So , after the long discussion with vin97 on "2 strokes vs 4 strokes" (masterpiece of topic pollution, one of my many, sorry I can't help it), I decided to ask to somebody with real knowledge on the subject. The question I asked was very simple (to avoid skewing his opinion), something like: "modern 2 strokes engines for bikes ? they seems to be good for naval, so ...". The person in question is a friend, so I got a fairly detailed reply.
His very 1st sentence in the reply was (translated to english):
"2 strokes are perfect on paper but to make them work well, you have to make them as complex, heavy and expensive as 4 strokes. Only exception is the Orbital thing, but it is dead anyway."
Other details:
- Naval 2 strokes are something totally different: head valves, uniflow scavenging (as opposite to cross or loop scavenging on bike's engines), 2-20 RPM. Thanks to the head valves (and the very low RPMs), they have less lubrication issues, hence the reliability. High speed (RPM) 2 strokes are something totally different.
- Basic two strokes have very limited potential for control (on 4 strokes you have cam timings and profiles, injection timings, etc). More complex 2 strokes may allow more, but are ... well, more complex.
- Serious troubles with emissions: direct injection solves them partially.
- Another issue is noise: they need a large exhaust to produce the necessary resonance (which is also an issue on bikes in terms of space). This alone makes them not suitable for road use.
- Modern 2 strokes (without talking about valved ones, which would be as heavy, complex and expensive as an equivalent 4 strokes) have applications for 1 or 2 cylinder racing engines, especially for limited usage (low number of Km before a serious maintenance). They are preferred in Enduro races (300cc) and somebody speculates a modern 250-300cc 2 strokes could be on par with 450cc 4 strokes in motocross.
- MotoGP ? No, don't think so. Could be better for the show and could give the doctors and paramedics more work, but technically doesn't seem the way to go.
- Honda, despite having produced the most advanced 2 stroke ever (used for the Dakar race) decided to stop producing 2 strokes. End of the story.
MaX.
Ok, I didn't know naval engines used other scavanging.
What does he mean with "very limited potential for control"? There are obviously no cam timings but you can also have injection timings.
Hmm about the exhaust, I don't think size is the problem....
(http://i616.photobucket.com/albums/tt245/jeteye/IMG_20746330311386.jpeg)
And why should the bikes be any less ridable in MotoGP with the traction control they have nowdays.
I get the impression that he doesn't take DFI seriously, with this you don't need to make the engine as complex and heavy as four strokes (add exhaust valve) in order to have an efficient engine.
Quote from: vin97 on June 17, 2014, 02:00:03 PM
Ok, I didn't know naval engines used other scavanging.
Me neither, but I was pretty sure about my assumption: a naval engine is totally different than a motorbike engine (scales aside I mean).
Quote from: vin97 on June 17, 2014, 02:00:03 PM
What does he mean with "very limited potential for control"? There are obviously no cam timings but you can also have injection timings.
For control purposes, the more controls you have the happier you are.
Quote from: vin97 on June 17, 2014, 02:00:03 PM
Hmm about the exhaust, I don't think size is the problem....
The problem is not the size of the terminal part (the silencer ?) but the size of the resonant chamber you see under the engine and under the tail (before the silencer).
It's a problem in particular for 4 cyl (or more) engines. You need space for that, space you could use otherwise.
Quote from: vin97 on June 17, 2014, 02:00:03 PM
And why should the bikes be any less ridable in MotoGP with the traction control they have nowdays.
I guess he was mostly pointing the typically brutal 2 strokes power curve.
True enough, you can work on it to make it less brutal (but it will cost you in peak power) and also traction control will help, but even a good traction control is more challenged by a brutal power curve.
Quote from: vin97 on June 17, 2014, 02:00:03 PM
I get the impression that he doesn't take DFI seriously, with this you don't need to make the engine as complex and heavy as four strokes (add exhaust valve) in order to have an efficient engine.
Well, I can't reply to that. He says DFI alone is not enough (by design in a 2 stroke you can't be as neat in handling exhaust gasses as in a 4 strokes), you think it is.
But then I still don't get it: DFI is there (and in principle it is not rocket science), why don't we have nice 2 strokes engines right now for road bikes ?
I think it's because there are other issues that DFI alone can't solve.
MaX.
What would you put there?
I think this RG 750 shows that fitting the exhaust pipes is no problem, even without having to do some crazy snake pipe routing.
DFI is working the problem is that the engines are not producing as much power as regular carbed two strokes because there is still a lot of work to do.
Right now, a DFI two stroke would not have a lot of power-per-displacement more than a four stroke. There is a little bit of weight loss but that's not enough for most people to go ahead and buy a new bike.
