Endoscopic Combustion Visualization for Spatial Distribution of Soot

Article pubs.acs.org/EF

Endoscopic Combustion Visualization for Spatial Distribution of Soot and Flame Temperature in a Diesohol Fueled Compression Ignition Engine Yeshudas Jiotode and Avinash Kumar Agarwal* Engine Research Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India ABSTRACT: A variety of research studies have been investigating the compatibility of primary alcohols as alternative fuels for diesel engines, in view of depleting crude oil reserves and resulting environmental degradation. Pollutants are formed in the engine combustion chamber and are mostly controlled using exhaust gas after-treatment technologies. However, active characterization of harmful pollutants such as soot and oxides of nitrogen (NOx) can be done in situ, at the locations of their formation in the engine combustion chamber by employing optical diagnostic techniques. In this study, the high temperature industrial endoscopy technique was adapted to a single cylinder diesel engine for in situ optical diagnostics of engine combustion for diesel and diesohols (diesel−alcohol blends) such as E20 and M20 fuels. In-cylinder combustion images were acquired by the endoscopic system and processed further for spatial soot and flame temperature distributions. Spatial distribution of in-cylinder soot was evaluated from “R intensity” values, and flame temperature distribution was evaluated by the correlated color temperature (CCT) method. The image distortions because of endoscopic lens were removed for the first time in this study. Luminous regions in the endoscopic images indicated soot radiations and therefore the locations where the soot is formed inside the combustion chamber, whereas the soot radiation temperature represented the flame temperature for diesel combustion. Both diesohol blends showed superior characteristics in terms of combustion and emissions compared to baseline mineral diesel.

1. INTRODUCTION Compression ignition engines are known for their popularity in the transportation, agriculture, and automotive sectors because they are mechanically strong, and durable and economical in terms of fuel consumption. However, harmful emissions of NOx and soot from diesel engines have severe adverse impacts on the human health and the environment. NOx produced in the engine combustion chamber is predominantly responsible for smog, acid rain, and ground level ozone, while soot exposure promotes cardiovascular diseases.1−3 Taking into account the increased rate of environmental degradation due to such harmful engineout pollutants, globally emission legislation to control these emissions is day by day becoming stringent. Petroleum origin fuels are the largest source of transportation energy, but highly volatile supplies and escalating costs have prompted researchers to explore decisive countermeasures for the future. Biofuels such as vegetable oils, biodiesel, primary alcohols, etc. have attracted the attention of researchers to fulfill the demand for cleaner energy and environment. Among these alternative fuels, primary alcohols such as ethanol and methanol can be used in compression ignition (CI) engines by blending up to 20% (v/v) with mineral diesel;3−6 these fuels are called “diesohols”. Both ethanol and methanol possess several physical and combustion properties similar to mineral diesel (Table 1), and that is why their blends with diesel can be used directly in CI engines without any significant engine design/hardware change. However, lower energy density remains one of the major drawbacks of alcohols. Methanol and ethanol have much higher octane ratings and evaporation rates. Their blends in diesel engines result in lower soot and NOx emissions due to higher fuel oxygen content and lower heating value compared to baseline mineral diesel. Emission species generated by alcohol fueled © XXXX American Chemical Society

engines have lower reactivity in the atmosphere and produce lower amounts of ozone, which is a precursor for smog formation. These are renewable fuels and can be produced from various carbon based agricultural feedstocks, coal, sugarcane, biomass, waste products, etc., which are abundantly available throughout the world.7,8 It has been reported that some of the toxic compounds, such as formaldehyde, acetaldehyde, etc., are emitted in higher concentrations by alcohol fueled engines.9,10 Both methanol and ethanol blends have shown satisfactory results for regulated emissions compared to baseline mineral diesel. Particulate matter (PM) and NOx emissions reduced significantly from diesohols compared to baseline mineral diesel.11−15 However, ethanol emitted higher NOx compared to methanol. CO and CO2 also reduced when using these alcohols blended with diesel in CI engines.11,14−19 However, emission of hydrocarbons (HC) was higher for both methanol and ethanol blends compared to baseline mineral diesel.11,14,15,20 Zhang et al. (2010) investigated regulated and unregulated emissions from a fourcylinder, diesel engine fueled by diesel−methanol blends.15 Significant reduction in emissions of NOx, particulate mass, particulate number, ethylene, ethane, and 1,3-butadiene in raw engine exhaust were observed, whereas HC, CO, NO2, BTX (benzene−toluene−xylene), unburned methanol, formaldehyde, and organic fractions in particulates increased with increasing methanol content in the blends. This literature review suggests that the blending percentage of alcohol should be within 20% (v/v) for superior combustion and emission characteristics of the unmodified engine. These studies Received: July 13, 2016 Revised: October 4, 2016

