ASTM D6812 tandard Practice for Ground-Based Octane Rating Procedures for Turbocharged/Supercharged Spark Ignition Aircraft Engines
10. Procedure
10.1 Engine Break-In:
10.1.1 If the test engine is new, remanufactured, or recently overhauled, break it in prior to conducting initial octane ratings. Conduct the break-in in accordance with the engine manufacturer's recommendations. Conduct the break-in with a fuel that does not contain metallic additives.
10.1.2 Start the engine, and follow the engine manufacturer's warm-up procedures. Perform a magneto check in accordance with the engine manufacturer's recommendation.
10.1.3 Operate the engine at the manufacturer's recommended power settings, and record the oil consumption until either oil consumption is stabilized or 10 h of engine operation is attained. Oil consumption stabilization should conform to the engine manufacturer's recommendation.
10.1.4 During the engine break-in, maintain the engine operating temperatures and oil temperature in accordance with the manufacturer's recommendations.
10.2 Power Baselines:
10.2.1 After oil consumption is stabilized, perform three separate power baseline tests of the engine. Each test requires measuring the power developed at combinations of every hundred rpm and 50 mmHg (2 inHg) MAP increments from takeoff power to low cruise power (that is, 55 % normal rated power). This ensures that proper power is being developed.
10.2.2 Record engine parameter data at a rate of at least 1 full channel scan for every 10 s of engine operation, and attach the results to the octane rating data.
10.2.3 Perform the installation of the knock sensing equipment after the power baselines have been performed.
10.2.4 If this knock sensor installation alters the cylinder/ignition system in any way, such as drilling of cylinders, use of longer reach spark plugs, or use of modified spark plugs, then perform the three power baselines again, after the knock sensor installation. Retest the same MAP and rpm settings tested in 10.2.1, and record the results. Record the engine parameter data again at a rate of at least 1 full channel scan for every 10 s of engine operation.
10.3 Cylinder Compression Check:
10.3.1 After power mapping the engine, perform cylinder compression checks, and record the results.
10.3.2 The engine shall be warm when the cylinder compression checks are performed.
10.3.3 Perform cylinder compression tests using the proper tooling and methods specified by the engine manufacturers.
10.4 Octane Rating:
10.4.1 After the cylinder compression checks have been performed, perform the octane rating.
10.4.2 For this test, record the engine parameter data at a minimum rate of 1 full channel scan for every 10 s of engine operation. The knock data sample rate shall be equal to or greater than at least 1 pressure sample per cylinder for every 0.4° of crankshaft rotation. Knock data shall encompass at least 100 consecutive engine cycles per power setting for each cylinder monitored.
10.4.3 Start the engine on house fuel. Allow the engine to warm up. Ensure all instrumentation indications are within proper range. Conduct ignition systems check.
10.4.4 Adjust the induction air temperature to maintain the engine manufacturer's limiting MAT within +/- 3°C (5°F) of the manufacturer's recommended maximum limit.
10.4.5 Adjust the cooling air pressure to maintain the maximum CHT within +/- 6°C (10°F) of the manufacturer's recommended maximum limit. Maintain all CHT within 28°C (50°F) of the maximum CHT. Maintain these settings throughout the octane rating.
10.4.6 Maintain the oil temperature within +/- 6°C (10°F) of the engine manufacturer's recommended maximum limit throughout the octane rating.
10.4.7 Set maximum rated engine rpm, full throttle, and full rich mixture conditions on the house fuel. (Warning - Verify knock-free operation after each and every test point condition is adjusted on the house fuel. The engine shall be knock-free on the house fuel prior to switching to the PRF/RF fuels. Switching fuels during a knocking condition may result in inaccurate ratings.)
10.4.8 Select appropriate PRF or RF to be tested. Begin the test sequence with a PRF or RF of octane quality that is likely to result in no knock.
10.4.9 Each time a different PRF or RF is selected, or the engine power setting is changed, allow conditions to become stable. Allow time for the selected fuel to enter the engine and for the conditions to stabilize. (Warning - Under moderate to heavy knock conditions, temperature stability may not be attainable. Do not allow the engine to operate under an unstable thermal condition for extended periods of time as this may cause severe and irreversible engine damage.)
10.4.10 Record knock data, and determine the combustion condition (normal combustion, light knock, moderate knock, or heavy knock). See Annex A1 for more detail. Do not allow the test engine to operate under heavy knock for extended periods of time.
10.4.11 If knock occurs, select the house fuel, reduce the engine power, and return to 10.4.7 with a higher octane quality PRF or RF.
10.4.12 If no knock occurs, select the house fuel and set the power on the test engine to the recommended climb power setting, or maximum continuous power setting if appropriate. Leave the mixture at the full rich setting.
10.4.13 Select the PRF or RF with the same octane quality as the fuel selected in 10.4.8.
10.4.14 Record knock data, and determine the combustion condition.
10.4.15 If knock occurs, select house fuel and return to 10.4.7 with a higher octane quality PRF or RF.
10.4.16 Ifno knock occurs, select house fuel and set the test engine to the maximum recommended cruise setting where the mixture can be leaned. Leave the mixture at the full rich position.
10.4.17 Select the PRF or RF with the same octane quality as the fuel selected in 10.4.8.
10.4.18 Record knock data and determine the combustion condition.
10.4.19 If knock occurs, select the house fuel and return to 10.4.7 with a higher octane quality PRF or RF.
10.4.20 Ifno knock occurs, calculate a 5 % reduction in fuel flow from the reading obtained from the maximum recommended cruise full-rich mixture setting.
10.4.21 Hold the throttle and engine speed fixed at the maximum recommended cruise settings. Starting with full-rich mixture settings, lean until the fuel flow decreases by the 5 % increment just determined. The engine should still be operating on the PRF or RF.
10.4.22 Allow conditions to stabilize after adjusting the mixture.
10.4.23 Record knock data, and determine the combustion condition.
10.4.24 If knock occurs, select house fuel and return to 10.4.7 with a higher octane quality PRF or RF.
10.4.25 If no knock occurs, continue leaning by the 5 % increment determined in 10.4.20 at the maximum recommended cruise position until either knock occurs, peak EGT in any cylinder is reached, or limiting TIT is attained. The engine shall be stabilized and knock data recorded after each 5 % increment change in fuel flow. (Warning - Excessive leaning may cause engine damage. All cylinder EGT's should be monitored so that the mixture is not leaned past peak EGT for any cylinder or past the limiting TIT. Rapid lean mixture adjustments may propagate heavy knock and may lead to engine damage.)
10.4.26 If peak EGT in any cylinder or the limiting TIT has been reached without experiencing a knock condition, enrich the mixture, and select a lower octane quality PRF or RF. If that lower octane quality PRF or RF has already been found to produce a knock condition, then the octane requirement is the last knock-free PRF or RF tested.
10.4.27 It is suggested, when choosing a lower octane quality PRF or RF, to choose a rating fuel of only one number (MON or AN) lower so as to minimize any potential engine damage. Continue testing with a lower octane quality PRF or RF until a knock condition is encountered. The octane requirement is defined as one number higher than the highest number that produced a knock condition.
10.4.28 At the conclusion of the test, select the house fuel, gradually reduce the power setting to idle, and allow the engine to cool.
10.4.29 After shutdown, make sure the fuel selector valve does not leak. If the valve leaks, repair the valve and repeat the test to ensure the PRFs or RFs were not contaminated.
10.4.30 Fig. 1 illustrates, in flow-chart form, the octane rating test procedures.
