Effects of Diluents on Knock Rating of Gaseous Fuels

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587 Effects of diluents on knock rating of gaseous fuels S O Bade Shrestha and R Rodrigues Department of Mechanical and Aeronautical Engineering, Western Michigan University, Kalamazoo, Michigan, USA The manuscript was received on 16 November2007 and was accepted after revision for publication on 29 May2008. DOI: 10. 1243/09576509JPE554 Abstract: Concerns on energy security, emissions, and the recent hike in the price of fossil fuels have prompted the rapidly growing interest in the use of various alternative and renewable fuels, including low heating value fuels such as land-filled gases, biogases, coal bed methane gases, and others. Generally, the low heating value (btu) fuels contain substantial amounts of diluents such as carbon dioxide, nitrogen, water vapour, and other trace gases in the fuel composition. The present contribution describes the results of the investigation of knock in a single-cylinder variable compression ratio (CR) spark-ignition engine fuelled with gaseous fuels, such as natural gas, methane, and hydrogen, in the presence of different amounts of diluents, specifically carbon dioxide and nitrogen, in the fuel mixture in order to represent closely the general composition of land filled and biogases in practice. The knock characteristics of the fuels were quantitatively evaluated in terms of the methane number using various methods. Generally, the addition of either diluent carbon dioxide or nitrogen in the fuel mixtures augmented the knock resistance characteristics extending the engine operational limits. With every 10 per cent increase of carbon dioxide in the fuel mixture, the CR was increased by one point, whereas for a 25 per cent of nitrogen content in the fuel mixture, the CR was augmented by a half point in the operating conditions considered. Keywords: landfill gases, biogases, knock rating, low-btu gases, alternative fuels, methane number 1 INTRODUCTION Knock in spark-ignition engines occurs when the fuel–air mixture in the end gas region ignites ahead of the flame front that originates from the spark of the spark plug. As a result, multiple flame fronts collide creating a shock wave [1] that reverberates in the combustion chamber, which thereby creates a characteristic metallic ‘pinging’ sound. The high pressure waves JPE554 © IMechE 2008 Proc. IMechE Vol. 222 Part A: J. Power and Energy

Transcript of Effects of Diluents on Knock Rating of Gaseous Fuels

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587

Effects of diluents on knock rating of gaseous fuelsS O Bade Shrestha∗ and R RodriguesDepartment of Mechanical and Aeronautical Engineering, Western Michigan University, Kalamazoo, Michigan, USA

The manuscript was received on 16 November2007 and was accepted after revision for publication on 29 May2008.

DOI: 10. 1243/09576509JPE554

Abstract: Concerns on energy security, emissions, and the recent hike in the price of fossil fuels have prompted the rapidly growing interest in the use of various alternative and renew -able fuels, including low heating value fuels such as land-filled gases, biogases, coal bed methane gases, and others. Generally, the low heating value (btu) fuels contain substantial amounts of diluents such as carbon dioxide, nitrogen, water vapour, and other trace gases in the fuel composition. The present contribution describes the results of the investigation of knock in a single-cylinder variable compression ratio (CR) spark-ignition engine fuelled with gaseous fuels, such as natural gas, methane, and hydrogen, in the presence of different amounts of diluents, specifically carbon dioxide and nitrogen, in the fuel mixture in order to represent closely the general composition of land filled and biogases in practice. The knock characteristics of the fuels were quantitatively evaluated in terms of the methane number using various methods. Generally, the addition of either diluent carbon dioxide or nitrogen in the fuel mixtures augmented the knock resistance characteristics extending the engine operational limits. With every 10 per cent increase of carbon dioxide in the fuel mixture, the CR was increased by one point, whereas for a 25 per cent of nitro gen content in the fuel mixture, the CR was augmented by a half point in the operating conditions considered.

Keywords: landfill gases, biogases, knock rating, low-btu gases, alternative fuels, methane number

1 INTRODUCTION

Knock in spark-ignition engines occurs when the fuel–air mixture in the end gas region ignites ahead of the flame front that originates from the spark of the spark plug. As a result, multiple flame fronts col-lide creating a shock wave [1] that reverberates in the combustion chamber, which thereby creates a char-acteristic metallic ‘pinging’ sound. The high pressure waves resonance in the cylinder is heard as a distinct metallic ‘pinging’ sound. The resonance frequencies of these pressure oscillations are usually between 4 and 10 kHz, depending on the engine application [2].

