Octane Ratings

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The issue

Is there a single grade of petrol which will properly meet the requirements of all petrol-engined motor vehicles in Kenya;  and, if so, what grade should that be?

The answer

Unleaded premium motor spirit with an octane rating of not less than 95 (RON) will deliver near-optimum and always adequate performance for all modern and older petrol engines, without significant loss of performance or fuel economy, and without risk of engine damage.

A slightly higher rating (up to 98) might offer marginal benefit to a few of the highest compression engines, but is not essential for their proper operation and would be unnecessarily expensive (with no additional benefit) to the majority.

A slightly lower rating (circa 93) would be adequate for the majority of petrol-engined vehicles in Kenya, but would increase the risk of  damage to high performance engines with a compression ratio of 9.5:1 or higher, especially when operating at sea level or if supercharged or turbocharged.

Octane rating is most critical at sea level.    Engines become slightly more tolerant of lower ratings at higher altitudes (where the fuel:air mixture enters engines at lower atmospheric pressure, and is therefore at proportionately lower pressure when compressed).

Although most modern engines have generally high compression ratios, requiring higher octane rating to avoid risk of detonation, many are now better protected against that risk by computerised engine management systems which have knock-sensors that adjust the engine (induction and timing) instantly and automatically.

World patterns

A survey of 50 countries from all continents shows that octane 95 (RON) is overwhelmingly the most common grade, with few countries offering lower grades and many using high grades.  Those offering only one grade use either 95 or 98.   The lowest grade premium fuel offered worldwide was 92 (India), but 95 is now being introduced in that market also. 

Technical background

Octane rating

This measures the resistance of a fuel to “spontaneous detonation” under extreme pressure.

All petrol engines are designed to induct a mixture of fuel and air into their cylinders, compress the mixture into a much smaller combustion chamber, and then ignite it with a spark – at a very precise moment relative to the movement of all engine parts.

Fuels with low octane ratings are liable to detonate spontaneously – too soon, and before the spark – as they are compressed.  This is most likely when the car is accelerating (when the greatest density of fuel:air mixture is being inducted and therefore the highest compressed pressure results).  This early detonation is known as pre-ignition.

Basic “petrol”(a mixture of “tops” and “naphtha”,  and also known as gasoline or motor spirit) produced by oil refineries has a very low octane rating (between 70 and 80) and has to be modified with additives to reduce the risk of spontaneous detonation.  These additives, known as “octane enhancers” or “anti-knock” chemicals, are expensive and thus increase production costs and pump prices.

While it can be extremely costly (in performance loss or engine damage) to use a cheaper petrol with an octane rating that is too low, it is also uneconomic to use a petrol with an octane rating that is unnecessarily high. 

The extra grade and price gives no extra benefit of any kind.  Octane rating refers only to the fuel’s resistance to spontaneous detonation, and does not represent or affect any other quality of the fuel.  Usually, in all other respects, 80 and 90 and 100 octane petrols are identical.  Any variations in quality are related to fuel purity in the supply chain, or to a number of other additives (oxygenates, antioxidants, anti foam agents, lubricants, corrosion inhibitors etc) which are not related to or reflected in octane grades.   

Engine knock

Marginal pre-ignition causes a “pinging” sound; engine power declines and fuel consumption increases.

If detonation is severely out of synchronization with the movement of pistons, it causes a loud “knocking” sound as the detonation force meets piston force in opposing directions, delivering hammer-blow shocks to fast moving engine parts.  Loss of engine performance is immediate and severe, and sustained engine knock can quickly distort and destroy engine components (especially con rods, bearings and valves).  Bear in mind that ignition/combustion takes place in the engine several thousand times per minute.

Compression ratio

This is the ratio between the volume of the whole cylinder where the fuel:air mixture first enters an engine under low pressure, and the volume of the combustion chamber into which the mixture is compressed (very high pressure) for ignition by a spark.

Early engines had relatively low compression ratios of 7:1 or less; limited by the quality of materials and engineering precision at that time…and the relatively low octane rating of early fuels.  

Compression ratios have since been increased to the highest levels that progressive advance in materials, engineering and fuel octanes allow, because higher compression enables higher performance.  In simple terms, a higher density of fuel:air mixture means a greater quantity of energy is released when the mixture is detonated.  In turn, higher performance (if not used to the maximum) translates to better fuel economy – an increasingly important global issue for many reasons.

Most modern petrol engines have compression ratios of about 9:1 and as high as 11:1.  This creates a much greater potential for of pre-ignition, and octane ratings have had to be improved accordingly.

For many years the most readily available, cheap and effective octane enhancer was a compound of lead.  Since the 1990s, lead has been phased out for health reasons and because it is incompatible with catalytic converters (exhaust gas neutralizers), and alternative additives are now used in “unleaded” petrol. 

Engine design

The optimum “octane rating” for fuel used by modern petrol engines ranges from 90  to 100, depending on the compression ratio of various engine designs.   The higher the compression ratio, the higher the octane rating needs to be, to ensure optimum performance in power and fuel economy, and to avoid engine damage.

There are no mechanical ill effects from using an octane rating higher than the specified optimum. An engine designed to use 90 octane petrol can readily use 100 octane and perform properly without loss of performance or risk of damage. 

Potential problems arise from using an octane rating that is too low. An engine designed to use 100 octane petrol cannot use 90 octane; this could cause engine “knock” (spontaneous detonation of fuel under pressure), significant loss of performance, and  potentially severe engine damage.

Supply system

In most countries, petrol of several grades is available at different prices at all stations, enabling each motorist to choose the lowest cost grade that will deliver adequate performance for a particular engine.

Offering many grades is less complex than might be expected:  by producing just two grades – one very high and another very low – a full range can be available using pumps which draw a specific proportion from each storage tank to deliver the average rating desired.

However,  this system still requires major investment in at least doubling production channels, bulk storage facilities at depots, and underground storage tanks and pump equipment on forecourts.  It also complicates shipping, pipeline and tanker delivery logistics.

Kenya has, in the past, had “regular” and “premium (super)” options, later used to retain choice in the transition from  “leaded” to “unleaded”.  Today, leaded fuel is not available, and just a single grade of unleaded is supplied.  This is adequate, providing the single grade offered is just high enough to cater for the highest compression engines, but still within a reasonable economic range for the majority of users. 

Worldwide and local study indicates an octane rating of 95 (RON) satisfies those two criteria with good assurance.

The possibility of offering more than one grade at all stations, or distributing 95 octane in low altitude areas and 93 octane in high altitude areas, is a matter for cost-benefit analysis in the supply system, not directly an issue of automotive technology factors – engine performance, fuel consumption, exhaust emission and mechanical surety.

It is a matter of systems investment, and supply and stock logistics,  versus production savings and pump price impact.   This option has no effect on vehicle performance.

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