Quote from: vin97 on June 17, 2014, 03:35:53 PM
What would you put there?
I think this RG 750 shows that fitting the exhaust pipes is no problem, even without having to do some crazy snake pipe routing.
You could probably hinge the engine lower if you hadn't had the big chambers below.
And what I'd put under the tail ? Nothing at all, if I can.
Can't remember where exactly but I've recently seen a pic on this forum with a 2 strokes having the resonant chambers where you typically (on a road bike) have the tank.
It's doable (albeit could be not very comfortable on the road), but then ... you'll have to put the tank elsewhere.
Quote from: vin97 on June 17, 2014, 03:35:53 PM
DFI is working the problem is that the engines are not producing as much power as regular carbed two strokes because there is still a lot of work to do.
Right now, a DFI two stroke would not have a lot of power-per-displacement more than a four stroke. There is a little bit of weight loss but that's not enough for most people to go ahead and buy a new bike.
Which kinda proves the point that many of the potential advantages of a 2 strokes over a 4 strokes (weight, simplicity, power-per-displacement) are lost as soon as you try to reach the same level of performance and emissions as a 4 strokes. Maybe Honda and the others just saw this a long time ago and took the (a posteriori) savvy decision to go 4 strokes.
MaX.
Quote from: HornetMaX on June 17, 2014, 03:49:50 PM
Can't remember where exactly but I've recently seen a pic on this forum with a 2 strokes having the resonant chambers where you typically (on a road bike) have the tank.
It's doable (albeit could be not very comfortable on the road), but then ... you'll have to put the tank elsewhere.
Done on the 1984 Honda NSR500. The fuel tank was under the bike. The design was abandoned on the later models. After that, all the japanese had 2 exhausts under the seat and 2 exhausts on the side of the bike (one each side, or both on the same side).
(http://2.bp.blogspot.com/_Q62H7x3hWII/S9FBYn-AhlI/AAAAAAAABgw/39yJe6YqYxc/s1600/IMG_6131.JPG)
The problem with exhausts is that you want them to be generally bigger than on that RG 750 prototype. And there are a lot of space issues. It was especially a big issue on inline 4 engines (the TZ750 being a good example, it forced the pipe to go under the fuel tank in front of the carburetors).
Thx EdouardB, not the pic I've seen, but as good as.
Now vin97 you see what I mean with "space problem", the chambers are huge.
MaX.
Ok, just for fun, here is the formula for a good 2 stroke GP exhaust (this one won at least 15 world championships). Calculate Dmax if you want, and try to put 4 of them on a bike, you'll see :P
(http://www.apriliaforum.com/forums/attachment.php?attachmentid=221626&d=1368826375)
The overall COG of the V4s is still a lot lower than the COG of a 600cc supersport bike.
There is not much too be gained by removing the pipes because at some point the COG will get too low and that is what happened with the "Dummy-tank NSR 500".
I agree, as soon as the displacement per cylinder exceeds 200cc or you have more than four cylinders, you will not be able to make the optimum pipe anymore, meaning you have to move the powerband to a lower rpm range, which is not necessarily a bad thing because in Motocross (where you have big, single cylinder two strokes) you need a lot of low end.
For a 500cc V4 race bike (and any other cylinder configuration doesn't really make sense as it has been proven), making good pipes is no problem.
It just proves that there needs to be development done by more than one or two companies.
The weight is not increased by DFI (probably even decreased because you don't need the carburetors anymore) and of course the engine will not be so simple because now you have to use a notebook to tune the engine but this is standard today and not really that complex. It's just a matter of working out the perfect cylinders to get the right gas flow/scavanging and fuel mixture.
One thing to add, the snowmobile engines with 160 hp at 800cc are just two cylinder engines, there has yet to be a DFI V4.
Edit:
I know where you got those pipe drawings :D
One of the best resources for two stroke knowledge that exists.
I have just found this article: http://articles.superhunky.com/4/106
It's very interesting to see how the real experts think about two strokes.
EDIT
Here is another good read: https://mega.co.nz/#!uUoUmayQ!BLEPDus1F5gd4aIFjEZh1TFTm6y-iwz6WKxUullf8_Y
Quote from: vin97 on June 19, 2014, 05:38:03 PM
I have just found this article: http://articles.superhunky.com/4/106
It's very interesting to see how the real experts think about two strokes.
First, the guy is speaking from what seems to be a strictly cross / offroad point of view: for these bikes I already said that there are right now good 2 strokes (thought you had already read that, just in case: http://forum.piboso.com/index.php?topic=1206.msg14764#msg14764 (http://forum.piboso.com/index.php?topic=1206.msg14764#msg14764)).