Knock is a phenomenon that is affected by various factors such as spark timing (ST), compression ratio

∗ Corresponding author: Department ofMechanical and Aeronau-

tical Engineering, Western Michigan University, Parkview Cam-

pus, 4801 Campus Drive, Kalamazoo, MI 49008, USA. email:

[email protected]

(CR), equivalence ratio, fuel type, mixture temperature,

end gas temperature, combustion chamber pressure and volume, heat transfer, and others. Knock is exten-sively researched as it is a major barrier to achieving higher thermal efficiency and increased power output in spark-ignition (SI) engines. Knock due to auto- ignition causes abrupt pressure changes in the cylinder and generates extreme temperature and pressure spikes due to rapid combustion of the gaseous mixture. Auto-ignition in an SI engine can cause potential damage to pistons, piston rings, connecting rods, head gaskets, bearings, spark plugs, and cylinder heads [3].

1.1 Knock rating

In spark-ignition engines, knock is a harassing trait characterized by a pinging sound. Knock results in loss of engine efficiency, performance, increased emis-sions, and potential damage to engine components. The resistance of the fuel to the incidence of knock is a critical factor in the consideration of fuel selection,

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and consequently an experimental procedure for rating fuels was established through the use of octane number (ON), methane number (MN), and the less widely used butane number (BN) [4]. Hence, knock rating refers to a numerical classification of motor fuel antiknock characteristics that is established empiri-cally with a specialized set of operating conditions. ON is typically used to characterize the knock resis-tance quality of gasoline, whereas MN or BN is used for the characterization of gaseous fuels.

1.1.1 Methane number

Gaseous fuels, such as natural gas, landfill gas, or bio-gas, when utilized to run in an internal combustion (IC) engine instead of gasoline or diesel can create entirely different variations in knock parameters. MN is defined as a measure of the resistance of a gaseous fuel to auto-ignition (‘knock’) when ignited in an SI engine. The reference fuels utilized to establish the MN are methane and hydrogen, where 100 per cent methane equals ‘100 MN’ and 100 per cent hydrogen equals ‘0 MN’ [5].

1.2 Knock intensity

To accurately determine the MN of a specific fuel blend, an established measure is required to quan-tify the onset and intensity of knock to identify a knock condition. This parameter is acknowledged as the knock intensity, and the empirical value employed as the threshold to define a knock condition varies from authors to applications and it is subjective.

2 EXPERIMENTAL SET-UP

An ASTM-CFR (cooperative fuel research) engine was utilized for knock investigation. This engine is approved by the ASTM and is specifically designed and extensively used throughout the world for research and testing of liquid fuels for the IC engines. The most important features of this type of engine are a single- cylinder, a variable CR, a variable ST, and a constant speed [6]. The ST can be varied over a wide range from 40◦ before top dead centre (BTDC) to 40◦ after top dead centre (ATDC). The engine geometric details are given in Table 1. The engine runs at constant speed of 600 r/min. The fuel under research can be tested at various CRs (4: 1–16:1) and various STs.

The engine utilized was originally designed to research and conduct experimental tests on liquid fuels. For the current research, the engine intake and fuel delivery system had to be modified to allow for the ability to operate on gaseous fuels.

Table 1 CFR engine details [7]

Make Waukesha

Compression ratio 4:1–16:1Cylinder bore 82.55mm (3.25 in)Stroke 114.3mm (4.5 in)Connecting rod length 254mm (10 in)Displacement volume 0.611l(37.33 in3

)

2.1 Metering panel

The air metering panel was designed separate from the fuel and diluent metering panel as the air flow control requirements were distinct. Air flow control consisted of an electronic mass flowmeter to mea-sure the intake flow and a 0.075 m3 (20 gal) surge tank (Fig. 1) to reduce the high-intensity pressure pulsations generated during engine operation.

The fuel and diluent metering panel was developed to safely and accurately meter the desired mixture ratios at the engine intake. The panel consisted of four sets of electronic mass flowmeters to regulate the desired concentrations of four gases, especially methane, hydrogen, carbon dioxide, and nitrogen. The Omega FMA series and TSI 4000 mass flowmeters uti-lized were specifically calibrated for the individual gases. Additionally, the panel consisted of flashback arrestors, gas filters, regulating needle valves, pressure gages, check valves, and a mixing manifold (Fig. 1).

