Potential engine mod? Otto to Atkinson conversion

[wwest40]

So, the Atkinson cycle engine is ~23% more efficient via elongating the "burn" cycle, more complete combustion of the A/F charge before entering the exhaust manifold, yet there is still enough energy in the "flow" of the exhaust gasses to spin a turbo.

NOT...!!

There is a pretty radical difference between air pressure waves that are simply created via alternating flow restriction and actual alternating reversal of flow.

[wwest40]

Turbocharging...

In order to take advantage of turbo boost the engine's "native" cylinder compression ratio must be "derated". Boost, once it arrives, will then raise effective compression ratio back up into the 13:1 range. So the 13:1 "native", off-boost, compression ratio of an Atkinson cycle engine would likely be reduced to something in the range of 10:1 or less.

Obviously that would result in the lowering of the performance of an Atkinson cycle engine to an ever lower level. That, in turn, results in even less exhaust flow/energy for spinning a turbo up quickly and thus the onset of turbo boost would become even more intolerable than with an Otto engine.

[wwest40]

Originally Posted by Christ

[/B]
You have the same thing with a 6 or 8 cylinder engine. Even 5 cylinder engines, and 10 cylinder engines will always have a cylinder on intake at the same time that one or more cylinders is on power/exhaust/compression. The only exception to the rule is the single cylinder engine, and any variant of rotary engine.

The only difference is the speed and timing at which the next piston draws back the unused (expelled) intake mixture.

2 cylinder engine, pistons are working 180 from each other (normally)
3 cylinder engine, pistons are working 120 from each other (normally)
4 cylinder engine, pistons are working 90 from each other (normally)
5 cylinder engine, pistons are working 72 from each other (normally)
6 cylinder engine, pistons are working 60 from each other (normally)
8 cylinder engine, pistons are working 45 from each other (normally)
10 cylinder engine, pistons are working 36 from each other (normally)
12 cylinder engine, pistons are working 30 from each other (normally)

In every iteration of a standard balance multi piston engine, each cylinder is working opposite another one such a way that the expelled intake mixture would be recovered by another cylinder.

MechEngVT - What I meant by bunk was more like "Wasteful". I mean this because the cam causes outflow of intake mixture back into the exhaust manifold. This causes issues with intake harmonics, etc, and screws up ideal mixtures in just about any gasoline engine. What's to say that the mixture pushed out wasn't the richest part of a layered homogenous mixture? Now the mixture in the cylinder is lean! Subsequently, the engine has had another pulse of air through the induction system, which in carb'd apps already has gas in it, or in FI apps has fuel added as it flows into the cylinder, but either way - the new cylinder's mixture is ideal, but it's sucking in the richest part of the last cylinder, so now it's rich! Now you have a lean cylinder, and a rich cylinder. Who knows what part of the rich cylinder will end up in the outflow from the second piston's compression event, and so on/so forth. See what I mean? In the Atkinson Cycle engine, this isn't a concern, as there is no outflow.

Ideally, to use the Atkinson cam in place of the Atkinson engine, the outflow would either have to be precisely controlled, or fuel precisely metered AFTER the ouflow event. This is where GDI comes into play. Gasoline Direct Injection engines are similar to diesel engines in that they can directly inject the fuel into the compressed air already in the cylinder. Therefore, the Atkinson cam's outflow event would only expel unnecessary air, and the mixture could be leaned by the GDI computer (ECU) to account for the outflow from the Atkinson cam, so that each cylinder could be individually fed it's own fuel supply without an outflow of fuel from the previous cylinder to suck in.

In that sense, the Atkinson cam is less complex, and easier to create/manufacture, and wins. In any other type of engine, I don't see it being a very reliable system. (Apparently, neither do many auto mfgrs.)

wwest40 - Turbocharger exhaust turbines don't require exhaust gasses to expand to reach full spool. They only require the flow of exhaust gasses. Regardless of the expansion ratio being too large or too small for the engine, the expansion of the exhaust gasses being complete, etc... there will still be exhaust flow and heat energy (not expansion, heat... the exhaust flow will still be hot.), so the turbo will still spool and create the feedback loop that turbochargers create in order to reach maximum speed.

Guess what happens if you have a turbo running off an engine's exhaust, but feeding it's compressed air to something other than the engine that it's leeching exhaust gas pressure from? The turbine will never reach full speed, and the compressor will never reach max potential boost. Turbochargers require a feedback loop to reach their full potential, except in extreme circumstances. (Obviously, if you put a T3/T4 hybrid on a submarine engine, it's going to hit 100,000+ RPMs without the feedback loop.) So I guess I should restate to include the obvious - a turbocharger sized properly for the engine will not fully spool without the feedback loop it creates.

That essentially means that your theory on why you don't see Atkinson Cycle or Atkinson Cammed engines with turbos is a myth, and holds no water in logical thinking or applied science.

If you don't believe that turbos don't require expansion of gasses (as opposed to flow) to operate, put a vacuum cleaner nozzle on one some day. I bet you a dollar it starts to spool up.

