The Homogeneous Charge Compression Ignition (HCCI) and Spark Induced Compression Ignition (SICI) combustion processes constitute advanced operating modes for automotive engines implying both high thermal efficiency and low NOx and soot emission thanks to the combustion of lean or diluted mixtures featuring a relative homogeneity.
They are part of the different strategies involved in Low Temperature Combustion (LTC) engines, and present as well some similarities with the MILD combustion concept.
Spark induced compression ignition (SICI) appears as a relatively new combustion control technology and a promising combustion mode in gasoline engines with high efficiency.
Spark assistance operates by initiating a reaction kernel before the main combustion event; the propagating flame then consumes a portion of the charge and releases fuel energy so the remainder of the charge auto-ignites earlier than it would have otherwise.
The control of self-ignition mechanisms implied in such operating modes as well as the limitation of the maximum heat release rates constitute major challenges for practical applications to engines.
Rapid Compression Machines (RCM) are relevant devices to study such modes of combustion.
Those featuring large optical accesses – like the one used in the present study – provide ideal conditions for studying both aerothermal and combustion processes by means of optical diagnostic methods.
For instance, thermal heterogeneities generated during the compression of inert mixtures were characterized by using Planar Laser Induced Fluorescence (PLIF) measurements on the toluene molecule.
The same device was also employed to study HCCI combustion of light or heavier hydrocarbon-air mixtures.
Several studies were devoted to the delineation between combustion regimes, called mild and strong ignition, during HCCI combustion in such devices, see for instance.
These regimes are closely related to the local mode of combustion (e.g. autoignition front or deflagration) and further distinctions between these regimes were proposed in depending on the influence of mixing on the reactivity gradients before auto-ignition.
The coexistence of deflagration and auto-ignition is also inherent to SICI combustion mode.
In this framework, optical diagnostic methods are specifically developed in the present work to analyze the HCCI and SICI reactive phenomena from a local point of view.
If knocking combustion was the subject of past RCM studies, this is not the case of SICI combustion.
The work of Boumehdi et al. can be cited here, as the authors investigated the effect of nanosecond surface dielectric barrier discharges (SDBD) with different voltages on the auto-ignition of mixtures containing methane and butane in a RCM.
In particular, no significant effects of the discharge were reported below a threshold value applied to the high voltage electrode and auto-ignition occurred in these conditions.
For higher voltage values, flame propagation was evidenced by the authors, but it did not lead to the auto-ignition of the end gas.
Therefore reported combustion mode was probably more representative of SI (Spark Ignition) combustion than SICI.
As the combustion mode is different, and since SICI results are provided below with the use of an innovative – high energy – igniter, the present work provides new results and is complementary to the study of Boumehdi et al.
The case of premixed isooctane – air mixtures, which are relevant to both Spark Ignition (SI) and SICI applications, is under consideration here.
Self-ignition is achieved in a two-stage process in the investigated conditions.
After compression stroke, a first rise in pressure is observed coupled with very low light emission, which can only be observed by using an intensifier device.
This cool flame is the prelude to the main stage of combustion, e.g. hot ignition.
Considering that (i) the post-compression temperature fields feature heterogeneities and that (ii) self-ignition mechanisms exhibit strong temperature dependence, a better understanding of the various aerothermochemical processes requires multidimensional measurements.
Optical diagnostics are employed to this end considering that they must be applicable to the severe conditions reached in the present study, i.e. about 700 K and 2.7 MPa after the end of the compression stroke, before the onset of heat release.
Formaldehyde is a combustion intermediate species, which is formed in the first stage of hydrocarbon oxidation.
Since formaldehyde is completely consumed during hot ignition, it is well-suited as a tracer species of cool flame regions.
In previous studies, detection of formaldehyde was performed in diffusion flames using laser-induced fluorescence (LIF) by excitation of the strong A-X401 vibronic band at 355 nm.
Brackman et al. studied the overlap between the laser profile and the absorption lines of formaldehyde, saturation effects and the potential occurrence of laser-induced photochemistry and applied the technique to an internal combustion engine.
