Design Methodology of Camshaft Driven Charge Valves for Pneumatic Engine Starts

Abstract

-Idling losses constitute a significant amount of the fuel consumption of internal combustion engines. Therefore, shutting down the engine during idling phases can improve its overall efficiency. For driver acceptance a fast restart of the engine must be guaranteed. A fast engine start can be performed using a powerful electric starter and an appropriate battery which are found in hybrid electric vehicles, for example. However, these devices involve additional cost and weight. An alternative method is to use a tank with pressurized air that can be injected directly into the cylinders to start the engine pneumatically. In this paper, pneumatic engine starts using camshaft driven charge valves are discussed. A general methodology for an air-optimal charge valve design is presented which can deal with various requirements. The proposed design methodology is based on a process model representing pneumatic engine operation. A design example for a two-cylinder engine is shown, and the resulting optimized pneumatic start is experimentally verified on a test bench engine. The engine's idling speed of 1200 rpm can be reached within 350 ms for an initial pressure in the air tank of 10 bar. A detailed system analysis highlights the characteristics of the optimal design found.Re´sume´-Me´thodologie pour le design des valves de chargement ope´re´es par arbre a`cames -Les pertes a`vide repre´sentent une partie essentielle de la consommation des moteurs a`combustion interne. La mise a`l'arreˆt du moteur pendant la marche a`vide peut, par conse´quent, en ame´liorer son efficacite´ge´ne´rale. Pour eˆtre accepte´par le conducteur, le rede´marrage du moteur doit eˆtre rapide. On peut re´aliser ce de´marrage rapide du moteur, moyennant un de´marreur e´lectrique puissant conjointement avec un accumulateur approprie´, solution retenue par exemple, pour les ve´hicules a`syste`me hybride e´lectrique. Cependant, ces derniers augmentent le couˆt et le poids. Une alternative consiste dans le de´marrage pneumatique du moteur en utilisant de l'air comprime´stocke´dans un re´servoir sous pression et injecte´directement dans les cylindres. Cette e´tude pre´sente le de´marrage pneumatique du moteur en utilisant des valves de chargement commande´es par arbre a`cames. On pre´sente une me´thodologie visant a`une consommation de l'air optimale en mesure de respecter des exigences diffe´rentes. La de´marche propose´e s'appuie sur le mode`le d'un processus repre´sentant l'ope´ration pneumatique du moteur. La ve´rification expe´rimentale du de´marrage pneumatique est re´alise´e et optimise´e sur un moteur 2 cylindres sur banc d'essai. Avec une pression initiale de 10 bar dans le reservoir d'air, la vitesse de rotation a`vide de 1 200 tr/min peut eˆtre atteinte en 350 ms. Une analyse de´taille´e confirme les caracte´ristiques du syste`me optimise´.

System Design and Analysis of a Directly Air-Assisted Turbocharged SI Engine with Camshaft Driven Valves

“…The second plot shows the time duration t s until the desired engine speed is reached. These results were obtained with the process model described in [13].…”

“…Consequently, the CV profile, which is defined by the opening instant, the closing instant, the valve lift, is fixed in each mode and needs to be carefully chosen. Cam design methodologies have been proposed for the boost mode [6] and the pneumatic start mode [13]. They cope with the various constraints, such as a lower limit for the tank pressure to prevent the flow of air-fuel mixture to the tank.…”

“…Hence, this mode can be used as a stop/start system and to eliminate the idling consumption. As shown in [13], the cam profile of the start mode is also sized according to a performance criterion, such as to reach an engine speed of 1200 rpm in 350 ms. There is also a lower limit for the tank pressurep t,s , which should not be underrun in order to fulfill the desired start performance.…”

“…The optimal CV design presented in [13] is used, i.e., p t,s = 10 bar. Figure 3 shows the results for various tank pressures.…”