The air within that vacuum OUTLET hose is COMPRESSED, and absent the ability to EXPAND somewhere "downstream" there would be NO FLOW.

So yes, turbocharging cannot, will not, be operational absent the ability for the compressed gasses exiting the cylinder to continue their EXPANSION.

[Christ]

Originally Posted by wwest40

Turbocharging...

In order to take advantage of turbo boost the engine's "native" cylinder compression ratio must be "derated". Boost, once it arrives, will then raise effective compression ratio back up into the 13:1 range. So the 13:1 "native", off-boost, compression ratio of an Atkinson cycle engine would likely be reduced to something in the range of 10:1 or less.

Obviously that would result in the lowering of the performance of an Atkinson cycle engine to an ever lower level. That, in turn, results in even less exhaust flow/energy for spinning a turbo up quickly and thus the onset of turbo boost would become even more intolerable than with an Otto engine.

I think you need to study more. Or study to begin with.

Who ever said you couldn't use a turbo on a 13:1 engine?

Then again, you're working with theoretical values to begin with... who ever said the Atkinson engine had to be 13:1? Or the Otto 10:1? Who said the Atkinson's static (correct terminology) compression ratio had to be 30% more than the Otto?

First of all, Compression ratio is determined by comparing the volume at BDC to the volume at TDC, assuming 100% VE on the intake stroke. It has nothing to do with expansion of gasses or combustion in itself.

Therefore - If the Atkinson's only difference is that the combustion stroke is longer, then the static compression is measured using the intake stroke. This would mean that an Atkinson engine and an Otto engine could (and must) be compared using the same compression ratio. I think you've lost this bit of data. Since the static compression ratio of both engines is the same, and the Atkinson engine is capable of drawing more power from the power stroke than the Otto engine is, Then the Atkinson engine is more efficient. That's all there is to it.

Couple that with the broadened efficiency of turbocharging, on a linear scale, and the turbocharger only makes the gap between the Otto and the Atkinson that much more obvious.

Please, read a book about these things. Arguing the matter is pointless if you don't have valid data or knowledge.

[Christ]

Originally Posted by wwest40

The air within that vacuum OUTLET hose is COMPRESSED, and absent the ability to EXPAND somewhere "downstream" there would be NO FLOW.

So yes, turbocharging cannot, will not, be operational absent the ability for the compressed gasses exiting the cylinder to continue their EXPANSION.

You're mixing words... the expansion you were originally talking about is heat expansion, and now you're talking about pressure expansion not caused by heat.

Yes, the vacuum is compressing the air at the inlet of the vacuum motor, which causes flow, which imparts kinetic energy to the turbine, which causes the turbo to spool.

There is no heat expansion there.

To compare this to an engine with no heat energy left at the bottom of its power stroke (which is what you're claiming happens in an Atkinson engine...).
You must simply think in terms of the power stroke being at BDC when the heat expansion is finished, and the piston forcing the expanded gasses out of the cylinder. Therefore, there is still pressure and expansion occuring in the exhaust tubing.

Have we finished?

[wwest40]

Physics 101..

Can a gas be compressed without becoming HOTTER..??

Can the temperature of a gas in a fixed volume container be raised without it becoming compressed...??

[wwest40]

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.

*** 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.

[Christ]

Originally Posted by wwest40

*** 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.

You seem to lack knowledge of the real world, logical application of any of the things you're theorizing about. Had you presented these loosely drawn together conclusions to any place other than an internet forum, you'd have been ignored completely by now. I'm not trying to be mean here, rather honest. You really and I can't stress this enough, need to learn what comparison means, and actually gain a working knowledge of the things of which you speak, rather than attempting to apply sophomoric logic to that which you don't fully understand.

Intellect is a good thing, and I'm not saying you're short of it. Proper terminology is definitely a thing that displays intellect. Using proper terminology properly displays experience.

Given some experience, you might have some really good contributions to the mechanical world, and the physical engineering and design aspects of many fields. Without proper use of terminology, you only serve to confuse yourself and others, which will eventually lead to people ignoring you on the whole.

[wwest40]

So, now the response is to attack me....

Well done.

[Christ]

No - that wasn't an attack.

That was simply a suggestion that you get your facts straight before attempting to upset the physical science of any specific engine design. It's great to have questions, which is how this re: chain started, but when you don't accept the answers to those questions, no matter how many times they're repeated, then you ask the same question in a different way, so as to make yourself seem right regardless of the situation or scenario, that's sociopathic.

I'm not going to take it any further, nor will I allow it to go on, at least not of my own accord.

If someone else wishes to continue the ill-fated discussion of incorrect pre-tenses, lack of proper terminology, and unending charade of base-less replies, they may. I will not. I have better things to do with my time, like my taxes. :wink:

I reiterate - this, in addition to the last post, were not attacks on you personally. There was actually a compliment in that last post, thinly veiled by constructive criticism and a small point of advice from one intellectual to another, if you consider yourself that.

I must insist, however, that you do a little more research on the subjects at hand before continuing this (or any other) discussion of them.