A

DOI: 10.1021/acs.energyfuels.6b01585 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Important Properties of Test Fuels22

a

test fuel

formula

enthalpy of vaporization (kJ/kg)

lower heating value (MJ/kg)

ON/CNa

oxygen content (% w/w)

methanol ethanol diesel

CH3OH C2H5OH C10H20 to C15H28

1098 838 349

20.08 26.82 44.8

99 (ON) 100 (ON) 40−55 (CN)

49.93 34.73 −

ON, octane number; CN, cetane number.

Figure 1. Schematic of the experimental setup.

pilot injection of liquid diesel with natural gas.25 Other researchers used the technique to study the effect of changing engine parameters on ignition sites, fuel−air mixing, and intake air flow patterns for different fuels.26−30 In this paper, visualization of in situ combustion of diesohols (methanol and ethanol blends with diesel) was accomplished by applying an endoscopic visualization technique to a customized single cylinder diesel engine. Recorded images were corrected for image distortions due to curved lenses and then analyzed for time-resolved in situ soot and flame temperature distributions so that efforts can be made to reduce the formation of pollutants in the engine combustion chamber.

characterize engine-out bulk emissions of promising alternative fuels such as ethanol and methanol blended with diesel; however, in order to understand the mechanisms and conditions responsible for production of these pollutants in situ in an engine combustion chamber, optical diagnostics techniques are essential. In the field of internal combustion (IC) engines, optical research engines and industrial endoscopes are the only possible tools for investigating in-cylinder combustion. Limited operating range in terms of engine load and speed restricts the use of optical research engines;21 therefore, endoscopy is the best solution for optical diagnostics of engines across a wide range of engine operating conditions, from low load to full loads, from low speeds to high speeds. Endoscopes can be installed with ease in an engine without any major engine modifications. Initially, endoscopes were used for surgery and investigation of internal organs of the human body. Later, with their use for visual inspection of complex mechanical parts, researchers in the field of IC engines began using them for engine investigations. Many researchers have used engine endoscopy for investigating injection parameters, air flow inside the combustion chamber, and in-cylinder combustion. Alam et al. (2005) investigated spray characteristics of diesel and diglyme blends with diesel using endoscopic visualization.23 Jeon et al. (2014) visualized combustion in a single cylinder engine, fueled by dimethyl ether (DME), and investigated temperature distribution and fuel−air mixture formation.24 Similarly, Mtui et al. (1996) investigated combustion duration and ignition delay for

2. EXPERIMENTAL SETUP For the present investigations, a four-stroke single cylinder, naturally aspirated compression ignition engine (DAF 10, Kirloskar) was suitably modified to install an endoscopic system for visualizing in-cylinder combustion of diesel and diesohols (E20 and M20) fuels. Figure 1 shows the schematic of the experimental setup. During the tests, engine speed was maintained constant at 1500 rpm (rated speed of the engine) and varying loads (0, 1.5, and 3.0 kW) were applied with the help of a resistive load bank, which was connected to the engine via an ac alternator. The engine cylinder head was modified and a piezoelectric pressure transducer was mounted flush with the cylinder head in order to record the combustion chamber pressure vis-à-vis piston position in the stroke. A charge amplifier was used to convert the electrical charge signal generated by the piezoelectric pressure transducer to a proportional voltage signal. A high speed combustion data acquisition system (MeDAQ; Hi-technique) recorded the time-resolved in-cylinder B


Article

Energy & Fuels Table 2. Specifications of the CCD Camera maximum frame rate exposure/shutter time pixel resolution (h × v) pixel size (h × v) shutter mode

7.3/13.5 fps (12/25 MHz, normal) 11.7/21.6 fps (12/25, center) 5−60 μs 1392 × 1040 pixels (normal) 800 × 600 pixels (center) 6.45 × 6.45 μs global (snapshot)

