A dial gauge was used to monitor the adjustable CR of the CFR engine. The dial gauge was mounted between the cylinder head and crankcase. Adjustment of the cylinder height was shown as a distance reading on the dial gauge, and could be related to a reference value for CR. A liquid fill method, as described in the ASTM manual for rating motor, diesel and aviation fuels, was used to establish a reference value for cylinder height and CR (ASTM, 1971).
The knock pickup sensor, mounted in the roof of the combustion chamber, was removed. A calibrated burette was used to fill the combustion chamber with 140 mL distilled water. With the piston at TDC, the CR of the CFR engine was increased, such that the meniscus of the water reached the flat top of the cylinder head. This process was repeated three times, and the dial gauge reading at this cylinder height was monitored. According to the ASTM manual for rating motor, diesel and aviation fuels, the dial gauge reading measured in this position, correlates to a CR of 5.496 (ASTM, 1971).
This measured relationship between cylinder height and CR was used as a reference point. The change in combustion volume correlating to a change in cylinder height was calculated using measurements for the bore and stroke of the CFR engine in Table 1. Figure 45 shows the calculated relationship between
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cylinder height, measured with the dial gauge, and combustion volume, filled with distilled water. The liquid fill method was used to verify the measured relationship at various points.
Figure 45: Liquid fill method results
CR was calculated from combustion volume using the swept volume of the CFR engine (bore and stroke measurements). Figure 46 shows the relationship between CR and dial gauge reading.
0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 VOLUME DIST ILL ED WAT ER [L]
DIAL GAUGE READING [inch]
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Figure 46: CR / Dial gauge relationship
The curve in Figure 46 was used during octane measurement to determine the CCR at which a fuel sample reaches a certain knock intensity. KI was measured using a detonation meter and displayed on an analogue KI gauge. The KI/CCR relationship was calibrated using iso-octane. Under RON standard ASTM operating conditions, the gauge was adjusted to reach mid-scale knock intensity, 50 ± 2 divisions, at a dial gauge reading of 0.454 inch, correlating to a CCR of 7.770 (ASTM D2699, 2016). This calibration was verified regularly during the testing phase of this project, and calibration data and test dates are included in this report in Appendix C. The characteristic curve of the CFR engine was used to relate CCR to the octane number of a sample fuel (CCR method, section 2.5.4.). This curve was calibrated following upgrades and modifications to the engine setup. Figure 47 shows the characteristic curve calibration. PRF blends (iso-octane and n-heptane), ranging in O.N. from 50 to 100, were tested under RON standard operating conditions.
4 5 6 7 8 9 10 11 12 13 14 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 COMPRE SSION RAT IO
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Figure 47: Re-calibrated CR curve
It was noted that the re-calibrated characteristic curve (Supplier 1) was not consistent with the curve used by Swarts (Swarts, 2006), before upgrades and modification of the engine. The curve used by Swarts (Swarts, 2006) is referred to as “Literature” in Figure 7. It was suspected that the primary reference fuels used for this calibration were contaminated. RF’s from a different supplier were purchased, and the calibration test was repeated (Supplier 2). Figure 47 shows that the characteristic curve produced by RF’s from Supplier 2 followed the original curve from literature more closely. It can be seen that the curve is lower, verifying the improved compression of the engine after upgrades and modification, shown in Figure 37. With increased compression, it is expected that a fuel sample with a certain O.N. will reach mid-scale KI at a lower CR. A sample of n-heptane from supplier 1 was analysed at the department of Process Engineering at Stellenbosch University. The sample was found to be 33 % pure, in contrast to the supplier specified purity of 99.7 %. A chemical analysis report is included in Appendix H. The characteristic curve produced by RF’s from
5 5.5 6 6.5 7 7.5 8 30 40 50 60 70 80 90 100 110 CO MPR ES SIO N R A TIO RON
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Supplier 1 was therefore discarded, and the characteristic curve produced with RF’s from Supplier 2 was used for octane measurement in this project.
The effect of different chemicals, and their respective combustion properties, on measured KI and octane results was investigated. Figures 48 and 49 show the measured CCR related to defined O.N. for a variety of fuel samples used in this project. The RON and MON values in Figures 48 and 49 respectively are by PRF blending definition, TSF blending definition or carefully scrutinised literature values from ASTM appendices (ASTM D2699, 2016; ASTM D2700, 2016).
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Figure 48: RON - PRF and TSF characteristic curve Figure 49: MON - PRF and TSF characteristic curve 5 6 7 8 9 10 11 40 60 80 100 120 140 COMPRE SSION RAT IO
RON BY PRF/ TSF DEFINITION OR LITERATURE VALUE
Toluene, Iso-octane, n-Heptane Toluene, n-Heptane Toluene, Iso-octane Ethanol, Iso-octane
Ethanol, Toluene Ethanol, n-Heptane
Iso-octane, n-Heptane 5 5.5 6 6.5 7 7.5 8 8.5 9 50 60 70 80 90 100 110 COMPRE SSION RAT IO
MON BY PRF/ TSF DEFINITION OR LITERATURE VALUE
Toluene, n-Heptane Toluene, n-Heptane, Iso-octane Ethanol, Iso-octane Ethanol, n-Heptane
Ethanol, Toluene
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One data point in Figure 49, at approximately 91 MON, is considered an erroneous outlier. It is shown in Figures 48 and 49 that data points overlap, and the characteristic curve, calibrated with PRF’s in Figure 7, is followed regardless of chemical composition.