Fuel Octane Selection
GOAL:
To assist students in becoming wiser consumers of gasoline and enable them to select the proper octaned gasoline resulting in an energy savings as well as a monetary savings.
OBJECTIVES:
Students will:
1. Realize that proper fuel octane selection can change MPG.
2. Recognize the economic benefit from proper fuel octane selection.
3. Understand that it takes more raw crude to refine higher octane fuels.
LESSON/INFORMATION:
Fuel octane requirements for gasoline engines vary with the compression ratio of the engine; diesel cetane requirements also vary with the compression ratio. Engine compression ratio is the relative volume of a cylinder from the bottom most position of the piston's stroke to the top most position of the piston's stroke. The higher an engine's compression ratio, the greater the amount of heat generated in the cylinder during the compression stroke.
Posted octane numbers on gasoline pumps are a result of testing fuel performance under laboratory and actual operating conditions. The higher the octane rating on fuel the less volatile (evaporative qualities) and the slower the fuel burns. Higher octane fuel contains more POTENTIAL energy but requires the higher heat generated by higher compression ratio engines to properly condition the fuel to RELEASE that higher potential energy. In the refining process, fewer gallons of higher octane fuels are yielded from a barrel of raw crude.
If fuel octane is too low for a given compression ratio, the fuel prematurely and spontaneously ignites too early and the fuel charge EXPLODES rather than BURNS resulting in incomplete combustion. The net effect is a loss in power and possible engine damage. The operator hears an audible "knock" or "ping", referred to as detonation. Detonation may vary from a faint noise on light acceleration to a constant, deep hammering noise while driving at a constant speed. Improper timing adjustments, vacuum leaks, or excessively lean fuel mixtures may also cause detonation.
Many vehicle owners believe that higher octane fuels are better for their vehicles since they are labeled "PREMIUM." The logic is that since it is a premium fuel it must be better. In reality, the premium label originates from the higher cost to refine and the resultant higher retail cost. Some refiners label their high octane fuels "SUPER." Some owners think that these fuels will make their vehicles more powerful. Only engines with high compression ratios can deliver all the potential energy from higher octane fuels! Always consult the manufacturer's octane recommendation to determine the proper octane requirements for any given vehicle. Generally, engines with compression ratios of 9.3 : 1 or less will safely operate with unleaded 87 octane fuel. Engines with higher compression ratios usually require higher octane fuels.
Many owners who operate vehicles designed to operate on 87 octane fuel experience ping and knock. They usually "fix" this problem by purchasing the higher priced, higher octane fuels. Most owner's manuals indicate that some light and intermittent ping is normal but that heavy or sustained ping or knock should be attended to by either purchasing the correct octane fuel or servicing the engine.
Most fuel refiners blend fuels for geographic areas and adjust their blends seasonally. These blending techniques compensate for the decrease in oxygen content with an increase in altitude and compensate for volatility during the warmer or cooler seasons. Significant ambient temperature changes (40 degrees Fahrenheit) or altitude changes (4,000 feet) may cause some serious engine detonation. This problem is usually corrected by filling the tank with "local" fuel that has been properly blended for season and altitude.
ACTIVITY:
1. From the owner's manual, determine the octane requirements for a personal or family vehicle. If the owner's manual is not available, telephone, write or visit a franchised dealer for that vehicle to determine the appropriate octane fuel for that vehicle.
2. Make sure that the tank level is at ¼ or less before beginning this activity. Perform the "MPG test as outlined in the "Fuel Mileage Calculation" guide with the highest octane (91+ octane) fuel available for two sequential tanks. Calculate the two tank MPG. On the work sheet, note any extraordinary engine noises or performance problems during this test and note the cost per gallon.
3. Repeat step "2" for the next lower octane fuel (89 octane). On the work sheet, note any extraordinary engine noises or performance problems during this test and note cost per gallon.
4. If applicable, repeat step "2" a third time with lowest grade fuel (87 octane). On the work sheet, note any extraordinary engine noises or performance problems during this test and note cost per gallon.
5. For each grade of fuel, use the MPG and cost per gallon to project the fuel cost to operate this vehicle for 10,000 miles.
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