How to Understand the Relation of Compression Ratio and the Octane Number?

Question

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?

Answer 1

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.

Answer 2

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.