RON95 or RON98

[Jonathan Kay]

.. the sourcing of additives to increase the octane of fuel for the Merlin engine shortly before hostilities is an often overlooked and underestimated critical factor in Battle of Britain. It's very unlikely that the Hurricanes and especially the Spitfires tangling with the Bf109s would have been able to hold their own without the power boost the higher octane delivered. I have the story somewhere in my military history library...

Setright is very good on the technology, including the 100 Octane fuel upgrade.

I reread the Merlin chapter in Hooker but it doesn't add much.

There's a discussion on WW2AircraftNet with some of the relevant official papers on the change.

What do you recommend on the strategic and geopolitical aspects of fuel and additive availability and selection?

Thanks

Jonathan

PS: And my congratulations on not assigning combat superiority to the British fighters in 1940! 

[john milner]

I would not go below the minimum rating. I believe pre-detonation can crack valves.

Going too high at worst is going to harm performance and waste a few quid.

[Bottrill]

I know the OP states Duratecs but for those K-Series owners my handbook states in a table that CC recommend 98 for standard R300, 400, 500 and 95 for all other K engines. 

Ive always ran my R300 on V-power (it's the closest petrol garage for a starters). What's the deal with a lack of recommendation for this fuel? I have read it a few times now but no explanation. 

Lee.

 

[john milner]

If your engine can tolerate a range of octanes I would try a few to see what happens. Basically high octane fuel does not ignite as easily as low octane. This allows higher octane fuel to be compressed into a smaller space so when it does ignite it will have a bigger bang due to a higher fuel concentration. In theory this will produce more power and result in better consumption and more power. The problem is the better consumption is more than lost due to higher price and the power increase is too small to notice on the road. Having said that I have tried high octane in my wifes tin-top that has a 2.0 Duratec and it did seem to run a bit smoother but whether this was just wishful thinking I can't say.

As for the cleaning properties of posh petrol I think you would need to be petro-chemist to see them. High octane does not necessarily mean better additives but it probably does have them even if you don't need them to help justify the cost.

If you put low octane fuel into an engine tuned to only run on high octane the fuel can ignite just on compression and before the spark. That is while the piston is still moving upwards. This can result in something nasty.

[Jonathan Kay]

This allows higher octane fuel to be compressed into a smaller space so when it does ignite it will have a bigger bang due to a higher fuel concentration. In theory this will produce more power and result in better consumption and more power.

At a given compression ratio I don't think that happens. (But higher octane fuel does allow a higher compression ratio.)

(There's a separate question of different available energy from a given mass of fuel for fuels of different composition.)

Jonathan

 

[Jonathan Kay]

Ive always ran my R300 on V-power (it's the closest petrol garage for a starters). What's the deal with a lack of recommendation for this fuel? I have read it a few times now but no explanation. 

I don't understand... lack of recommendation by whom?

Jonathan

[Bottrill]

Jonathan, it's the second time I've read this recommendation about Shell. One of the "two Steves" I believe. Unless it's Chinese whispers so to speak. Just wondered what the reasoning was behind it.

Lee

[Jonathan Kay]

Thanks.

I just wish there were some studies...

Jonathan

[john milner]

At a given compression ratio I don't think that happens. (But higher octane fuel does allow a higher compression ratio.)

I think that is the point I was trying to make. High octane fuel makes higher compression feasible which should result in a bigger bang. For equal compression rates the higher octane fuel will give a slightly smaller bang than low octane as it is less explosive than low octane. This is why an engine tuned to run only 95 (standard K series) runs a bit flat with high octane fuel.

[Jonathan Kay]

High octane fuel makes higher compression feasible which should result in a bigger bang.

Higher compression ratio allows a difference between engines, but even that isn't because of a bigger bang, it's because the higher expansion ratio (which is the same as the compression ratio in conventional engines) allows more of the energy released in combustion to do useful work as opposed to being wasted as heat.

For equal compression rates the higher octane fuel will give a slightly smaller bang than low octane as it is less explosive than low octane.

I don't think that's the case for ordinary fuels of different octane rating in the same engine. And explosiveness isn't the same as available energy per unit mass. But I'm open to evidence.

This is why an engine tuned to run only 95 (standard K series) runs a bit flat with high octane fuel.

I don't think it does, but, as above, happy to be proved wrong.

