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contd.....
The engine operating conditions for the research and motor methods are given in Table 8.4. During the test, the air-fuel ratio of the engine with the sample fuel is adjusted for maximum knock intensity. Then on these settings, the engine compression ratio is varied and the knock intensity obtained with the test fuel is bracketed by two blends of the reference fuels that differ by not more than 2 ON units. The octane number of the sample fuel is obtained by interpolation between the knock intensity readings for the two blends of the reference fuels and their octane numbers. The octane number above 100 ON is determined by using isooctane and varying amounts of TEL (tetraethyl lead) in terms of ml TEL/ per litre. Tetraethyl lead is an antiknock additive. ASTM standards have specified octane numbers to the blends of isooctane and TEL.
| Table 8.4 |
| Engine Test Conditions for ASTM Research and Motor Methods |
| Operating Conditions |
Research Method |
Motor Method |
| Engine Speed, RPM |
600 |
900 |
| Inlet temperature, °C |
52 |
149 |
| Inlet Pressure |
Atmospheric |
Atmospheric |
| Humidity, kg/kg of dry air |
0.0036-0.0072 |
0.0036-0.0072 |
| Coolant Temperature, °C |
100 |
100 |
| Spark Advance, °CA btdc |
13 (constant) |
19-26 (varies with CR) |
| Air-fuel ratio |
Adjusted for maximum knock |
Adjusted for maximum knock |
The test operating conditions used in Motor Method viz., higher intake mixture temperature and higher spark advance are likely to produce more knock and thus, the method is more severe. For the most fuels motor octane number is lower than the research octane number. The difference between RON and MON is known as fuel sensitivity:
| Fuel Sensitivity = RON – MON |
(8.1) |
The fuel sensitivity for fuels is mostly in the range 0 to 12 octane units. The reference fuels being paraffins, the paraffinic hydrocarbons have low octane
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