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.

CULMINATING ACTIVITY:: 1. Prepare a short report highlighting the need to determine octane for the selected vehicle.; 2. Using your MPG data for each grade of fuel, compare the operating costs for ten years at 10,000 miles per year. Assume fuel costs $1.25 for 87 octane, $1.45 for 89 octane, and $1.65 for 91+ octane.

FUEL OCTANE WORK SHEET

1. Octane recommendation (source: owner's manual): _______

2. Fill tank with 91+ octane fuel; odometer reading: ______

3. Fill tank with 91+ octane fuel; odometer reading: ______
Gallons purchased: ______
Traffic Mix: ________________________

4. Fill tank with 89 octane fuel; odometer reading: ______
Gallons purchased: ______
Traffic Mix: ________________________

5. Fill tank with 89 octane fuel; odometer reading: ______
Gallons purchased:______
Cost per gallon: ______
Traffic Mix:________________________

6. Fill tank with 87 octane fuel; odometer reading: ______
Gallons purchased: ______
Cost per gallon: ______
Traffic Mix: ________________________

7. Fill tank with 87 octane fuel; odometer reading: ______
Gallons purchased:______
Cost per gallon:______
Traffic Mix:________________________


Type Fuel	Cost/gallon	MPG	Cost for 10,000 miles
91+ octane
89 octane
87 octane

INFORMATION CHECK: Place a "T" before the statements that are true and an "F" before the statements that are false. After each false statement, explain why it is false.; _______1. 91 octane fuel has more potential power than 87 octane fuel.; _______2. To release all the potential power of 91 octane gasoline, it is necessary for an engine to have a compression ratio higher than 9.3 : 1.; _______3. Any gasoline engine will efficiently burn any octane fuel.; _______4. The engine's compression ratio is the most important factor in selecting gasoline octane.; _______5. Posted fuel octane ratings are a result of testing fuel in the laboratory and under actual operating conditions.; _______6. Diesel engines require a low octane fuel.; _______7. Engine knock or ping is always a result of poor quality fuel.; _______8. Gasolines are seasonally and regionally blended.

TEACHER'S NOTES: This is an important energy saving activity for students. Over the years, our society has somehow gotten the misnoma that a "Premium" grade fuel is better for a vehicle than a "Regular" grade fuel. Most vehicle owners never refer to their Owner's Manual to determine the acceptable octane fuel for their vehicle. Additionally, when a vehicle begins to knock and ping under acceleration, many owners apply a "quick fix" by switching to premium grade fuel. It is true that excessive and continued pinging and knocking is harmful to an engine and this MUST be attended to immediately or serious engine damage will result. Very few recently manufactured vehicles require premium, high octane fuel. To operate these vehicles with lower than required octane fuel could lead to immediate and serious internal engine damage. Most engines that do ping or knock on light acceleration do not need premium fuel; these engines need proper servicing such as timing adjustments, repairing vacuum leaks, or servicing emissions control valves. To operate a properly tuned engine designed for 87 octane with 89 or 91 octane fuel will only increase the cost of operation. No additional power, fuel economy, or durability will result from the higher octaned fuel.

ANSWERS TO INFORMATION CHECK:: 3. False. To efficiently burn any fuel of any octane, the compression ratio of the engine must be high enough to release all the potential fuel energy.; 6. False. Diesel engine fuels are rated in cetane numbers, not in octane numbers.; 7. False. Engine knock or ping may be a result of too low an octane rating for that engine's compression ratio; most engine knocking and pinging is a result of an out-of-tune engine or emissions control servicing.

RECOMMENDED READING:: Ellinger, Herbert E. Automechanics, Fourth Edition. Prentice Hall, Englewood Cliffs, New Jersey. 1988.; _____ United States Department of Energy. Energy Conservation Information for Vehicle Owners. U.S. Government Printing Office: 1991-289-606.; Shell Oil Company. How to Get More Miles Per Gallon. The Better Mileage Book. 1991.

Comments or questions to: TechAsmt@LA.GOV

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