Jonathan

[john milner]

"expansion ratio" and "energy released" in my book are the same as bigger bang.

Low octane ignites easier than high octane this why it is unsuitable for high rates of compression. High octane fuels also often contain miracle additives which mean less fuel per litre.

I have tried high octane in my standard K. The engine sound was muffled and acceleration was down. Curiously the consumption improved quite a bit so for a time I thought I was on to something good. After a while I realised the sluggish performance had affected my driving style and it was my driving that had improved consumption.

[Jonathan Kay]

"expansion ratio" and "energy released" in my book are the same as bigger bang.

The expansion ratio in a conventional engine is a feature of the mechanical design and isn't affected by combustion. If you never ignite the fuel or totally stop injecting fuel into a cylinder there's still an expansion ratio for the gas in the chamber.

The heat generated by combustion depends on the mass and composition of the fuel.

The fraction of that heat that can be turned into useful work is determined by the expansion ratio.

When an engine delivers more power because it's given more fuel the heat generated is greater and the pressure on the piston is greater but the expansion ratio remains the same.

Jonathan

[Steve_K]

In engineering terms JK has it right. The compression ratio is a key term in determining thermodynamic efficiency of an engine. The higher the ratio, the higher the energy available to do work. The problem with high ratios is the tendency for the fuel/air mixture to detonate, i.e. explode (and in relative terms it is an explosion rather than the usual 'burn') before the spark is passed. To counter this detonation the higher octane fuels are actually harder to ignite by compression. Note that the amount of chemical energy available from the various octanes is more or less the same in engineering terms.

Steve

PS FWIW the theoretical relationship for the thermodynamic efficiency is given by (1-1/R)^(Ɣ-1) where R is the compression ratio and Ɣ is the ratio of specific heats or Poisson constant. I think this is right but its been a long time since this came up last.  Don't bother working it out for your own engines as the real world is a lot more complicated than the theory implies - gamma isn't constant. BTW Petrol's star rating relates to the delay period between the spark discharge and the start of the pressure rise. More stars, lower delay period.

[john milner]

So what did I get wrong? Low octane ignites easier, high octane is harder to ignite so can be compressed into a smaller space to create a bigger bang. More high octane can be squeezed into a small space before it self ignites. Try squeezing the same amount of low octane into a small space and it can ignite before the spark. Therefore you can get more power from high octane as it allows for higher rates of compression.

Bigger bang = more power

Given the choice I would stick with low octane as it is cheaper, easier to find and the benefits of high octane for me would be too small to notice. Low octane may even be better at starting when very cold but that is very much a theory of mine and may be blx.

[Jonathan Kay]

Therefore you can get more power from high octane as it allows for higher rates of compression.

"More" in a comparison between different fuels in the same engine, or between two different engines with different compression and expansion ratios?

Jonathan

[p.mole1]

My car is supposed to use 98 octane, it does not seem to have an overly high compression ratio so I put 95 octane in and I noticed a marked reduction in performance! never heard any pinking or detonation.

I have had other cars which were supposed to use 98 octane but I often used 95 and never noticed one jot of difference, however all these cars have had knock sensors.

[ScottR400D]

That's the point isn't it? We can't change compression ratios and whilst higher octane can allow for a higher compression, in practical terms it also allows for advanced ignition which gives a longer more complete burn, releasing more energy?

If our cars are able to advance ignition enough to use the potential of higher octane fuel then it would be worth using. Question is, are they?

[Jonathan Kay]

... in practical terms it also allows for advanced ignition which gives a longer more complete burn, releasing more energy?

Do you mean at the time an engine is designed or dynamically during operation?

Jonathan

[ScottR400D]

Well, at design both ignition timing and compression would be set based on the recommended fuel, I guess.

In use, the knock sensors will dynamically retard the ignition if detonation is sensed, as could be if a lower grade of fuel was used.

If an engine was designed with parameters based on 95 octane then there'll be no detonation whilst using it. However, it's hard to see how there would be any gain in performance when simply using a higher octane fuel, unless the ignition was advanced, which wouldn't happen dynamically.

I do stand to be corrected on this but that's how I've understood what I've researched.

