Sigma® on knock phenomenon control of Flexfuel engines

Brazil, pioneer in the mass production of alcohol vehicles in the 80'decade, blunts in the last two years with a new large scale program, a new Flexfuel vehicles generation that will crosses 60% sales in the end of 2005. These vehicles use in the same engine, any mixture in any proportion of the ethanol or gasoline. Simultaneously to the Flex fuel vehicles, the search of Natural gas, allied the construction of gas pipelines Bolivia-Brazil, made available this another fuel alternative in the market, with a unprecedented growth, the country has already the second largest vehicles fleet powered by natural gas of the planet.

These alternatives bring great challenges for combustion engine improvement to reach his efficiency due to great diversity in fuel physical proprieties, to be used in the same engine. To increase the engine thermodynamic efficiency and generate torque, mechanical solutions were adopted as, for instance, a high compression ratio in a never used level. The correct ignition adjustment for each fuel in a same engine presents a fundamental setup to assure the engine durability and thermal efficiency for mass production for simultaneous use of different fuels.

This study presents the Sigma _® technology, the fifth Magneti Marelli's generation to control the knocking phenomenon, used on FLEX vehicles in production and also on concept vehicles named TETRAFUEL. The Sigma _® assures the optimum ignition point for each individually cylinder, in any type of fuel, in the whole engine operation. It allows to reduce or to increase the ignition spark adjustment in the threshold of engine mechanical resistance and maximum possible thermal efficiency. Recovering great part of the engine mechanical limitations being possible the use in a same engine the four available fuels in the Mercosul, Hydrated alcohol, Gasoline mixed with 22% of anhydrous alcohol, Pure gasoline and Compressed natural gas.

Knock detection and regulation at limited controlled levels, is a method to optimize the thermodynamic efficiency of IC engines increasing the generated torque and reducing the fuel consumption. The necessity to tune the spark advance very close to the knock limit and increase the combustion efficiency requires developing efficient methods to detect knocking and to measure the associated energy. Additional requirements for knock control derive from new engines technologies like Direct Injection and Variable Valve Timing, that can influence sensors signal quality and detection of phenomena.

Here in the following we propose an approach to knock detection and regulation based on the use of standard automotive accelerometer sensors, associated to the estimation of knock energy by a statistical parameter. The experimentation of the method shows a strict correlation between estimated and in-chamber measured knock indicators. As a consequence the knock control algorithm operates on a physical basis to regulate the objective of knock intensity and frequency.

A significant advantage of the method is a simplified calibration procedure that allows reducing the effort of tuning phase.

Furthermore an architectural analysis demonstrates the potential to realize the signal elaboration process by digital technology, fulfilling the required function performances. The algorithm is compatible with standard automotive micro controller providing a dedicated fast conversion acquisition channel.

The sigma was applied on the TETRAFUEL vehicle and due his characteristics to quantify the knock intensity was possible to improve the engine efficiency recovering engine to engine differences as well fuel variation from pure gasoline, passing to Brazilian gasohol (22%E), pure ethanol and CNG - Compressed Natural Gas.