Hello, and Good day to you Steemians, I'm glad to be with you again.
I've come today to discuss something many of us see almost everyday, but either do not know it's exact significance or have the wrong idea about it. However, before I go into depth of the discussion, let me tell you what aroused my interest in the topic.
In my country, petrol is "officially" referred to as Premium Motor Spirit (PMS), at least that is what our journalists, academicians and major petrol marketers and service stations tell us. In fact, the first time i took a Fluid Mechanics class, the lecturer's idea of determining the brilliant students was by asking to know those that know this "official" name of petrol. I don't think I need to tell you where I'm from, because if you happen to run Google Search on the phrase "Premium Motor Spirit", in less than 3 seconds you'll know my country. If you're my compatriot, the next paragraph may shock you, for if you've even taken pride in knowing that official name, your bubble is about to burst.
Three days ago, I decided to check the origin of the phrase, for I was interested in what the British or Americans (or whoever else coined it) were thinking, when they chose the phrase for petrol. Alas, I realized that I'll have to look inwards to my compatriots, as the phrase is only used in my country, in fact chances are that if you find it in any article outside my country, the author is still my compatriot. Why? Simple answer : it is not, and it never was petrol's official name.
So, how did it come to be what lecturers in my country will tell you is the scientific name of petrol? Well, it becomes easier to answer that question if you remove the "Premium" and you are left with "motor spirit". Well this took me down the history of petrol in Britain, when it became the fuel for internal combustion engines. The marketers then for a very short while referred to it as "motor spirit", but later, they all adopted the name petrol, such that the term became extinct (in Britain, at least). Now we know about that right.
Where along the line did "Premium" join the story though? The answer is simple and many living outside my country will even understand it better than those living here : Premium was supposed to describe the grade of the petrol, in the United States for example, you'll find Regular, Plus and Premium grade gasoline (gasoline is another name for petrol, don't worry, this one is actually official).
By now, I'm sure you're beginning to remember those times you've had to make a choice between these grades if you live in countries where petrol's are graded, since there is a significant difference in the price of these grades. However, have you wondered the basis upon which these petrols are graded and what each grade offers, whether you shouldn't even bother about paying the extra cost? Well, let's say I did the wondering for you, just come with me as I tell you what I learnt in "wonderland".
Here is a straight fact : In the United States Petrols are graded as either Regular, Plus/Mid or Premium grade based on the Octane number of the petrol. Some refer to it as Octane rating. Depending on whether your country sells petrol by their grades (My country doesn't), you probably would have seen something like this at your service stations.
What really is Octane number? How did it come about, what is it's significance, does it also apply to diesel fuel, what do you benefit from knowing all these? These and some other questions will be answered here. Please stay with me.
To easily understand Octane number, we need to quickly take a class in Spark Ignition Engines (otherwise called petrol engines) operation. These engines work by burning fuel in the engine cylinder, the burning takes place when a spark is released into the combustion chamber at the right time during the compression stroke of the piston. If you want to know more about these process,please see this post by @rharphelle to understand better.
Notice that there was emphasis on the right time, that is because the engine runs in a timing which determines when the spark plug fires during the compression stroke, any combustion other than this timed combustion causes problems. Now, you will be forgiven for thinking the spark from the plug is the only cause for conbustion in petrol engines, well, that is not true. Two scenarios can occur inside the engine where combustion takes place without the spark from the plug. Check them out :
- Knocking or Detonation : Please if you have ever been told that an engine is knocked when the moving parts become stiff due to lack of lubrication, please drop that notion here. You remember that for combustion to take place, the spark from the plug ignites the fuel and air mixture, now the engine is designed such that this flame spreads uniformly to burn the air and fuel mixture, such that uniform power will be transferred to the piston.
However, it sometimes occur that some parts of the air/fuel mixture ignite separately on their own, before the flame propagation from the spark plug reaches them, such that you have two flame fronts, and the piston doesn't receive uniform power, instead there is an imbalance in power transferred to the piston as the engine is not normally designed to function that way. This is knocking. The name comes from the "knock" sound caused by the shock waves that the irregular combustion of the air/fuel pocket had on the piston.
Remember, the detonation didn't come from the spark of the plug
- Pre Ignition : People usually confuse this with knocking, but there is a technical difference, Pre-ignition happens before the spark plug ignites the mixture, not after. That is the air/fuel mixture burns or ignites even before the spark plug fires.
How bad are these two cases? For one, knocking, if continued will eventually spoil your engine, but it takes time to do so, since it occurs at the same time combustion is expected to take place, albeit abnormally.It also leads to a reduction in engine performance.
However, Pre-ignition will damage an engine very fast, because the timing of the ignition is way out of sync with proper ignition timing, so the engine easily and quickly feels the damage.
Now, let's make a summary, something is common to both processes, the fuel attained ignition itself without help from the spark plug. So you can see that without help from the spark plug, petrol can still ignite itself. This is caused by many reasons, including the compression of the fuel by the piston which has raised its pressure and temperature, sometimes aided by incandescent carbon deposits, or a spark plug that is too hot for that engine etc. However, something else was seen, they are both undesirable in Engine operation, they will reduce engine performance and eventually damage it. Well, the problem is solvable.
It is only fitting then that the adage that best suits this matter is there is no flame without fire, and in this case we have located the fire to be the ability of the petrol to ignite itself, at least all we now have to do is put out the fire, well we didn't get here first. Engine manufacturers noticed these problems much earlier and sought a way to address it. In conjunction with Petroleum refiners, they found a solution, that solution created a system of rating that gave birth to OCTANE NUMBER
Since the main factor behind the self ignition of the petrol is the compression of the fuel by the piston, and different engines have different compression ratios( high performance engines have higher compression ratios, thus the fuel experiences higher pressure for the combustion to produce more power).
It is then logical that if you have two engines, say engine A, a regular engine and another one, engine B, a high performance engine. If you put the same fuel in both engines, you may experience knocking and pre-ignition in engine B without experiencing it in engine A, why? because that same fuel is subjected to more compression and subsequently more pressure in Engine B than in Engine A. So, we can conclude that the fuel can withstand the pressure in Engine A without igniting itself, but the same fuel will ignite under the pressure of Engine B.
Yes, depending on the constituents of petrol grades, they can withstand different levels of pressure before igniting themselves. It was discovered that the more Iso-Octane (IUPAC name - 2,2,4-Trimethylpentane) you have in your fuel, the less likely it is to ignite itself. However, the more of Heptaneyou have in your petrol, the more likely it is to ignite itself.
So a system of rating was made for petrol to tell the ratio of both compounds present in the petrol, so that you can know which suits which engine best.
If Engine B cited in the example above uses the right grade if petrol, there will be no knocking or pre-ignition
On the rating system, Heptane is given Zero(0) points and Octane given hundred (100)points, such that when you are told that petrol has an Octane number of 85, it means that the fuel has the same resistance to knocking as a blend of 85% Iso-octane and 15% Heptane does.
What if the Octane number is 100, it would mean that the petrol has the same resistance to knocking as a blend of 100% Octane and 0% Heptane does. It means this fuel can withstand more pressure without knocking or pre-igniting than the one with 85 Octane number.
Well, now I am comfortable to give you the dictionary definition of Octane rating trusting that you will understand it even if you're not very familiar with engines. I really hope you do. According to Merriam Webster Dictionary .
a number that is used to measure the antiknock properties of a liquid motor fuel (such as gasoline) with a higher number indicating a smaller likelihood of knocking
Now we know what Octane number is, however there are serious misconceptions about the Octane number that we need to clear :
Does a low Octane number mean that the fuel is bad?
NO, not necessarily. As indicated above, the Octane number is only telling you the strength of that particular fuel against antiknock, so if it is the range that can withstand your Engine's pressure, it is not bad. For this reason, in the manual of every car, you will find the Octane number of the petrol grade your vehicle should use, that is the best grade for your vehicle, to perform optimally, it doesn't mean that petrol with lower grade is bad. Your neighbor, for example, may be using a vehicle with lower compression ratio, thus be able to use the fuel with lower Octane rating than yours for his own engine's optimal functioning.
Does a high Octane number mean that the fuel will make your engine perform better?
NO. So long as you're using the grade of fuel recommended for your vehicle, if your engine is not performing optimally, it is not due to the fuel's Octane number. To make it clearer, let's say the recommended Octane number for your vehicle is 85, using a fuel with an Octane number of 95 adds nothingto the performance of your engine.