 

 

 

[Clousta]

 

Slightly off topic.  I use 98 Octane for my SAAB 9-5 Aero estate. 11 days ago I took a trip with my gilder trailer from near Luxembourg to south east of Prague in the Czech Republic. At the Czech border I had to buy an Czech road atlas (cheaper than a TomTom down load of maps for the whole of Europe) and a ten day motorway vignette. I refueled with Shell Racing 100 Octane (the other alternatives being 95 or 95 with ethanol) which I though was an amusing option.

When I had dropped off the trailer at my destination I drove to my hotel.  The car was more spritely (and not just because there wasn't a trailer on the back) withe racing 100 octane fuel.  In addition the mpg on the return journey through Germany on a weekday, with the trailer on the back, was improved despite having to accelerate to overtake trucks on the motorways (Monday return rather than the Sunday outbound journey).

However, the SAAB Aero has a full turbo with the Trionic 8 engine management system that adjusts the boost in conjunction with the knock sensor and many other parameters, including ignition timing, to get the most out of the fuel and optimise the performance of the engine.  So octane rating is of interest for the Aero. Now... if one could package a SAAB 2.0 or 2.3 turbo engine into a Seven ...

Saab 9-5 Aero

Trionic 8

 

 

[Roger King]

Clousta, bearing in mind that your engine management has a knock sensor and can vary both boost and ignition timing to suit the fuel in use, your experience is exactly what would be expected i.e. better performance and economy on higher octane fuel.

However, for non-turbo engines and particularly those without a knock sensor, higher than specified fuel octane is nothing but a waste of money.

And we shouldn't get carried away with the idea that the more the ignition advance the better; this is a common misconception, but isn't true. For any given engine design (and assuming the lack of turbo or knock sensor) there will be an optimum timing figure (I'm talking here about full advance at full throttle for simplicity, but the same actually applies across the full ignition map). Quite often on a dyno, you can find the best ignition timing figure for max power, but then find that you could actually advance the timing more, without ill effect but also without gaining any power. This means that you have already found the optimum setting and pushing the advance any further merely increases the risk of detonation damage which is pointless. Eventually, adding even more advance will cause detonation with consequent damage. In theory, this means that the engine could have been built with a higher compression ratio, but manufacturers will always leave a little safety margin to allow for dodgy fuel batches or exceptional temperatures, etc.

If you have to run higher octane fuel because your engine's compression ratio requires it, you will very likely actually need less ignition advance. This is because the higher the compression ratio, the faster the fuel burns. In an ideal world, fuel would burn instantly, but in practice it takes a little time and this is the reason for ignition advance - so that the fuel is fully burnt and generating maximum pressure in the combustion chamber at around TDC. In one way it's actually a disadvantage to have to light the fuel before TDC, because the pressure generated resists the turning of the engine until TDC is reached. It's just that the gains of having ignition advance far outweigh the losses. Likewise, four valve per cylinder engines have much more efficient breathing, atomisation and a shorter flame travel than a two valve engine; all of this leads to much faster combustion so typically a four valve engine needs around ten degrees less ignition advance, despite generally producing more power, better economy and lower emissions.

If you ever want a practical demonstration of increased efficiency of a four valve engine, try decoking an exhaust valve from a Crossflow and then one from a BDR or Zetec, etc. The Crossflow does a relatively poor job of converting heat into kinetic energy and as a result runs a much hotter combustion chamber than the more modern engines - the carbon is baked on to valve and is seriously difficult to remove. But the four valve engine is more efficient (still terrible though) and runs cooler due to more efficient conversion of heat to kinetic energy - the carbon (and there's less of it than on a Crossflow) just falls off with gentle application of a wire wheel.

[Clousta]

 

Roger,

I concur fully. There is no point using higher octane fuel if the engine can't use it.

I previously had a 1998 P38 Range Rover where the GEMS engine management system adjusted for fuel quality (knock sensor and ignition advance) so I use 98 Octane.  It ran on lpg  and I invested in the RPi Engineering chip set (ignition and fueling) to make best use of the octane rating of LPG, which is typically 104-106 octane.  The chips overrode the GEMS limits on ignition advance to match the octane rating of the fuel.

Sadly I had to let the Range Rover go as the annual tax in Belgium for the Range Rover was €2500 p.a. and paying for my son's University education is higher up the priority list.  Ironically the SAAB 9-5 Aero at 250 bhp is 25 bhp more than the Range Rover but the Belgium road tax is one fifth that of the Range Rover!

 

[Clousta]

 

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