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In my country are sold gas with different octane number (RON: Research Octane Number): 88, 90, 92, 95, 97, 98, and the latest is 100. RON-88 is still sold since still many old cars, motor bike, and agriculture machines which are using low spec gas. RON-90 is intended for transition, as alternative in case customer want to move forward using better gas but not so expensive. In our country, the higher RON, the more expensive.

As we know, every engine is designed with a specific compression ratio, and that RON is very related to the compression ratio. But many times I read explanation from automotive services claiming that higher RON will be better for car. Not specifically explained which car and what kind of car. Indeed we know that most cars are now having compression ratio 10:1, which according to the petrol producer that kind of is car suitable using petrol with RON 92-95. In my understanding, the compression ratio is related to the Top Dead Center (TDC), which it will produce specific compression that will compress the petrol inside ignition chamber, so it will increase temperature just before ignited by the spark. That compression made engine got hotter. If an engine with specific compression ratio is fed with petrol with RON-88, the petrol will be burned before TDC. Vice versa, if that kind engine is fed with RON-100 petrol, than the petrol will NOT be completely burnt. Hence, the automotive service claim is wrong.

So my question are: 1). am I right with my understanding? 2). for that 10:1 engine, if it fed with lower or higher RON, what will be the impact to the engine?

AirCraft Lover
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  • It is the air-fuel mixture that is compresed and that is mostly air as liquids are not compressible, which is why you get hydraulic lock. – Solar Mike Nov 13 '18 at 04:44
  • Have you searched on here and the Motor Vehicle Maintenance and Repair Stack? If not, you should before posting. – Solar Mike Nov 13 '18 at 04:46
  • See https://engineering.stackexchange.com/q/12421/10902 – Solar Mike Nov 13 '18 at 04:59
  • Also https://mechanics.stackexchange.com/q/498/10976 – Solar Mike Nov 13 '18 at 05:02
  • I didn't see the answer from both the links. The first is talking about fuel price related to its RON. The second is talking about engine, but not discussing about the impact of "wrong" octane number to an engine. – AirCraft Lover Nov 13 '18 at 10:52
  • I posted those as examples of what you MAY possibly find if you did a search on both Stacks.... I see you still say that you "compress the petrol inside the ignition chamber ..." Did you not follow the diagram about the knock resistance? – Solar Mike Nov 13 '18 at 11:32
  • Here is the answer I got from another thread. This answer I have posted to reply the below comment. – AirCraft Lover Nov 13 '18 at 11:36
  • The question you asked in the title has a lot of information in this answer :https://mechanics.stackexchange.com/a/508/10976 – Solar Mike Nov 13 '18 at 11:36
  • As this question also I posted in another thread, then I got answer as every engine is design with specific CR, so the fuel must be match to the design. According to the thread, higher RON will leave slag due not all the fuel is burnt (as higher RON will required higher temperature to burn). But feed the engine with lower RON will make knocking/detonation. This explanation seems make sense as the petrol producer said the CR for every petrol they sell. – AirCraft Lover Nov 13 '18 at 11:36
  • "the petrol producer said the CR for every petrol they sell" really? Don't think so as some petrol is used in engines with an 8 to 1 compression ratio and also in other engines with a 12 to 1 compression ratio... I'll let you figure it out.... Oh and each engine may be designed with a compression ratio, but it is often and easily altered either during production runs or later. – Solar Mike Nov 13 '18 at 11:38
  • Initially, since 2000 to 2010, the petrol producer is advertised that **higher octane number is better for new car. The km (mileage) will be some for the same money expensed**. That was when they only sell RON92. But more time, more car with new/higher CR. The petrol producer also sell petrol with RUN 95, then later RON 97. Some people with common car which the CR is mostly 10:1 to 11:1, feed their car with higher RON, RON97. Then many complained as the the claimed is not true that the mileage will be same for the same money and more dame complain. Then the produces rectify the claim in 2012. – AirCraft Lover Nov 16 '18 at 04:06

2 Answers2

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Almost all gasoline-fueled engines have ignition systems which under high speed conditions will spark the charge in the cylinder before top dead center, so as to furnish enough time for the charge to burn to completion before the exhaust valve begins opening. So the issue is not whether or not the spark plug goes off before TDC, it is whether or not the fuel will combust smoothly or explode (detonate) with sudden violence.

The octane number is a measure of how resistant the fuel is to detonation, which is very bad for an engine that is designed to run on gasoline and so must be avoided. High compression ratios strongly favor detonation, so engines with high compression ratios must use higher-octane fuel.

In engines with lower compression ratios, detonation is still possible when the engine is running hot and under a heavy load, so there is a slight advantage to using higher octane fuels in any engine. However, most engines these days have automatic systems in them called "knock detectors" that detect the onset of detonation and retard the spark timing to block detonation under those conditions- so buying higher octane fuel for an engine that does not require it is mostly a waste of money.

niels nielsen
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  • So, can I say that what is claimed by the automotive service that it is better using higher RON than lower RON is correct whereas the petrol producer consistently said that every car has specific petrol requirement? – AirCraft Lover Nov 13 '18 at 10:49
  • As this question also I posted in another thread, then I got answer as every engine is design with specific CR, so the fuel must be match to the design. According to the thread, higher RON will leave slag due not all the fuel is burnt (as higher RON will required higher temperature to burn). But feed the engine with lower RON will make knocking/detonation. This explanation seems make sense as the petrol producer said the CR for every petrol they sell. – AirCraft Lover Nov 13 '18 at 11:33
  • "According to the thread, higher RON will leave slag due not all the fuel is burnt" Nonsense – Greg Locock Dec 22 '22 at 21:59
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If the fuel octane is low for an engine, it will self pre-ignite too soon in the compression stroke and cause knocking . This is not good for the rod bearings. However , a specific engine may tolerate it. Also it reduces relative power output especially at at higher throttle settings. On the other hand, it is not necessarily fatal. Long ago , I had an 1964 Olds 394 cu.in ( roughly 6.4 L) with factory 10.5 compression. It was badly rusted and I planned to junk it so I used low octane . ( During the oil embargo, gas was expensive and I had a long commute). It knocked like crazy, when pulling away from a light people would look around to see what the noise was. I drove it several thousand miles like that; low octane knock apparently did not hurt that engine. When I pulled in to the junk yard, the guy came out to check serial numbers and could not believe the engine was running , it was so quiet. Of course this was long before knock sensors.

blacksmith37
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