ASTM D6424-99
(Practice)Standard Practice for Octane Rating Naturally Aspirated Spark Ignition Aircraft Engines
Standard Practice for Octane Rating Naturally Aspirated Spark Ignition Aircraft Engines
SCOPE
1.1 This practice covers ground based octane rating procedures for naturally aspirated spark ignition aircraft engines using primary reference fuels.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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An American National Standard
Designation: D 6424 – 99
Standard Practice for
Octane Rating Naturally Aspirated Spark Ignition Aircraft
Engines
This standard is issued under the fixed designation D 6424; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3.1.5 knock condition, n—for octane rating, when the
knock intensity in any cylinder is light knock or greater as
1.1 This practice covers ground based octane rating proce-
described in Annex A1
dures for naturally aspirated spark ignition aircraft engines
3.1.6 knock number, n—for octane rating, a numerical
using primary reference fuels.
quantification of knock intensity.
1.2 This standard does not purport to address all of the
3.1.7 motor octane number of primary reference fuels from
safety concerns, if any, associated with its use. It is the
0to100—the volume % of isooctane (equals 100.0) in a blend
responsibility of the user of this standard to establish appro-
with n-heptane (equals 0.0).
priate safety and health practices and determine the applica-
3.1.8 motor octane number of primary reference fuels above
bility of regulatory limitations prior to use.
100—determined in terms of the number of millilitres of
2. Referenced Documents
tetraethyl lead in isooctane.
3.1.9 naturally aspirated aircraft engine, n—aircraft piston
2.1 ASTM Standards:
engine that breathes without forced means from either turbo-
D 2700 Test Method for Motor Octane Number of Spark-
chargers or superchargers.
Ignition Engine Fuel
3.1.10 no-knock condition, n—for octane rating, when the
3. Terminology
combustion instability in all cylinders is less than light knock.
Refer to Annex A1 for description of knock intensity.
3.1 Definitions:
3.1.11 peak EGT, n—for octane rating, as the mixture is
3.1.1 engine motor octane requirement—one full motor
manually leaned from a state rich of stoichiometric, the exhaust
octane number greater than the maximum motor octane num-
gas temperature will increase with the removal of excess fuel.
ber that results in knock (graphic knock level descriptions can
As the mixture is continually leaned, a peak temperature will
be seen in Annex A1). For example, a test engine knocks on
be attained, after which continued leaning will result in lower
primary reference fuels with 96 and 97 motor octane numbers.
exhaust gas temperatures.
The test engine does not knock on a primary reference fuel
3.1.12 primary reference fuels, n—for octane rating,
with a 98 motor octane number. The maximum motor octane
blended fuels of reference grade isooctane and n-heptane.
number that results in knock is 97, so the motor octane
3.1.13 stable engine conditions, n—for octane rating, cyl-
requirement is 98.
inder head temperatures change less than 5°C (9°F) during a 1
3.1.2 full rich—condition in which the mixture control is at
min period. Any changes or minor adjustments to throttle,
the full stop position with the fuel flow within manufacturer’s
mixture, or engine conditions mandate restarting the clock for
recommended settings.
determining stable conditions.
3.1.3 house fuel, n—for octane rating, an unleaded, straight
3.2 Symbols:
hydrocarbon fuel used for engine warm-up and all non-octane
3.2.1 CHT—cylinder head temperature.
rating testing.
3.2.2 EGT—exhaust gas temperature.
3.1.4 knock, n—in an aircraft spark ignition engine, abnor-
3.2.3 inHg—inches of mercury.
mal combustion caused by autoignition of the air/fuel mixture.
3.2.4 MAP—manifold absolute pressure.
3.2.5 mmHg—millimeters of mercury.
3.2.6 MON—motor octane number.
This test method is under the jurisdiction of ASTM Committee D-2 on
3.2.7 PRF—primary reference fuel.
Petroleum Products and Lubricants and is the direct responsibility of Subcommittee
3.2.8 psig—pounds per square inch gauge.
D02.J on Aviation Fuels.
3.2.9 rpm—revolutions per minute.
Current edition approved June 10, 1999. Published August 1999.
