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Lab Methods

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Research and Motor Octane Ratings

  • Method ASTM D2699/ASTM D2700
  • Repeatability 0.2 octane numbers
  • Reproducibility 0.7 octane numbers
  • Blending non-linear
  • Additive Correction Yes

This laboratory test method covers the quantitative determination of the knock rating of liquid spark-ignition engine fuel in terms of Research and Motor O.N., including fuels that contain up to 25 % v/v of ethanol. However, this test method may not be applicable to fuel and fuel components that are primarily oxygenates (with adjustment of the carburetor it is possible). The sample fuel is tested using a standardized single cylinder, four-stroke cycle, variable compression ratio, carbureted, CFR engine run in accordance with a defined set of operating conditions. The O.N. (Octane Number) scale is defined by the volumetric composition of PRF blends. The sample fuel knock intensity is compared to that of one or more PRF blends. The O.N. of the PRF blend that matches the K.I. of the sample fuel establishes the Research O.N

A fuels octane rating is measured as its resistance to knock. To get a better understanding of what knock and octane values are we must look at the combustion process. First Air and Fuel is drawn into the cylinder through the intake valve as the piston moves down. The piston then travels up and compresses the mixture. Once the piston has reached the top the spark plug ignites the fuel and pushes the piston back down in sync with the crank shaft. Finally the exhaust gasses are expelled through the exhaust valve on the next stroke up before coming back down drawing in more fuel and air. This is the way a normal combustion event works if the octane of the fuel is properly suited for the engine. Now if the fuel is very low octane when the piston travels up on the compression stroke the heat and compression can cause the fuel to ignite before the spark plug is set to fire. This will cause the piston and rod to push back against the crank shaft at the wrong time and create a knocking or pinging sound with a resulting loss of power. If this happens repeatedly it can cause catastrophic damage to the crankshaft or rods which will destroy the engine.

There are two different measurements of octane value, Ron and Mon. This was created to give a more robust octane value since the usage parameters can vary. The Research Octane Number was designed to simulate a vehicle traveling down the highway at steady speeds under a light load. The engine parameters feature lower rpm and fixed spark advance during the test. The Motor Octane Number was designed to simulate stop and go driving, acceleration, or pulling heavy loads. The engine parameters feature higher rpm with variable timing based on the compression. Once both values are obtained they are averaged to give the R+M/2 rating. A typical regular octane gasoline would be 82.0 Mon 92.0 Ron for an 87.0 R+M/2. This is normally displayed on the pumps for domestic gasoline. Some other countries only rely on the Ron and do not display the Mon.

These methods are performed by first making two different fuel type blends PRF (saturate blend) and TSF (aromatic/saturate). These fuels are used to tune the engine and ensure that no matter what type of sample(aromatic, saturate, or olefin) is being run it will be properly rated. The engine compression is adjusted by raising or lowering the head to the correct height for the fuel blends. Next the carburetor is selected that contains the fuel. The engine will then begin to knock and the needle will move on the knock meter. The air fuel ratio is then adjusted until the maximum knock value is reached and the knock meter will be tuned by adjusting the meter (starting point) and spread (sensitivity). This is then switched to the second standard and the air fuel ratio is optimized for the knock. This time instead of adjusting the meter the temperature controller for the intake heater (ron) or mixture heater (mon) is adjusted to line up the motor with the first standard. After this is complete the sample runs through the same process but the compression is adjusted by moving the head to align the knock with the first two standards and the value taken. This is the way compression ratio works. Bracketing is similar and is used for low and high values.