The emissions arguments against two strokes is no longer true and comes from memories of loop scavenged two strokes.
Two strokes produce alot more torque for a given BMEP (they only have to produce half as much BMEP to produce a given level of torque relative to a 4 stroke).
The biggest challenges they faced (at least when it comes to diesels) is the fact that left over residual limited volumetric efficiency (the thinking at the time during the 60s was that left over exhaust residual was a bad thing- no longer the case), and the fact that they needed 'exotic' Turbo charging- which was expensive at the time also. Both issues have changed dramatically.
The biggest challenge a two stroke diesel faces these days is that there is not enough time for gas exchange. This is not an insurmountable problem.
Would be interesting to know if there is any new 2 stroke development based on the fundamental design of the EMD supercharged / turbo-supercharged locomotive engines aka same overall flow design, but more modern injection system..
Amazing that an engine designed and went into production around 1938 is still one of the world's more impressive prime movers..
It is true that some of the problems of the 2-stroke have gone away, but some are worse now.
It is also true that we accept EGR now, but it now has to be a precisely controlled EGR which is much easier to do outside the cylinder. Built-in EGR of an unknown quantity is unacceptable for meeting emissions and performance requirements.
Unburned hydrocarbons would remain a major problem, worsened by the fact that the emissions standards are extremely low now. This can be resolved by catalysts, but at considerable expense in increased catalyst sizing and to develop the required controls and OBD that would be required.
Any engine that utilized oil in the fuel for lubrication would be flat out for automotive applications, and soon for almost any other application. Among other issues, that oil will cause enough sulfur emissions to wipe out modern catalyst strategies. Oil also has (and picks up) heavy metals that, although they are not regulated right now, it seems likely they will be regulated in the not-too-distant future.
Power density is great, but fuel economy will suffer. The world is changing such that fuel economy is king, but for some applications power density will remain favored.
Gas exchange may not be insurmountable, but for someone to go into the expense to figure it out, the application would have to at least look promising and achievable.
"It is true that some of the problems of the 2-stroke have gone away, but some are worse now.It is also true that we accept EGR now, but it now has to be a precisely controlled EGR which is much easier to do outside the cylinder. Built-in EGR of an unknown quantity is unacceptable for meeting emissions and performance requirements."
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This is obvious- and externally fed EGR could compliment the internal EGR- the point is that the remaining residual is no longer the big deal it was once thought. To ascertain the Internal EGR the scavenging ratio must be known- which can be measured these days, using sophisticated techniques- such as skip firing.
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"Unburned hydrocarbons would remain a major problem, worsened by the fact that the emissions standards are extremely low now. This can be resolved by catalysts, but at considerable expense in increased catalyst sizing and to develop the required controls and OBD that would be required."
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why would unburned HCs be any higher on a non loop scavenged uni flow scavenged two stroke than a 4 stroke?
Let's put it this way, I HAVE SEEN measured dyno data, with emissions data- showing HCs, NOx, Co, Soot (incredibly low soot) and smoke that was better than competitive - even compared to the Duramax, Cummins ISB and Ford Scorpion 6.7. Given- it was an R and D engine and non manufacturable.
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"Any engine that utilized oil in the fuel for lubrication would be flat out for automotive applications, and soon for almost any other application."
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A two stroke engine doesn't automatically mean Loop scavenged- I mentioned that already. This is the knee jerk reaction from everyone.This seems to be a prejudice- particularly those that aren't willing to open themselves past 4 strokes.
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"Power density is great, but fuel economy will suffer. The world is changing such that fuel economy is king, but for some applications power density will remain favored.Gas exchange may not be insurmountable, but for someone to go into the expense to figure it out, the application would have to at least look promising and achievable. "
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Why would fuel economy suffer?
There is no reason why a 2 stroke engine couldn't get impressive Indicated Thermal Efficiency values- approaching just under 50% with the right combustion chamber shape that implements a fast burn and good air utilisation.
Picture the old Detroit Diesel arrangement - inlet ports around the base of the cylinder, cam operated exhaust valves in the head.
One of the problems is that if you get lots of oil to the piston rings, some of it will get into the ports and then into the intake charge. A small amount of that will go out the exhaust ports (but these days, even tiny amounts like that can be an issue). The rest of it will go through the next combustion cycle, but this is combustion happening in a rather uncontrolled manner. Getting less oil to the rings reduces the oil consumption, but can starve the rings of oil. It is a very fine balance, but it appears that even that fine balance results in engine-out emissions that are too high, because Detroit Diesel has abandoned two-strokes (I still love 'em, but that's another matter.)