Anyway, your article is from 2008 ... we are 6 years after ... where is that good KTM 2 strokes engine for road bikes ?
I've seen plenty of 4 strokes from them for their road bikes but no 2 strokes ... same for Aprilia.
Quote from: vin97 on June 19, 2014, 05:38:03 PM
EDIT
Here is another good read: https://mega.co.nz/#!uUoUmayQ!BLEPDus1F5gd4aIFjEZh1TFTm6y-iwz6WKxUullf8_Y
The Orbital thing (that I also mentioned before you posted this) is nowadays almost dead: the paper is from 2001 ...
Also, the relevant data is mostly for very small engines: 50- 100cc - 180cc.
For these small engines 2 strokes are OK. But if you think that you can easily scale this to a 500cc 4 cylinder with the kind of power we want (> 100hp), then good luck.
Now you're free to think that is dead because Honda/Yamaha/whoever hates 2 strokes, but that is a bit hard to believe.
Some people at KTM seems to have a good opinion of 2 strokes potential: if the orbital thing was that good, why didn't they go forward with that ?
Just to please Honda (or whoever) ? Hmmm ...
MaX.
I didn't say that I agree on everything that is being said in the articles but it proves that the switch to four strokes was not done because they are the better engines with more potential.
One thing I didn't think of was the idea to build large displacement two strokes with DFI.
So not a highly tuned 500cc DFI V4 but a very mildly tuned 1000cc DFI V4 that has more horsepower and is still a lot lighter and cheaper than a 1000cc four stroke engine (and very reliable).
What I find really interesting is that DFI two strokes are actually 'greener' than fuel injected four strokes with cats used in today's bikes.
I gave enough reasons why two strokes have been abondened but since you didn't want to believe that Honda's influence on it was quite significant, I posted these articles.
Quote from: vin97 on June 19, 2014, 11:19:41 PM
I didn't say that I agree on everything that is being said in the articles but it proves that the switch to four strokes was not done because they are the better engines with more potential.
And the proof is where ?!?!
Quote from: vin97 on June 19, 2014, 11:19:41 PM
One thing I didn't think of was the idea to build large displacement two strokes with DFI.
So not a highly tuned 500cc DFI V4 but a very mildly tuned 1000cc DFI V4 that has more horsepower and is still a lot lighter and cheaper than a 1000cc four stroke engine (and very reliable).
You do think scaling something up or down is trivial, that's understood.
Quote from: vin97 on June 19, 2014, 11:19:41 PM
I gave enough reasons why two strokes have been abondened but since you didn't want to believe that Honda's influence on it was quite significant, I posted these articles.
I do believe Honda influenced the shift a lot, but that's totally normal" they were the #1 maker taking a very strategical decision. It's normal their opinion counts.
But I do believe they had very good technical reasons to do so (on top of business ones, of course). Everybody else followed, and not because they are Honda-slaves or stupid. Because it made sense. To all.
Honda, almighty as they are, can't oblige Yamaha, Suzuki of whoever to make a worse engine if a better one is doable.
At the time of the switch 2/4 strokes, emissions regulations killed the two strokes, not Honda (unless you mean that Honda asked for the introduction of the emissions regulation, so that they could put in place their evil "no 2 strokes" plan, which, honestly, sounds very suspect).
If you absolutely want to scream against somebody for the death of 2 strokes, talk with the law-makers in any decent country. But your argument is gonna sound pretty weak: the common image of motorcycling is already poor as it is, without complaining about emissions regulations cars are submitted too (and in a more stringent manner).
In any case, the situation is damn simple: wanna have honda eat his own hat ? Put on the market a 2 stroke that outperforms the 4 strokes.
Honda will have the choice: fight improving the 4 strokes, or embrace the change and and fight with 2 strokes. Whichever direction they take, we (the users) are happy.
Until then, people can recall the 2 stroke era drinking whisky and saying "it was better before", but that's small talk from romantic guys.
MaX.
P.S.
I worked for years in a domain where change is really hard to be embraced by decision makers (and by really hard I mean borderline freakin' impossible).
When I joined that company, my predecessor has been spending years to finalize some innovative theoretical stuff, but had zero success in getting any approval for it to fly for real.
So she dropped it, switched position and I took her place.
I gave "just another presentation" of that stuff to the big bosses, with the only difference with respect to my predecessor that I took the time (a lot of time) to accurately simulate the results (in that domain, full scale real tests are not an option) and quantify the benefits. In terms of money, not with something like "I think it has potential, if we invest enough in it".
3 years later (for that industry, 3 years is extremely fast) the thing was flying (with the predicted benefits) and immediately after it was the standard way of doing it.
