To meet future CO₂ emissions limits and satisfy the bounds set
by exhaust gas legislation reducing the engine displacement while
maintaining the power output ("Downsizing") becomes of
more and more importance in the SI engine development process. The
total number of cylinders per engine has to be reduced to keep the
thermodynamic disadvantages of a small combustion chamber layout as
small as possible. Doing so new challenges arise concerning the
mechanical design, the design of the combustion system concept as
well as strategies maintaining a satisfying transient torque
behavior. To address these challenges a turbocharged 2-cylinder SI
engine was designed for research purposes by Weber Motor GmbH and
Robert Bosch GmbH. The design process was described in detail in
last year's paper SAE 2012-01-0832.
Since the engine design is very modular it allows for several
different engine layouts which can be examined and evaluated. This
paper shows a comparison between three major combustion system
concepts: - Gasoline Direct Injection, - Advanced Gasoline
Port-fuel Injection, and - Compressed Natural Gas Port-fuel
Injection.
As a first step the engine behavior was examined using different
turbochargers on the test bed to find the best match for a
2-cylinder engine application, especially with respect to low-end
torque and transient behavior. Subsequently the 2-cylinder engine
was equipped with three different combustion system concepts and
tested on the engine test bed using the standard measurement
equipment including high and low pressure indication. The concepts
will be evaluated on the basis of their low-end torque and
transient performance. Therefore a measurement method and a key
performance index were defined. Additionally the influence of the
combustion system concept choice on fuel consumption and CO₂
emissions will be evaluated using drive cycle simulations. For each
concept the advantages and challenges will be highlighted and
discussed.