Direct Injection - Carbon Build Up

[HungaryGK]

VTC exchange!

[TnTkr]

Thank you for an excellent picture, HungaryGK! That is really enlightening. For me it looks like the valve guide seals are leaking oil, which have caused crust build up.

Anyway, the picture makes me think, that the carbon build up problem in this case is not so much about the quality of fuel or oil, but more about short trips and light load of the engine (and maybe leaking valve guide seals), so that the carbon is not burning away.

The cars are nowadays so powerful, that in many areas it is difficult to have full load on engine for any longer times within legal speeds (except in Germany) or loads. Or at least you need full load inside and some mountain roads to climb.

The carbon build up cases in US L15B engines, that I mentioned before, have been carbon build up on fuel injectors, and that is most likely related to the low quality of fuel, or at least additives not suitable to direct injection engines.

[jazzaro]

Thank you for an excellent picture, HungaryGK! That is really enlightening. For me it looks like the valve guide seals are leaking oil, which have caused crust build up.

Anyway, the picture makes me think, that the carbon build up problem in this case is not so much about the quality of fuel or oil, but more about short trips and light load of the engine (and maybe leaking valve guide seals), so that the carbon is not burning away.

The cars are nowadays so powerful, that in many areas it is difficult to have full load on engine for any longer times within legal speeds (except in Germany) or loads. Or at least you need full load inside and some mountain roads to climb.

The carbon build up cases in US L15B engines, that I mentioned before, have been carbon build up on fuel injectors, and that is most likely related to the low quality of fuel, or at least additives not suitable to direct injection engines.

I disagree.
Pics both show interesting facts:
1- camshafts, rocker arms and other lubricated parts are very clean, this means that the oil was good and did not create burned varnishes and clogghings, so I presume good quality oil and in-time oil changes.
2- this car is a 1.5 GK, introduced in Europe in 2018 so it's quite new: 2 years and 80000km mean that the car hasn't been softly used, so we can hardly talk about short trips and low loads.
3- the position of the carbon buildup on the valve shows that the valve face is clean, this is because the injector sometimes sprays some petrol on it when the valve is opened. Carbon is only on the valve stem, and this can be for several matters:
a) valve guide leaks, as you correctly said.
b) low quality fuel. I don't know present conditions, but in the past East Europe was not known for having high quality petrols. In south Italy, sometimes fuel stations are used to illegal disposal of chemical waste, we have several case of common rail diesel engines with broken injectors by contaminated fuel. In USA, petrol commonly have higher quantities of sulphur than european fuels, and this is not good for carbon buildup.
c) high amount of blowby gas recirculation, so much oil vapour wetting the valve stem.
d) wrong cam timing, since the forumer is changing a broken VTC and this could have caused some fuel mixture backflow or incorrect flux of air and fuel spray.
One, more than one or all this matters could have caused the clogghing.
It'd be interesting to read european HRV forums, since the L15B with direct injection is standard on that model. Do owners complain carbon buildup on their petrol engines?

[TnTkr]

Mostly good points but I disagree with some:
2- Relatively high kilometrage is not excluding light load and it can be driven in urban driving accumulating from short trips, slow speeds, many cold starts and lots of idling, which all can expose to crust.
b): As the fuel is direct injected to combustion chamber, the quality of fuel cannot be the cause to crust in valve stem.

[culzean]

Mostly good points but I disagree with some:
2- Relatively high kilometrage is not excluding light load and it can be driven in urban driving accumulating from short trips, slow speeds, many cold starts and lots of idling, which all can expose to crust.
b): As the fuel is direct injected to combustion chamber, the quality of fuel cannot be the cause to crust in valve stem.

Short journeys also bad for EGR IIRC,  which is strange because EGR does not open below about 3000rpm, but maybe it is the fact the it never opens that crusts up the stem and when it does open one time ( maybe the careful owners foot slipped on accelerator and went over 3000 LOL ) the shaft sticks and stops it from closing again.

Well if valve is not affected by DI how come it gets a carbon build up on head and stem when DI is used ? Bad fuel can contain many impurities that do not burn and more likely to cause build up than good fuel.

[TnTkr]

I would assume, that with direct injection carbon build up on head and stem is coming from EGR, possible oil mist in crank chamber breathing and possible stem seal oil leakage.

[culzean]

I would assume, that with direct injection carbon build up on head and stem is coming from EGR, possible oil mist in crank chamber breathing and possible stem seal oil leakage.

Well non-DI engines have had EGR for a long time now and never seen build up on valves...  Don't modern DI engines have the injection split between port and direct injection to prevent the valve build up.

[TnTkr]

( maybe the careful owners foot slipped on accelerator and went over 3000 LOL )

*off-topic warning* IMO it is misuse of 1,5 Jazz if rpm range is not used up to redline. That's where the fun is. 

[TnTkr]

Don't modern DI engines have the injection split between port and direct injection to prevent the valve build up.

Honda L15B3 has direct injection to combustion chamber only.

[jazzaro]

Mostly good points but I disagree with some:
2- Relatively high kilometrage is not excluding light load and it can be driven in urban driving accumulating from short trips, slow speeds, many cold starts and lots of idling, which all can expose to crust.
b): As the fuel is direct injected to combustion chamber, the quality of fuel cannot be the cause to crust in valve stem.

2- possible but not probable, the forumer could tell us about his habits.
b) It can surely be. In diesel engines, fuel injection comes in the last part of the compression time so, considering no valve leaks, no fuel can go back to inlet manifolds.
In petrol engines using direct FI, something of similar happens only at low loads and low-medium revs, and it's called stratified charge; in this mode petrol is injected after inlet valve closing and you would be absolutely right, but for high loads and in some other particular conditions (depending from each  model) the engine will shift in homogeneus charge, and petrol will be sprayed inside during the intake timing, so when one or two* intake valves are open. I-VTEC Honda engines can vary some intake  timing parameters, but they cannot avoid at low and medium revs some little backflow of air-fuel mixture, and engeneers want this to keep valves clean. If you have bad fuel, injectors can work bad getting dirty ann clogged, so the spray won't be nice and homogeneus, but its direction and drop dimension could vary. Considering this matters, can happen to find fuel in the intake manifolds even in a direct injection engine.

* First Vtec engines we use to think about  were very high power Civic 1.6vti, or 1.8 Integra Type R... marvellous pieces of engeneering where the Vtec was used to gain very high revs and high power, joined to a good torque at low revs. Phase and timing pattern had two configurations, one for low revs and another for high revs, both with two intake valves opened.
Our engines have a "different purpose", less sportier, so the Vtec system is used to open one or two intake valve depending from what the driver needs. If you look at cam profiles, you will see that lateral cams are different, one is quite circular, the other is standard, so in "vtec mode 1 only one valve will open, the other will stay closed or will slighty open just to help the swirl. On vtec mode2, when the pin locks the inner rocker arm with the two outers, bothintake valves will open and the engine will have a tumble mixture motion. A single intake valve gives more turbulence and better firing at low revs, two valves give power increasing the amount of air entering in the cylinder.
So the Vtec is used for opening one or two intake valves and for changing the lift, while the VTC is used to change continuosly the phase.

[jazzaro]

Don't modern DI engines have the injection split between port and direct injection to prevent the valve build up.

Sometimes they have both systems (Volkswagen TFSI for instance), but not the L15B3.

[TnTkr]

Even if small amount of fuel mist may get out of the combustion chamber when injecting intake valve open, there is no that amount of backflow that could cause fuel impurities causing carbon build up on valve stems as seen in the picture from HungaryGK.

It is true, that the intake valve closes when the piston is already on the way up, but the flow direction through valve is definitely towards the combustion chamber. That is, in modern car engines, which we are now discussing. Otherwise the fuel economy and hence CO emissions could not be in the level required from new cars.

[TnTkr]

Just for your interest, a member in fitfreak.net forum just disassembled a GK5 Jazz L15B1 engine, and he had this picture presenting nicely the hole of the fuel injector: https://cimg1.ibsrv.net/gimg/www.fitfreak.net-vbulletin/1297x975/20200622_054021_d1484ca1ba1fc07eeeb5f5460f649d52a180f1d3.jpg.

There's also a picutre of piston showing the cavity where the fuel injector points: https://cimg7.ibsrv.net/gimg/www.fitfreak.net-vbulletin/1297x975/20200621_002248_a56272ae2307b4b04f3dd3dcea6690ce675e3614.jpg.