2.2 Data acquisition

The electronic mass flowmeters were coupled to the engine data acquisition system and monitored via software. This set-up allowed the accurate delivery of fuel–air mixture to the engine intake. The data acqui-sition system was also utilized to analyse and monitor various other engine related parameters such as crank angle, in-cylinder pressure, intake temperature, and exhaust temperature.

A BEI model HS35 incremental optical rotary encoder mounted on the crank shaft of the engine was utilized to record the crank angle data. The encoders’ disc resolution of 4096 enabled data logging of in- cylinder pressure transmitted by the Kistler model 7061B flush-mounted pressure transducer for every 0.088◦ of crank angle. All devices were connected to the National Instruments SCB-68 terminal block which was coupled to the computer using a PCI-MIO-16e-4 data card and Labview software as the data acquisition interface.

3 EXPERIMENTAL PROCEDURE

Biogas and landfill gas composition varies with timeand geographical location. Therefore, in order to

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Fig.1 Schematic diagram of experimental set-up

closely represent the landfill and biogas compositions, experiments were conducted for various compositions of fuel mixtures and different percentages of carbon dioxide and nitrogen (Table 2). The experiments were performed to determine the effect of carbon dioxide and nitrogen on knock characteristics of the fuel mix-ture, as it accounts for a significant portion of the diluents in these fuels.

To investigate the onset of knock in the presence of diluents, the knock limited spark timing (KLST) and knock limited compression ratio (KLCR) had to be determined with different volumetric composi-tions of diluents. This involved the variation in ST for KLST while keeping all other parameters such as equivalence ratio, CR, and intake temperature con-stant. Unless otherwise specified, the KLST values were determined for a constant CR of 12:1, equiva-lence ratio of 1.0, and an intake temperature of 303 K. The ST was varied from 40◦ BTDC to 40◦ ATDC, which is based on the fuel–diluent composition and operating conditions. All the tests were conducted with full open throttle.

Table 2 General composition of landfill gas [8]

No. Gas Volume (%)

1 Methane 45–602 Carbon dioxide 40–603 Nitrogen 2–54 Oxygen and other NMOCs <1

Similarly, the CR was varied for KLCR while keep-

ing all other parameters such as equivalence ratio, ST, and intake temperature constant. Unless otherwise specified, the KLCR values were determined for a con-stant ST of 13◦ BTDC, equivalence ratio of 1.0, and an intake temperature of 303 K with full open throt -tle. The CR was varied from 4.5 to 16.0 for the various fuel–diluent compositions and operating conditions until the predetermined knock intensitywas achieved.

The initial sets of experiments conducted were to determine the baseline knock characteristics, specif-ically KLST and KLCR of methane and hydrogen of MN 0, 20, 40, 60, 80, and 100. The baseline was criti-cal to the determination of the optimum ST and CR to cover the range of ST and CR parameters to conduct useful analysis. With the baseline data generated, the knock parameters were further investigated with the presence of 5, 10, 15, 20, 25, 30, 40, and 50 per cent CO2 or N2.

3.1 Knock detection

The accurate determination of the MN of a fuel relies on a reliable and repeatable knock detection method. Several methods exist in industry and research that can be employed, such as human ear, engine vibra-tions, and in-cylinder pressure, which are some of the more common methods. Additionally, less sparingly used knock detection methods are ion current sensing and wall thermal losses [9].

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Unlike peak pressure data acquisition, setting up a knock detector is challenging as it requires high-end transducers capable of high-frequency pressure res-olution. Also, a high level of expertise in frequency analysis and acoustic mode detection is required to separate knock from other common engine noises.

For the current research and with the experimen-tal set-up, the in-cylinder pressure knock detection method was adopted as a feasible option. After

conducting a significant literature survey stating the direct link between knock and pressure oscillations [4, 5, 7, 10–13], the in-cylinder pressure knock detec-tion method was adopted as shown in Fig. 2 as it provides a quantifiable metric not always seen in other methods.

