftermarket ECU's. This system has custom injector blocks, each having 2 injectors. The ECU fires the injectors in two batches, let's call them bank A and bank B. Each bank fires 3 times in each distributor rotation, so there is a total of 6 injection firing events in each distributor rotation. Each of the injector blocks in this installation has one injector wired to bank A and one injector wired to bank B. So, each injector in an injector block is alternately fired, three times each, in one distributor rotation. This system is not a sequential injection system, which would require a more advanced ECU, a means of monitoring the cam position, and manifolds that place injectors directly in each intake port; this system is based on a throttle body injection system. The unit is programmable by a laptop PC with aftermarket software. So, Ric is using the GM ECU "thought process", but has changed the values to reflect the needs of the Austin Healey 6 cylinder engine. The ECU's chip (the program resides in the chip) is then removed and can be reprogrammed (with the new values) through a process called "burning". The resulting chip is then reinstalled in the ECU. Despite the complexity of the ECU, it is generally the most reliable component of an EFI system, as long as you have it mounted in a good location. The ECU should not be in direct proximity to engine or exhaust heat. Since temperatures should not exceed 150 degrees Fahrenheit, it would be best to mount the ECU in the cockpit area.