Premium Versus Regular Gasoline
Within the petroleum industry, what we commonly call gasoline is referred to as "motor fuel." All motor fuels are blends of various hydro-carbons, which are long chain molecules consisting as the name implies of hydrogen atoms and carbon atoms, that are bonded in various ways, there are heptanes containing 7 atoms, pentanes containing 5 atoms, and octanes which naturally contain 8 atoms. Actual fuels are more complex and bear names like 2,2,4-trimethylpentane.
The refinery chemists who blend the actual fuel stocks do not know, nor do they need to know the exact ratios of heptanes, pentanes, etc, in a given batch of motor fuel, the fuel stocks are combined in whatever proportions are needed to secure the desired octane number and the desired vapor pressure.
Vapor pressure determines whether a batch of motor fuel is a summer or a winter blend. Summer blends have high vapor pressure so that the fuel doesn't boil off when the car is left baking in the sun and the temperature in the fuel tank rises to 120 degrees or more. Summer blend gasoline is sold all year round at high altitudes. Winter blend motor fuels have low vapor pressure. Most automobiles will get slightly better gas mileage when burning Winter blend.
Early automobile engines burned gasoline, which was a condensate that collected in natural gas pipelines. Gasoline often had an octane number as low as 50 which means that it had a tendency to explode or detonate and so early gasoline engines had low compression and could only run at low rpm (900-1400 rpm was typical). In order to create more compact fuel efficient engines it was necessary to increase both compression and rpm. The change to blended motor fuels allowed refinery chemists to produce various grades of gasoline with standard octane numbers.
Higher octane fuels burn more slowly and with less tendency to detonate in the engine cylinder. Remember that at cruising rpm the spark occurs about 15 degrees before the piston has reached top dead center. Also bear in mind that the piston is at a complete stop at top dead center and is moving at a slow speed as it starts is descent on the power stroke. You can see why a fuel that explodes creates an enormous and destructive pressure above the piston. This is accompanied by a distinctive knocking sound.
The actual octane number is arrived at by creating blends of two known hydrocarbon fuels, 2,2,4-trimethylpentane which is better known as isooctane which has good antiknock tendencies and is arbitrarily assigned the value 100, and heptane which has poor anti-knock qualities and is assigned the value 0. A blend of 60% isooctane and 40% heptane would have an octane number of 60. A blend of 80% isooctane to 20% heptane has an octane of 80. The actual octane of a batch of motor fuel is determined by burning it in a special test engine whose compression can be varied. The compression ratio is adjusted until knocking is detected and this point can be compared with known blends of heptane and isooctane.
There are six methods of defining the octane number of a motor fuel. The two that are used to define automotive fuel are the Research Octane Number (RON) and the Motor Octane Number (MON). The number that appears on the fuel pump is called the anti-knock index number and is the average of the RON and the MON and can be expressed as RON+MON/2.
Typical automobile engines up through the 1980's were designed to run well on 87 octane motor fuel. High performance engines featured higher compression ratios and thus require high test motor fuel. Today, the picture is more complicated. Engine makers have known since the 1920's that the easiest way to increase fuel efficiency of an engine is to raise the compression ratio. Modern automobile engines feature higher compression ratios and operate at much higher rpm. The Smart Car for example runs on expensive premium motor fuel.
The most sophisticated engines have complex computer-based fuel management systems that alter ignition timing and valve timing for maximum efficiency. These engines can often be run on either regular or high test motor fuel but will get better fuel efficiency on high test.
During WW II, hi-performance aviation fuels were made with octane numbers as high as 140-145. Currently the highest octane aviation fuels have octane numbers of 110-115.
If your automobile engine operates without knocking or pinging on regular grade fuel, there is absolutely no advantage to purchasing higher test fuel. You should only use a higher octane motor fuel if you engine knocks and pings on regular. So long as the octane number is the same, all brands of motor fuel are the same. You can safely use "bargain brands". What you wish to avoid is water in the fuel and fuel containing sediments which will clog your fuel filter. Go to stations that have high volume sales and a final filter on the pump offers some assurance against sediment contamination. Happy motoring!