The knock was detected and quantified by using an algorithm developed by Checkel and Dale [ 2, 14], which calculates the first, second, and third

Fig. 2 Average of 100 cycle pressure curve for knock and non-knock at compression ratio 14, sparktiming 25◦ BTDC, intake temperature 303 K, intake pressure 98 kPa, and 600 r/min

Fig. 3 First derivative of the average of 100 cycle pressure curve for knock and non-knock con -ditions at compression ratio 14, spark timing 25◦ BTDC, intake temperature 303 K, intake pressure 98 kPa, and 600 r/min

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Fig. 4 Second derivative of the average of 100 cycle pressure curve for knock and non-knock conditions at compression ratio 14, sparktiming25◦ BTDC, intake temperature 303 K, intake pressure 98 kPa, and 600 r/min

Fig. 5 Third derivative of the average of 100 cycle pressure curve for knock and non-knock con -ditions at compression ratio 14, spark timing 25◦ BTDC, intake temperature 303 K, intake pressure 98 kPa, and 600 r/min

differential of in-cylinder pressure (Figs 3 to 5), and then in addition verified by the audible pinging sound heard during a knock condition. It is a known param-eter that during a knock condition the in-cylinder pressures rapidly rise to significantly higher levels, providing a large positive curvature in trace diagrams followed by a narrow pressure peak with a large negative curvature (Figs 3 and 4). Since the second dif-ferential represents the curvature of the signal, a rapid

change from positive to negative curvature would be associated with a large negative value of the third dif-ferential (Fig. 5) that can be directly related to end gas ignition [2, 14].

The knock indicator algorithm used was a cubic spline fit differentiator, which uses nine points around a centred point to calculate each point’s derivative as a local slope of the best fit line [2]. As with any digitized data, a certain degree of inherent noise always exists,

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Fig. 6 Third derivative of one (1) actual cycle displaying a knock condition at compression ratio 12, spark timing 10◦ BTDC, intake temperature 303 K, intake pressure 98 kPa, and 600 r/min

and therefore the knock indicator algorithm also included a low-pass filter to attenuate low-frequency noise.

The 100 cycle in-cylinder pressures were then acquired for knock and non-knock conditions and were used to calculate the respective third derivatives using the knock indicator algorithm. The resulting third derivative data were then filtered using a low- pass filter and plotted against crank angle to determine the negative amplitude. With the knock indicator algorithm functioning, the knock threshold value had to be determined and validated to ensure accurate and repeatable results.

3.2 Knock validation

A knock condition was validated using the knock indi-cator algorithm by running a number of iterations to determine the maximum negative amplitude of the third derivative of pressure in a normal non-knock, slight-knock, moderate-knock, and heavy-knock con-dition run. This information was then coupled with the audible pinging noise of knock and was used to establish the threshold value for a knock condition per cycle, which was determined to be −750 kPa/◦CA3. To validate the knock analysis, displayed in Fig. 6, is an audibly confirmed knock cycle exceeding the threshold value which depicts a knock condition.

Also, in order to acquire repeatable and accurate results, a knock limit of 20 per cent was chosen arbi-trarily to define a condition of knock after conducting series of experiments and confirming through hearing of pinging noise for the engine employed. Therefore, if

within the 100 consecutive cycle run, 20 values exceed the threshold value, the run would constitute as being a knock condition. Figure 7 displays graphically such an instance in a run containing 100 cycles where such a condition exists.

To further validate the knock indicator, data were generated for six consecutive (100-cycle) runs by operating the CFR engine at an established parame-ter, which would provide a 20 per cent knock limit. Analysing the data, a knock variation from 20 to 29 per cent was observed (Fig. 8) with a 14 per cent coefficient of variation of knock occurrence, which is

Fig. 7 A 20 per cent of knocking cycles displayed from a 100 cycle run in the CFR engine with methane operation at compression ratio 12, spark tim-ing 10◦ BTDC, intake temperature 303 K, intake pressure 98 kPa, and 600 r/min

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Fig. 8 Knock variation per six consecutive runs in the CFR engine with methane operation at com-pression ratio 12, spark timing 10◦ BTDC, intake temperature 303K, intake pressure 98 kPa, and 600 r/min

acceptable due to the cyclic variations and the non- predictive nature of combustion itself. The variation can also be attributed to the minute fluctuations in the flow of gaseous fuel from the high-pressure cylinders into the engine intake. Furthermore, as the number of knocking cycles increased in a run, the heavier was the knock with a loud pinging noise. Depending upon the levels of the pinging noise, knock could be classified as a slight knock, a moderate knock, or a heavy knock condition, which was somewhat subjective.