Figure 3. Start of combustion and combustion duration variations with engine load. It was not possible to capture the entire combustion event in a single engine cycle due to the limited frame rate of the camera. During investigations, it was assumed that combustion in every cycle would be quite similar for any given engine operating condition and there will not be cycle-to-cycle variations. An Arduino microprocessor based circuit was used for triggering the camera after a 0.5° crank angle degree position in every fifth combustion cycle. A suitable trigger pulse was generated by the camera control circuit, which triggered the camera for capturing the image. Optical access to the combustion chamber was gained by the 70° rigid industrial endoscope, which has provision for a cooling air supply, which helps it sustain even in high temperature conditions. Dry air at 6 bar was supplied continuously through the cooling passages in the endoscope, which removes the heat gained by the optics via radiation and conduction and helps cool the endoscope and maintain its temperature within safe design limits. Additionally, the viewing tip of the endoscope was protected from the high temperature and pressure environment inside the engine combustion chamber by an optical window, made of high quality quartz, which can sustain maximum pressure up to 60 bar, thus protecting the endoscope. The CCD camera was mounted at the other end of the endoscope, i.e. the ocular of the endoscope with the help of a C to F mount. The camera was aligned with the field of interest (near the injector tip) for each engine operating condition, prior to the start of the experiment by illuminating that region with the help of an external light source, which shines the light onto the region of interest in the combustion chamber. The images captured were distorted because of the curvature of the optical window and curved optical lens arrangement in the endoscope, which needs to be corrected. Captured images were therefore corrected using GIMP and ImageJ software, which removed spherical aberrations [distortions caused by endoscope lenses] in order to evaluate the captured images correctly.31 Corrected images were then processed in MATLAB using standard algorithms and a model given in the open literature (explained in section 3) for evaluating time-resolved in situ soot and flame temperature distributions for the test fuels under different engine operating conditions. Taking into account the safety of the optical window, endoscopic experiments were performed at 0, 1.5, and 3.0 kW engine loads.

Figure 2. In-cylinder pressure and HRR vis-à-vis crank angle position. pressure history data to evaluate the combustion characteristics of the test fuels under varying engine operating conditions. Several modifications were done in the engine cylinder head in order to gain optical access in to the combustion chamber. Two holes were drilled in the cylinder head at appropriate locations: one for inserting the endoscopic access system into the combustion chamber and the other for making access to illuminating light from a light source into the engine combustion chamber. The combustion chamber area near the fuel injector was chosen as the field of interest because combustion occurs mostly in this zone. The endoscopic access system consisted of a charge coupled device (CCD) camera, a 70° rigid endoscope, an quartz optical window, protection and pressure sleeves, a C to F mount for a connector lens, and the cooling air system for the endoscope. Combustion events were captured using a CCD camera having a limited frame rate of 13.5 fps (Table 2) and were stored in the computer using Camware software.

3. RESULTS AND DISCUSSION The optical diagnostics of a single cylinder CI engine was performed for diesel, E20, and M20 using the endoscopic visualization technique. The engine was operated at the rated C


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Energy & Fuels

Figure 4. In-cylinder combustion visualization for the three test fuels at different crank angle positions.

speed of 1500 rpm and variable loads (0, 1.5, and 3.0 kW), while maintaining a constant fuel injection pressure of 200 bar.

Soot formed inside the combustion chamber in situ and spatial distributions of flame temperature were evaluated from the D


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Energy & Fuels

Figure 5. In-cylinder soot concentration for the three test fuels at different crank angle positions.

endoscopic combustion images captured; the results were validated using in-cylinder pressure data. 3.1. In-Cylinder Pressure and HRR Investigations. Figure 2 represents the in-cylinder pressure and heat release rate (HRR) with respect to the crank angle position for diesel, E20, and M20, obtained at different engine loads.

