Firstly, keep in mind that a gasoline’s octane rating is simply a measurement of the fuel’s ability to resist engine knocking. It does not refer to a substance or to the quantity of energy or power in the fuel. More correctly, an octane rating is often called an “anti-knock index”. When unburned gasoline vapours spontaneously explode in the cylinder before the expanding flame in the combustion cylinder reaches them, it actually causes two simultaneous explosions (the other is from the spark plug). This results in a knocking or pinging sound, and when an engine knocks the result is a reduction in the power it delivers. The higher a fuel's octane number, the higher its resistance to engine knock. Technically there are three different "octane numbers" associated with every gasoline. The Research Octane Number, or RON, is measured under fairly easy test conditions. The Motor Octane Number, or MON, is a tougher test measured at higher engine speed and temperature. The value that relates most closely to actual driving conditions is the average of these two values: Road Octane Number = (RON + MON)/2. This Road Octane value is the one referred to in Shell stations: Shell Bronze gasoline has an octane rating of 87, Shell Silver is 89 and Shell V-Power is 91. Occasionally, less scrupulous Canadian gasoline outlets will use the confusion of these different octane measurements to exaggerate their octane rating claims, by advertising their fuel's Research Octane Number - which will be higher than the Road Octane Number. It is also a common practice in many European countries to advertise the Research Octane Number on their pumps, so you may see unexpectedly high octane values when travelling abroad. In Canada, motorists should always be sure that the octane number a vendor advertises is its Road Octane value, not its RON. Engines in vehicles built for sale in North America are designed to a specified octane requirement to make sure they don't knock or ping (engine knocking reduces the amount of power it can deliver to turn the wheels). Once that octane level has been met, in normal instances your car will not experience more power or better mileage if you use a higher octane fuel. In other words, if your Canadian vehicle owner's manual specifies an octane rating of 87, running on gasoline with an octane rating of 89 or 91 or more will normally not make a difference. However, there are some exceptions to this, and it must be noted that if the owner’s manual specifies using gasoline with an octane rating of 89 or 91, then that’s what you should use because that’s what the engine is designed and tuned for. One - and a very important - exception is that when a vehicle gets older, the normal build-up of fuel- and lubricant-related deposits in the engine can increase the fuel octane number a car requires to prevent engine knock. For this reason, if a car more than a couple of years old experiences engine knocking, the problem may be solved simply by moving to the gasoline with the next-higher anti-knock index. You will also benefit from a higher octane rating if:
- the engine is knocking on the gasoline you are using, or
- you have knock sensor ignition that can take advantage of higher octane values at times of peak demand, or
- high summer temperatures, mountain driving, pulling heavy loads (including loaded roof racks) has temporarily increased your engine’s anti-knock index requirements.
While fuel with a higher octane rating cannot of itself deliver more power, in the above circumstances it can give an engine a greater ability to resist knocking, thereby helping it to deliver the full measure of power it is designed and tuned to provide. In these cases, where drivers require their vehicles to continue providing peak performance despite challenging driving conditions, for improved responsiveness we recommend using a mid-grade fuel like Shell Silver (octane rating 89) or a premium fuel like  Shell V-Power™ (octane rating 91) gasolines. One other exception is found in high-performance vehicles. The use of more complex computer algorithms in their engine control systems to control spark timing using one or more knock sensors, enables better performance on higher-octane fuel. Owner’s Manuals for most of these vehicles will specify use of 91 octane fuel. Is it safe to clean things with gasoline? Never! Repeat: Never! Gasoline is tricky stuff, and Shell cares about its safe use. Gasoline vapourizes very easily, and the invisible vapours are heavier than air. They can spread across the floor of a garage to a water heater, an electric motor, a car's engine or something else that can swiftly ignite them. Never use gasoline as a cleaner, charcoal briquet starter or as a solvent. Don't use it for anything except as a fuel for a gasoline engine. Remember: the vapours from just one cup of gasoline have the explosive force of five sticks of dynamite. There is a separate section on gasoline elsewhere in this site. There is a more extensive look at fuel safety elsewhere in this site. Is it okay to continue filling my gas tank to the brim after the nozzle automatically shuts off the flow? Not really. When your tank is full at, say, $29.22 and you keep filling it to $30, you may actually overfill it. An over-filled tank may leak from the filler neck due to heat expansion, or spill over from the stop-and-go movement of the car in traffic. Is there a difference between the gasoline sold in summer and that sold in winter? Yes. There is a difference in the volatility, and Shell adjusts the volatility of its gasolines many times a year to meet changing climatic conditions. The more volatile a petroleum liquid like gasoline is, the lower its boiling point and the greater its flammability. The volatility of a petroleum product can be precisely determined by its evaporation rate. Volatility that is too high can cause "bubbles" of vapour in the fuel, which in turn can cut off the fuel supply entering the carburetor or injector. It can also lead to stalling on hot spring and summer days (it is then known as vapour lock). Volatility that is too low can create problems when starting and warming a cold engine, since it is more difficult to ignite the less-volatile fuel. Low volatility can also cause excessive engine deposits. Altering the volatility of summer and winter gasolines is important in controlling cold engine start-up, general driveability (including idle quality, stalling and hesitation), vapour lock, carburetor icing and fuel economy. When I first start up my car, I can smell gasoline, and then the smell goes away. Why does this happen? Can it be dangerous? This is likely caused by poor vapourization of a portion of the fuel entering your car's carburetor. In order to produce a quick vapourization of gasoline, the fuel is sprayed into the air flow passing through the carburetor. The gasoline is vapourized into small droplets to mix it with the air. During startup, the carburetor must deliver a very rich fuel/air mixture of about nine parts air to one part gasoline. Because of cold engine temperature and poor air flow, some gasoline will not vapourize properly; it remains a liquid and produces the distinctive gasoline vapour you have noticed. The situation isn't dangerous, but if it persists or becomes more noticeable it could mean the carburetor is in need of adjustment or overhaul. When I fill up my car, I notice that there's sometimes a little glass sphere somewhere on the side of the pump. The label says it must be full both before and after using the pump. What is it? It's called a "sight glass". The federal Weights and Measures Act requires that gasoline must be circulated through it so motorists can see that the gasoline they are pumping into their tank is a quality product free of air, sand and other contaminants - and that it is actually moving.
| 
