Interesting definition, but contradictory, incomplete and incorrect.

If the pre-ignition is being caused strictly by compression as they state, how would this be controlled by retarding the ignition?

Such compression related events are better known as 'dieseling'. Contrary to detonation where the fuel 'explodes', dieseling is normal combustion or 'burning' of the fuel albeit far too early but is far less destructive.

Again, understanding the difference between burning and exploding is essential.

In a practical sense, dieseling events can be deliberately induced on an engine (without necessarily inducing detonation) by advancing the ignition past the point at which the engine achieves peak performance. The effect will be a proportional reduction in performance as noted above with the discussion of pistons trying to go backwards.

The point of this is if dieseling/pre-ignition/detonation was an issue on these engines it would be felt, heard and measured by their owners. In the 13 years that the 4.2L engine has been around or 16 years for the 4.0L, not one case has been reported. On the contrary, there's plenty of first person evidence showing that either deliberate or accidental use of low octane fuel does not result in any visual or audible signs.

This infers that the engine management system is more than capable of coping with any detonation events. The additional fact that no mileage penalty is seen (as would be expected by retarding the ignition), suggests that the engine is not prone to detonation in the first place.

On the other hand, the point gets raised frequently about using fuel rated higher than specified by Jag- ie greater than 95RON/91AKI. Some, usually those that believe the sky will fall if less than 95RON is used, think that higher octane fuel will result in increased performance. If this was true, it could only occur through changes in the ignition curve (more advance). If this were the case, it would mean that the engine is undergoing detonation while being operated on it's rated fuel. If it's dangerous to operate an engine on sub-optimal fuel and have to rely on knock sensors to save it, then the users of 91 AKI are just as guilty as the sinners who use 87.


It is true that detonation and pre-ignition can destroy an engine in short order but this is seen only on old school non-computer controlled engines or on modern engines that have been modified to the point that no safe guards are sufficient to protect them.

Just not an issue on stock, street driven cars.

 

I think the PCM looks at the knock sensors and uses data tables to set the timing of each coil's spark. Each table has a max and min correction of the timing.

BTW it does NOT do this (on our cars) under all driving conditions e.g. it does not do it at WOT.

Using the expected fuel will get values within min & max.

Using higher RON/AKI fuel would get other values. I'm curious what effect they have on MPG, performance, and whatever else makes sense to wonder about.

Using too low RON/AKI gets inadequate values or marginal ones which is why the handbook warns about what can occur. On an occasional basis it sounds survivable but what's the effect of (knock or whatever one calls it) on MPG, engine life, performance and any other parameter that's of interest?

(I'd like to know, out of curiosity. I won't be hunting around for such fuel, which is not especially easy to find here and anyway I like to stick within OE recommendations and to me min means min.)

 

The PCM would select other values only in response to detonation events. Is there evidence of this occurring while using the recommended 95RON/91AKI fuel?

Those who have deliberately dared to use low octane fuel or used it out of ignorance (many cases of this, at least on the S-types) report no apparent effect.

I bought a used SUV a few years ago with the 32V DOHC 4.6L Ford modular engine. Same 11:1 compression as our AJ engines. The car was titled in the father's name but driven by the university age son. About a month after buying it, I found a few old gas receipts which showed the vehicle had been fueled with 87AKI, vs. the recommended 91AKI. The inside of the gas cap door was clearly stated 'use premium fuel only'. The son's explanation was that he had indeed used premium fuel which he interpreted to mean 'top tier' brands and nothing to do with octane rating. It's estimated that he put 100K KM on the vehicle with the wrong fuel. It still runs like a charm but now gets fed 91 as a special treat.

There is merit to the comments about massive and swift engine destruction, but again these are memories of the bad old days of non-computer controlled non-EFI engines. In looking back, I'm amazed I didn't blow more of them up myself.

 

Well as I've already said pre ignition can occur from things other than static compression. BUT HIGH STATIC COMPRESSION AND LOW GRADE GAS IS A COMBINATION THAT WILL CAUSE IT! Oh and know that if you're hearing it ping, it's also pingIng at a level only the computer and the knock sensors can pick up. Your hearing it is usually because the computer is maxed out trying to control it. But mikey you stick to your low grade and save them pennies...For now anyway..LOL

 

I made no mention of static compression. The article you linked to and my comments discussed dynamic compression. Understanding the difference is essential.


You still seem confused by the differences between normal combustion and detonation. The pre-ignition you are (overly) concerned about would not produce the characteristic 6400hz signal the knock sensors are looking for.

I've never heard my Jag (or any computer controlled car) ping when using low octane gas, so really don't follow your comment.

You should also be aware that if an engine is not experiencing detonation events, the probability of it suffering 'dieseling' described above is about zero.

The sky is not falling.

 

In 1987, I experienced a catastrophic engine failure in a rental car in Europe after putting in "Regular" fuel instead of "Premium" in Italy. The knock put a hole in the piston. It was a computer controlled engine and it was not a high compression, performance engine. How do you explain that if the lower octane fuel wasn't an issue?