Collin et al. proved its efficiency when detecting formaldehyde emissions at the start of low temperature reactions in an HCCI Engine and enhanced the detection of more formaldehyde as the low temperature reactions progressed.
Kim and Gandhi monitored formaldehyde PLIF signal in the case of a light load HCCI combustion with various air-fuel equivalence ratios and found that formaldehyde was uniformly formed within the combustion chamber during the first-stage ignition.
In order to characterize the flame structure and its transient behavior, Hultqvist et al. used high-speed chemiluminescence imaging enabling time resolved observation of light emissions in an engine where cool flames were found to exist some 20 crank angles degrees before Top Dead Center (TDC) using intensified high-speed camera.
Different studies aimed at the simultaneous measurement of formaldehyde and OH signals within HCCI engines, see for instance where formaldehyde traces cool flame regions while OH traces the principal heat release.
It has been shown that OH is strongly correlated to high temperature chemistry of premixed combustion.
It is as well detected in the burned gases at high temperatures.
One may notice other research works with more academic devices correlated the local heat release rate to the convolution of OH and formaldehyde signals obtained by fluorescence within counterflow premixed laminar flames.
In this work, the ignition and combustion phases for both HCCI and SICI modes are investigated with single or double-pulse planar laser-induced fluorescence on formaldehyde combined with chemiluminescence imaging.
The first objective is to analyze the phenomenology of HCCI processes in the presence of two stage ignition with a fuel highly resistant to auto-ignition, in complement to previous works.
The second objective is to investigate SICI combustion mode in severe conditions, e.g. close to the limits of existence of this mode.
A very lean mixture and a high energy igniter are used for this purpose.
The aim is to analyze the topology and dynamics of reaction fronts, as they are closely connected to the heat release rate.
The results are part of an experimental database devoted to HCCI/SICI combustion and may be useful for the assessment of combustion models used in numerical simulations.
The study focuses firstly on phenomenological aspects with the prospect of complementary information from both diagnostics performed from early to late stages of the both HCCI and SICI combustion processes.
Instantaneous 2D distribution of formaldehyde is compared with corresponding integrated chemiluminescence signal.
The first one is necessary to analyze the end of the combustion process, while chemiluminescence is found particularly efficient to monitor the onset of high temperature reactions.
The phenomena are analyzed at the light of velocity and temperature measurements reported in a previous study.
In a second phase, a more quantitative approach is considered through the estimation of apparent velocities of reactive fronts.
Homogeneous Charge Compression Ignition (HCCI) and Spark Induced Compression Ignition (SICI) of a lean iso-octane air mixture are investigated through simultaneous measurements of planar laser-induced fluorescence at 355 nm and high-speed chemiluminescence in the parallelepipedic combustion vessel of a rapid compression machine (RCM).
The fluorescence images show the presence of intermediate species produced during the cool flame with a significant contribution of formaldehyde.
These species are consumed at the high temperatures characteristic of either hot ignition or flame propagation.
This two dimensional diagnostic method brings useful information about the location and shape of reactive fronts, and is efficient until the latest instants of combustion.
By contrast, high-speed chemiluminescence enables time resolved investigation of these reactive fronts with spatial integration along the combustion chamber depth.
Furthermore, the first ignition kernels can be detected as the full volume of the chamber is visualized after TDC.
The coupling of these two diagnostics provides complementary information about the transient three dimensional combustion processes.
Phenomenology of HCCI combustion is investigated by these means: original results were reported through temporal evolution of PLIF images during the first stage of ignition.
They put into evidence the first reactive phenomena are driven by the effect of thermal stratification on cool flame pre-reactions: initiated in the hot zone, the cool flame then propagates within the initially colder region.
For short ignition delays, fast consumption of formaldehyde is observed during the second stage of ignition.
It is associated with a steep pressure rise.
For long ignition delays, relatively slow propagation of wrinkled reactive fronts is reported along with a smooth pressure rise.
In agreement with previous studies, the rate of pressure rise, e.g. the heat release rate, is strongly correlated to the local mode of combustion: deflagration or auto-ignition fronts.
