How to Set Octane Testing Engines Intake/Exhaust Valves Clearances
The octane testing engines use a four-stroke cycle with two crankshaft revolutions for each complete combustion cycle. The two critical valve events are those that occur near TDC, intake valve opening and exhaust valve closing.
Intake valve opening shall occur 10.0°±2.5° after-top-dead-center (atdc) with closing at 34° after-bottom dead-center (abdc) on one revolution of the crankshaft and flywheel. Exhaust valve opening shall occur 40° before bottom-dead-center (bbdc) on the second revolution of the crankshaft and flywheel, with closing at 15.0°±2.5° atdc on the next revolution of the crankshaft and flywheel.
Setting Octane Testing Engines Intake and Exhaust Valves Clearances
Measurement of valve clearance is made using a feeler gauge of the prescribed dimension inserted between the flat surface of the half ball and the top surface of the valve stem. Adjustment is made using the adjusting screw on the end of the rocker arm over the push rod.
Cold Setting: Cold settings should be made at room temperature before starting the octane testing engines. Setting the valves loose at 0.25 mm (0.01 in.) will allow proper break-in of valve faces and seats during initial warm-up. This allows the valves to open and close fully during break-in. Once the octane testing engine has run one hour and conditions have stabilized and come to equilibrium, it will be necessary to adjust the valves (intake and exhaust) to 0.20 mm±0.025 mm (0.008 in.±0.001 in.). ■Intake Valve …………………………………………… 0.25 mm (0.01 in.) ■Exhaust Valve ………………………………………… 0.25 mm (0.01 in.) NOTE: ASTM Research and Motor Method specifications state that the controlling condition for valve clearances shall be the setting with the octane engine operating under the following conditions: ■Fuel: 90 octane number primary reference fuel blend ■Operating Conditions: Standard at equilibrium temperatures ■Fuel-Air Ratio: Adjusted to provide maximum knock intensity
Hot Checking: With the octane testing engines operating and at equilibrium temperature, measurement of valve clearance must be performed quickly as follows: A. Measure the clearance using a feeler gauge. B. If the clearance for either valve is not 0.20±0.025 mm (0.008±0.001 in.), make the necessary adjustment. Loosen the lock nut, reposition the adjusting screw, and then retighten the lock nut. C. Re-measure the valve clearance and continue adjustment until an acceptable measurement is obtained. NOTE: Adjustment of valve clearances with the octane engine hot is a trial and error process.