UC Berkeley leading project investigating partial fuel stratification and microwave-assisted sparkplugs for LTC engines
1 July 2013
|The LTC research project has three proposed components. Click to enlarge.|
The University of California Berkeley is partnering with MIT, Lawrence Berkeley, Lawrence Livermore and Sandia National Labs and Ricardo to investigate the the use of partial fuel stratification (PFS) in compression-ignition (CI) engines as well as the combination of PFS and microwave-assisted spark plug (µWASP) technology in spark-ignition (SI) engines to enable Low Temperature Combustion (LTC) engines to operate over the full load and speed range.
The 3-year, $1.65-million project is supported by the US Department of Energy (DOE) and National Science Foundation as part of their collaborative research program for advanced combustion engines. (Earlier post.) The project is being led by UC Berkeley professors Prof. Robert W. Dibble and Prof. Jyh-Yuan Chen as co-principal investigators, with Dr. Samveg Saxena of UC Berkeley and Lawrence Berkeley National Laboratory, and Prof. Wai Cheng (Co-PI), Massachusetts Institute of Technology (collaborating institution). One focus of the project will be centered on light- and heavy-duty vehicles running on gasoline/ethanol blends in order to improve engine efficiency and lower emissions.
The fundamental physics behind flame propagation through a stratified charge will be studied to obtain a comprehensive description of the PFS ignition process. These insights will be applied, through fundamentally based phenomenological models, to enable a significant engine efficiency improvement (25-40% from current practice) and to achieve low emissions across a wide range of load and speed.
Among the goals of this research is to provide a detailed understanding of the chemical kinetics that cause pressure-sensitive intermediate temperature heat release and equivalence ratio sensitivity in gasoline—phenomena that enable high load boosted LTC with PFS.
For the µWASP technology, this program is intended to provide understanding of the interactions between the spark discharge, the microwave field and the stratified mixture in the flame kernel formation process in order to better understand the PFS ignition process.
Ricardo will provide its testing and simulation capabilities for the second phase of the project, which will kick-off in early 2014 and operate out of the company’s Detroit Technical Center. During the testing phase, Ricardo will use its Extreme Boosted Direct Injection (EBDI) engine technology, which can deliver up to a 30% fuel economy and CO2 improvement compared with existing engines.
|Operating principle behind an engine using PFS. Premixing can be accomplished either with port fuel injection (PFI) as indicated in the figure, or with early direct injection (DI). Credit: Saxena and Bedoya (2013). Click to enlarge.|
CI PFS. The team is collaborating with Sandia National Laboratory on the compression-ignition PFS work. In the partial fuel stratification (PFS) strategy, a majority of the fuel is injected in the intake manifold or directly injected in the cylinder very early to allow premixing, while a small amount of fuel (up to a max of 20%) is directly injected late to create areas with locally higher equivalence ratio.
As a result of the φ-sensitivity (a measure of the impact of varying fuel-air equivalence ratio on combustion timing when all other factors are held constant) of certain fuels, a sequential ignition event is created as richer regions ignite earlier. Ignition propagates toward the leaner regions to create smoother heat release as compared with conventional homogeneous charge compression ignition (HCCI). This sequential ignition event allows lower peak pressure rise rates, and thus lower ringing intensity, thereby allowing more fuel to be injected to achieve higher load.
(Ringing refers to high amplitude pressure oscillations from within the combustion chamber due to high heat release rates, and is a constraint governing high output power limits. Ringing is analogous to knocking in spark-ignited engines.)
PFS with gasoline. In a newly published review of fundamental phenomena affecting low temperature combustion, Saxena and colleague Iván D. Bedoya noted that:
Although PFS has been shown to be effective for highly φ-sensitive fuels with two-stage ignition, there are two limits which constrain the usefulness of PFS. First, when overly delayed DI timing is used in PFS, distinctly separate ignition events can occur between the premixed charge and the DI charge. Under these conditions, the maximum pressure rise rate of the premixed charge ignition will constrain the operating limits. Second, when too high a DI fuel fraction is used in PFS, the heat release rate of the DI portion can be too high and/or high local temperatures may lead to excessive NOx formation.
