Flexible Fuel and manufacturing dispersion 2008-36-0310
Back to the non Flex Fuel vehicles, the knock control system was designed and calibrated to absorb differences between engines (mainly compression ratio) and to protect the engine against knock damage (a correction up to 4 degrees BTDC was usually enough). But now, two new variables get in the scene: Flexible Fuel strategy, working from E22 to E100 (all blends in between) and small displacement (1.0 liter) high compression ratio engines. In this new scenario the system must be capable of correcting all spark advance differences, once knock control system acts as a safety feature, protecting the engine even if the fuel learning shows some deviation.
In addition to that, we have the compression ratio variation between minimum and maximum limits. Since the engine is small (as well its combustion chamber), each tenth of a millimeter difference during manufacturing process, results in an important final compression ratio variation. This variation requires an important spark reduction based on the same base calibration.
And finally, to meet customer requirements, another 3 degrees BTDC must be corrected as a safety factor. So, when we put together the different fuels, compression ratio dispersion and test requirements, it is clear that the actual calibration is obsolete and a new borderline and knock control calibration methods must be developed / implemented.