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.