Effect of RON rating on Hybrids

At the launch of the Jazz Hybrid by Honda Malaysia yesterday, I had the chance to chat with its LPL Mr. Shingo Nagamine. One of the topics I raised concerns the petrol grade requirement for the Jazz Hybrid. This relates to the issue of having the choice of either RON95 or RON97 petrol in Malaysia. 

Firstly is the question of the petrol grade the Jazz Hybrid has been set-up to use. For this, Nagamine-san says that models like this Jazz Hybrid are imported into Malaysia directly from Japan but they are not completely identical to the JDM units. There are some work done to ensure the car optimizes into the local environment and that includes the quality of petrol available here. In this case, Nagamine-san says that Honda tuned the Jazz Hybrid to work best with the RON95 petrol that we get at our fuel pumps.

But there is another quite common question and that is whether RON97 petrol will deliver any difference. My understanding is that once an engine is designed to work with a specific octane petrol, then the use of higher octane petrol should not deliver any advantage. The octane, or RON rating as we use here in Malaysia are a rating for the degree of resistance of the petrol to combustion and is needed for high specification engines in order to avoid knocking/detonation and pre-combustion. There is a common misconception that higher octane petrol somehow contains some 'magic' which allows engines to deliver more power and economy the higher the octane rating. This is not true. 

But there is still the fact that I have gotten countless feedback from readers that they did get better mileage and better 'response' (i.e. power) when they use RON97 petrol even on cars which supposedly don't need it. Like the Honda City. Or the Accords and regular Civics. This I can even correlate as I myself have often obtained slightly better mileage whenever I switch to RON97 from the RON95 I am regularly using for my 1G Jazz VTEC.

So I put this question to Nagamine-san. And I was surprised and enlightened by his response. 

Nagamine-san explains that the use of RON97 petrol on the Jazz Hybrid for e.g., may give slightly better fuel economy than with RON95 which the Jazz Hybrid has been tuned to use. The reason is of course the reduction of detonation. When ECU receives signal from the knock sensor that it has detected detonation from the engine, it will retard ignition timing. This moves the engine away from its optimal performance range and consequently has a negative effect on fuel consumption and also power. 

But then the Jazz Hybrid is specifically tuned to use RON95 so why does the engine still knock ? 

The issue is this tuning ensures optimal operation over a wide range of conditions but there will still be the very exceptional extreme conditions, very rare occurrences but still possible, that we put the engine under enough stress that it encounters mild detonation. Mild but still enough to cause the ECU to react by retarding ignition timing. Under real-life usage, it is simply impossible to ensure such conditions will never occur. And it is unproductive to accomodate for all extreme conditions. So the tuning for RON95 ensures that under all practical and foreseeable conditions, the engine will run fine without detonation. But on the very rare occasion where we might be forced to struggle up a very steep hill at very low speeds in a very low gear on an extremely hot day, there will still be a chance that the engine detonates, probably mildly. 

It is these exceptional and extreme conditions where the use of the higher RON97 petrol will help avoid detonation. However these conditions should be extremely rare for the majority of driving conditions. Thus the statement that RON97 may give slightly better fuel economy and power on the Jazz Hybrid. 

How does this information apply to us readers ? Each individual will have to analyze his or her own unique environment and understand the kind of conditions he or she will tend to encounter in daily driving. Again, the exceptional and extreme conditions will deal with issues like trying to force power out of the engine in very stressful conditions. Examples would be like steep uphill climbs and at very low engine rpms, usually 2,000rpm and below. The prevailing environment will have to be very hot. The ECU can compensate for these conditions but there is a limit and beyond those limits the engine will detonate. Usually we won't be able to hear it but engines like the LDA used in the Jazz Hybrid are equipped with knock sensors which are specifically designed to detect these detonations. And the method used to stop detonation is to run richer and retard ignition timing - both of which will adversely affect fuel economy and power output.

So the final decision of whether to use RON95 or RON97 petrol depends heavily on how much stress we put on our engine in our normal drives. RON97 petrol is a lot more expensive than RON95 so a balance needs to be striked. It might also be prudent to use RON95 regularly and to switch to a tankful of RON97 if we expect to encounter stressful driving conditions for an upcoming trip.