Why 98 + needed for a mitsi GDI?

So would I be right in thinking that the real problem is the EGR rather than the octane!Also is it more likely that problems will occur with toodling about around town rather than open road running!I think I saw somewhere that cleaning out the intake system sort of fixes the problems for a while - is that correct or is more serious work required!

Brought & a month later returned a GDI Galant to teh dealer a few years ago
Had plenty of power but once it got up to full running temp, the revs would surge a lot when stationary.
Was fine when cold, was a bit thirsty for an 1800cc.
Dealer wasn't happy to pay out 2K+ to get the manifolds etc cleaned so swapped that for a different vehicle.

Correct the issue is EGR as that is where the carbon is comming from. And correct long idle times and light throttle angle will have alot of EGR %
Cleaning the intake will fix lack of power and surging issues.

Yep half a day to strip the covers etc / 30 minutes to clean the manifolds / half a day to reassemble ( seems that way)

No absolutely not.

The car is trying to set up a really tricky ultra lean burn which dosn't work properly when running low octane. (I can give you the long winded technical explanation if you want).

Running 91 causes incomplete combustion which creates the carbon in the first place, the EGR just sucks that carbon back through again making things worse. (which is why people think its the EGR when really its just the messenger)

This is what people don't get, the EGR is just a secondary issue and altering it is a stop gap. Why is it so very hard to just run the fuel that the manufacturer designed the engine to run!

There is ZERO I repeat ZERO cost advantage in running 91 because your economy suffers substantially.

Blow it, I will give the long winded explanation because people don't seem to get it.

Basically you can't understand why 91 Octane causes carbon unless you understand how Direct Injection works. But before that you need to understand the challenges of tuning a regular engine for efficiency, and how the Direct Injection system gives you an advantage.

Basically the idea is to get the most efficiency out of an engine you want to to optimise several things, two of those are:

1. You want to run as high a compression as possible to facilitate as much expansion as possible from your charge.

2. You want to run as lean a mixture as you can to use as little fuel as possible.

Now. with a normal car you can only optimise these things so far before you run in to trouble.

Firstly, compression, as you compress the fuel air mixture there comes a point where it will self ignite from the heat generated by the compression process. This is how a diesel works without spark plugs. So, you can only wind the compression up so much before the fuel fully ignites while the piston is still traveling upwards. Obviously, that's a bad thing. The higher the octane of the fuel the hotter it can get before it ignites, that's why engines designed for high octane can run higher compression. Higher octane fuel also takes longer to burn and is arguably more controlled in its burn. The amount of energy in the fuel is the same, but because you can run a higher compression and earlier ignition timing you can get more of the energy out of the fuel. How much more efficiency depends on how much the engine is optimised to run high octane.

Secondly, while you can burn a very lean mixture its hard to ignite a lean mixture. as anyone who has lit a gas torch or Bunsen burner can tell you. So in a normal engine you can only lean the mixture out so far before it won't ignite reliably.

This is where direct injection helps. In a normal engine the fuel is injected behind the valves which then open and release it to be sucked into the chamber along with air as the piston travels down. On the way up of course it compresses ready for ignition.

With direct injection the fuel is injected directly into the chamber at incredibly high pressure. This in itself dosn't do much, what does the magic is the fuel stratification or the fs part in fsi (VW's term for the system).

Strata by the way basically just means layer and stratification obviously layered. Which is how the magic happens. Instead of the fuel going in at the bottom of the stroke where it can compress and heat up with the air, it goes in as close as possible to the top of the stroke. This helps you to run a higher compression ratio.

The really neat part though is how the fuel is injected. Initially a burst of fuel is injected and given time to disburse evenly in the chamber, this is an ultra lean mixture, we are talking up towards 3 times as lean as you could get a normal engine to run. Then at the last moment right before the plug fires another burst of fuel is injected right under the spark plug. This gives you a small patch of much richer mixture, which is what starts the ignition and the trick to igniting what is still overall a very very lean mix.

This whole process is real knife edge stuff because the mixture is unbelievably lean compared to a normal engine. If you try and do it with 91 octane it makes it more difficult as you can have various issues like ignition starting as soon as the richer ignition mix starts to gather causing incomplete combustion.

