Page 1 : Octane Rating, Energy Density, Better Performance?
The recent introduction of the RON 100 Euro 4M fuel has ignited yet another debate over the octane rating and how it affects the performance and fuel efficiency of our cars. According to the age-old claim, a higher octane rating delivers better performance and fuel efficiency.
When challenged to prove that claim, supporters of the higher octane rating fuel will point to the fact that high-performance cars use them to achieve better performance. Are they correct? Let’s find out…
Octane Rating Primer
First of all, RON is short for Research Octane Number. It is a ROUGH indicator of the maximum compression ratio at which a particular fuel can burn without spontaneously detonating. You don’t want your fuel detonating because that damages the engine, while greatly reducing performance and fuel efficiency.
RON is actually just one of TWO octane ratings used to measure the fuel’s ability to withstand spontaneous detonation in the engine. The other one is MON (Motor Octane Number). The RON rating is based on a low engine load, while the MON rating is based on a high engine load. A fuel’s MON rating can be up to 10 points lower than the RON rating.
In Asia and Europe, the number you see at the petrol stations are RON numbers. In the United States, they use the average of the RON and MON numbers using the formula of (R+M)/2. So if a fuel has a RON of 95 and a MON of 85, it will be listed as a RON 95 fuel in Asia and Europe but a 90 octane rating fuel in the United States.
This doesn’t mean that the fuel in America is of a lower RON rating or performance. They are the SAME FUEL. It’s just how the SAME octane numbers are reported. In Asia, we only report the RON rating, while the Americans report the average between the RON and MON ratings.
Petrol (or gasoline to the Americans) have an energy density (or specific energy content) of about 42.4 MJ/kg. That is how much energy is in the fuel, not how much it delivers. 70-75% of that energy is lost as heat generated during the engine combustion, leaving only 20-30% of that energy to actually move your car.
The energy density of the fuel at the pump can be higher or lower by up to 4%, depending on the fuel blend used by the refinery, and regulations set by the country. In some countries, the blend changes with the season, yielding slightly better or poorer performance and fuel efficiency with a corresponding increase or reduction in pump prices.
Note that this has NOTHING to do with the octane rating of the fuel, just its blend. That is because the octane rating is NOT a performance indicator. In fact, it does not even correlate with the energy density of the fuel blend. You can have a denser fuel blend with a low octane rating, and a lighter fuel blend with a high octane rating.
Higher Octane Rating = Better Performance?
If you drive a sports car, it will likely use a high-compression engine. The high compression delivers more power and better fuel efficiency by stuffing more air into the cylinders. However, such high-compression engines require higher octane fuels. Not because they have more power, but because such fuels will not spontaneously detonate in the cylinders from the high compression pressure.
In such sports cars, you will usually find two RON numbers – a minimum octane rating, and an optimal or recommended octane rating. The Audi RS4 Avant, for example, requires a minimum of RON 95 but works best with the RON 98 fuel.
For maximum power and fuel efficiency, Audi RS4 drivers should use the RON 98 fuel. Using the RON 95 fuel will result in a slight reduction in power and fuel efficiency at high engine loads. This is NOT because the lower octane fuel is less powerful, but because the RS4 engine will change its timings slightly to prevent the lower octane fuel from spontaneously detonating at high engine load.
The Audi RS4 can actually use even lower RON fuels – as low as RON 91. However, this will result in reduced performance and the car should be driven gently. Remember that the RON number is based on low engine loads. At higher engine loads, you will need a higher RON rating to avoid engine knocking.
Not all sports cars require high octane fuels though. This is a popular misconception. Take the Nissan 370Z, for example. It only requires RON 95 fuel to perform optimally and can run on RON 91 fuel in an emergency. Again, when using RON 91, it should be driven gently to avoid engine knocking.
Most other cars, however, will only list a single octane rating, which is both the octane rating they are tuned for and the minimum octane rating you should use. You can use higher octane fuels but you will not derive any performance benefit because the engine is not capable of higher compression ratios that would take advantage of the higher octane.
Unless your car manufacturer specifically tells you that your car is optimised for a higher octane rating, you are just wasting your money buying a higher octane fuel. It will not make your car go any faster.