Improvement of a High-Performance CNG-Engine Based on an innovative Virtual Development Process 2011-24-0140

Methane as an alternative fuel in motorsports? Actually this solution is well known for the reduction of CO₂ emissions but apparently it does not really awake race feelings. At the 2009 edition of the 24-hour endurance race on the Nürburgring the Volkswagen Motorsport GmbH, in addition to vehicles powered by gasoline engines, introduced two vehicles powered by innovative turbo-charged CNG engines for the first time. The aim was to prove, that also an "environment-friendly" concept is able to provide the required efficiency, dynamic and reliability for a successful participation in motorsports. After the success in the 2009 edition the engagement has been continued also in 2010, this time exclusively with CNG vehicles.

Focusing on the CO₂ emission, reclusively the higher hydrogen content of methane which represents the main component of NG leads to a CO₂ reduction during the combustion of about 20% compared to gasoline. Thanks to the laminar burning speed of methane which is approximately maximal for a stoichiometric mixture, CNG engines do not require a mixture enrichment at WOT operating conditions, so that the fuel consumption decreases. In addition the very high knock-resistance of natural gas allows a further efficiency increasing by using a higher compression ratio. Conclusively the CO₂ reduction of the CNG version ranges from ca. 30% using natural gas up to 80% for bio-gas. On the other hand gas injection in the intake manifold causes a loss of charge due to both the low mass density of natural gas and the absence of heat of vaporization. The latter also produces a temperature level of the exhaust gas at the turbine which is more critical. Another drawback of CNG engines is the homogenization of the air-fuel mixture. This process is more critical because even high gas velocities at the injector nozzle cause a very low fuel penetration. Therefore, mixture homogenization or stratification depends much more on charge motion as usual for liquid fuels. For this reason the design of the intake system and the combustion chamber is a crucial step for the optimization of a CNG engine.

In this paper the engine development process has been performed mainly in a virtual context. The implementation of an innovative 3D-CFD tool (QuickSim) that has permitted full-engine simulations of this turbocharged CNG race-engine has allowed, within short time (few months), to remarkably increase the engine performance. The virtual engine development process has started with the 3D-CFD analysis of the fluid motion of the basic engine. Based on this analysis, many engine-design modifications have then been virtually tested, so that at the end only a few promising solutions have been "concretely" realized and tested at the test bench. Since the results at the test bench have finally confirmed the expectations from the simulation results, following this procedure it has been definitively possible to speed up the engine development process even by limiting the budget.