4 RESULTS AND DISCUSSION

The first sets of experiments conducted were ori-ented towards determining how knock varies with ST. The ST is a critical factor in preventing engine knock and also reducing power loss, gas consump-tions, and emissions. Using 100 per cent methane as a fuel at CR 14 and an equivalence ratio 1.0, the ST was varied in a range that would provide condi -tions of no knock to 100 per cent knock occurrence. The third-order polynomial trend line was employed to accurately reflect the knock occurrence as the ST was gradually advanced, which displays that by advancing 12◦ CA, the engine knock occurrence pro-ceeded from a knocking-free condition to 100 per cent knocking conditions (Fig. 9). Unlike CR, ST can be easily manipulated and therefore modern day vehi-cles feature an electronically control spark advance which is designed to maximize engine efficiency. The spark advance can have a significant effect on performance and also on hydro-carbon and nitro-gen oxide emissions, and it is not necessary that the optimum ST will coincide with the minimum emissions.

Fig. 9 Knock limit versus spark timing for 100 MN oper-ations at compression ratio 14, equivalence ratio 1.0, intake temperature 303 K, intake pressure 98 kPa, and 600 r/min

The knock limited equivalence ratio (KLEQR) for binary mixtures of CH 4 and H2 displays that for a constant CR and spark advance, as volumetric concentration of H2 was increased, the onset of knock was observed at significantly lower equiva-lence ratios (Fig. 10). The fast flame propagation of H2 is responsible for creating high temperature at the end region causing detonation and there -fore drastically decreasing the KLEQR [15]. It can also be observed that with the addition of small amounts of H2 to CH4, the operation limit is sig-nificantly reduced. These data generated are con-sistent with the results reported by Karim and Li [15], which show a similar trend when operated at CR 12, ST 12◦ BTDC, intake temperature 311 K, and 900 r/min.

Fig. 10 Variation of KLEQR with fuel composition for binary mixtures of CH4 and H2 at compres-sion ratio 12, spark timing 10◦ BTDC, intake temperature 303 K, intake pressure 98 kPa, and 600 r/min

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4.1 Knock limited ST

The variations in KLST for binary mixtures of hydro-gen and methane were determined in the presence of varying percentages of diluents. Two sets of data were generated for the addition of the individual dilu-ents, which are carbon dioxide and nitrogen. The KLST results at a constant CR of 12:1 are displayed in Figs 11 and 12. With the addition of either diluent the KLST decreases for increasing percentage of H2 additive to a point, spark retarded beyond 40◦ ATDC, which was the maximum limit at which the ST could be varied on this experimental set-up.

Generally, the addition of incombustible diluent advances the KLST point since a part of the fuel com-position is replaced by the diluent, thereby, lowering

the heating value of the fuel and also due to the change in overall concentration of the fuel, which affects the chemical kinetics. Therefore, under con-ditions of higher diluent, greater spark advance would be required to incite a knock condition compared with a fuel composition without diluents. It can be noticed from Figs 11 and 12 that with the addition of CO2 dilu-ent, a 2–3◦ CA advance in KLST is observed while N2

diluent addition yielded a 1◦ CA advance for every 10 per cent increase in the diluent concentrations for a given mixture composition when evaluated against the baseline curve.

When KLST is plotted with respect to either dilu-ent (CO2 or N2 ) in the fuel and diluent mixture it can be observed that the KLST advances 20–45◦ CA for incremental CO2 addition depending on the MN

Fig.11 Variation of KLST for binary mixtures of CH4 and H2 in the presence of varying percentage of CO2 diluent at constant compression ratio of 12:1, intake temperature 303 K, intake pressure 98 kPa, and 600 r/min

Fig. 12 Variation of KLST for binary mixtures of CH4 and H2 in the presence of varying percentage of N2 diluent at constant compression ratio of 12:1, intake temperature 303 K, intake pressure 98 kPa, and 600 r/min

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Fig. 13 Variation of KLST with varying percentage of CO2 diluent for 0–100% H2 (100–0 MN) fuel at constant compression ratio of 12:1, intake temperature 303 K, intake pressure 98 kPa, and 600 r/min

Fig. 14 Variation of KLST with varying percentage of N2 diluent for 0–100% H2 (100–0 MN) fuel at constant compression ratio of 12:1, intake temperature 303 K, intake pressure 98 kPa, and 600 r/min

Fig. 15 Variation of KLCR for binary mixtures of CH4 and H2 in the presence of varying percentages of CO2 diluent at constant spark timing of 13◦ CA BTDC, intake temperature 303 K, intake pressure 98 kPa, and 600 r/min

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Fig. 16 Variation of KLCR for binary mixtures of CH4 and H2 in the presence of varying percentages of N2 diluent at constant spark timing of 13° CA BTDC, intake temperature 303 K, intake pressure 98 kPa, and 600 r/min

