The auto industry is constantly striving to explore cleaner and more sustainable forms of combustion engine to support both conventional and hybrid electric powertrains – and in collaboration with Volkswagen, Empa, ETH Zurich and Poznan University of Technology, icardo’s software engineers are creating tools that will enable the development of a new form of compact, lean-burn natural gas engine offering diesel-like power and performance and extremely low NOX.
Compressed natural gas (CNG) has ong been recognized as an attractive alternative to gasoline or diesel as a transportation fuel. It is typically less expensive, exists in more abundant reserves, and provides lower overall greenhouse gas (GHG) emissions when burned. In addition to fossil resources, there are also significant supply chains being developed for biomethane, the renewable equivalent of CNG derived from agricultural operations, domestic refuse disposal, or water treatment works, and power-to-gas energy conversion.
Coupled with a mature distribution infrastructure, including widespread existing filling station availability, it is understandable that CNG continues to attract significant interest. As a road fuel for use in an internal combustion engine, CNG has some further very practical operational advantages. It is comparatively resistant to knock, making it ideal for boosting
and downsizing, and it is amenable to the use of higher compression ratios to further improve efficiency and reduce CO2 emissions.
Further efficiencies are possible when lean combustion is employed, as heat losses are reduced. But lean operation also brings with it some inherent limitations. To date, most passenger-car CNG applications have been based on conversions of spark-ignited gasoline
platforms relying on port injection and the use of a broadly homogenous charge. This approach places a restriction on the extent of lean operation possible if combustion stability is to be maintained, thus restricting the fuelsaving and emissions reduction potential of light-duty automotive CNG operation.
In heavy-duty gas engine applications for large commercial vehicles and power generation, this limitation can be overcome through the use of a pre-chamber: this enables combustion to be initiated in nearstoichiometric conditions in the vicinity of the spark plug through the use of a pilot injection process. The flame front emanating from the pre-chamber nozzles then provides the basis of a more stable ignition of the leaner, port-injected charge within the main cylinder.
The precise conditions close to the spark plug at ignition, such as the spatial distribution of turbulence intensity and fuel concentration, play a pivotal role on the flame development within a scavenged pre-chamber equipped gas engine. A successful pre-chamber therefore needs to be very carefully designed to ensure that it delivers the required level of combustion stability to the main cylinder. Firstly, scavenging needs to be effective to ensure that unburned hydrocarbons and exhaust products are expelled; the flow structure and injection characteristics and the timing need to be arranged in a manner that delivers the required conditions close to the spark plug electrode and, finally, the spark needs to initiate
combustion in a stable manner.
As the flame front propagates, it needs to move in a manner that avoidsexcessive wall quenching, requiring the nozzles to deliver a reliable flame jet into the cylinder to initiate combustion of the main charge. In addition to the engineering challenges of achieving the above, the inherent constraints of packaging a miniaturized pre-chamber (including its spark plug and gas injector) within the space available in an automotive cylinder head have limited the use of this approach for passenger car applications.
The GasOn project
In order to help realize the full potential environmental and emissions benefits of CNG combustion in the passenger car sector, the multi-partner EU Horizon 2020 project GasOn has been aiming to develop advanced CNG mono-fuel engines which improve on the current state of the art based on converted gasoline platforms. Within the project, Ricardo is partnering with Volkswagen, Empa, ETH Zurich and Poznan University of Technology, on the development of a new form of pre-chamber ignition (PCI) system that is capable of extending the lean limit of automotive CNG operation, while also enabling the adoption of diesellike compression ratios. The combustion system and corresponding hardware is developed by work package leader Volkswagen, with ETH Zurich providing fundamental experimental capability, advanced analysis models and engine control system, and Poznan University of Technology and Empa respectively conducting investigations of developed
systems in a single cylinder research engine and full-scale engine. Ricardo’s role within the project is focusing in particular on the development of the computational fluid dynamics (CFD) tools that would enable the future design of this type of engine.
“There has been an increasing interest in the potential of PCI CNG engines as a potential substitute for diesels in future automotive products,” explains Evgeniy Shapiro, Ricardo Software development manager for the VECTIS CFD package. “If we are able to design PCI systems that deliver stable CNG combustion significantly beyond current lean limits,
then with higher compression ratios it should be possible to offer performance broadly similar to diesel but with much lower NOX emissions.”
In order to enable PCI systems to be designed effectively within the time and resource constraints typical of automotive product development, accurate and fast CFD modelling of
the mixture formation and early flame kernel development in the pre-chamber are essential. “A review of the physical models currently available in commercial CFD codes highlighted a gap in the technology currently available,” continues Shapiro. “This is not least because the
initial stages of ignition in spark-ignited engines typically occur at time scales, temperatures and geometries falling outside of the scope of conventional CFD techniques.”
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