Both in-cylinder pressure and HRR increased with increasing engine load for all test fuels. The initial part of combustion takes place in premixed charge mode and the later part of the combustion is in mixing controlled mode in a compression ignition engine. Rapid heat release in the premixed combustion zone was due to increased injection fuel quantity injected at E


Article

Energy & Fuels higher engine loads, which also led to higher in-cylinder pressure and peak HRR values. E20 showed higher in-cylinder pressure compared to baseline mineral diesel. Alcohols are oxygenated fuels. Ethanol and methanol contain 34.73 and 49.93% (w/w) oxygen in their molecular structures, respectively. This inherent oxygen in alcohols helps in faster combustion and heat release compared to baseline mineral diesel, especially in the premixed combustion mode. This could be the reason for observing the highest HRR in the case of E20 in the premixed combustion mode. Despite having the highest percentage of oxygen in the case of methanol, M20 showed the lowest in-cylinder pressure and HRR in the premixed combustion phase for a given engine operating condition. Tutak et al. (2015) reported that the mass of methanol injected had to be higher than that of ethanol due to its lower heating value.32 On the other hand, the enthalpy of vaporization of methanol (1098 kJ/kg) is higher than that of ethanol (838 kJ/kg), which results in a reduction of the overall in-cylinder temperature as obtained in the case of M20. Therefore, the cooling effect due to vaporization of M20 was significantly higher compared to that of E20, resulting in lower in-cylinder pressure and HRR for M20. HRR was higher for M20 in the diffusion combustion phase possibly due to the same cooling effect of methanol. 3.2. Start of Combustion and Combustion Duration Investigations. In this study, the crank angle position at which 10% mass burn fraction (MBF) occurs is considered as the start of combustion (SOC) and the crank angle position at which 90% MBF occurs is considered as the end of combustion (EOC). It was observed that SOC (Figure 3) was 1−1.5 CAD (crank angle degrees) and 2−3 CAD earlier for E20 compared to baseline mineral diesel and M20, respectively, except at no load. Being oxygenated fuels, ethanol and methanol burn relatively earlier than mineral diesel. However, combustion in the case of M20 was observed to be delayed because of the excessive cooling experienced in the case of methanol, since its latent heat of vaporization was highest among all test fuels. With increased engine load, SOC advanced by 1−1.5 CAD for all test fuels. The reactivity of the fuel−air mixture increased with increasing engine load due to higher temperature of fuel−air mixtures, which in turn reduced the ignition delay and advanced the SOC. The crank angle duration between SOC and EOC is considered as the combustion duration (CD). It increased for all test fuels with increasing engine load. E20 showed the highest CD, followed by baseline mineral diesel and then M20. The earlier the SOC, the higher was the CD. 3.3. In-Cylinder Combustion Visualization. Figure 4 shows the in-cylinder combustion events inside the engine combustion chamber for the three test fuels, captured at different piston positions (after every 5 CAD from 5° before top dead center (BTDC) to 30° after top dead center (ATDC)) by the endoscopic access system. The illuminated regions in the field of view, which emit light in the visible wavelength range, were captured by the CCD camera. The images were then corrected for spherical aberrations, as indicated earlier. CI engines are known for heterogeneous threedimensional (3-D) combustion, and this behavior was clearly observed in the endoscopic combustion images captured in this study. Combustion started at several ignition sites, and no flames were observed for any of the test fuels. It was observed that flame luminosity for all test fuels increased with increasing crank angle and then decreased after attaining maxima near the TDC position. The flame luminosity was not very high in the beginning of combustion due to largely premixed combustion.

Figure 6. Average in-cylinder soot concentration vis-à-vis crank angle position.

This was followed by mixing controlled combustion, wherein heterogeneous combustion dominated and combustion luminosity increased, which was visible in the images captured. It can be stated from the in-cylinder pressure curves and endoscopic combustion images that flame luminosity has a direct correlation with in-cylinder pressure as well. Flame luminosity increased with in-cylinder pressure and vice versa. Luminosity in the combustion images increased with increasing engine load for all test fuels, because at higher engine loads, more test fuel quantity was injected into the combustion chamber, and a greater fraction of this fuel underwent mixing controlled combustion (Figure 2). Among the test fuels, baseline mineral diesel showed the highest flame luminosity, followed by E20 and M20 at any given engine operating condition. This was due to the fact that the presence of oxygen in the fuel molecules enhances premixed combustion and the light emitted during the premixed combustion is not in the visible wavelength range. This trend is also observed in the HRR data (Figure 2). 3.4. Spatial Soot Distribution. The spatial distribution of in-cylinder soot was evaluated from distortion-free combustion images in MATLAB. Huang and Zhang33 suggested that the “R intensity” value (from the RGB matrix) at any pixel of an image, where the hue number (for every RGB combination of image, there is a unique corresponding hue number) lies between 1 and 80°, expresses the soot-induced digital coloration and represents the soot particle concentration at that location.33−35 Figure 5 shows contours of “R intensity”, which represent the concentration of soot within the visualized area of the engine combustion chamber. White regions in a contour indicate the presence of thick soot clouds (highest concentration of soot). Spatial distribution of soot and its in situ concentration inside the combustion chamber increased with increasing engine load because of higher fuel quantity injected at higher engine loads. F


