DFI drawbacks

Just have a couple quick thoughts about DFI. I've wondered about the effects on intake valves such as carbon build up due to the lack of fuel to constantly keep them clean. Also the effects on the engine oil due to any present blow by that would force even more fuel into the oil, creating fuel contaminated oil. Think of this in terms of a diesel engine that can almost never have uncontaminated oil even after you change it. Wouldn't DFI increase the compression ratio slightly as well? Or would it be the exact opposite of the above by providing a cleaner complete burn. Just some random thoughts....care to share?

It is the fuel sprayed behind the valves in a port-injected engine that causes the build-up on intake valves, which is not carbon. Carbon builds up inside the combustion chamber. DFI doesn't increase fuel content of the oil either; all DFI does is change the point at which fuel is injected. DFI by itself does not increase the compression ratio, but most engines employing DFI get a compression boost during the redesign, because the fuel injected directly into the combustion chamber is cooler (because the hot intake valves haven't heated it), and this allows a higher compression ratio to be tolerated with a given level of anti-knock in the gasoline, increasing power. DFI has two potential drawbacks. First, the super high pressure fuel pumps sometimes make a lot of noise. Second, the super high pressure fuel lines can be a maintenance complication, and a safety hazard if handled improperly.

I've heard that DFI engines often don't sound as good, but don't quote me on that.

Very interesting question. I hadn't really thought about it. I think blueone covered your questions very well. I wonder if maybe we'll see some effect of heat on the intake valves, valve guides and intake ports, but I sort of doubt it. As with any new technology or product evolution, there are bound to be unanticipated drawbacks. Like blueone said, the high pressure fuel rails may prove to be maintenance items over time and I wonder how much fuel may be blasted out of them in the event of a serious crash.

A complaint commonly levied against the 997.2 , yes, but it has been pointed out in every review I have read that the 987.2 does not suffer the same fate. Is that down to engine placement and less use of sound deadening material? Who knows, but we'll find out in a few months when they start to arrive. The biggest problem is probably what blueone pointed out above. I follow the BMW boards as well and they have had some significant issues with the 335i's high-pressure fuel pump failing and needing replacement. Seems like it's the sort of product that will fail early or not at all, but I guess that, too, we will see with time.

All else being equal (and it probably isn't), the answer would be that the higher fuel pressures would be a higher risk. This would be primarily due to the increased misting effect that is made possible by the higher pressures - bigger vapour cloud, higher peak ignition pressures due to the better atomization. There would be ways of mitigating that increased risk but it would take a much closer look at the system design to understand what, if anything, has been done. Just lending a safeguarding approach to this question that is utilized in chemical/refining process design. ie. minimize volumes and minimize pressures where reasonable.

Actually it is gasoline's heat of vaporization (the heat energy absorbed converting the atomized fuel into a vapor) that creates the cooling effect allowing for a slightly higher compression ratio. Regards, Joe

re: reliability of the new DFI hardware, I would guess that Porsche doesn't make any of this. It is likely a Bosch product and Bosch has been making DFI units by the millions for years.

High pressure DFI or just DFI? HDFI would use smaller injection orifices, run higher combustion temperature, require more computing power and control parts, etc from what I can see. I was surprised to see DFI has been around for years in gas engines (aviation first, then Mercedes in the 50's) but these appear to be entirely different. Then HDFI appears to have shown up seriously about 10 or so years ago, but OMC basically went bankrupt with their design (2 stroke though), and even the success of the Audi R8 in LeMans can't vouch for its reliability in consumer cars (unless you carefully prepare the fuel for your car and rebuild the engine every 24 hours!). That being said, a Google search didn't come up with any recent HDFI issues and there have been more and more of them on the road in the last several years. I would hate to be a young guy though, buying one of these used in 10 years - trouble shooting has to be a pita!

Just to add a little more confusion to this thread, I believe the statement that there are fewer parts in the new DFI engine referrs to the design of the bottom end (ie, no separate cradle holding the crank) rather than to theisystem. The elimination of the crank cradel should forever eliminate the RMS leaks.