An analogy for further help, if you're pumping water into the overhead tank in your house, does pumping more water into an already full tank increase the quality of water you get? Certainly not,instead you will be wasting the power spent in pumping the extra water.
In the same way, buying fuel with a higher Octane number than is required for your vehicle won't increase the performance of your engine, it will however increase something- cost. And since you are getting nothing for the extra cost, that will be a waste.
To determine Octane ratings, fuels are tested by running them in test engines and comparing the values obtained with those obtained for a mixture of Iso-octane and Heptane. We have three types :
The RESEARCH OCTANE NUMBER : Usually abbreviated as RON is determined by running the fuel in a test engine running at 600 rpm. The engine will have a varying compression ratio in a controlled manner. The result is then compared to the results for the blended mixture of Iso-octane and Heptane.
The MOTOR OCTANE NUMBER : Usually referred to as MON. According to Dabelstein (2007), this is also determined in an engine similar to the one used for RON, however the engine speed is higher at 900 rpm, the fuel mixture is also preheated and the ignition timing is varied. All of these are done to further see the limit of the petrol's knock resistance.
If you live in Europe or in countries like Australia and New Zealand, the Octane number you see at the filling station is the RON. However, in countries like the United States, Brazil or Canada, what you find is the AKI
ANTI KNOCK INDEX : Usually abbreviated as AKI. MON values are usually higher than RON values by 8 to 12 units, so the AKI is an average of both values. You will either see it as AKI, (R+M)/2 value or PON (PON means Posted Octane Number).
Octane Numbers and Altitudes.
As you go higher above sea level, there is a decrease in atmospheric pressure. For this reason, the compression in your engine decreases as you travel on higher altitudes, so your fuel doesn't necessarily have to withstand as much pressure from compression as it would at lower altitude. So, a fuel of 85 RON can behave like a fuel of 87 RON at higher altitudes.
The principle of operation of Diesel Engines ( also Compression Ignition engines) is a bit different from that of Petrol Engines. Instead of relying on the spark from the spark plug to ignite the aur/fuel mixture, Diesel engines rely on the ability of the diesel fuel to ignite itself after it has been compressed.
If you have been following me, you will realise that what is causing the Petrol engine problems is actually the principle upon which Diesel engines work. So you will agree with me that having a diesel fuel that can ignite itself at lower pressure will even be a blessing. This is the reason why Octane ratings do not apply to Diesel Engines, instead the number for eating Diesel Engines is called Cetane Number.
So, the Cetane number measures how much a fuel delays before it ignites and the amount of compression it requires to ignite, due to the presence of Cetane (IUPAC name-calling Hexadecane). So the higher the Cetane number, the quicker the fuel ignites under compression. That shows it is the inverse of Octane number.
The Cetane number ranges between varying values depending on countries and regional regulations. However, they are all between the range of 40 and 60.
There are other hydrocarbons with better antiknock properties than Octane, so when rated on the Octane rating system, such fuel will have an Octane number higher than 100.
If you live in Lagos, Nigeria, for example, the RON of the fuel you will get from the filling stations is between a minimum of 90 and a maximum of 93, according to a research conducted by Ikeora et al 2015. However, some vehicles require Octane rating of up to 95, so in that case you may want to employ [Octane boosters].
We all have a lot to thank that Chemist named Russell Marker for. Through his development of the Octane rating system in 1926, he has saved a lot of engines, saved us cost, power and even stress. He is dead now, but everytime we see the Octane numbers and remember their significance, he comes alive in our thoughts.
REFERENCES
W. Dabelstein, A. Reglitzky, A. Schütze and K. Reders "Automotive Fuels" in Ullmann's Encyclopedia of Industrial Chemistry 2007, Wiley-VCH, Weinheim.doi:10.1002/14356007.a16_719.pub2
C. O. ikeora, H. N. Okoye. 2015. Analysis of Premium Motor Spirit (PMS) Distributed in Lagos Metropolis. Middle-East Journal of Scientific Research 23 (7): 1321-1326, 2015.
OCTANE RATING | SELECTING THE RIGHT OCTANE FUEL | CETANE NUMBER
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