The other thing is that if you need to have cam operated valves in the cylinder head anyway, and you need to have a scavenging blower (which the four-stroke doesn't), what are you really gaining in terms of simplicity? If anything, the two-stroke ends up more complicated because of the scavenging blower.
Marquis,
I have not seen great models or commercial algorithms to figure out what is left in the cylinder with enough certainty. I have seen an un-named Fortune 100 company try it, and not manage to succeed well enough yet.
UHC is far worse in every 2-stroke I've seen, partially due to the fact that the cylinder composition is an unknown. It's not good enough to be in the right place almost all the time. In today's emissions environment, a 10-second burp can dump an hour's worth of emissions. If the 2-stroke is sensitive to transients, etc., it will be tough to make it. I've not seen a real-time device for measuring UHC that matches the EPA method, so I'm at least a bit dubious of your claim that you've seen UHC measurements that were favorable to those engines (but I don't doubt your veracity - I just wonder if you were seeing what you thought you saw). However, it's certainly possible that someone worked this out.
I realize that a two-stroke can be made without oil in the fuel, I was just noting that because one of the common methods of making a two-stroke work is off the table.
There are at least two reasons that a 2-stroke can't get the same fuel economy. 1) your maximum actual compression ratio must be lower because you don't have enough crank angle degrees to optimize valve timing for compression, and 2) you have uncooled EGR in the cylinder which limits the amount of material you can compress to get peak temperature and increases your exhaust temperature (this hurts you on both ends of Rankine). Also, the hot EGR in the cylinder also reduces the power density, giving back a bit of the power density advantage you were gaining.
I'm not saying the issues are insurmountable, I'm just not going to hold my breath for a commercial 2-stroke product to hit the market, and I don't foresee a takeover of the market with 2-strokes. If that conventional thinking is wrong, then whoever proves it wrong stands to make a pretty penny.
Modern direct injected 2-stroke scooters have lower emissions than thier 4-stroke equal displacement catalytic converter toting compatriots and weigh less, further widening the efficiency gap.
A 4 stroke doesn't have a scavenging blower? What do you think the piston is doing between power strokes. Designing a 4 stroke is like juggling with 2 balls. Designing a 2 Stroke is like juggling with 5 balls. Its much easier to teach the diesel theory by describing it as 4 discreet strokes. A diesel anyway, consists of compressing a charge of air, injecting fuel (for the Pop) and expanding the force through the crankshaft. The difference is how you get the charge into and out of the cylinder. Turbo-charging blurs the issue quite a bit but that's another story.
Of course the piston acts as an air pump. My point is that a Detroit Diesel has a separate, discrete, additional device with additional rotors, shafts, bearings, and gears that adds a bunch of extra moving parts.
A 6 cylinder 4-stroke has (example) 1 crank 1 cam 6 rods 6 pistons 12 (or 24) valves. A 6 cylinder Detroit Diesel has all that, plus the scavenging blower on top of that.
Yes the uniflow 2 stroke has all those parts not to mention the added complication of getting ports into the liners. The medium speed 2 strokes have about the same bore and stroke of their rivals. The difference is that for the same power output the stress levels are half as much allowing it to get by with cast iron pistons and a lighter crankcase and so forth. I've always wondered about the stress on the 4 stroke piston pin. Sealing the head to the liner is another problem on 4 stroke's and in one case they resorted welding the liner to the cylinder head. The highest horsepower 4 stroke engine is a MAN model of about 35,000 hp while the largest 2 stroke is Wartsila at 109,000 hp. MAN makes those too. Size matters and Wartsila claims that their engine has a BSFC of .260 lb per hp-hr. Presumably on bunker C and that doesn't include the lube oil pumps and water pumps and fuel pumps and treatment equipment which are electric powered and get their power from the ships service generators.
To get back to the original question. $ strokes are simpler and easier to be build as long as they stay well within a reasonable specific power level. The trend however is to soup them up and they are going to have a lot of problems with material as well as emission equipment as any one that has to deal with that stuff will atest to.
For the record Sulfur is not required in lube oils and can technically be engineered out easily- especially for a 2T. Ensuring a S-free lube is used in the field for a given application may be another matter.
