Monday, January 25, 2010

Regular Car, Premium Gasoline

An increasing number of regular looking vehicles in the US are requiring high octane premium gasoline.  These vehicles include the tiny Smart fortwo and a variation of the Volkswagen Jetta.

Is this premium fuel requirement a stealth move for some auto manufacturers to gain market share?  Has this increased sales of high octane gasoline relative to regular octane gasoline in the US?

Octane Ratings Explained
Octane ratings are the numbers printed on a gasoline pump often labeled regular, midgrade and premium.  In the US regular gasoline is greater than 85 and less than 88 octane, midgrade is equal to or greater than 88 and less than 90, and premium is equal to or greater than 90. In Europe and Asia the number on the pump for equivalent fuel is usually 5 points higher.  In other words 87 on a pump in the US is equivalent to 92 on a pump in the UK.

Octane rating has little to do with the energy content of the gasoline.  It has all to do with how much volume of the gasoline can be squeezed, or compressed, into an engine cylinder without the gasoline igniting before the spark plug fires.  Gases heat up when compressed and gasoline engines are tuned to compress the gasoline as much as possible without compressing it so much the fuel burns without a spark plug.  The goal is for the timed and synchronized spark plug to ignite the fuel when a piston reaches the top of the combustion chamber.  If ignition due to compression (rather than the spark plug) occurs then an engine can be damaged. 

Many new cars requiring high octane fuel can tolerate slightly lower octane because their computers adjust for the less compression ignition resistant fuel, but there is typically a lower power output and one should only do this if a vehicle manufacturer says it is ok for your engine.

As gasoline with higher octane ratings resists compression more than regular can be added to the combustion chamber on each stroke of the engine.  This higher compression tolerance allows more more gasoline to be burned and more power on each stroke.  This is why racing cars use high octane gasoline.

Why don't all cars use high octane gasoline? There is a substantial incremental cost for a refinery/gasoline blender to raise the octane of gasoline.  Higher octane requires expensive gasoline blendstocks and consumers are sensitive to increased gasoline prices. 

The additional fuel cost to a driver for high octane gasoline compared to regular is around US$100-US$200 each year (based on 10,000 miles per year in a 25MPG vehicle).

Should one fill up with higher octane than a vehicle manual suggests?  No.  It will likely be a waste of money without any improvement in performance.  If your vehicle manual recommends regular octane then just use regular. 

Use high octane gasoline if you have a vehicle with a high compression engine and your vehicle manual recommends it.  However, as mentioned above, some vehicle manufacturers say their engine computers can adjust to handle lower octane rated gasoline.

High Octane Gasoline No Longer just for Sports Cars
In the past high compression gasoline engines were primarily used in sports cars and luxury high performance vehicles.  Now more vehicle manufacturers are putting high compression engines into what would not generally be perceived as being a sports car.

One reason vehicle makers are doing this because it improves the statistics consumers look at when purchasing a vehicle.  A high compression engine running on premium will accelerate a vehicle from 0 to 60 MPH faster than a low compression engine.  Consumers rarely ask a car dealer which octane rating of gasoline a vehicle burns.  They do ask how quick the vehicle acceleration is from a standing stop and on a test drive will notice the peppiness and responsiveness of a high compression engine. 

Has Demand for Premium Gasoline Changed?
In summary, the number of vehicles requiring premium (high octane) gasoline (rather than midgrade or regular) has been increasing in the US because vehicle manufacturers have realized that high compression engines can increase vehicle sales by enhancing the statistics consumers look at and the test drive experience.  One could surmise, therefore, that the percentage share of premium gasoline being sold in the US has been steadily increasing as these new vehicle owners dutifully follow vehicle manufacturers' recommendations. 

The data shows that most of these high octane requiring vehicles are being filled with lower octane fuels.  The percentage of premium relative to total gasoline sales has been steadily falling (see chart below): 

(click chart to enlarge)

It is likely the nod from some vehicle manufacturers that onboard computers can adjust newer high compression engines to handle lower octane gasoline that gives comfort to some owners.  Another way of looking at the data is that these new, often unsuspecting, high compression engine owners are attempting to save at the pump having hastily splurged on a sprightly engine.

(Why is "octane" the name used for the compression resistance of gasoline?  Read Oil 101, Chpt. 9)
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