ASTM D2885 Determination of Octane Number of Spark-Ignition Engine Fuels
ASTM D2885 Standard Test Method for Determination of Octane Number of Spark-Ignition Engine Fuels by On-Line Direct Comparison Technique
10. Basic Engine and Instrument Settings and Operating Conditions
10.1 Standard Operating Conditions:
10.1.1 Installation of CFR Engine Equipment and Instrumentation - Place the CFR engine on a suitable foundation and hook up all utilities in accordance with the specifications of the engine manufacturer. Assemble the supplemental automated analyzer system and fuel delivery system components in accordance with the instructions of the system manufacturer. All installation aspects are to comply with local and national codes and installation requirements.
10.1.2 Proper operation of the CFR engine requires assembly of a number of engine components and adjustment of a series of engine variables to prescribed specifications. These settings and adjustments are specified in the CFR F-1 & F-2 Octane Rating Unit Operation & Maintenance Manual and in the Basic Engine and Instrument Settings And Standard Operating Conditions sections of Test Method D2699 or Test Method D2700, or both, and are of the following types:
10.1.2.1 Conditions based on component specifications (Annex A2 and A3 of Test Method D2699 or Test Method D2700, or both).
10.1.2.2 CFR engine assembly settings and operating conditions.
10.1.2.3 Proper operation of the automated analyzer system equipment and instrumentation.
10.2 CFR Engine Assembly Settings and Operating Conditions:
10.2.1 Compensation of Compression Ratio for Standard Knock Intensity - Knock testing engines operating at sites where the barometric pressure is lower or higher than 29.92 in. Hg, standard pressure, will knock softer or harder respectively than the engines operating at standard pressure. To compensate for this effect, the engine compression ratio is adjusted proportional to the difference between the site median and standard barometric pressure. The range of barometric pressure experienced at any testing location is generally less than 1.5 in. Hg and the compression ratio compensation to cause essentially standard knock intensity at the location can be achieved using a fixed offset based on median barometric pressure for the site. This compensation can be made once by setting the offset between the two dials of the digital counter and using the compensated digital counter reading for the ΔO.N. measurement.
10.2.1.1 Determine the range of barometric pressure that typically occurs at the site for the year and calculate the median barometric pressure. If there are significant seasonal differences, it may be appropriate to calculate the median barometric pressure for each season.
10.2.1.2 Using the median barometric pressure and Table A4.4 or A4.5 of Test Method D2699 for research octane number units and Table A4.9 or A4.10 of Test Method D2700 for motor octane number units determine the compensation for guide table cylinder height (digital counter reading).
10.2.1.3 Set the digital counter so that the lower dial reading is compensated for the site median difference in barometric pressure from the 29.92 in. Hg standard pressure.
10.2.2 Selecting and Setting Compression Ratio for On-line Operation - On-line ΔO.N. measurement for a given pair of fuels is initiated by setting the engine compression ratio to the guide table digital counter reading that corresponds to the appropriate CRF assigned octane number from the tables in Annex A4 in Test Method D2699 or Test Method D2700, whichever is appropriate, for the AMS.
10.2.2.1 For systems that operate at a constant C.R., the barometric pressure at the site may change slightly with time and this will result in minor shifts in engine K.I. level. If the K.I. on the comparison reference fuel trends below 35 or above 65 the AMS may be taken off-line, for a short period of time, to reset the K.I. to 50 by adjusting the detonation meter -METER READING- dial before continuing on-line analysis.
10.2.2.2 For systems that operate at a constant K.I. by adjustment of compression ratio the barometric pressure at the site may change slightly with time and this will result in minor shifts in the digital counter reading. If the digital counter reading for the reference fuel trends more than 20 units, the AMS may be taken off-line, for a short period of time, to reset the K.I. to 50 at the CRF O.N. digital counter reading by adjusting the detonation meter -METER READING- dial before continuing on-line.
10.2.2.3 Typical minor shifts in either knock intensity or digital counter reading affect each of the fuels under test in the same manner and these shifts do not significantly affect the ΔO.N. measurement.
10.2.3 Span Determination and Adjustment - The span setting for the analyzer is critical for the accurate determination of ΔO.N. The engine spread for constant C.R. systems or adherence to guide table readings for constant K.I. systems at the octane range of the standard or prototype fuel must be accurately determined and reflected in the analyzer span.
10.2.3.1 For AMS operating at a constant compression ratio, the span setting (K.I./octane) is to be determined by the running of two PRF fuels with a difference of 1.0 more or less 0.2 O.N. on the analyzer as per the manufacturer's instructions. The difference between the two fuels' K.I. readings divided by the difference in the two fuels' O.N. will give the spread for engine at that octane. The spread for the engine at the octane range of the PRFs will then need to be entered into the analyzer software as the span per the manufacturer's instructions.
