Direct Injection, Fuel Dilution, and Ring Sticking

[Overfill]

Hey all,

If a GDI engine has fuel dilution, would most people regard that as something driven by:
-Poor piston rings, with the injection system behaving at its prime, just tend to let fuel pass.
-Direct injection systems, even when operating at their prime, will (with few exceptions, if any) always blow fuel past the rings. (Generally intrinsic to this injection system).
-Condition of the Direct injection system is additive but not the dependent factor; condition of the piston rings (stuck) primarily drive fuel dilution.

If there is fuel dilution, wouldn't the fuel in the oil act as a solvent to solvate varnish if the oil doesn't/ couldn't contribute to further dirty-ing of the pistons (assuming a nominal oil and appropriate change interval to prevent additional deposits)?
I try to emphasize that last part mainly to rule out "trying new oils" as a fix nor the object of discussion.

 

[Trav]

Gasoline even with ethanol does not act as a very good solvent for varnish. Easy to prove to yourself, take a carb like one from a OPE engine and try to clean the varnish off with gasoline, it will not even touch it. Gasoline turns to varnish when heated.

 

[Ws6]

Fuel dilution is always bad. Never good.
DI engines are not all equal. I had a CX5 with one, and took it to 106k miles, and had zero issues with the engine. Some DI engines (likely older ones) needed walnut media blasting of the valves every 30k miles or some nonsense.

 

[Triple_Se7en]

More fuel dilution issues with today's TGDIs, more-so than GDIs.
I wanted to consider either the AWD Chevy Equinox or the Honda CR-V, when I purchased my Santa Fe. Both had direct inject and turbos......... um.... no thanks!

There are times when I have 2nd-guessed my decision and instead purchased the RAV-4, which had neither direct inject or turbo.
Still debating if I made the right decision. The reason I chose the Santa Fe is because I found the long model, but the 3rd seat wasn't there. So I have space-galore back there..... much more than the RAV-4.

I looked at models like the Chevy Traverse.......a few others. But the prices are those were around $5-6K more.

 

[Overfill]

So maybe ring sticking due to varnish (initiated by fuel dilution) can become a bit of runaway issue where the varnish turns to coke/carbon, which is more difficult to solvate and transport away...

This is in contrast to my first supposition which was essentially "wait, wouldn't the fuel free the rings and stop further fuel dilution / it should fix itself."

 

[benjy]

manufacturers are ALL mum about direct injection issues but see that some are using both at xtra cost for sure says they are aware. modern technology has a learning curve + buyers are the beta testers witnessed by subarus long CVT warranty + the fact if replaced under warranty subaru wants it sealed for inspection + possible improvements, so add CVT's to the NO-NO list IMO

 

[KrisZ]

I also don't think fuel will have any effect at cleaning varnished and gummed up rings, if anything it may contribute more to it. And this is happening on brand new engines, imagine these engines with 150k, 180k, 200k miles on them. I would imagine these problems will only get worse as these engines start to wear out. So more fuel dilution and more blow-by would be my guess.

 

[Rand]

I wouldnt want a 10 year old 100000 mile one. Putting in the cheapest oil possible and/or extended oci.. its a recipe for disaster later down the road.
These TGDI engines are not tolerant of the wrong oil or long OCI.

The Tgdi 2.0 in my jeep is very responsive and peppy engine with more torque than the v6 options (50ft-lb more)

That being said they have a semi-intelligent oil life montitor that likes to stretch it out to near 10000mile oci in the summer.

The oil is thin and black between 5000-6000 miles.

I usually change the oil at 40%-50% remaining on the OLM.

 

[Propflux01]

My questions are what actually causes the blow by, after all, diesels are in a sense direct injected and don't seem to have a problem, and what to do after the rings stick and you need to get them unstuck without tearing the engine down?

 

[demarpaint]

Originally Posted by Rand
I wouldnt want a 10 year old 100000 mile one. Putting in the cheapest oil possible and/or extended oci.. its a recipe for disaster later down the road.
These TGDI engines are not tolerant of the wrong oil or long OCI.

The Tgdi 2.0 in my jeep is very responsive and peppy engine with more torque than the v6 options (50ft-lb more)

That being said they have a semi-intelligent oil life montitor that likes to stretch it out to near 10000mile oci in the summer.

The oil is thin and black between 5000-6000 miles.

I usually change the oil at 40%-50% remaining on the OLM.

Smart move, oil and filters are cheap. A Jeep 2.0L TGDI engine, not so cheap.

 

[Ws6]

Originally Posted by benjy
manufacturers are ALL mum about direct injection issues but see that some are using both at xtra cost for sure says they are aware. modern technology has a learning curve + buyers are the beta testers witnessed by subarus long CVT warranty + the fact if replaced under warranty subaru wants it sealed for inspection + possible improvements, so add CVT's to the NO-NO list IMO

Mazda directly addressed it with high temps on the intake valves and tuning. My cx5 had 106k miles with jo engine problems. The turbo cx9s likewise are now showing up at 100k plus with no issues. Mazda has spoken at length about prevention of carbon on valve issues and how they mitigate it by and large.

That said, I did get get the 10 year, 150k mile bumper to bumper warranty.

 

[Mad_Hatter]

Originally Posted by Propflux01
My questions are what actually causes the blow by, after all, diesels are in a sense direct injected and don't seem to have a problem, and what to do after the rings stick and you need to get them unstuck without tearing the engine down?

Wonder if it has to do with the fact that diesel fuel is so oily and (maybe) leaves behind a thin film of oil on the cylinder walls even after combustion?

Since gas is more refined and the fuel is injected directly into the cylinder on a GDI, as opposed to "normal" engines, how does the fuel vapor not have more of a stripping away effect on the upper cylinder walls protective oil film?

