Thank you OK for the links, some more late night reading for all of us who are interested in this subject.
You are quite welcome.
More of my thought and points:
On batch or even SMPI the injection period is not always into a 'forward' moving air mass (depending on load) and partial obstruction of an opening valve will compress then expand the charge which may cause a % of droplets to condense or fall out of suspension - but I agree that running λ = 1.0 at high throttling with the injection period during the valve opening event does promote good fuel atomization with mixture swirl or tumbling depending on valve actuation. This mixture will be exposed to the full area of the cylinder wall be it cold start or fully warmed. Any condensate will be wiped on the up stroke. We have all read reported dilution issue with SMPI for those who idle the vehicle at warmup - especially with starts and coolant below freezing.
Yes, and performance-oriented SEFI engines have been known to do a bit of dilution too, but not to the degree we are seeing with these TGDI engines. Ignoring carburetors, batch fire and TBI would be the least inclined towards proper homogenous mixture formation because of course there is going to be fuel being sprayed into air masses where the column or segments aren't moving. SEFI/SMPI, there is always SOME movement, even if it is relatively slow, like at low load, low RPM, which is going to promote a better blended intake charge.
It appears side-positioned D.I is at its best at economy cruise, say λ = 1.1 were the injection pulse width is >/= 10ms
and the fuel enters when the piston is at its upper 1/3 of compression stroke travel and the piston top contour - if so employed - will deflect fuel away from the opposite wall. In this scenario there is simultaneous charge phase cooling to prevent pre-ignition and a mostly concentrated charge away from distal surfaces to prevent detonation. An issue with D.I is when the fuel must be injected during the intake stroke and if side positioned and vectored it will impinge and likely wash the opposite wall - especially with long stroke narrow bore engines.
Actually, I'm glad you brought up small bore engines, because that's one of the things touched-on in some of the above material with respect to these small displacement, highly strung TGDI applications. The bore is small, so the high pressure spray of the DI nozzle gets a lot of fuel on the walls, and if the walls are cool, those form droplets that may be forced by the rings during the remainder of the compression stroke or as part of the blow-by gas as they are already condensed at the ring-wall interface, down the side of the piston.
Now maybe 1/3 of the ring circumference will be running up along a "dry" cylinder bore.
D.I. cannot inject to great effect under high load/high rpm during exhaust and compression "off--cycle" as SMPI or Batch can,
So we see high velocity/volume injection attempted in milliseconds during un-throttled high load scenarios.
Yes, and, at extremely high pressure. I'm curious how well the metering of the DI injectors works compared to say a present advanced port injector, which has a much lower nozzle pressure.
Again this is all specific to any particular design, geometry and timing.
Absolutely, and that's the important bit. Some DI engines seem to avoid dilution quite well, like the BMW B58 for example. Not sure about their 4-popper, but it may as well. Other engines, like the Honda 1.5L, seem to be notorious for it to the point where their reputation proceeds it. As far as I know, Ford has fared reasonably well with their DI engines, as they are also mentioned (well, the Ecoboost is) in the above material.
I see some BMW with the Bosch system use a center injector near the spark plug. I wonder what other engines use this approach.
This would be optimal along with SMPI if it doesn't impact valve sizing to a great degree.
Yes, the B58 is one of those, you can see the injectors between the valves here:
My FORD uses side positioned injectors, and is likely an older, sub optimal arrangement.
But I only say that due to my current dilemma.
Couple good pictures of a Ford DI engine with the side injection here:
The top of the piston design, clearly with the intention to deflect the DI charge, as you remarked on earlier, seems well thought-out:
I see one SAE paper above was presented by engineers from Jiangling Motors Co., LTD., China
The hotbed of research and development. Looks like a discussion of an early implementation. I don't disqualify it, I will read it.
Excellent, looking forward to your feedback.
On oil sampling, I agree without a proper lab test I would not have presentable data on my Jetta D.I. Turbo
fuel dilution tendencies.
But, after initial wear-in, I experienced no increase on the dip stick, no ugly appearing oil, and no engine racket
when warmed. Maybe Fat and Happy, but you see I only fret the discordant effects I see and feel.
Or, in other words, I don't go looking for trouble, it finds me all on its own
Good discussion.
- Ken
Good discussion indeed. I don't recall the VW engines being huge diluters either, FWIW. But of course early VAG engines had serious issues with IVD's.