BTW, the car had already driven from Belgium, to Holland, all the way down the Rhine and across Germany, through Austria and most of the way across northern Italy from Venice to Florence with no issues on the standard fuel in the other countries. The Italian gas had a lower octane and it was apparent shortly after filling up. Sounded like an old school Mercedes 240d. We made it to Nice before it gave up the ghost. Just a few hundred miles. One hell of a trip at 17 though...

 

Well mikey the difference here is what you've never HEARD, and what I've EXPERIENCED. I guess in your world XBox racing is the same as a day at the track too, mikey YOU! Just keep doing EXACTLY what the manual says and you'll be fine..LoL

 

An excellent idea. Here's what the manual says of relevance to 87 AKI octane


Note: Mid or regular grade gasoline with a CLC
or AKI octane rating of not lower than 87 may
also be used, but performance and fuel
economy will be reduced.


So the OP will be fine. Thanks!

 

As I recall one or other, maybe more than one, of the many Jaguar tech docs / guides / manuals describe the PCM as adjusting timing based on (whatever we call it, knock? ping?). Seems reasonable that it does this whatever the fuel is (since it doesn't know what the fuel is except by watching the various sensors when the engine is running).

So yes it does it with 95RON/91AKI.

(I've not seen anything saying it only looks for 6400Hz.)

I'm actually curious to know what the effect of higher octane would be (bearing in mind the algorithm appears to be to change timing till the knock sensors in effect say to back off the timing). Will it just use a bit less fuel? Less/more power? Something else?

It looks like too low octane means the PCM can run out of adjustment and (ping/knock/...) can result (plus lower mpg/etc). No biggie for me as I won't ever put in lower than 95RON fuel.

 

Let's presume that your reference material is correct, in that the ignition is set by the PCM looking for the onset of detonation and is constantly making adjustments to keep it right at the threshold.

This would fly in the face of those who believe that any detonation is bad detonation. It infers that all S-types detonate pretty much all the time, no matter what fuel is used.

Does the PCM run out of authority when low octane 87AKI fuel is used? Apparently not as no owner has reported any audible signs of detonation. I've tried, without success. For that matter, no owner has reported reduced performance and/or increased fuel consumption which would be expected with retarded ignition. Perhaps Jaguar's quoted caution in the manual is overstated.

On the other hand, no owner has reported improved performance or reduced consumption when using fuel greater than 95RON. Why not?

Is it because the ignition advance offered by use of 95RON fuel already permits the optimum setting and that further advances have no benefit? Again, more advance is not always a good thing, contrary to popular belief.

Is it because there is actually little or no detonation when operating on 95RON so the PCM gets no signal to try additional advance?

I can't help but again speculate that because of the absence of evidence of difference in performance or consumption when using different fuels that this engine is just not prone to detonation in the first place.

 

Who are you trying to convince Mike?..Yourself or trying to convert everyone to YOUR way of thinking?..Give it a rest with the last word on everything all the time...Premium Fuel, 91 octane, is what is minimally required for these cars. PERIOD..and nothing less should be used as per the manufactures recommendations..using anything less is asking for problems, whether you can or cannot hear them, feel them, or notice engine performance..WHO CARES..If you think you are smarter than the engineers that designed these engines, then put Canadian Club in it for all that matters...But stop trying to convince people that lower octane fuels are acceptable, otherwise it would have been issued in the operating requirements.

 

But it is....

Note: Mid or regular grade gasoline with a CLC
or AKI octane rating of not lower than 87 may
also be used, but performance and fuel
economy will be reduced.

If you having nothing to contribute other then trying to bully others, please move along. No obligation to read this or any other post.

 

Here you go again with your LAST WORD and 'Oh please don't bully me' whine..grow a pair and quit sounding like a little baby when someone disagrees with your distorted thinking....Right away it's bullying...

 

Fine, let me have the last word on this thread then: In my experience, regardless of the "base" cost of Regular, Premium Fuel costs $0.20 more per gallon, equating to about $3.20 extra per tank. Even if it is OK to run it on Regular, given the purchase price of our cars in the first place, and the sticker in the fuel flap telling us to put premium in it, why on earth would anyone even bother putting Regular in it? To save $3.20? Don't be so ****ing moronic.

 

I mean, your body can consume junk food every day, but you're going to have long term health issues and it runs a lot better when you feed it with healthy stuff that is recommended by dieticians. I look at my vehicles like I would one of our children, and you get out of it what you put into it.

 

I expect the workshop manual and technical guides (being aimed at professional techs etc) are at least as and probably more reliable about the car than the vastly shorter handbook (aimed at the average or so owner).

People's failure to mention what happens (after careful monitoring) on more than 95RON/91AKI fuel is just an absence of data and as I'm sure is well known tells you nothing.

I was rather hoping someone would know rather than guess.

 

There are a couple of jag techs on this forum (brutal, etc). Perhaps someone could direct them here and hear their thoughts.

 

Divorce the wife and buy premium.
Problem solved!

 

This guy wins the internet today

 

Can't say that I agree with your analogy. Premium is not Kobe beef and regular is McDonald's rejects.

The only difference between regular and premium is the octane rating. How the octane difference comes about during refining does not infer that one fuel is better quality than the other. For that matter, many refiners use the much hated ethanol to raise octane rating.

But that's another argument for another day.