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"You may have read or heard one of your favorite Car Tech editors talking about gasoline direct injection and how it's one of the "big technologies" that's helping to keep the almost 200 year-old internal combustion engine alive well into the 21st century. In this week's issue of the ABCs of Car Tech, I'm going to explain just what the heck gasoline direct injection is and why you should care if it's in your next car's engine or not."

What's so great about direct injection? (ABCs of Car Tech) | The Car Tech blog - CNET Reviews

Read down to the last sentence for Subaru info.
 

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never really stopped to consider the muck building up on the back of the intake valve(s).

I've considered a grimmspeed or similar air/oil separator to handle the PCV on my WRX - to try to reduce oil vapor gumming up TG valves but I guess EGR would still be a problem with direct injection?

thanx for posting the link.
 

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If you google direct injection problems, tons of carbon build up problems pop up.........In Audis, in particular........But none are immune. The oil separator on the PCV helps, but not as much as you might think. The Toyota system, with the combination of port/direct injection seems like the best stop gap solution. What will be the final answer is still in development, hopefully.
All diesels are direct injection. I am not sure why they don't suffer the same issue.
 

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All diesels are direct injection. I am not sure why they don't suffer the same issue.
interesting point - how do they handle PCV, EGR?
 

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The diesel - burns differently so could be a different result regarding this issue. Also the compression with the diesel is higher that could also play into this issue.

The BRZ is listed as having Toyota DI tech bolted to the flat 4 Subaru Engine. So it will be interesting to see if this is a problem with the BRZ after they get a few miles on them.
 

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Ford is selling the twin turbo DI V6 in nearly every major vehicle form the Flex, F150 to the Taurus - and its been out for a while now - so could be interesting to see if they start having major problems with this as these vehicles get some age and miles on them.
 

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Another probable reason (or 20 not mentioned for the higher fuel rail pressures is the need for much better atomization when injecting directly to the cylinder, and the need to get the fuel into all of the air in the cylinder.

Port injection is putting the fuel spray into a rapidly moving turbulent air stream that will prvide some mixing and vaporization, in many respects the air in the cylinder is much less active, and the fuel that is going to the 'far parts' of the cylinder with port injection is sprayed in along with the air that goes there.
 

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you can start a car witout a starter motor to rotate the crank with DI.

But i read that tests done by - IIRC Bosch - found it not to be practical.

It would save a lot of weight as you could delete the starter motor and use a smaller battery and smaller alt.

maybe someone else can make it work someday.
 

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The diesel - burns differently so could be a different result regarding this issue. Also the compression with the diesel is higher that could also play into this issue.

The BRZ is listed as having Toyota DI tech bolted to the flat 4 Subaru Engine. So it will be interesting to see if this is a problem with the BRZ after they get a few miles on them.
The compression ratio for gasoline DI engines is pretty [email protected] high, too. Some are over 14 to 1. Not diesel territory, but high for a spark ignition engine.
 

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Heck, with DI you could even make it run backwards!

The bearings won't be happy with the oil getting sucked out of 'em, though.
 

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The compression ratio for gasoline DI engines is pretty [email protected] high, too.
Yep, with DI, fuel/air charge is cooler than with port injection.
That inhibits detonation and allows higher compression ratios;
higher compression produces higher specific torque (lb-ft/liter)
and specific horsepower (hp/liter).

.
 

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All diesels are direct injection. I am not sure why they don't suffer the same issue.
Diesels do, and it can be pretty bad depending on the design.

VW used to have a bad problem with sludge in the intake manifolds and intercoolers of its TDI engines. The combination of very fine droplets of oil (PCV gasses) and exhaust particulates (from EGR) would form sludge that would build up on the walls of the intake manifolds and collect in the intercoolers.

That's largely been addressed through the latest technologies. Common Rail injection operating at 25,000+ psi and very precise fuel injectors that inject fuel 5-8 times for each injection cycle have reduced exhaust particle size and amounts. Reducing compression ratios and using urea to combat NOX emissions can greatly reduce EGR operation, reducing one element of the sludge. The PCV side of it...that really requires filters, which I haven't seen any automotive manufacturer use.

Someone else mentioned compression ratios of GDI engines being sky high. Very true. What I find interesting is that gasoline direct injection is allowing higher compression ratios in gas engines, while emissions requirements (particularly NOX) is driving diesel compression ratios downward. It used to be common for diesel compression ratios to be upwards of 22:1. The latest VW TDI engines have a 16.5:1 compression ratio. I read an article a while back that for overseas markets, Mazda was working on two more SkyActive engines...one gas, and one diesel, and the compression ratio was the same on both: 14.5:1. I could be off just a bit on that CR, but the article stuck out in my mind because it showed that gas engines and diesel engines are moving closer together in terms of compression ratios and some technologies, all in an effort to meet emissions and fuel economy goals.

I've read about the carbon buildup on GDI engines. I think if theres a way to change PCV, the carbon buildup will likely not happen, or at least be reduced. By change PCV, I mean do something else with it besides just dump it into the intake manifold. I tend to think that PCV is probably a large contributor to carbon buildup on intake valves of GDI engines.
 

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Yep, with DI, fuel/air charge is cooler than with port injection.
That inhibits detonation and allows higher compression ratios;
higher compression produces higher specific torque (lb-ft/liter)
and specific horsepower (hp/liter).

.
How does the charge stay cooler? Or is it the heat of vaporization of the fuel when it is sprayed in absorbing some of the heat?
 

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How does the charge stay cooler?
The combined effect of evaporative cooling and reduced contact time
between the fuel fraction of the charge and the hot surroundings, i.e.,
no exposure to hot intake runners, and possibly shorter contact with
the hot cylinder walls (depending on when in the firing cycle the fuel
is injected).

Although the evap cooling effect is pretty much the same for either
type of injection, postponing it until later in the cycle reduces the
time available for re-heating to occur.

.
 

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All diesel's are NOT direct injection. Many (older VW diesel, 6.9 Ford and many others) use a pre-ignition chamber where the fuel is injected.
 

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The "Preignition chamber" is still part of the combustion chamber, subject to the high pressures within, so it is still direct injection.
 

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Ideally, direct gas injection would allow for higher compression ratios without the need for high octane fuel. Per-ignition can be eliminated because there wouldn't be fuel in the combustion chamber yet during the compression stroke. Multiple injection events per cycle can help to burn more completely and reduce emissions. I have to admit that most of my dealings with DI involve diesel engines, though.

I have to agree with 4wheeldog -- although technically a bit different, pre-chamber injection on older diesels is essentially still direct injection, for all intents and purposes. Burn characteristics have been improved since they moved away from pre-chambers, though. Many diesels have gone to four valves per cylinder (not needed for air flow since they are turbocharged), in order to place the injector right in the center of the cylinder head.
 
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