4-strokes obsolete?
4-strokes obsolete?
(OP)
Why build a 4-stroke over a two stroke? Assuming both engines utilise direct injection a 2-stroke is fundamentally better is it not?
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RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
http://www.engineering-4e.com
RE: 4-strokes obsolete?
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.
www.auto-scape.com
RE: 4-strokes obsolete?
Amazing that an engine designed and went into production around 1938 is still one of the world's more impressive prime movers..
RE: 4-strokes obsolete?
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.
RE: 4-strokes obsolete?
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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.
www.auto-scape.com
RE: 4-strokes obsolete?
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.
RE: 4-strokes obsolete?
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.
RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
- Steve
RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
Do you have any association with orbital?
Regards
Pat
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RE: 4-strokes obsolete?
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.
RE: 4-strokes obsolete?
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.
RE: 4-strokes obsolete?
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.
RE: 4-strokes obsolete?
Untill a couple of days ago I hadn't even heard of Orbital. Please dont dismiss what I'm saying on the sole grounds that it sounds like a sales pitch.
RE: 4-strokes obsolete?
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
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RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
A turbocharger creates both the extra intake pressure and the extra exhaust manifold back pressure. But, it can't be the only scavenging device, if only because the engine can't be started using the turbocharger alone.
A lot of the Detroit Diesels used both the scavenging blower (inherent in the design) and a turbocharger.
RE: 4-strokes obsolete?
The turbo is gear driven through a one way sprag clutch to guarantee blow through at start up and at low rpm. Hence the name turbo-supercharger.
The intake and gear noise on these engines is impressive and very characteristic. a between the lower power
At idle there is a very characteristic pulsating gear whine as each cylinder fires on these slow turning engines. But as the rpm comes up it becomes only turbo whine.
The lower output engines are gear driven supercharger only.
RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
The last of the Foden 2 strokes were doubled up into V12's and turbocharged,they were used in landing craft.
RE: 4-strokes obsolete?
What about the case when the scavenging pump volume per engine cycle (assuming positive displacement) is appreciably greater than the engine's swept volume?
RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
Are you making the statement just for ported intake and port exhaust 2 strokes, or making the same statement for ported intake, poppet valve exhaust 2 strokes?
The Detroit Diesel Series 71's and the EMD 567,645 (and other displacements) use ported intake and poppet valve exhaust. In this case it would seem that the degree that a blower can pressurize the cylinder would be dependent upon the exhaust poppet valve closure timing versus the port closure timing..
EMD cylinder head with 4 poppet exhaust valves.
http
EMD (removable) cylinder liner showing ported intake and with cylinder head mounted...
http
Chart of EMD 645 (and others) supercharged (for blow through) and turbo supercharged on same displacement, higher output engines.
http://www.sdrm.org/roster/diesel/emd/history/
Comparison of same size engines for example the 16 cylinder 16-645E supercharged (for blow through) produced 2000 hp. The same displacement, same compression ratio, at same rpm but turbo supercharged 16-645E3 produces 3000 hp.
The DD Series 71s used the same flow concept (intake port and poppet valve exhaust)as the EMDs. All DDs use superchargers for guaranteed blow through as mentioned in a prior post and additional single or twin turbos on the higher output engines.
12 cylinder twin turbo, aftercooled detroit diesel
ht
RE: 4-strokes obsolete?
I think the EMD 710 is still being produced, and meeting the emissions requirements just fine. 4 strokes do offer better control over inlet and exhaust events, well easier to change I should say. Much easier to over come limitations like spring surge and valve float using a 2 stroke.
RE: 4-strokes obsolete?
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That's not entirely true: The Mazda RX-8 with its rotary engine uses oil for lubrication to lubricate the rotor seals and combustion chamber and is still being sold. (Granted oil is injected and not mixed with the fuel but the injected oil does take the same path).
And there may or may not be more applications for rotary engines (which will always depend on oil lubrication):
http:/
There was some talk about Honda possibly going back to employing 2-strokes a few years ago:
http:/
But then again: Even racing karts with 4 stroke engines are becoming increasingly popular (where 2 stroke engines have always been predominant).
RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
Regarding achieving a "supercharging" effect using the Detroit Diesel layout (ported intake and cam operated exhaust valves), of course it would be possible if the exhaust valves opened before the intake ports opened and closed before the intake ports closed. Possible, yes. But, this would either result in the effective expansion stroke being shorter than the effective compression stroke (unfavorable if taken too far), or require the exhaust valves to open and close in an even fewer number of crankshaft degrees (already a problem in those engines, and it would limit valve lift), or require the intake ports to take up a much greater portion of the cylinder thus reducing "trapped volume" i.e. the effective displacement (unfavorable).
Detroit Diesel got it right by using the scavenging blower to just scavenge, and a separate turbocharger to both "supercharge" and create exhaust system back-pressure to trap the extra charge in the cylinder ...
RE: 4-strokes obsolete?
Usually I figure everyone should have to play by the same rules, but perhaps the Wankel deserves a separate regulatory category since it's development seems to be interrupted by bars set for what's 'technologically feasible' for piston engines.
RE: 4-strokes obsolete?
- Steve
RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
But the main reason why the RX-8 has better emissions than the RX-7 is because the exhaust ports are located on the side-walls which increases the probability that the unburnt gases are being recycled: http
(Also, the RX-8 engine may be the world's only internal combustion engine which doesn't have an inlet/outlet overlap).
RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
Anyway the major difference between Euro 4 and Euro 5 is apparently a 25% reduction in NOx. Hydrocarbons and CO remain unchanged and CO2-emissions or fuel economy is not even part of Euro 5.
http://e
http:/
So at least according to this, the RX-8 must have problems reaching the required NOx limits or this car is simply not selling well enough and Mazda decided to stop selling the RX-8 in Europe and blamed someone else. Who knows.
RE: 4-strokes obsolete?
http://www
I have no association with Ricardo!
RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
ht
Maybe Satanetc. should consider a two-stroke petrol/SI engine with this type of port/valve layout. The main problem with these is that the camshaft must run at crank speed and thus work the valve gear very hard.
Possibly with four large diameter exhaust valves the lift could be very low and not overstress the valve gear? Maybe half-speed camshafts with alternate pairs of valves opening?
From memory - Bruce Crower had a two-stroke petrol engine like this.
RE: 4-strokes obsolete?
Slide 29 shows the port/poppet timing and, scavenging manifold, cylinder and exhaust manifold pressures versus time on a typical uniflow 2 stroke supercharged design.
http:/
RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
RE: 4-strokes obsolete?
Jay Maechtlen
http://www.laserpubs.com/techcomm
RE: 4-strokes obsolete?