Flexible Fuel meets high compression engines: The knock control challenge 2008-36-0309
Since the appearance of the first flexible fuel vehicle back in 2003, many improvements have been done in order to deliver a more reliable and more efficient engine package. The increase in compression ratio is one of the mechanisms used in performance pursuit and to guarantee the engine durability meeting fuel economy requirements, running with fuels from regular E22 to E100 under high compression, many challenges were faced. The pre-ignition running at low engine speeds and mid to high loads had to be controlled, maintaining a reasonable driveability. Increasing the engine speed across its useful band, a special knock event could occur. It is special because results in peak pressures up to 15.000 kPa, named “single strong knock” and is present mainly at highest closed-loop load operation. In addition of that, design limitation was the cause for cylinder #04 low sensitivity. This characteristic narrowed the spark correction band resulting in destructive knock activity. Transient condition (tip-in event) became an important condition and transient knock control had to be calibrated to avoid driveabillity issues. And finally, with all set, the knock system diagnostics was calibrated using fuels with different characteristics (again, E22 to E100) and, those differences, in a few cases resulted in failure been detected in healthy systems. In other words, the technical solutions and trade-offs for the challenges above and described in this paper leaded to customer satisfaction.