Traditionally, compression ignition (CI) engines are rich burn engines when compared to stoichiometric burning of air-fuel mixture in spark ignited engines.
Therefore, CI engines have the advantage of increased efficiency at the penalty of increased nitrogen oxide (NOX) and particulate matter (PM).
While the cost of the after treatment for CI engine is too expensive, researchers are making efforts for in-cylinder reduction of NOX and PM simultaneously.
Low temperature combustion (LTC) concepts are promising in reducing the emissions without compromising the efficiency.
Homogeneous charge compression ignition (HCCI) is one of the LTC concepts in which the vaporized fuel is well mixed with the in-cylinder air prior to combustion.
The resulting locally lean mixture suppresses the soot formation and in-cylinder temperature is lower, allowing to control the NOX formation.
However, it is very difficult to control the combustion phasing because the ignition timing is kinetically controlled.
Furthermore, HCCI combustion is challenging at high load due to increased pressure rise rate which makes it an ardent task to commercialize this combustion concept.
Given that the HCCI combustion concept is limited by the narrow operating range, another promising LTC strategy known as partially premixed combustion (PPC) was proposed.
PPC is intermediate between conventional CI combustion and HCCI wherein the fuel injection timing is earlier than CI combustion but later than HCCI combustion.
With PPC, combustion is stratified and the fuel-lean as well as the moderate fuel-rich zones reduce the NOX and PM emissions without compromising the efficiency.
In addition, the combustion phasing is controlled both by fuel injection timing and intake air temperature, though chemical kinetics still play an important role.
Therefore, there is an increasing attention on the investigation of PPC in recent years.
Kalghatghi and Johansson demonstrated gasoline PPC with early fuel injection timing and reported the advantages of lower emissions by utilizing lower exhaust gas recirculation.
They suggested that gasoline could be used for PPC as ignition delay is increased, which leads to increased premixed process and reduction of PM and unburned hydrocarbons (UHC).
PPC exhibits a mixture of premixed and diffusion flames typically initiated by the bulk auto-ignition.
As such, some local combustion pockets or high temperature zones are observed during combustion.
However, at low load operation, use of gasoline is problematic because of the overly lean mixture that leads to the misfire and combustion instability.
Recent research studies have found that low octane gasoline fuels are more suitable for PPC combustion.
In particular, research octane number (RON) 70 fuel is regarded as ideal choice and there have been a number of experimental investigations with primary reference fuel (PRF) 70 (70% iso-octane + 30% heptane) under PPC conditions, showing that auto-ignition kernels are found in the fuel rich regions and then the flame front propagates into the lean fuel regions.
FACE (Fuels for advanced combustion engines) are formulated recently and FACE I gasoline (RON = 70) was investigated in an optical engine at low load condition at HCCI and PPC combustion modes.
Computational fluid dynamics (CFD) simulations coupled with chemical kinetic reactions complement the experimental measurements by providing quantitative information and fundamental understanding on the fuel-air mixing process, fuel oxidation, auto-ignition processes, and pollutant evolution.
A numerical study investigated the ignition characteristics of PRF70 at PPC mode and reported two-stage ignition process at PPC mode.
The auto-ignition of stratified fuel/air mixture dominated the first stage ignition whereas the diffusion and chemical reaction both played an important role in second stage ignition [34,35].
For a mixed-mode combustion as encountered in PPC engines, identification and characterization of in-cylinder distributions between the premixed and nonpremixed combustion modes is important.
For this purpose, the Takeno flame index (FI), which has been commonly used in turbulent flames analysis, may prove to be useful in IC engine applications.
By definition, the flame index ranges from +1 to −1, and a positive (negative) value represents premixed (nonpremixed) modes.
The present study attempts to employ the flame index in engines operating in the HCCI and PPC modes as a means to investigate the effects of different combustion modes on the engine performance and pollutant formation.
Upon validation of full cycle simulations against full optical CI engine experiments using PRF70 fuels in HCCI and PPC modes, the simulation data are analyzed using FI to identify the spatial distributions of different combustion modes within the cylinder at different operating conditions.
Further attempts are made to relate the FI information with important combustion characteristics, such as the low and high temperature heat releases associated with the complex fuel chemistry through the investigation of the important intermediate species.
Full cycle engine simulations were conducted using the 3D CFD code CONVERGE to obtain a better understanding of the in-cylinder combustion process under HCCI and PPC modes of engine operation. A good agreement is achieved between the simulations and experiments. Overall, the experimental trends of combustion and engine-out emissions were satisfactorily reproduced by the current simulations. The major results and findings of the study are listed below:
- The experimental results of in-cylinder pressure, RoHR, CA10 and CA50 were satisfactorily reproduced in the simulations. The two stages of ignition behavior with LTR and HTR regions were evident in both experiment and simulation. The combustion phasing of CA50 was decoupled from the SOI at HCCI mode (zero slope of d(CA50)/d(SOI)) whereas CA50 was coupled with SOI at PPC mode (negative slope of d(CA50)/d(SOI)).
- The flame index could be used as an indicator for evaluating and quantifying the in-cylinder combustion development under HCCI and PPC engine operating conditions. Premixed flame and diffusion fuel/air mixtures co-existed in the reaction zones under PPC mode.
- The typical low temperature premixed combustion below 1600 K was achieved at HCCI mode with the earliest SOI of −180 CAD aTDC, while the PCCI mode with the in-cylinder temperature below 2000 K was reached with the SOI of −100 CAD aTDC where the diffusion index was nearly negligible. Both of the two LTC modes successfully avoided the formation of soot and NOx. The PPC mode with the in-cylinder temperature below 2500 K was achieved at the relatively late SOI of −40 CAD aTDC with the local equivalence ratio below 1, which generated only a low level of NOx emissions.
- Two peaks existed in the HO2 evolution profile corresponding to LTR and HTR, respectively. The formation of HO2 in LTR was stronger than that in HTR. HO2 could be used as an indicator for tracing the two-stage ignition phenomenon and evaluating the evolution of the premixed combustion together with the flame index.
In future research, the laser diagnostics will be used to obtain closer inspection of the LTR phenomenon by PLIF-CH2O together with the high-speed camera to obtain the in-cylinder distribution of OH radicals for the HTR region.
Reference: Y. An, M. Jaasim, V. Raman, F.E. Hernández Pérez, H.G. Im, B. Johansson, Homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC) in compression ignition engine with low octane gasoline, Energy. 158 (2018) 181–191. doi:10.1016/j.energy.2018.06.057.