This combustion mode depends on the ignition delay, as the pre-ignition temperature field results from the unsteady large scale aerodynamics and turbulent mixing.
Finally, RCM is found to be an interesting tool to analyze HCCI combustion mode for lean mixtures featuring a low fluctuation level of ignition delays.
In particular, the combustion process and the related temperature distribution observed for long ignition delays present some similarities with those encountered within engines.
By contrast, the existence of hot zones at TDC with very low gradients is more specific to RCMs and affects the combustion process for short ignition delays.
However, the front velocities are representative of practical applications, and finally this provides a unique opportunity to study auto-ignition front phenomena with a temperature distribution which is well controlled and more academic than in HCCI engines.
The combined diagnostics provide as well essential information for the phenomenological analysis of SICI combustion in severe conditions, e.g. for a very lean mixture at high pressure.
A high ignition energy level of 305 mJ was required for the flame kernel to resist to post-compression conditions, and in particular to the induced aerodynamics.
This value was also chosen to obtain a significant decrease in the auto-ignition delays of the end gas.
This makes the specificity of the investigated SICI conditions.
The interaction between three phenomena is put into evidence in these conditions: (i) the flame consumes a pre-reacted gaseous mixture, (ii) the flame front, initially laminar, is highly affected by the large-scale post-compression aerodynamics and in particular by corner vortices, and (iii) it becomes progressively wrinkled by turbulence.
Such phenomena influence the flame surface and its propagation velocity, and consequently the heat release rate, which is a key parameter for these combustion modes.
As evoked above, flame propagation is followed by the auto-ignition of the end gas.
In comparison to HCCI results, SICI experiments feature lower maximum values of the rate of pressure rise, for similar ignition delay values.
This tends to illustrate the relevance of SICI combustion for practical applications.
Large zones of formaldehyde disappear near the piston at the end of combustion process, and fast motions of light emission are detected on chemiluminescence records.
This confirms the occurrence of fast auto-ignition fronts at the last stage of SICI combustion process, which is in agreement with the relatively high value of the recorded rate of pressure rise at these instants.
In the HCCI case for short ignition delays, e.g. for a similar auto-ignition timing, fast fronts also propagate, leading to a higher rate of pressure rise.
They propagate in a large zone initially located at the top of the chamber.
These results suggest thermal gradients and their distribution in space represent a mean to control both the auto-ignition front velocity, and their area surface and thus heat release rate at the end of the SICI process, whereas the chemical composition, pressure and temperature significantly affect all the different stages of the SICI process.
From a more quantitative point of view, the cycle resolved analysis of reactive fronts by chemiluminescence enables estimating an apparent velocity at early stages of propagation.
The obtained values confirm the existence of two propagative modes, namely deflagration and auto-ignition in the HCCI cases at these instants.
Double-pulse 355 nm PLIF images confirm the propagation velocities are representative of deflagrations in the measurement plane at large ignition delays values in the HCCI case.
In conditions similar to the SICI case, this double frame diagnostic confirms the existence of fast autoignition processes during hot ignition of the end gas.
It suggests the presence of a more volumetric – but subsonic – consumption process, which is fairly different from the fast auto-ignition front propagating in HCCI mode for short ignition delays.
The gathered data are relevant for future assessments of combustion models for the simulation of HCCI/SICI combustion modes in lean conditions, and in particular in the presence of heterogeneities.
In future works, the phenomenology described here may be useful as well for the analysis of a larger experimental database devoted to SICI combustion.
In addition, the proposed double pulse formaldehyde PLIF technique coupled to fast chemiluminescence was found relevant to study the late stages of the combustion process, when complex three dimensional effects occur.
Such a time resolved planar diagnostic technique will be relevant for further parametric studies of the fast dynamics of auto-ignition fronts at these instants.
Reference: C. Strozzi, A. Claverie, V. Prevost, J. Sotton, M. Bellenoue, HCCI and SICI combustion modes analysis with simultaneous PLIF imaging of formaldehyde and high-speed chemiluminescence in a rapid compression machine, Combust. Flame. (2019) 58–77. doi:10.1016/j.combustflame.2019.01.002.