Octane

A gasoline's octane number is a measurement of its antiknock capability. Increasing the octane from 87 (regular) to 89 (mid-grade) or 91 (premium) does not increase power or miles per gallon, nor does it keep the engine's internal parts cleaner. Increasing the octane only increases a fuel's resistance to spark knock.

In 1985, when port fuel injection was new, the plugging of tiny passages in the injectors with fuel deposits became a serious concern. Because deposit control additives were not available in many regular-grade fuels, higher grades were sometimes recommended to keep injectors clean. Today, redesigned injectors are more resistant to plugging, and deposit control additives are generally used in all grades of fuels.

With today's fuels, there is absolutely no reason to use high-octane fuel except to reduce spark knock or pinging.




Spark Knock - Knock occurs after the spark and before all of the air-fuel mixture has burned smoothly across the combustion chamber. The increasing heat and pressure may cause spontaneous ignition of the last 3 to 5 percent of the air-fuel mixture before normal combustion consumes it. This mini-explosion causes a vibration that is heard as a ping or knock. Excessive spark knock over a prolonged period of time can damage the engine.

One reason most of today's engines do not need premium gasoline is the Ignition Control (IC) system. The IC system's knock sensor (detonation sensor) reports any pinging to the Powertrain Control Module (PCM), which retards ignition timing. Delaying the spark produces less build-up of heat and pressure - so the end gas does not explode, but burns properly. However, retarding the timing reduces engine efficiency; so the system operates only when necessary.

Another way to reduce knock is to use combinations of the basic hydrocarbons that offer more resistance to knock. Adding lead to the gasoline will also increase the octane but fouls catalytic converters. Another way to increase octane is to add "oxygenates," that is, chemicals composed of hydrogen, carbon and at least one oxygen atom. Increasing the octane enables the fuel to withstand greater pressure and heat. Under normal conditions, regular unleaded 87 octane fuel is sufficient for today's vehicles.

Octane Tests - Octane testing measures the compression ratio at which a gasoline will knock. The octane number is an average of the Research Octane Number (RON) and the Motor Octane Number (MON). Both tests use a single-cylinder variable compression-ratio test engine to compare a fuel sample to a known mix of two liquid hydrocarbon fuels (isooctane and normal heptane). Isooctane is arbitrarily rated at an octane number of 100 and normal heptane is rated at zero. A test sample of 87 percent isooctane and 13 percent heptane receives an octane number of 87. Any gasoline sample which matches the laboratory sample's performance in the engine will be rated 87 octane.

Reid Vapor Pressure (RVP) Test - The Reid Vapor Pressure (RVP) test indicates the fuel's tendency to evaporate early. The test also indicates how the engine may perform during a cold start and warm-up. The higher the RVP, the higher the volatility.

RVP is measured using a Reid vapor pressure tester. A gasoline sample at 32°F is placed in a sealed container that is attached to a pressure gage. The container is then placed in a 10°F water bath for two to five minutes. The gage reads the pressure in pounds per square inch (psi) exerted by the vaporized fuel within the sealed container. This test can be performed using the Saturn Service Fuel Test Kit SA9213NE (or equivalent).

According to more stringent standards effective on June 1, 1992, except in Alaska and Hawaii, the EPA limits RVP to 9.0 psi maximum between May and September 15. In "ozone non-attainment" areas in the southern and western U.S., the EPA limits RVP to 7.8 psi maximum between June 1 and September 15. The EPA permits these maximum vapor pressures to be 1.0 psi higher for gasoline-ethanol blends that contain (by volume) 9 to 10% ethanol.