After reading the detailed Press Information guide posted to the CC today,
The Cayman Club - Downloads - Porsche 997 Generation 2 Press Pack, it is clear that a lot of the stuff posted here about how DFI works was in error:
I guess you could say that DFI provides some sort of cooling, but the main advantage is in it's ability to handle multiple injection pulses in a single stroke, due to the position of the injector tip in the combustion chamber and the speed of the injection event.
With DFI the fuel distribution can be metered using multiple discrete pulses in a single stroke.
The engine designers can now control the flame propagation across the combustion chamber with great accuracy, virtually eliminating detonation. In fact, DFI engines almost start to behave like compression ignition engines in a crude sense, as there is much more flexibility in injecting fuel in concert with the valve overlap on the compression stroke. The bottom line is better control of the flame, cooler combustion chamber temperatures, and most importantly, better volumetric efficiency of the engine due to the substantially increased compression ratio.
Multiple injection pulses
only occurs in high load low rpm situations (see p. 17 of the press guide). So, although this post repeatedly cites multiple injection pulses as the "main advantage" of DFI, in fact this only occurs for a fairly limited range of operational parameters for the 997 DFI engine. That stuff about controling flame propogation and eliminating detonation is not true for the 997 DFI. The comments about the improved cylinder cooling leading to higher compression ratio are spot on.
With direct fuel injection, higher revolutions to attain the same torque that an engine with equal displacement (but a lower bore/stroke ratio) might attain are likely now possible with the use of less fuel per combustion cycle. Therefore, engines with higher bore/stroke ratios driven at higher RPM levels need not necessarily be less frugal in terms of overall fuel consumption because the primary combustion is now centered in a cone emanating from the injectors, and the fuel-air mixture around the chamber's circumference region (which doesn't burn off so efficiently anyway and only increases the cylinder wall temperature) is much lower than in port injection engines.
Except in a few circumstances, like the 1st few minutes of warm-up, the 997 DFI injects fuel during the
intake stroke of the engine. Thus, the combustion is not "centered in a cone emanating from the injectors", the combustion process will occur after the compression stroke, long after the fuel injection is finished.
The dynamics of the combustion process are indeed the dominant benefit of DFI, not the bore/stroke ratio, although this does contribute. I suspect that the ratio has been altered specifically to take advantage of the new fuel delivery process.
I think what should be added to this discussion is more detail on how DFI affects the combustion process. For the first time we now have the injector nozzle downstream of the intake valve, and this fact fundamentally alters the operation of a 4 stroke spark ignition engine. Because of the location of the injector nozzle we can now inject fuel during the compression stroke, whereas as before the only way to get fuel into the combustion chamber was during the intake stroke. This is a huge difference.
Injecting the fuel during the compression stoke now allows control of the fuel atomization and resulting combustion in the time domain, which is to say we can control the speed of the combustion process by using multiple injection events with varying fuel volumes. This has the effect of spreading the combustion event over a longer time period, which creates more torque. The combustion pressure acting on the piston is now longer in duration, as well as more evenly distributed over the surface of the piston.
This post draws a lot of conclusions from the incorrect view that fuel injection occurs during the compression stroke. It does not (see p. 16, for example, of the press guide). Fuel is injected during the intake stroke, and the combustion process will not be fundamentally changed.
The key point is, that cylinder temperatures in general are now somewhat lower (permitting a higher compression) as before under identical compression levels because combustion can be more focused toward the center, rather than also around the edges, and because more heat is extracted to vaporize the fuel inside the chamber.
The higher compression ratio is the key point. Again, there is no fundamental change in the combustion process, and no "focusing towards the center" that is in any way different than a conventional engine, at least for the 997 DFI.
BMW's 3.0 liter gasoline turbo engine, which recently won the Engine of the Year Award and produces roughly as much torque at 1300 RPM as the new Carerra 3.8 DFI engine does at higher revolutions, doesn't deploy vario-cam.
I don't see how you can compare the tubo BMW engine with the Cayman engine??? With forced induction lots of things change...
An excellent post - thanks! However, I think that Variocam will still be valuable. The amout of fuel that can be burned is limited by the amount of air that can be drawn into the cylinder. The intake system and cams of the engine are "tuned" to operate at a resonant frequency to pack more air into the cylinder than would otherwise be drawn in. The Variocam allows this resonant frequency to be varied with engine rpm, allowing for excellent volumetric efficientcy over a wide rpm range. DFI does not change this in any way.
I would say that my view of the continued importance of intake resonance tuning control through runners, variocam, flapper valves, etc. has been confirmed by the 997 DFI engine. Porsche has added
additional resonance control features in this engine - not removing them as obsolete, as several posters suggested would occur. The new engine has resonance control flaps in the
air filters!
It’s radically different, and we have to rethink the way 4 stroke engines operate, because they do not work the same way with DFI. When the fuel is injected in the compression stroke as opposed to the intake stroke, it’s a whole new ball game.
In a DFI engine, we have no fuel in the intake charge whatsoever. Fuel is not introduced until the valve is not only closed, but the piston has traveled virtually all the way (but not quite) to TDC of the compression stroke. At this point, the dynamics of the intake charge behavior is completely changed, the gas (air) is now under significant pressure as well as heat, and of course the volume is reduced to 1/12th of the original static volume.
In a DFI engine, optimizing of combustion is done with multiple bursts of fuel spray, timed to coincide with the piston travel and spark events. In effect, the fuel/air mixture is ignited progressively, and flame propagation and the resulting cylinder pressure can be controlled fairly precisely. Remember, the injection events can vary not only with time, they vary with volume as well, and usually, the first “shot” is quite small. In a sense, this is a stratified charge phenomena, albeit highly controlled.
Again, alot of conclusions based on the incorrect view that the DFI system would have multiple pulses of fuel in the compression stroke, near TDC. Niether of these are true for the 997 DFI.
2.) Fuel is injected after the valve closes and downstream of the valve in the compression stroke, not the intake stroke.
3.) The mixing of fuel and air is now totally de-coupled from the intake event.
Same comment.
I'm not trying to be prissy here - just setting the record straight. :cheers: