Study: gasoline GDI engines = lower CO2, but more soot

[MetroMPG]

Interesting factoids from a study from the University of Toronto about gasoline direct injection:

• between model years 2009 to 2015, the percentage of new vehicles sold with GDI engines jumped from 5% to 46%
• GDI engines emit lower levels of CO2, but they emit more black carbon

Clearly GDI has become a popular way to increase fuel economy and reduce certain emissions (eg. CO2).


But apparently airborne soot (black carbon) is also recognized as a contributor to climate warming, and...

in some cases, you need up to a 20 per cent improvement in fuel economy in order to offset black carbon emissions

Think a more fuel-efficient engine is the green choice? Maybe not - U of T Engineering News

[Frank Lee]

Ya always have to pay. :/

[acparker]

Soot filter, or, alter the combustion mix to lower soot production.

[me and my metro]

I understand that the direct injected engines build up carbon on the back of the intake valves. No fuel spray to keep them clean and cooler.

[MetroMPG]

I've read the same things about valves gunking up, and have seen first-hand that some people shopping for new cars are actively avoiding GDI's.

[rmay635703]

Originally Posted by acparker

Soot filter, or, alter the combustion mix to lower soot production.

Lean burn virtually eliminates soot, by oh the terrible Nox

On a related topic water mist systems can improve fuel economy, seems like that would also cure the valve coking issues.

[RustyLugNut]

Originally Posted by acparker

Soot filter, or, alter the combustion mix to lower soot production.

Carbon traps work fine for micro meter sized particles. GDI engines produce sub micro to nano meter sized particles. Standard traps would be largely ineffective.

Altering the combustion mix is not so much the issue as how and when the mix is injected. Also, how much EGR is being used. GDI engines usually inject well after all the valves have closed to provide maximum entrapment of air so no oxygen displacement occurs. But, this leaves little time for evaporation of the droplets and that leaves some small droplets in place when combustion starts resulting in fine carbon being produced. Also, side reactions become railroaded and thus you get hydrocarbons such as benzene slipping out of the tailpipe. Higher intake temperatures as well as an increase in mixture tumble and swirl help reduce carbon particles but they increase NOx.

Catalyst charged traps are in the works and this should just about eliminate the bulk of the black carbon soot. Also, increased fuel pressure and injector tip design will help reduce carbon formation much as has happened with diesels. At some point, lean NOx traps may make their way onto GDI engines to free up the design parameters.

[RustyLugNut]

Originally Posted by rmay635703

Lean burn virtually eliminates soot, by oh the terrible Nox

On a related topic water mist systems can improve fuel economy, seems like that would also cure the valve coking issues.

This might be true of PFI ( port fuel injected ) engines, but not for GDI. Many Asian market GDI engines have lean burn and suffer the same black carbon production. It is inherent to the injection and combustion profile.

Water injection at the ports will clean up the valve's back faces but will cause displacement of air and reduce the specific power of the engine which is the advantage of GDI in the first place. The injector will have to be in-cylinder to provide the power/emissions benefit but then there will be no valve cleaning effect.

[rmay635703]

Originally Posted by RustyLugNut

This might be true of PFI ( port fuel injected ) engines, but not for GDI. Many Asian market GDI engines have lean burn and suffer the same black carbon production. It is inherent to the injection and combustion profile.

I bet a little more compression, a longer stroke and lean burn would reduce carbon.

Also which Asian market GDIs go lean? Just curious.

Originally Posted by RustyLugNut

Water injection at the ports will clean up the valve's back faces but will cause displacement of air and reduce the specific power of the engine which is the advantage of GDI in the first place. The injector will have to be in-cylinder to provide the power/emissions benefit but then there will be no valve cleaning effect.

Who said water mist has to be on all the time and always the same volume?

Could easily have it shut off near WOT. and be on during lean burn partial throttle.

[RustyLugNut]

Originally Posted by rmay635703

I bet a little more compression, a longer stroke and lean burn would reduce carbon.

Also which Asian market GDIs go lean? Just curious.



Who said water mist has to be on all the time and always the same volume?

Could easily have it shut off near WOT. and be on during lean burn partial throttle.

It is more about time of injection. If there is not enough evaporation, large enough droplets will be available at combustion and these provide the basis for the carbon soot. Compression and stroke have little to do with this. Look at modern diesels. They always run lean and they produce soot. And they have strokes and compressions far above what a GDI is talking about. But, the soot formation in a GDI is more similar to the diesel than to a PFI spark ignition engine.

A source in Korea has told me that some Hyundai models as well as others have the capability of lean burn depending on the target market. Many countries care little about NOx.

And yes, you are correct, you can limit your water injection to only part throttle. Of course there are in cylinder injection schemes that are worth the effort. You will not be able to leverage that with port water injection.