Annual Book of ASTM Standards, Vol 05.01. 3.2.10 TDC—top dead center.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6424–99
4. Summary of Practice location and proper material selection. EGT probes are usually
installed within 5 cm (2 in.) of the exhaust stack flange. The
4.1 A recently overhauled, remanufactured, or new, natu-
EGT probes shall be accurate to within 1 % of full scale.
rally aspirated aircraft engine is octane rated, using PRFs, to
6.1.1.7 Engine Speed Sensor—The dynamometer or propel-
determine the minimum motor octane requirement. Minimum
ler stand shall measure the engine shaft speed to determine
motor octane requirement is defined as one number above the
power development. The engine speed sensor shall be accurate
highest MON in which knock was detected. The engine is
to within 6 5 rpm.
tested at three or more of the worst power points subject to
6.1.1.8 Fuel Flow Meter—If the device is calibrated for a
detonation behavior. These points usually involve high mani-
particular fuel, then the device shall be recalibrated for each
fold pressures. At the very least takeoff power, a maximum
different and subsequent fuel. Data should be reported in mass
continuous or climb power, and a cruise configuration shall be
flow units.
tested. Takeoff power and climb power are tested under full
6.1.1.9 Fuel Pressure Transducers—Locations of fuel pres-
rich mixture conditions, and cruise power is tested under full
sure transducers shall conform with those recommended by the
rich and lean mixture configurations in 5 % increment reduc-
engine manufacturer. One transducer is required for the me-
tions from full rich fuel flow. Engine operating temperatures
tered fuel pressure, if necessary, and another is required for the
and oil temperatures are kept at maximum allowable limits,
pump pressure. The fuel inlet pressure shall not fall below the
while induction and cooling air temperatures are maintained at
minimum specified by the engine manufacturer during the
extreme hot day conditions for severe case testing.
rating process.
4.2 Octane ratings are determined under stable engine
6.1.1.10 Induction Air Pressure Transducer, located so as to
conditions using PRFs of known MON.
4.3 Knock sensor installation and knock quantification are measure the pressure of the induction stream prior to the
throttle plate.
described in Annex A1.
6.1.1.11 Induction Air Temperature Sensor, located so as to
5. Significance and Use
measure the temperature of the induction stream prior to the
5.1 This practice is used as a basis for determining the
throttle plate.
minimum motor octane requirement of naturally aspirated
6.1.1.12 Knock Sensors—The referee method for knock
aircraft engines by use of PRFs.
detection is described in Annex A1. This method requires flush
5.2 Results from standardized octane ratings will play an
mounting piezoelectric transducers. At the very least, the four
important role in defining the actual octane requirement of a
cylinders with the highest CHTs shall be monitored. These
given aircraft engine, which can be applied in an effort to
transducers are connected to charge amplifiers and shall be
determine a fleet requirement.
capable of measuring combustion pressures under a high
temperature environment.
6. Apparatus
6.1.1.13 Oil Pressure Transducer—Location of pressure
6.1 Instrumentation:
measurement shall conform to engine manufacturer’s specified
6.1.1 The engine shall be equipped with the following
location.
instrumentation, which shall be accurate within 6 2 % of full
6.1.1.14 Oil Temperature Sensor—Location of temperature
scale unless noted otherwise.
measurement shall conform with manufacturer’s specified
6.1.1.1 Absolute Manifold Pressure Transducer—Location
location.
of MAP sensor shall conform to engine manufacturer’s speci-
6.1.1.15 Torque Meter—The dynamometer or propeller
fied location. Manifold pressures shall be measured with an
stand shall measure the torque to determine power develop-
accuracy of less than 2.5 mmHg and recorded to ensure proper
ment. The torque measurement shall be accurate to within 1 %
engine behavior and repeatability.
of full scale.
6.1.1.2 Cooling Air Pressure Transducer, located so as to
6.1.2 The engine should be equipped with the following
determine the pressure within the cowling.
instrumentation, which shall be accurate within 6 2 % of full
6.1.1.3 Cooling Air Temperature Sensor, located either
scale unless noted otherwise.
within the cowling or at the entrance to the cowling. If a
6.1.2.1 Induction Air Flow Meter—Data should be pre-
thermocouple is utilized, it should extend at least a third of the
sented in mass flow units.
way across the measured area.
6.1.2.2 Induction Air Humidity Sensor, located in either the
6.1.1.4 Crankshaft Angle Encoder, if required for knock
induction air plenum or induction air duct. Data should be
detection. The encoder shall have a sample resolution of at
presented in absolute, rather than relative, quantities.
least 0.4° of crank shaft rotation. The encoder TDC pulse shall
6.1.2.3 Outside Air Temperature Sensor, capable of measur-
be aligned with the TDC of cylinder number one prior to
ing both the ambient wet bulb and the dry bulb temperatures
octane rating.
prior to any engine testing.
6.1.1.5 Cylinder Head Temperature Sensors, installed in
6.2 Data Acquisition:
each cylinder. The sensing locations and types of thermo-
couples shall conform to the engine manufacturer’s recommen- 6.2.1 The instrumentation listed in 6.1 shall be scanned and
dations. The CHT measurements shall be accurate to within the data recorded at least once every 15 s by an automatic data
1 % of full scale. acquisition system. The data shall be stored in a universal
6.1.1.6 Exhaust Gas Temperature Sensors, on all cylinders. format (for example, comma separated values (CSV) for IBM
Installation shall conform with manufacturer’s recommended compatible machines) that can be retrieved at a later date.