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Energy & Fuels

Figure 7. In-cylinder flame temperature distribution for the three test fuels at different crank angle positions.

of M20 was the lowest, followed by E20 and baseline mineral diesel. Although M20 has the highest oxygen content (% w/w), it showed a higher soot concentration than E20 at no load, possibly because M20 cannot burn completely due to greater cooling effect and higher latent heat of vaporization of methanol.

Diesel showed higher spatial soot concentration, followed by E20 and M20. Soot was generated inside the combustion chamber, when there was incomplete combustion due to localized fuelrich regions. Since diesohols have higher fuel oxygen content (% w/w) than baseline mineral diesel, the in situ soot distribution G


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Energy & Fuels

for M20. Flame luminosity and soot radiation temperature (which represents flame temperature) for M20 were also lower than for E20. In summary, endoscopic combustion analysis was performed for diesohols compared to baseline mineral diesel, which showed the in situ formation of soot and high temperature zones in the engine combustion chamber.

To investigate the in-cylinder soot distribution quantitatively, the average soot concentration was evaluated from combustion images at each crank angle. Figure 6 shows that the average soot concentration increased for all test fuels with increasing engine load. Baseline mineral diesel showed the highest soot concentration among all test fuels at any given engine operating condition. 3.5. Spatial Flame Temperature Distribution. The spatial distribution of the flame temperature was evaluated in MATLAB using the correlated color temperature (CCT) method.36−38 Combustion in a CI engine is primarily dominated by radiation emitted by soot particles, formed inside the engine combustion chamber.39 Soot particles emit light of different colors, depending on their surface temperature. Hence, CCT evaluated from soot radiation closely represents in situ flame temperature distribution in a CI engine combustion chamber. Figure 7 shows the spatial distribution of flame temperature for all test fuels evaluated at different crank angle positions. The area of CCT contours first increased and then decreased for all test fuels at any given engine operating condition. CCT contours have the highest area near the TDC, where combustion flames cover the maximum area inside the engine combustion chamber. With increased engine load, soot covered a higher area inside the combustion chamber, which is why the area bound by the contours of CCT also increased. As oxygenated fuels, diesohols (M20 and E20) produced relatively lower soot inside the combustion chamber, compared to baseline mineral diesel, and therefore, radiation from soot particles (which represents the combustion flame temperature) and the area bound by CCT contours were also relatively lower.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91 512 2597982. Fax: +91 512 2597408. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS In-cylinder combustion visualization was performed in a single cylinder CI prototype engine, fueled by diesohols (M20 and E20) and baseline mineral diesel at a constant engine speed of 1500 rpm and variable engine loads (0, 1.5, and 3.0 kW), using the endoscopy technique. Results obtained from in-cylinder pressure data and endoscopic visualization were analyzed and the following conclusions can be drawn: • E20 was superior compared to M20 and baseline mineral diesel in terms of different combustion characteristics. E20 showed the highest in-cylinder pressure and HRR in the premixed combustion phase. • Due to a higher latent heat of vaporization, M20 showed a greater charge cooling effect prior to combustion. This resulted in retarded SOC and relatively higher HRR in the diffusion combustion phase, leading to lower peak in-cylinder pressure. Despite having delayed SOC, the combustion duration was relatively lower for M20. This indicated that M20 spray droplets were relatively more difficult to ignite; however, once they ignite, they undergo rapid combustion. • Combustion images acquired using endoscopic access clearly showed that CI engine combustion was initiated at several ignition sites, unlike spark ignition engines, and there is no flame front present in the combustion chamber. • Soot luminosity in the case of baseline mineral diesel was higher compared to diesohols. Subsequent analysis for spatial soot and temperature distribution indicated higher in situ soot distribution and flame temperature distribution for baseline mineral diesel compared to diesohols. • Among the two diesohol fuels, in situ soot formation characteristics for M20 were superior to those for E20. Soot produced inside the combustion chamber was relatively lower H


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Endoscopic Combustion Visualization for Spatial Distribution of Soot

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