The ability of using PFS with gasoline fuel has been a recent subject of study, and at naturally aspirated conditions PFS is ineffective for reducing ringing since gasoline is not highly φ-sensitive at these operating conditions. Interestingly, as the intake pressure is increased in gasoline HCCI engines, gasoline becomes highly φ-sensitive since more intermediate temperature heat release (ITHR) reactions occur. Thus, at high load conditions which utilize high boost pressure, PFS can be used with gasoline fuel to reduce ringing and allow higher efficiency.
The potential for using PFS to lower heat release rates has also been studied with other fuels. PFS with [neat] ethanol shows no performance improvements because ethanol is not φ-sensitive, however other fuels like hydrobate and iso-pentanol demonstrate significant improvements with PFS. However, the φ-sensitivity behavior of these fuels with changing intake pressures is different from gasoline, therefore the engine control strategy when using PFS will have to adapt to the fuel properties.—Saxena and Bedoya (2013)
Saxena said that although neat ethanol shows no performance improvements with PFS, gasoline, however, is very equivalence ratio sensitive at high pressures. Lower blends of gasoline and ethanol (i.e. E10) are also equivalence ratio sensitive at high pressures. Performance for higher ethanol fractions—e.g., E85—is still a question.
In the review, they also noted that another strategy allowing higher load is spark-assisted HCCI (SA-HCCI). SA-HCCI exploits the slower heat release of flame propagation to avoid the excessive pressure-rise rates that lead to ringing. SA-HCCI allows substantially improved load over HCCI without requiring intake pressure boosting, and provides other benefits such as improved low load operation and improved cycle-resolved control of combustion timing.
Facilitating further high load operation will require better understanding of the fundamental phenomena occurring at high load conditions, particularly when using engine boosting. For instance, better understanding is required of the chemical kinetic processes leading to intermediate temperature heat release in certain fuels and better explanations are required to understand the reported qualitative observations that fuels with LTHR or ITHR tend to exhibit more φ-sensitivity. For spark-assisted HCCI, a clear gap in the literature exists in understanding the complex interacting phenomena for boosted SA-HCCI operation.
As ringing is one of the principal constraints that dictate the high load limits, better fundamental understanding is required of this phenomena. Research efforts to improve the ringing intensity correlation are underway, and better quantitative understanding of how ringing impacts heat loss is required.—Saxena and Bedoya
µWASP. Researchers at UC Berkeley have been investigating the use of microwave assisted spark plugs (µWASP) over a number of projects. A 2011 SAE paper by Saxena and colleagues explored the extension of the lean stability limits of gasoline-air mixtures using a microwave-assisted spark plug, for example. Ongoing work in the UCB Combustion Modeling Lab is investigating the ignitability range of fuel-air mixtures with an eye to facilitating ignition in high-pressure mixtures.
In 2010 in a presentation at the DOE’s DEER conference, Dr. Dennis Assanis, then at the University of Michigan, presented results of investigations by a university consortium on high pressure, lean burn (HPLB) engines, including and exploration of the benefits of a µWASP approach. (The new project can be thought of as an extension to the work of this previous HPLB consortium, Dr. Saxena noted.)
Using an igniter from Imagineering, Inc., the researchers found that stability was improved near the lean limit (φ ~ 0.65). Under spark-assisted compression ignition (SACI) conditions (high preheat) µWASP should permit more dilute operation and wider range of control, they suggested.
The current project may take the team into the SA-HCCI regime, but that’s not necessarily the case, said Dr. Saxena.
It’s entirely possible that all the mixture will be burned via flame propagation (as opposed to with compression ignition) for all the operating points that we explore. It’s tough to say until we run the experiments as to whether we’ll venture into the SA-HCCI regime.—Samveg Saxena
Samveg Saxena, Iván D. Bedoya (2013) Fundamental phenomena affecting low temperature combustion and HCCI engines, high load limits and strategies for extending these limits, Progress in Energy and Combustion Science, doi: 10.1016/j.pecs.2013.05.002
V. H. Rapp and et al. (2013) Extending Lean Operating Limit and Reducing Emissions of Methane Spark-Ignited Engines Using a Microwave-Assisted Spark Plug. Journal of Combustion, vol. 2012, no. 927081, p. 8 pages doi: 10.1155/2012/927081
A. DeFilippo, S. Saxena, V. Rapp et al. (2011) Extending the Lean Stability Limits of Gasoline Using a Microwave-Assisted Spark Plug, SAE Technical Paper 2011-01-0663 doi: 10.4271/2011-01-0663
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