When one proves something for real, people buy it. When one talks about "potential" or when one builds a 50cc and then says "I could make a 500cc easily", people just say "Hmm, OK".
Quote from: HornetMaX on June 20, 2014, 12:14:08 AM
And the proof is where ?!?!
In the quotes given in the first article.
Quote from: HornetMaX on June 20, 2014, 12:14:08 AM
You do think scaling something up or down is trivial, that's understood.
I said somewhere else that you won't be able to build the 'optimum pipes' for a 1000cc two stroke V4 but that is the point, you want the maximum power at very low rpm (max. 8000 rpm) so that the engine is very reliable.
Quote from: vin97 on June 20, 2014, 11:56:21 AM
Quote from: HornetMaX on June 20, 2014, 12:14:08 AM
And the proof is where ?!?!
In the quotes given in the first article.
These are opinions, it's slightly different. And by the way, it also points the index on the emission regulations.
Quote from: vin97 on June 20, 2014, 11:56:21 AM
Quote from: HornetMaX on June 20, 2014, 12:14:08 AM
You do think scaling something up or down is trivial, that's understood.
I said somewhere else that you won't be able to build the 'optimum pipes' for a 1000cc two stroke V4 but that is the point, you want the maximum power at very low rpm (max. 8000 rpm) so that the engine is very reliable.
Sounds simple, I wonder why nobody is doing it though ...
MaX.
What do you want then if you won't even accept statements from Lotus and Aprilia engineers?
Quote from: vin97 on June 20, 2014, 01:07:58 PM
What do you want then if you won't even accept statements from Lotus and Aprilia engineers?
I'd like the Aprilia (or KTM or whoever is in favour of 2 strokes) to show they can do what they say is doable, instead of moaning about Honda's decision to go full 4 strokes.
I can understand many people in the racing world were upset by the decision at the time (maybe because it came from Honda, likely to have muscled the switch thanks to its dominant position).
But if you state today that 2 strokes are the way to go, then you have to show it. Orbital tried, with no success at all (for the kind of engines we're discussing here). But kudos to them for at least trying (openly), as the other makers didn't even bother (as far as we know, then in the secrecy of their R&D labs it's another story).
If they criticize the decision from a technical point of view, then the only way to support their speech is clear to everybody: go build it.
If they criticize it for other reasons (political, ethical, or cost containment) then fine, but that's something else (and for sure does not support the thesis "a 2 stroke would be better").
MaX.
How many times do I have to explain why big manufacturers won't develop a new two stroke from zero?
It's only little companies like Orbital who would do it but they lack the money and time to get a proper result.
engineering wise two stroke a lot better samiish power smaller block and cheaper to produce, Honda for sure pushed 4 stroke in racing but the FIA also have emissions limits, California was the first place to ban two stock for emissions then brussle jumped in as well.
Reason why you wont see a two stroke for a while is not the motor design but the fuel and oil, Once there a lubicante that burns clean I think they come back for a while until electric take over.
Quote from: vin97 on June 20, 2014, 03:05:49 PM
How many times do I have to explain why big manufacturers won't develop a new two stroke from zero?
It's only little companies like Orbital who would do it but they lack the money and time to get a proper result.
It's still unclear why they don't do it though, if the outcome is so sure and better.
Quote from: RBp on June 20, 2014, 04:50:57 PM
engineering wise two stroke a lot better samiish power smaller block and cheaper to produce, Honda for sure pushed 4 stroke in racing but the FIA also have emissions limits, California was the first place to ban two stock for emissions then brussle jumped in as well.
Reason why you wont see a two stroke for a while is not the motor design but the fuel and oil, Once there a lubicante that burns clean I think they come back for a while until electric take over.
No lubricant burns clean.
Also, with DFI you have less issues, but then it requires a complete lubrication circuit, like the one of 4 strokes (so losing part of the advantages).
MaX.
Quote from: HornetMaX on June 20, 2014, 05:30:11 PMAlso, with DFI you have less issues, but then it requires a complete lubrication circuit, like the one of 4 strokes (so losing part of the advantages).
You will have an oil pump and an injector into the crankcase, the oil will still be burned but it can be accurately aimed at the bearing to use as few oil as possible (this is how it is done in the Rotax DFI snowmobile engines).
It doesn't differ much from autolube systems used in almost all two strokes and you certainly can't compare it to the oil sump of four strokes.
It's possible to make oil that burns very cleanly it just needs to be engineered to suit the heat the specific engine.
Everything is possible for the one that does not have to do it.
They day we have a 2 stroke that is on par on all the aspects and 10% better on at least 2 aspects (weight, power, torque, fuel efficiency, emissions, longevity) I'll be the 1st one to say it's better.
Waiting for that day ...
MaX.