It is no more customary to use conventional diesel fuel for CI diesel engines and gasoline for SI engines and engineers through modification and manipulation of fuel composition as emulsions develope new enhanced fuel chemistry, which can be responsible for extra power demand and emission control policies in a global scale.
In this framework, adding oxygenated fuels as a substitute with diesel sounds like a pragmatic method to achieve a desirable outcome.
Emulsification of different fuels is an idea to balance out the ratio of H–C–O in a blended structure of fuel to give the features of interest.
Methanol and Dimethyl ether (DME) are recognized as alcohols in the form of renewable power source in internal combustion engines (ICEs) with diverse features that can be replaced by diesel and control the reactivity of air-fuel mixture.
Basically, methanol has a very low cetane number and high ignition delay that can be compensated with the high cetane number of DME.
Dimethyl ether has low-ignition temperature and can be easily evaporated with high oxygen content and no C–C bonds contributing to smokeless combustion.
HCCI engines are a new generation of engine systems characterized by low NOx and high thermal efficiency inheriting the two main capacity of CI and SI engines where there is no direct controlling mechanism over the ignition timing such as injection or spark timing.
The use of blended fuels in this kind of engine is more highlighted in HCCI engines since it gives a leverage to determine the ignition timing by modification of alternative fuel fraction with higher/lower flash point thereby tuning the auto-ignition point.
The following lists a number of significant studies implemented on the scope of either blended fuels or alternative fuels application in HCCI engines.
Since the operation of HCCI is limited by the ability to control the extreme burning of the homogenous charge and avoiding knock, Li et al.
studied the knock phenomenon and cyclic variation within the HCCI engine with a blend prepared by n-butanol and n-heptane.
The obtained results indicated that the volume fraction of n-butanol can greatly influence the knock as an increase in butanol lessens the knock probability.
Researchers use blending as an outlet to harness the combustion phasing in HCCI ignition mode, for example, Turkcan et al.
used a blend of alcohol-gasoline blends in HCCI-DI engine to investigate the impact of second injection timing role on combustion and emission.
It was pointed out that increasing methanol fraction would raise the NOx and peak pressure (Pmax), while an increase of ethanol content works oppositely.
Zhang et al. in a study considered exergy losses of the auto-ignition process for DME and alcohol blended fuel.
The work indicated that the addition of methanol and DME increases the exergy losses by the H2O2 reaction.
Wang et al. considered the combustion of polyoxymethylene dimethyl ether (PODE) in an HCCI engine under various EGRs and charge mass equivalence ratios.The detailed data from the study showed that PODE lean HCCI combustion engenders ultra-low NOx and soot content, meanwhile, CO emission decreases with EGR.
Yousefzadeh and Jahanian used another alternative fuel in the HCCI engine and proposed compressed natural gas (CNG) to control the combustion phase.
Their main findings concluded that hydroxyl radical serves as a robust factor in combustion phasing determination leading to a yet better response time.
Nishi et al. were able to set a new HCCI combustion through EGR and engine speed variation.
The DME fuel was used in the engine and detailed chemical kinetics was incorporated with a single-zone model.
It was shown that higher EGR ratio is required to reach the quasi-steady state.
Khandal et al. also tried alternative fuels combustion in HCCI engine.
These alternative fuels were Honge biodiesel, cottonseed biodiesel, and hydrogen gas.
Their results showed that diesel/biodiesel fuel powered HCCI has 67% lower smoke and 99% lower NOx at 80% load, although 3.4% lower thermal efficiency was reported compared to CI mode.
In another recent and relevant study , diesel-biodiesel powered HCCI engine was tested that was designed for off-road applications.
In brief, the BMEP (brake mean effective pressure) of HCCI without EGR is lower than HCCI with EGR and the BMEP value is the highest for CI engine.
Recently, Yao et al. performed a critical review on the diesel-methanol application in CI engines.
It was summarized that methanol fumigation can decrease emissions in the diesel engine.
The methanol addition to diesel increases the radical pool and the radicals such as H2O2 require higher activation energy .
Exergy reviews in combustive systems such as engines draw more and more attention due to its potential in identifying the possible agents in reduction of waste energy.
In HCCI engines, there are records of exergy study on blended fuels where the secondary fuel fraction affects the second law efficiency and heat/work exergy.
From the above review, it can be acknowledged that in HCCI engines it is paramount that the technicians and experts can bring the ignition and combustion phasing under control.
So, adopting two alternative fuels with different flammability characteristics, which can substitute diesel by different ratios in a blend seems a logical resolution.
Furthermore, EGR introduction in HCCI is practiced to manage the high burning rate of HCCI homogenous charge combustion.
Moreover, new exergy terms are used to measure the feasibility of methanol and DME blending in diesel in HCCI mode.
The effect of addition of methanol and DME on diesel in HCCI mode combustion is investigated from the second law of thermodynamics view.
The numerical work is performed by AVL-FIRE CFD code and then the results of engine modeling are employed for exergy analysis within the framework of in-house developed code.
This is a rare work on HCCI engine control by varying DME/methanol on base diesel fuel, which gives flexibility over the ignition and combustion of the engine, meanwhile, a detailed exergetic performance is evaluated with advanced parameters as EGR and secondary fuel fraction changed.
It is of great interest to concoct and synthesize fuel blends that can cope with the demand to both increase the engine power and reduce the emission at the same time. This study investigated four fuel blends of D50M30DME20, D60M10DME30, D70M20DME10, and D80M20 in an HCCI engine for with/without EGR application. As the following remarks would confirm, D50M30DME20 is an ideal solution to boost the energetic performance, while D60M10DME30 dominates in terms of exergy performance and lower irreversibility:
i. The D50 powered HCCI engine with EGR = 20% has the maximum ID and minimum RPR, while increasing diesel fraction in blend decreases ID monotonically and increases RPR. When no EGR is used, D60 (representative of highest DME fraction in the blend) has both maximum ID and RPR. It shows that EGR alters the pattern of ID and RPR trends for proposed multi-component fuels.
ii. Due to higher cetane number of DME (∼60), the blend with the highest ratio percentage of DME (D60) represents the highest ID (4.546 CA) and RPR (3.177 bar/deg CA), which results in high HRR. These are owing to the low viscosity of DME and longer ID, which remarkably improves the mixture uniformity. When EGR is involved, the scale of intense burning rate for this blend is suppressed.
iii. The case of D60 has the lowest irreversibility rate and ranks second in exergy terms that leads to identifying it with the highest EPC. On the other hand, D80 with the highest portion of diesel fuel in its composition (80%), shows the worst energetic and exergetic performance among the blends.
iv. It was revealed that in engine performance terms (IMEP, IP, IT, efficiency), D50 among other blends is dominant in both 1400 rpm and 2000 rpm speeds. As known EGR would decrease the engine’s performance and efficiency and higher engine speed definitely increases the engine output power. The closure statement is that there found a potential candidate solution (i.e. D50 powered HCCI engine) that can simultaneously increase the engine efficiency and decrease the irreversibility.
References: H. Taghavifar, A. Nemati, J.H. Walther, Combustion and exergy analysis of multi-component diesel-DME-methanol blends in HCCI engine, Energy. 187 (2019) 115951. doi:10.1016/j.energy.2019.115951.