Maybe that's not an issue because the rings are carrying with them a sufficient layer/film of oil on the ensuing compression stroke? I know Hyundai/Kia has issued a TSB recommending "thicker" oils not long ago for a couple models if I recall correctly. Somebody posted the TSB here on BITOG a few days back.

 

[MCompact]

A 7500 OCI using M1 5W-30 served my MS3 quite well- it was going strong at 158k miles when I flipped it for my F22. UOAs indicated that a 10k OCI was possible.

 

[KrisZ]

Originally Posted by Mad_Hatter
Originally Posted by Propflux01
My questions are what actually causes the blow by, after all, diesels are in a sense direct injected and don't seem to have a problem, and what to do after the rings stick and you need to get them unstuck without tearing the engine down?

Wonder if it has to do with the fact that diesel fuel is so oily and (maybe) leaves behind a thin film of oil on the cylinder walls even after combustion?

Since gas is more refined and the fuel is injected directly into the cylinder on a GDI, as opposed to "normal" engines, how does the fuel vapor not have more of a stripping away effect on the upper cylinder walls protective oil film?

Maybe that's not an issue because the rings are carrying with them a sufficient layer/film of oil on the ensuing compression stroke? I know Hyundai/Kia has issued a TSB recommending "thicker" oils not long ago for a couple models if I recall correctly. Somebody posted the TSB here on BITOG a few days back.



Diesels have very high compression ratios, generally twice as high as gasoline engine. The piston rings on those are high tension, so they provide a better seal, that is why diesels don't have that problem, unless they start losing compression. Also, their piston design is much different than gasoline. They have a cavity in the middle of the piston and when the piston is at TDC, the fuel is injected directly into that cavity, so there is no over spray onto the cylinder walls.

Gasoline is not injected as vapor, but rather as finely atomized mist. With port injection, that mist has to travel down, past the intake valves and into the cylinder during the intake stroke, so it has plenty of time to vaporize when the engine is fully warmed up.
With direct injection the mist has no travel time at all. So to compensate, the fuel injection system is run at very high pressures so that the injectors can atomize the fuel into smaller droplets, but they are still droplets, not vapor. Since there is very little time for the fuel droplets to absorb the heat inside the cylinders and vaporize, a good portion of the injected fuel never does. Plus, since the fuel is directly injected into the cylinder, some of it simply ends up on cylinder walls.
Gasoline in liquid form doesn't burn, so the leftover fuel gets pushed past the rings during the combustion stroke.

 

[Bryanccfshr]

When speaking of direct injection and phase changing of gasoline I believe there is a lot of science in the design by engine makers. The timing and the placement of the injection of fuel is usually localized in the center of the chamber. Any fuel not ignited during the initial spark has the complete cumbustion/power stroke and the compression stroke to vaporize and ignite, incomplete combustion would result in soot, not liquid fuel on the cylinder walls.
I surmise the process is less efficient on cold start resulting in actual Unburned fuel in Greater amounts than in port injection applications. This is where the additional fuel dilution originates and short tripping with complete cool down between sessions worsens the issue. Other causes could be faulty injectors or poor spray pattern by deposits or unforeseen consequences of design.

 

[KrisZ]

This is the usual field injector location in GDI engines, the spark plug center location is more important I guess.

When I said some fuel may end up on cylinder walls, I meant during intake down stroke, when fuel is sprayed, not after combustion.
Then, during the compression stroke, fuel has greater chance to get past the rings.

And fuel can still be left over after combustion has occurred. If there was more fuel than oxygen for a complete combustion, then the remainder will be a mix of soot and raw fuel. That's how a lot of modern cat converters get warmed up quicker, by sending unburned fuel down the exhaust.
But now that I think about it, I don't think unburned fuel would contribute much to oil dilution, as most of it would exit through the exhaust valve.

So most likely it is the fuel sprayed during the intake stroke, and it would be worst with the engine cold.

 

[Bryanccfshr]

There are many different designs and concepts with today's engines. One thing that makes the start up process quicker is water cooled
/heated exhaust manifolds reducing the need for a rich mixture to get the Catalyst activated.

Another cool concept and application is as part of load mapping many direct injectors don't open until the compression stroke. This could mean the mixture is just right in a small space of the cylinder directly by the spark plug but outside of that it is lean.

 

[Bryanccfshr]

Here is one of the better explanations I have come across.

https://youtu.be/66C4YIiwRbM

 

[bobdoo]

Direct injection vaporizes fuel far more effectively than port injection, or carbs. That's one reason *why* auto mfrs use it.

I know my CX-5 dumps a lot of fuel at startup, to force the cat con to warm up faster. That contributes to fuel dilution.

The other big problem is the low-tension piston rings. They, again, contribute to MPG (in the fed testing regimen), but allow massive blow-by.

Improper break-in doesn't help. Those rings require high throttle bursts when the engine is new, to properly bed them to the cylinder wall.

 

[BMWTurboDzl]

Originally Posted by Propflux01
My questions are what actually causes the blow by, after all, diesels are in a sense direct injected and don't seem to have a problem, and what to do after the rings stick and you need to get them unstuck without tearing the engine down?


As I understand TGDI engines go through periods where they have to run rich in order to reduce combustion temps. This extra fuel with the addition of higher compression ratio causes some fuel to wash down into the crankcase. It's not "stuck rings" but possibly low tension rings which may exacerbate it.


Depending on the model some diesels have a little or a lot of fuel dilution when they're set up to inject extra fuel for DPF regen but outside of that scenario the big difference is that diesels use EGR not extra fuel to reduce combustion temps.


Mazda Sky-Active gas engines use EGR to reduce combustion temps.