Abstract
We review developments in the understanding of cycle-to-cycle variability in internal combustion engines, with a focus on spark-ignited and premixed combustion conditions. Much of the research on cyclic variability has focused on stochastic aspects, that is, features that can be modeled as inherently random with no short-term predictability. In some cases, models of this type appear to work very well at describing experimental observations, but the lack of predictability limits control options. Also, even when the statistical properties of the stochastic variations are known, it can be very difficult to discern their underlying physical causes and thus mitigate them. Some recent studies have demonstrated that under some conditions, cyclic combustion variations can have a relatively high degree of low-dimensional deterministic structure, which implies some degree of predictability and potential for real-time control. These deterministic effects are typically more pronounced near critical stability limits (e.g. near tipping points associated with ignition or flame propagation) such during highly dilute fueling or near the onset of homogeneous charge compression ignition. We review recent progress in experimental and analytical characterization of cyclic variability where low-dimensional, deterministic effects have been observed. We describe some theories about the sources of these dynamical features and discuss prospects for interactive control and improved engine designs. Taken as a whole, the research summarized here implies that the deterministic component of cyclic variability will become a pivotal issue (and potential opportunity) as engine manufacturers strive to meet aggressive emissions and fuel economy regulations in the coming decades.
| Original language | English |
|---|---|
| Pages (from-to) | 366-378 |
| Number of pages | 13 |
| Journal | International Journal of Engine Research |
| Volume | 16 |
| Issue number | 3 |
| DOIs | |
| State | Published - Apr 27 2015 |
Funding
In the internal combustion engines of interest in this review, the historical record shows that both stochastic and deterministic effects on cyclic variability are always present. Depending on the conditions studied, the literature has two sets of distinct data and conclusions, and both explanations can be correct. Stochastic variations are always present, but deterministic features (i.e. cycle-to-cycle memory) tend to be visible only when operation is near some type of global transition point (tipping point) and when heat, mass, or momentum feedback opportunities are significant (e.g. with high residual gas or EGR). Thus, it should not be surprising that studies where combustion was far from extinction (e.g. not dilute) and/or involving no significant feedback of species or heat would not be able to detect significant levels of determinism. Both physical mechanisms can be important over wide ranges of operation in modern engines, so it is always important when evaluating or planning experimental studies of CV to recognize how close the operation is to a tipping point and also what the opportunities are for feedback. Fortunately, progress in identifying and characterizing determinism in engine dynamics over the past few decades affords the opportunity to control the engine dynamics to mitigate CV effects. In the short term, coarse-graining techniques such as symbolization and low-dimensional modeling hold promise for on-board, real-time interactive control, but practice will increase sophistication of these tools. As engine operation is increasingly pushed into less stable combustion regimes to meet aggressive emissions and performance targets, increased understanding and complexity of control strategies will be expected or required. Feedback control to exploit deterministic effects near unstable boundaries should be an integral part of engine and vehicle design in coming years. Declaration of conflicting interests The authors declare that there is no conflict of interest. Funding The aspects of this work have been sponsored over the years by the Vehicle Technologies Office, Office of Energy Efficiency & Renewable Energy, US Department of Energy, Gurpreet Singh, Ken Howden, Leo Breton, managers. This article has been authored by UT-Battelle, LLC, under Contract No. DE-AC0500OR22725 with the US Department of Energy. The US Government retains and the publisher, by accepting the article for publication, acknowledges that the US Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this article, or allow others to do so, for the US Government purposes.
| Funders | Funder number |
|---|---|
| US Department of Energy | |
| UT-Battelle | |
| Office of Energy Efficiency and Renewable Energy | |
| Vehicle Technologies Office |
Keywords
- Cyclic variability
- complex systems
- dilute combustion
- nonlinear dynamics