Gasoline vs Diesel Direct Injection

[Food Scientist]

Did some research online and couldn't really find anything.

I read about how some people are having problems with gasoline DI getting deposits on the intake valve because gasoline isn't constantly washing off any deposits. With the deposits then the valve can't fully seat and you run into misfire problems.

Why doesn't this happen with diesel engines that have been using DI for 20+ years now?

 

[KrisZ]

It does happen, and has been a problem ever since the EGR was introduced to diesel engines. But in North America the diesel market for passenger and light duty vehicles is around 3% of the market share. It`s hardly a blip on the charts.

 

[demarpaint]

DI is not a problem in diesel engines, it's the PCV, EGR, and DEF system that has been problematic for a lot of diesel owners. Like KrisZ said it only makes up for very small market share so it doesn't get that much attention.

 

[tcp71]

Airflow in the diesel keeps the valves from heating up as much and "baking" deposits on like the gasser with its throttle plate slowing air velocities. Some direct injection engines (GM?) have an extra injector in the throttle body to, occasionally, spray fuel over the valves to keep them clean.

 

[badtlc]

Originally Posted By: tcp71
Airflow in the diesel keeps the valves from heating up as much and "baking" deposits on like the gasser with its throttle plate slowing air velocities. Some direct injection engines (GM?) have an extra injector in the throttle body to, occasionally, spray fuel over the valves to keep them clean.


Are you sure about that? Mazda said the cool intake valves cause the vapors to condense on the valves and that keeping the intake valves above 400F was the critical point for gas.

It would make sense that if it is lesser of an issue for diesels it would be due to higher intake valve temps and not less air flow, although that does help increase the temps. Either way, i don't think deposits are "baked" on.

 

[440Magnum]

There was another thread on this recently, but short story:

- Some newer diesels with a lot of EGR do have intake valve deposit issues.

- Most diesels don't have intake deposit issues because there's no throttle- the intake valve just flows all the air it can flow every time it opens. Gasoline engines are throttled, which means that at part throttle and (especially) idle, there's very little air flowing past the intake valve as well as a lot of retrograde flow OUT of the cylinder and back into the intake during part of the piston's motion. That PCV-rich (oil and blow-by), partially exhaust-laden air with it which puts deposits on the valves.

 

[KrisZ]

Just do a google image search for "diesel intake manifold deposits". The results are much more severe than anything I've seen for GDI.


Originally Posted By: 440Magnum
- Most diesels don't have intake deposit issues...


You have to be more specific. In heavy duty or industrial application I agree with you on the "most" part, but in light duty and passenger cars, it is quite a problem. It's all has to do with the type of usage, not the throttle plate.
Another reason why most diesel owners don't notice any driveability issues until the entire intake is almost clogged, which takes a very long time and perhaps is also the reason it is not reported on so much, is forced induction. Most cars don't have a boost gauge, so owners would not notice increase in boost pressures and the turbo will compensate for any losses in the low to mid-range power requirements.

I have a feeling that gasoline DI engines with forced induction will also not see any driveability issues and perhaps they may never have any driveability issues to begin with, simply because gasoline engines do not produce enough soot to clog the entire intake manifold.

 

[Olas]

There's nothing wrong with DI and it doesn't cause deposits, EGR and PCV cause deposits.

 

[Rand]

Originally Posted By: Olas
There's nothing wrong with DI and it doesn't cause deposits, EGR and PCV cause deposits.


I'd agree but that's fact by omission.

With DI there is no fuel flowing over the valves to clean them. So while DI doesn't "cause deposits"

It also doesn't clean them in the way MPFI did.

 

[OneEyeJack]

Higher levels of magnesium in diesel oil has been effective in reducing valve deposit in compression fired engtines. Chevron teamed with Detroit Diesel because their 60 series engines were experiencing valve deposit problems.

The magnesium in the diesel oil combines with the deposits on exhaust valves making them unstable. These deposits decompose and do not remain in place. The remaining deposits are thinner, allow better valve cooling and help to extend valve life and prevent catastrophic failures.

This was very important work was done by Chevron with their RPM oils for other applications as well, like emergency power generators with low duty cycles and was important in solving valve problems as well as certain Detroit Diesel over the road engines.

Chevron was the first oil company to solve the problems of valve deposits in this way lead by J. A. Mc Geehan, W. Alexander, P. R. Ryason, and B. L. Schmidt. You can read their papers on line with a bit of searching.

 

[demarpaint]

Originally Posted By: Olas
There's nothing wrong with DI and it doesn't cause deposits, EGR and PCV cause deposits.


I'd say in diesel engines it is the EGR and PCV causing the problems. I'm not so sure with gas engines though, I'd be willing to bet the EGR and PCV isn't helping matters though.

 

[Olas]

Originally Posted By: Rand
Originally Posted By: Olas
There's nothing wrong with DI and it doesn't cause deposits, EGR and PCV cause deposits.


I'd agree but that's fact by omission.

With DI there is no fuel flowing over the valves to clean them. So while DI doesn't "cause deposits"

It also doesn't clean them in the way MPFI did.


There wouldn't be any need to clean anything if it weren't for EGR and PCV polluting our inlet tracts. You're right that wet manifolds are cleaner than dry manifolds when those devices are in place.

The answer is to re engineer the emission systems. EGR can be mapped out very easily, PCV can be vented to exhaust very easily, and optimum power/efficiency can be maintained almost much longer while reducing maintenance costs.
Or retrofit an engine that predates emission laws