D6424–99
6.2.2 If in-cylinder pressures are recorded to determine sheets, and any other documentation issued by the original
knock intensity, the pressure data shall be sampled at a rate of equipment manufacturers or repair overhaul shops before any
at least 1800 samples per pressure cycle per cylinder. octane rating tests are performed.
6.3 Power Absorption—The testing is to be performed in a
8.2 Only the engine accessories required to operate the
ground based test cell using either a dynamometer or propeller
engine shall be installed on the test engine when conducting the
test stand that shall be capable of maintaining a constant speed
octane ratings.
to within 6 10 rpm.
8.3 The exhaust system employed shall not induce a back
6.4 Fuel System:
pressure greater than the back pressure called for in the engine
6.4.1 The fuel supply shall have a disposable or cleanable
manufacturer’s specifications.
filter. The filter shall allow the proper minimum fuel flow.
8.4 If the test engine’s fuel system is designed to recirculate
6.4.2 The fuel selection valve shall be capable of selecting
fuel to the tank, provisions shall be made to ensure that no fuel
at least two different fuel sources without the possibility of
is recirculated to the containers with the PRFs.
cross contamination of either source.
8.5 The idle mixture setting and full rich fuel flow rate shall
6.4.3 The fuel supply system must comply with federal,
be set in accordance with the engine manufacturer’s recom-
state, and local regulations related to fire, hazards, and health
mendations.
issues.
8.6 The idle stop and full throttle throw positions shall be
set in accordance with the engine manufacturer’s recommen-
7. Reagents and Materials
dations.
7.1 The MON of PRFs is confirmed by using Test Method
8.7 Before any octane rating, and after all break-in and
D 2700. All fuels used for the initial engine octane ratings are
power baseline runs have been performed, a cylinder compres-
PRFs that consist of blends of reference grade isooctane and
sion test shall be performed on all cylinders and the results
n-heptane. The PRFs will be prepared in increments of one
recorded.
MON.
8.8 Prior to testing, the integrity of the fuel selection system
NOTE 1—Warning: PFR is flammable, and its vapor is harmful. Vapors shall be confirmed and the system flushed. The engine fuel
may cause flash fire.
selector apparatus shall be checked to ensure no leakage.
8.9 All engine settings shall be checked after the break-in
7.1.1 Isooctane (2,2,4-trimethylpentane) shall be no less
period and before any octane rating. As a minimum, this shall
than 99.75 % by volume pure, contain no more than 0.10 % by
include fuel pressures, oil pressure, fuel flow, and magneto
volume n-heptane, and contain no more than 0.5 mg/L (0.002
timing.
g/U.S. gal) of lead.
8.10 A systems check shall be performed, as per specific
NOTE 2—Warning: Isooctane is flammable, and its vapor is harmful.
aircraft engine manufacturer’s recommendations, prior to start-
Vapors may cause flash fire.
ing the test engine. As a minimum, this shall include the
7.1.2 n-Heptane shall be no less than 99.75 % by volume
following: idle throttle stop, wide open throttle throw, mixture
pure, contain no more than 0.10 % by volume isooctane, and
cut-off position, and mixture full rich position.
contain no more than 0.5 mg/L (0.002 g/U.S. gal) of lead.
8.11 A systems check shall be performed after starting the
NOTE 3—Warning: n-Heptane is flammable, and its vapor is harmful. test engine. This shall include as a minimum the following
Vapors may cause flash fire.
items: oil pressure, magneto ground check as per the engine
manufacturer’s recommendations, and instrumentation indica-
7.1.3 A sample shall be taken of each primary reference fuel
tions within normal ranges.
and subjected to Test Method D 2700 for motor octane
verification.
9. Calibration and Standardization
7.2 Fuels used for operations other than octane rating (for
example, warm-up) shall consist of unleaded hydrocarbons and
9.1 The engine shall be set up in accordance with the
should be capable of satisfying the test engine’s octane
manufacturer’s specifications. The ignition timing shall be set
requirement under the conditions for the fuel to be used.
within 6 1° of the recommended setting. The fuel flows shall
be set within 6 2 % of the recommended fuel flow (or 6 5%
NOTE 4—Warning: These fuels are flammable, and their vapor is
of the recommended pressure when appropriate).
harmful. Vapors may cause flash fire.
9.2 Instrumentation shall be calibrated and checked to
7.3 Engine break-
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