Troels Dyhr Pedersen.indd - Solid Mechanics

5 Introduction
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5.1 About HCCI combustion
Homogeneous charge compression ignition, or HCCI, is generally regarded as a
promising technology for future power train concepts, since it allows an engine to operate
with high thermal efficiency and very low emissions of particulate matter and nitric
oxides. There is however some drawbacks that still keep the principle from being used in
commercial engines. These include a limited load capability, excessive engine noise
caused by knocking combustion and the problem of controlling combustion phasing and
heat release rate. These areas are the main focus points of the current research.
Several variations of the original HCCI concept have been developed to overcome the
problematic issues. Hence names such as PCCI (premixed charge compression ignition),
PCI (premixed compression ignition), CAI (controlled auto ignition) etc. are commonly
used to denote specific variations of premixed combustion. A thorough description of the
various principles is given in [4]. The common feature of all these concepts is that they
are all low temperature combustion processes which result in lower levels of nitric oxides
and particulate matter than the SI and DI combustion principles.
Proprietary names are also emerging as the technologies are implemented in commercial
vehicles. Volkswagen have the GCI (gasoline controlled ignition) and CCI (combined
combustion system), which are implementations of gasoline and diesel HCCI variants.
CCI have been demonstrated in their Touran model, but are not ready for production.
General Motors have developed a technology called Ecotec, which is currently tested in
the US model Saturn Aura. The engine uses variable valve timing, direct injection and
electric cam phasing to achieve proper timing of HCCI combustion with gasoline.
Mercedes have demonstrated an HCCI engine and calls their technology DiesOtto, which
is a gasoline engine with the ability to switch to auto ignition combustion mode. The
prototype engine, which was presented in 2008, is a very complex HCCI engine with
features such as variable compression ratio, variable valve timing and variable geometry
turbo charging.
It is obvious that engines with the abovementioned technologies will be quite expensive,
so the question is if the benefit in terms of modest fuel savings can really justify the
additional cost to the consumer. It may one of the reasons that despite the efforts to
commercialize HCCI, development appears to be slow within the automotive industry. It
may be that the interests in commercializing the concepts are not yet large enough to
justify the efforts. When looking into the potential fuel savings, figures from 15 - 30 % in
part load operation are common guesses. Such savings are fully possible with hybrid
solutions as well, although these tend to be even more costly and increase the weight and
complexity of the vehicle.