Quote:
Originally Posted by MechEngVT
wwest40:
I don't think any engine's combustion event has fizzled out before BDC on the power stroke, by which time the exhaust valve has already opened.
Agreed, but I think we can also agree that with the Atkinson cycle the combustion event is more "fizzled out" than with an Otto engine.
Even at partial throttle there is enough air/fuel mixture in a cylinder for very substantial expansion as the fuel vapor is combusted.
Agreed, absolutely. But with partial throttle with the Atkinson cycle is there enough, at what point is there enough, "substantial expansion" left to justify the power stroke's additional travel..??(***)
Expanding the combustion cycle in the Atkinson engine doesn't "waste" movement/momentum on the elongated power stroke, it merely extracts more energy from the expanding gases.
Again, agreed.
And you don't just compress it to a "native" 13:1 because of detonation. ESPECIALLY at partial throttle. While it is full-load spark knock that will kill an engine in a heartbeat, it is part-throttle pinging
I suspect you are referring to pinging due to engine "lugging". That has not so much to do with compression ratio but is moreso the result of the too much engine load and thereby the piston not being able to move downward as fast as the flame front is expanding.
that will kill an engine with 10,000 paper cuts. Until we get to high anti-knock index fuels (methanol) you'll be stuck at 11-12:1 max with naturally aspirated Otto cycles.
You seem to be saying that somehow keeping the compression ratio constant as a function of the level of cylinder charge would be detrimental. By that standard how would an engine with half the displacement volume work..??
Christ is right re: turbos' operation, only the reason that you'll never see a turbocharged Atkinson engine is that a "turbocharged Atkinson" is called a Miller engine.
Okay, I hereby revise my statement. "You will never find a Miller cycle engine using a turbocharger.
Buffering pressure pulses: ever heard of the analogy between intake air flow and acoustics? The intake "manifold" is somewhat of a muffler on the intake side of the engine. On ALL engines, not just Atkinson, there is unsteady intermittent flow going into each cylinder and when the air mass in the intake ports flows it gains momentum. When the intake valve closes the momentum of the port flow stops and sends a pressure wave reverting up the port opposite the "flow" direction. These pressure pulses leave the intake port into the plenum, which is essentially an open volume common to all (or a good number of) the intake ports and is fed by the throttle body or carb. Since not all reversion pressure waves occur at the same time and each of them is small in magnitude relative to the volume or mass of air in the plenum the plenum pressure is more consistent than the port pressure. Therefore, the plenum acts as a resonant chamber or "buffer volume" to help equalize the effects of the unsteady flow between the cylinders. In the Atkinson engine there is actually a small amount of reversion mass flow instead of just inertial pressure. This reversion flow is small relative to the inlet charge and therefore small relative to the port volume. Port volume is typically small relative to plenum volume, so in this respect both the intake port and plenum act as "buffers."
Used to LOVE the intake sounds at WOT out of my Ford 390 V8 4 barrel carb with the intake filter removed.
Christ:
I think you're reading too much into the intake reversion flow. Contrary to popular opinion port fuel injection systems typically inject fuel into the intake port onto a CLOSED intake valve (to give more time to inject fuel and to ensure full vaporization). The head of combustion/exhaust of the preceding cycle starts to vaporize the liquid fuel droplets and the turbulence of the intake event homogenizes the mixture. In a homogeneous mixture there is no "rich" or "lean" part as that would indicate it is non-homogeneous. Since fuel is injected toward the valve the "rich" part of the inlet charge would be drawn in first and turbulence would mix it well or if not the rich portion would swirl near the piston during the intake stroke. With the Atkinson-cam it is the last portion of inlet charge that is expelled. This would either be fully mixed near stoichiometric or worst-case be the "lean" portion of the charge. Since the expelled volume is small relative to the intake port most of this air should remain near the valve of the same cylinder to be re-used during the next intake event in that cylinder. Over multiple cycles the closed-loop ECU control system would learn from the O2 sensor how much fuel needs to be injected PER CYCLE which would already account for the expelled mixture from the previous cycle. Engines and electronic control systems are pretty robust in this regard; they can run sub-optimally pretty well and can be adjusted incrementally.
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*** Take the Mazda CX-7 DISI (Direct Injection Spark Ignition) turbocharged I4, for instance. Wouldn't you agree that keeping the non-boost compression ratio at the DFI "standard" of 12:1 instead of the current ~10:1 would be a good idea...?? Then use the delayed intake valve closing technique to (gradually??) lower the effective compression ratio as boost comes up..??
Transition, "smooth" transition, from Otto mode to Miller cycle mode..??
Or how about DFI with a native cylinder compression ratio of 15-16:1 and running in Atkinson cycle for an effective ratio of 12:1 and then a reduction to as low as 8:1 as boost rises (still maintaining 12:1 "net") with even higher throttle openings...??
Not that I think any of this would be viable absent an engine driven, via an e/CVT, positive displacement SuperCharger. An e/CVT would allow the SC to provide "boost" in accordance with engine throttling, no need for a throttle plate.