Once this technology got into production engineers quickly realised that trying to set up these fuel stratified charges under all running conditions and with all fuels you got issues. This is where the famous GDI problems come from. So later systems like VW's which dosn't generally suffer these problems got around it by having the cars computer engage the FSI mode when running conditions were more ideal i.e warm engine, correct octane fuel etc

I'm impressed jazz, shame you ended it with "etc" though.
You keeping well!(o:

Cont.
Now to give you an idea how important octane is in achieving a complete burn of a properly stratified charge. If you run a VW FSI on even just 95 instead of 98 octane it will only achieve suitable conditions and implement FSI mode 10% of the time. On 98 Octane it can achieve FSI mode 60% of the time.

Now consider the Mitsubishi which is in forced FSI mode all the time (as far as I am aware anyway) running 91 is not even going to be close to giving it suitable running conditions ever, let alone most of the time. So of course you wind up with incomplete combustion. That causes the soot which is then sucked back in through the EGR which actually starts an even worse spiral because this sooty exhaust gas in the mix takes you even further away from achieving a complete burn.

So. does that make sense now!

Hit me up with questions if it doesn't.

BTW, you could nit pick my explanation somewhat, for example there is a bit more to achieving fuel stratification like piston design. Technically in a normal engine you may start fuel ignition while the piston is stilltraveling up etc. But you have to draw the line somewhere to make it readable. Feel free to have a go yourself at further explanation if anyone wants.

Hopefully, that helps a bit with the understanding and no one got lost or bored along the way.

Good thanks!
Sorry that wasn't the end, TM only lets you write so many words in a post. something I am ashamed to say I know a little to much about!
Glad you enjoyed the read.

Kinda following on from that - as I understand it injecting directly into the cylinder into what is an already highly compressed environment, and getting more compressed as the cylinder continues on up, is why direct injection cars run very very high fuel pressure delivery systems. Otherwise the fuel simply wouldn't go in.

That's the down side of direct injection, fuel pumps that cost your first born when they go bad, and injectors that will be cut to shreds with any water in the system.

Sorry to have interrupted you, Am enjoying the read, thanks

High fuel pressure is also required to set the layers up properly and in time, this all happens in the blink of an eye.

High pressure fuel pumps don't have to be overly expensive, I hear that Mitsubishi's is, VW's is not end of the world (have noticed it in my travels) but as of yet I don't know of anyone who has had to replace one.

Alfa direct injection injectors are known to poo themselves occasionally like Mitsi ones but mine seem O.K so far. Not end of the world to replace. I am yet to come accross anyone first hand who has wrecked a VW injector. Possibly due to better filtration!

Given the massive economy advantages of FSI and that systems like VW's can clearly be reliable and affordable its well worth the effort in my opinion. Just look at the Golf, what other automatic 2.0 car built in 2004 can do 0 - 100 in 8.5 seconds yet if driven sensibly on the open road you can get it to dip below 5l/100km. That's just incredible!

If you are doing typical km's and say that overall you are 2.5l/100km better off than a non fsi equivalent you are looking at being about $800 better off a year.

No prob's, great to hear you are enjoying it. It makes the effort worth while!

Not that I dont agree that running 95+ will reduce carbon Jazz but in my experriences the quality of the engine oil playes a larger role in inlet manifold carbon. And I have spent a large number of hours under the bonnet of D4 engines so it is my experences not just a speel from Toyota lol.

I agree there are other factors at play, and what is true for one direct injection engine is not necessarily true or completely true for another.

However you are not supposed to choose to do somethings properly and not others. Its not a case of you can run 98 or use a quality oil, you are supposed to do both! (If that is what the engine is designed for)

I just don't get why people seem to think octane ratings are something they can choose at whim like what they eat for breakfast. Most engines are designed for a specific octane and with direct injection engines its pretty imperative to run the right one. Even if the likes of the VW engine is designed to stop you having a major failure from running the wrong octane dosn't mean you are free to choose.

BTW, what's the go with D4 engines, do they only drop into FSI mode when conditions are suitable or do they try and force it all the time, or is it a case of it depending on the age of the engine!
What octane are D4's actually supposed to run!

Wow, thanks Jazz - your posts have cleared up a lot of things I did wonder about direct injection on gas engines, cheers :) "All you ever wanted to know about FSI but were too afraid to ask"

Otherwise known as. "the dangers of giving Jazz a soap box".

Glad it helped, thanks for letting me know.

Jazz some of what you have googled is correct, alot is rubbish. Carbon is a by product of combustion as someone who claims to know a lot about GDI technologyyou would know GDI combustion chamber shape is less then ideal fo NOX emissions so too is a lean high temperature combustion environment. Hence the tolerance of EGR, the CAUSE of manifold carboning.

With out searching through some of my older posts do you know the tolerance of EGR compared to a MPI engine!

Fuel octane rating plays a negligible part in manifold carbon so much so some of MMC??

Eh, giving you a soap box is ok, at least you consistantly do your best to deliver helpful information. I tend to swing wildly from partially useless to whinging at trolls, I have to start working on the useful bit sooner rather than later!

EGR is not injected at all during idle, this is and always has been true for both GDI and MPI engines

Yes tell us why, we are learning here.

Once again Jazz to the rescue.Thanks for that I think I can now get my head around the problems.I agree if the engine is designed for 98 then that is what should be used.i also tend to agree with the bit about the marketing people saying it will run on 91 or whatever.They are right it will run but for how long and how well!

So apart from the high costs of parts (if you happen to need them) the Mitsi GDI is not the plague that some people make it out to be.I can think of lots of engines that have some odd ball curse that is mostly operater invoked.

Once again thanks to all

Clearly you missed the part where I said it would be easy to be cheap and pick holes in what I said as it had been simplified to make it readable. I have taken significant liberty's and generalisations, there is a lot more in there yet someone could pick holes in (like injector cycles etc).

As I have pointed out in past, my experience is not with GDI engines, I am not really a fan and have no interest in them to be honest. My experience lies with VW and to some degree Alfa's versions of the same technology.

The text is intended as a general overview explaining the challenges of direct injection in general and why its imperative to run the correct octane, the latter part of course is the point of the thread. not whether EGR is a contributing factor to carbonisation.

There are better qualified people to specifically judge Mitsubishi's GDI than me. In general as a concept the technology fuel stratification technology is fantastic. Unfortunately a bit like Alfa with their selespeed the first off the blocks sometimes don't get things perfect. VW and no doubt some others though have shown that its entirely possible to produce a reliable version (almost all petrol VW's have been direct injected for some years)

Certainly I think a lot of the problems are operator related as you surmise. However I think its safe to say there are some design issues as well, lie combustion chamber design, high use of EGR etc. Personally, I would take on a GDI if it had good history and I otherwise preferred the vehicle for some reason.

Its worth noting BTW, Volvo used Mitsubishi's GDI engines as an option likes of the S40. Those vehicles were and to some degree still are rated as some of the most reliable proper cars of their time. I suspect that may say something about the Volvo driver being more inclined to do things by the book. But that is only an educated guess.

Thanks for your kind thoughts.

Reading back what I have said it does sound like I have made the octane rating out to be the one and only source of carbon deposits and problems with GDI's. So at the risk of sounding like I am back tracking, I will happily concede that is not necessarily the case, there are definitely other factors at play too. I can see now reading it why some may have got the pip with what I said.

The other thing I should clarify is when I say I suspect the Misubishi trys to run FSI mode all the time, I mean whenever engine rev's/load etc permit, as opposed to VW's which monitors the combustion cycle more closely and will hop out of FSI mode if say low octane fuel is causing incomplete combustion even if the other operating conditions allow for lean burn. Obviously you can't run a super lean mixture all the time, not even close anyone who has disconnected a carburetor pump or choke on an old engine understands that.

Anyhow, my message is simple, run the correct octane, sure the GDI has design issues as well but there is no point in making things much worse by deliberately working outside Mitsubishi's design intentions. Anyone who read my novel above will hopefully now understand what this does.

That's about all I have to contribute, I will leave it now for others to pick apart or whatever they wish to do.