A video from YouTube channel ML Studios has been making the rounds yesterday, and it was to do with the subject of a fuel’s RON rating what the effects of using the ‘wrong’ type of RON rating might be for any given type of petrol engine. It starts out promising.
About a minute into this video, presenter Mas Faiz Hakim starts to discuss the correlation between a vehicle’s engine capacity and the RON rating of the petrol used. The presenter says that lower RON fuels are more suited to smaller displacement engines, while engines of larger capacities should be fed higher RON fuels.
Faiz elaborated by saying that using higher RON petrol in a relatively small engine will require it to ‘work twice as hard’, causing it to be over-exerted and eventually require repairs of servicing ahead of schedule. Conversely he adds that a lower RON petrol used in a larger discplacement engine – such as 2.0 litres and above – will cause premature combustion, more commonly known as knocking.
While it is correct that the lower the RON rating, the more easily the petrol will combust, it would be inaccurate to draw a correlation between a fuel’s RON rating and the capacity of engine it is used in. The Research Octane Number, which is what RON stands for, refers to the fuel’s ability to resist premature ignition, which is when the fuel-air mixture combusts upon compression before spark is initiated.
When premature combustion, or knocking occurs, this is manifested as an unusual tapping sound from the engine, hence the term. This is due to a number of factors; once of which could be the presence of excess carbon deposits, which ignites in the combustion chamber and sets off the rest of the air-fuel mixture earlier than it should in the compression stroke.
Another reason knocking can occur is when a lower RON petrol is used in an engine with a high compression ratio, and it is this factor rather than the engine’s displacement that is of greater consequence when observing the outcomes of using different RON ratings.
A small displacement engine does not necessarily use a low compression ratio, and similarly a larger engine won’t always have a higher compression ratio than that of the smaller engine. So then, which kinds of engines tend to use higher compression ratios? These will typically be naturally aspirated engines – without forced induction such as exhaust-driven turbochargers or mechanical superchargers.
For instance, the 107 hp/150 Nm 1.6 litre S4PH VVT engine in the 2019 Proton Persona uses a compression ratio of 10:1, while the 185 hp/163 Nm 1.6 litre B16B DOHC VTEC engine in the EK9 Honda Civic Type R has a compression ratio of 10.2:1. Generally speaking, a higher compression ratio enables more power to be extracted from a given capacity, though this also generates more heat.
In the case of the EK9 Type R, the Honda also benefits from lighter – therefore also more expensive – engine internals for a higher RPM limit. These are often partnered with parts such as a more efficient cooling system and the use of more durable materials, which compounds the additional cost and inherently limits their application to more specialist, high-performance vehicles.
On the other hand, forced induction engines such as the 227 hp/350 Nm 2.0 litre TSI turbocharged engine in the Volkswagen Golf GTI uses a compression ratio of just 9.6:1, as the positive pressure or boost has already compressed the intake air, and therefore the air-fuel mixture before the compression stroke.
Back to NA engines – what happens when an engine with a relatively low compression ratio uses a higher RON petrol? Our colleagues Farid (who authored the BM story on this topic) and Durrani from paultan.org/bm carried out this exact test, with a pair of 1.3 litre Proton Sagas with CVT gearboxes. In short, the exercise found that the RON 97-fuelled car actually used slightly more fuel than the RON 95 car, though RON 97 isn’t actually detrimental to the engine’s health.
On the flipside, what if lower RON petrol is used in a high-compression ratio engine, such as the 1.5 litre Skyactiv-G with a 14:1 ratio in the Mazda 2? This isn’t a worry in modern engines with electronic fuel and ignition management as well as knock sensors. If lower than ideal RON petrol is detected via the knock sensors, engine management will advance the ignition to counteract the premature combustion and prevent damage as a result.
Of course, overall engine output will also be reduced, which leads to the misconception that higher RON petrols ‘give more power’ – what higher RON petrols do is enable high compression ratio engines to perform closer to its potential than a lower RON petrol can, all other factors constant. Gains are there for the taking, though this is from reduced losses.
Perhaps closer attention is also in order; Faiz points out an excerpt in the Perodua Axia manual which states “use only unleaded fuel with it’s Research Octane Number 95 or higher”, which is immediately followed by ‘guna (RON) 95 sahaja, kerana itu yang direka khas untuk kereta jenis Axia’ (“use RON 95 only as that is what the Axia is designed for.”)
Again and as mentioned earlier, higher RON petrol isn’t actually bad for your car, it just may not make financial sense for everyday applications. The presenter is however, correct in advising users to consult the owner’s manual for correct operation.
Now, what if you’re willing to pay the premium for even higher-rated petrol, namely, RON 100? We’ve put Petron’s Blaze100 to the test against its Blaze95 rangemate, sampled with cars and bikes collectively spanning several segments and employing rolling roads for good, objective measure. Watch the video below to find out more.
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