10.2.3.2 For AMS operating at a constant knock intensity the span setting (C.R./octane) is to be determined by the running of two PRF fuels with a difference of 1.0 more or less 0.2 O.N. on the analyzer as per the manufacturer's instructions. The difference in the C.R. between the two fuels divided by the difference O.N. will give the span for the engine at that octane. The span for the engine at the octane range of the PRFs will then need to be entered into the analyzer software as per the manufacturer's instructions.
10.2.4 Fuel-Air Ratio Characteristic - With the engine operating at a cylinder height that causes knock, variation of the fuel-air mixture has a characteristic effect, typical for all fuels. This test method specifies that each stream sample and CRF shall be operated at the fuel-air ratio that produces the maximum K.I. To maintain good fuel vaporization, a restrictive orifice or horizontal jet is utilized so that the maximum knock condition occurs for fuel levels between 0.7 and 1.7 in. referenced to the centerline of the carburetor venturi. The mechanics for varying the fuel mixture can be accomplished using various approaches.
10.2.4.1 Fixed Horizontal Jet - Variable Fuel Level System - Fuel level adjustments are made by varying the float reservoir in incremental steps. Selection of a horizontal jet having the appropriate orifice size establishes the fuel level at which a typical sample fuel achieves maximum knock.
10.2.4.2 Fixed Fuel Level - Variable Orifice System - A fuel reservoir, in which the fuel can be maintained at a prescribed constant level, supplies an adjustable orifice (special long-tapered needle valve) used in place of the horizontal jet. Fuel mixture is changed by varying the needle valve position. Typically, the constant fuel level selected is near the 1.0 level, which satisfies the fuel level specification.
10.2.4.3 Dynamic or Falling Level System - A fuel reservoir, filled to a higher level than that required for maximum K.I., delivers fuel through either a fixed bore or adjustable horizontal jet. With the engine firing, the fuel level falls as fuel is consumed. Fuel level changes at a specifically selected rate that is established by the cross-sectional area of the fuel reservoir and associated sight glass assembly. Maximum K.I. is recorded as the fuel level passes through the critical level.
10.2.5 Intake Air and Mixture Temperature Setting Practices:
10.2.5.1 Motor Method:
(1) Intake Air Temperature - 38 more or less 2.8°C (100 more or less 5°F).
(2) Intake Mixture Temperature - 149 more or less 1°C (300 more or less 2°F) maintained within 1°C (more or less 2°F) when C.R. or K.I. results used for a delta octane measurement are recorded.
10.2.5.2 Research Method:
(1) Intake Air Temperature - 52 more or less 1°C (125 more or less 2°F) is specified for operation at standard barometric pressure of 101.0 kPa (29.92 in. Hg). IATs for other than standard barometric pressure conditions need to be adjusted to compensate for the site median barometric pressure.
(2) Determine the site median barometric pressure (see details previously given under Site Compensation of Compression Ratio for Standard Knock Intensity).
(3) Use the site median barometric pressure and Table A4.4 or A4.5 of Test Method D2699 to determine the applicable intake air temperature.
(4) Adjust analyzer measurement system settings to deliver the compensated intake air temperature and this temperature shall then be maintained within more or less 1°C (more or less 2°F) when C.R. or K.I. results used for a delta octane measurement are recorded.
10.3 Proper Operation of the Automated Analyzer System Equipment and Instrumentation:
10.3.1 Sample Stream Sampling Systems:
10.3.1.1 Cyclic and Continuous Fuel Sampling Techniques - AMS can determine the knock characteristic measurement using either a grab sample or continuously flowing sample.
10.3.1.2 For the continuously fowing sample approach, fuel is continuously delivered to the CFR engine carburetor while knock measurement is in progress, and any unconsumed fuel is removed from the AMS.
10.3.1.3 For the intermittent or grab sample approach, a carburetor device isolates a portion of either the stream sample or CRF, then performs the knock measurement sequence on that sample.
10.3.1.4 The system needs to operate on each fuel for a minimum of 4 min. The time periods spent on each fuel can be set based on engine operation and site requirements.
10.3.1.5 The system must be rating the sample stream for a minimum of 50 % of the cycle time.
10.3.2 Sample Temperature - Deliver the CRF and sample fuel to the knock-testing unit critical carburetor components at the same nominal temperature. This temperature shall be greater than 0°C (32°F) but not exceed 10°C (50°F).
10.3.3 System Alarm Functions - AMS systems for unattended operation utilize sensors, control logic, and other devices designed to protect the system and facilities from abnormal conditions. Some typical sensors are: low crankcase oil pressure, loss of jacket coolant, loss of sample stream pressure or flow, or both, excessive C.R. as evidenced by cylinder height limits, indication of system measurement instability as evidenced by out-of-limit repeatability measurements for comparison reference fuel, the presence of hydrocarbon vapors at the unit, the presence of carbon monoxide in the room atmosphere, and so forth. Some alarm functions are active and result in system shutdown. Other alarms are passive and simply indicate an operating characteristic that is out of performance limits.
