The EC of removed water vs. microwave time is shown in Fig. 5(b). When microwave time was increased from 2 min to 12 min, the measured EC initially increased and then decreased, and the peak value 62.8 μS/cm appeared at a microwave time of 6 min. EC primarily depends on the ionic concentration of Cy7 NHS ester . EC of a solution with i kinds of ions can be calculated by the equation: K = ∑Aici, where Ai is the molar EC of “i” kind ion, and ci is the molar concentration of “i” kind ion. Thus, the EC of removed water was calculated using the main anions (SO42− and Cl−) and main cations (H+ calculated by pH, Na+, Ca2+, and K+) in removed water. Ai values of H+, SO42−, Cl−, Na+, K+, and Ca2+ were 349.7, 160, 76.3, 50.1, 73.5, and 119.0 S/cm2, respectively. The calculated ECs are shown in Fig. 5(b). The calculated ECs attributed to H+, SO42−, Cl−, Na+, K+, and Ca2+ were 30.46, 17.37, 4.73, 1.66, 1.37, and 0.95 μS/cm, respectively. The calculated EC had the same variation tendency as the measured EC with prolonged microwave time. However, the calculated EC accounted for 73–94% of measured values because contributions of some ions with little or trace amount (i.e., Mg2+, Fe2+, Al3+, and F−) and undetected ions to calculated ECs were not considered. Moreover, the largest biggest contributions of ions to EC came from H+, SO42−, and Na+ in removed water. Thus, EC mainly resulted from the initial increase and decrease in total ion concentration in removed water.Fig. 3. Comparing the power and thermal efficiency in gasoline and gas engines.Figure optionsDownload full-size imageDownload as PowerPoint slide
The higher thermal efficiency of the engine with natural gas compared to gasoline HA14-1 likely to be ascribed to the higher heating values of CNG as compared to gasoline (45 MJ/kg and 42.75 MJ/kg natural gas and gasoline respectively). Also, because of higher octane number of natural gas  and , advanced spark timing was used which led to improve the thermal efficiency. For gaseous fuel, MBT spark timing was used for all operation conditions, while in gasoline mode, to avoid knock, the spark timing was retarded. Spark timing for both cases are excretory system presented in Table 5.
Spark timing for gas and gasoline engine.Engine speed (RPM)Spark timing (BTDC)Gasoline (Knock limit)CNG (MBT)15001.819.820005.224.025009.024.0300010.825.8350013.525.8400014.628.1450016.130.0500018.031.8550020.631.9Full-size tableTable optionsView in workspaceDownload as CSV