(Fig. 13). Similarly, incremental N2 addition can only provide about a 6–23° CA increase in KLST based on a particular MN (Fig. 14), which is due to the relatively low impact of the diluent on the fuel mixture that was mixed at the intake manifold of the engine with air that is composed of 79 per cent N2. The higher impact of CO2 can be attributed to its higher heat-ing value and it may also participate in chemical dissociation reaction. The lower the MN the greater the KLST increase was observed for either diluent. In both cases, the KLST span figures are determined from a range of 100–20 MN (0–80 per cent H2) beyond which at 0 MN insufficient data were available due to heavy knock and a KLST value beyond 40° CA ATDC.

At 0MN operation, CO2 diluent presented KLST advancement, while even 40 per cent N2 diluent provided no improvement. Readers should note the missing curve for 0 MN fuel with N2 addition, which was beyond the experimental limits of 40° CA ATDC (Fig. 14).

4.2 Knock limited CR

The variations in KLCR for binary mixtures of hydrogen and methane were also determined in the presence of varying percentages of diluents. The KLCR results at a constant ST of 13° CA BTDC as per the ASTM method are displayed in Figs 15 and 16. Similar to the KLST experiments, two sets of data were generated for the individual diluents, which are carbon dioxide and nitrogen.

For binary mixtures of CH4 and H2 only, the KLCR decreases for increasing percentage of H2 as a fuel additive. The decrease in KLCR can be accounted to the fast flame propagation of H2, which is responsible

Fig. 17 Variation of KLCR with varying percentage of CO2 diluent for 0–100% H2 (100–0 MN) fuel at constant spark timing of 13° BTDC, intake tem-perature 303 K, intake pressure 98 kPa, and 600 r/min

for the high temperature at the end gas region known to cause detonation. As either diluent is added and the percentage of CO2 or N2 is increased, the KLCR is increased for both0 and 100 MN fuels (Figs 15 and 16). The addition of diluent to the binary fuel mixture reduces the combustion temperature as it acts as a heat sink being an incombustible gas and the end gas chemical reaction rates. Furthermore, the diluent CO2

might dissociate under these conditions, increasing its heat sink effects in comparison to N2, which is essentially not active.

From Fig. 17, it can be asserted that for every 10 per cent increase of CO2 in the fuel diluent mixture, the CR was increased by a value of one point up to 20 per cent of CO2 and increased even more drasti-cally with the addition of higher percentage of CO2. Similarly, with the addition of up to 25 per cent N2

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diluent, the CR was increased only by a value of 0.5, beyond which the increase was significantly higher (Fig. 16). The higher effectiveness of CO2 addition is attributed to the higher heating value of CO2 and its likely participation in the chemical reactions.

As it can be seen in Fig. 17 for pure H2 mixtures, the KLCR could be found for higher CO2 contents otherwise unattainable at these operating conditions. It clearly showed that, as mentioned earlier, CO2 acted as a knock suppression agent by reducing the temper-ature of end gas and altering the chemical kinetics in the end gas region.

5 CONCLUSION

An addition of either diluent extended the operating limits of a particular fuel composition in the SI engine employed. The CO2 diluent was very effective to allow controlled combustion at lower MN fuel compositions that ordinarily would have a high knock. The N2 diluent also displayed a similar tendency but at a lower effec-tiveness level than the CO2 diluent. With the addition of CO2, the KLCR can be increased significantly for the range of fuels from 0 to 100 MN. It was found that for every 10 per cent increase in CO2 concentration in fuel diluent mixtures, the CR was increased by one point at the operating conditions considered. Similarly, with the addition of up to 25 per cent N2 diluent, the CR was increased only by a half. It was also observed that the KLST advanced by 1–3◦ CA for CO2 and about 1◦ CA for N2 for every 10 per cent increase in the diluent concentration in fuel diluent mixtures at these operating conditions. Additionally, it was found that increase of either diluent concentration in pure H2 operations enabled increasing KLCR and advancing KLST in the operating conditions employed.

These data can be applied to engine applications that utilize landfill gas, biogas, or H2 as a fuel for oper-ation, so that the CR and/or ST can be increased to achieve greater efficiency.

ACKNOWLEDGEMENTS

The authors would like to thank Pete Thanhauser and Guruprasath Narayanan for their support provided in building the experimental set-up and data acquisition implementation.

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