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The current discussion about possible limitation of CO 2 emissions makes improvement of fuel consumption a central topic for gasoline engine development. Various technological solutions are available to realize this improvement. Concepts featuring direct fuel injection, engine downsizing and unthrottled control of engine load with variable valvetrains are currently considered the most promising ways to achieve this goal. Further concepts that are under development include Controlled Auto Ignition (CAI) and homogenous lean burn combustion as well as certain combinations of these technologies. Within the European market, direct injection is currently the most popular solution. The drawback is that a very expensive exhaust gas aftertreatment system is necessary to keep exhaust emissions within legal limits. Downsizing is another very effective solution to increase fuel efficiency, but may lead to customer acceptance problems due to reductions in both engine displacement and the number of cylinders. The use of variable valvetrains is a very attractive way to reduce fuel consumption in gasoline engine applications, as a minimum because exhaust emissions can be effectively treated with conventional exhaust gas aftertreatment systems. Controlled auto ignition is a promising future combustion process concept by which fuel consumption as well as NO x emissions can be significantly reduced, but only within a limited engine map area. In combination with a supercharged combustion system, tuned for enhanced lean burn capability, homogenous lean burn combustion enables a significant increase in compression ratio together with a significant reduction in fuel consumption. Within this paper, various technology concepts for future combustion processes that achieve a reduction of fuel consumption in gasoline engines through valvetrain variations are discussed. To utilize the highest degree of flexibility, the investigations were conducted on an engine equipped with a electromechanical valvetrain (EMVT), featuring both port fuel injection and gasoline direct injection. The investigations show that the electromechanical valvetrain is an excellent platform for developing the Controlled Auto Ignition process. Controlled Auto Ignition has been realized with port fuel injection (PFI) over a speed range between 1000 and 4500 rpm and over a load range between approximately 1 and 6 bar BMEP, depending on the engine speed. The fuel consumption at part load operation could be reduced by up to 18% compared to the conventional base engine. Tests performed with gasoline direct injection have shown a significant extension of the maximum possible load range for CAI by 1 bar. Additionally, the reduction in fuel consumption can be improved to 22% during CAI operation with direct injection. Due to the cycle-synchronous control of valve timings, injection and ignition offered by the EMVT, transitions between the CAI and SI operating modes can be realized without misfire or negative torque response. Based on the investigations with the EMVT engine, the necessary variations in the valve train as well as the requirements for transient behavior can be identified, thus giving distinct targets for a mechanically variable valve train that will enable CAI. |
