Evaluation of an Octane Analyzer
THE OCTANE NUMBER associated with a motor fuel is a measure of the performance of that fuel. The number does not correspond to the concentration of any one constituent in the fuel, but rather to the preignition properties of the fuels as compared with standard fuel mixtures. The term “octane number” is derived from the fact that the standard fuels contain isooctane.
The ASTM Standard Procedures for determination of octane numbers require the use of two Waukesha co-operative Fuel Research (CFR) engines (Waukesha Engine Div., Dressier Industries, Inc., Waukesha, WI). The Research Engine runs at 600 rpm, with a controlled air mixture and temperature as defined in ASTM D 2699. The Motor Engine runs at 900 rpm, with a controlled air mixture and temperature and controlled fuel temperature as defined in ASTM D 2700. These engines, or “knock engines,” necessarily consume the fuel sample and a series of standard fuels during the test procedure.
The octane number of a fuel is defined by the compression at which the knock intensity of that fuel is 50% of the maximum knock intensity for the engine. Pure isooctane is defined as having an octane number of 100, while heptane is defined as having an octane number of 0. A series of standards containing heptane, isooctane, and toluene are used to define a scale between these two bounds. The research octane number (RON) of a fuel is the octane number measured on the Research Engine. Similarly, the motor octane number (MON) is the number determined by the Motor Engine. The octane number typically posted on the pump at a service station is the average of the RON and MON (called antiknock index [AKI], or [R +M]/2).
In a traditional test, gasoline samples are collected from various filling stations and brought back to the laboratory for testing on CFR engines. The testing is necessarily time consuming and expensive. For this reason, many states have begun using the ZX 101C to perform on-site octane screening. The analyzer makes octane determinations in seconds and eliminates the tremendous expense of CFR testing. An octane analyzer
The ZX 101C (Zeltex, Inc., Hagerstown, MD) is a portable, battery-powered octane analyzer for use with gasoline. It consists of three primary components: the analyzer, a sample container, and a light shield. The entire package in a carrying case weighs less than 5 kg. The instrument performs an octane number determination in less than I min and does not require the use of standard samples. The measurement is completely nondestructive.
The analyzer measures octane number via near-infrared (NIR) transmission spectroscopy. The instrument contains a patented solid-state optical system comprising 14 near-infrared emitting diodes (IREDS) with narrow bandpass filters, a silicon detector system, and a fully integrated microprocessor. Figure 1 shows a schematic representation of the analyzer. The sample holder is a scaled, flat-sided, reusable glass container with an optical pathlength of 75 mm. The sample volume is approx. 225 mL. To make an octane number determination, the user acquires a background signal from the empty sample chamber, measures the absorption spectrum of the sample twice, then acquires a second background signal. The entire process requires less than I min and can be performed by untrained, unskilled personnel.
NIR spectroscopy
The NIR spectral region is usually defined as that portion of the spectrum with wavelengths in the range 700-2500 nm. In this spectral region, overtone frequencies of molecular vibrations absorb light quite readily. Because the overtone absorption bands are typically wide and overlapping, spectroscopists cannot merely measure peak heights to perform quantitative analysis. Instead, multivariate regression analyses are utilized to correlate spectral features with concentrations or physical properties of interest.
The ZX101C operates in the short-wavelength NIR, from 800 to 1100 nm wavelength. The instrument is factory calibrated to predict octane number (RON, MON, and PON) from the absorption spectra of the fuels being tested. This prediction is accomplished through the use of a multivariate regression equation of the form: Octane number = K0 + K1 (OD1) + K2 (OD2)…+ K14 (OD14) + K15 (Ta) where K0 is a bias term, K1 through K15 are slope coefficients, OD1 through OD14 are the absorbencies measured at each of the 14 wavelengths, and Ta is the ambient temperature at the time of the test. The instrument can store up to 10 calibration equations and is factory calibrated for RON, MON, and PON of blended gasolines.
Analyzer features
The analyzer has several features that safeguard against erroneous readings: high variance warning – triggered if repeat measures for a given sample exceed some preset value; temperature warning – triggered if the analyzer is used outside of the recommended temperature range (15-45 degrees Celsius): out-of- range warning – triggered if the sample being tested is outside the calibration range of the analyzer, and curve checking – triggered if the optical spectrum of the current sample is atypical of gasoline samples stored in the unit. Limitations imposed by CFR engine testing Quantitative NIR spectroscopy is a secondary technique, in that results are generated through regression equations that model the output of some primary method of analysis. In the case of octane testing, the primary method of analysis is the CFR engine.
A secondary method can never show better accuracy than the precision of the primary method. This is true because accuracy cannot be determined beyond the bounds set by the primary method. In the case of CFR engine testing, the limitations are as summarized in Table 1. In this context, reproducibility describes the ability of an engine to generate the same result for a given’ fuel after being shut down, then restarted and used by a new technician. Repeatability describes the ability of an engine to generate the same result for a fuel sample in successive tests. The 95% confidence interval corresponds to roughly two times the standard deviation of measurement for large numbers of retests.
