2 stroke-4 stroke

[Recovered]

If we could go back to the bit about forced induction, someone said that the two-strokes de-facto forced induction helps it against four-strokes.  Not really.

When a 4T takes in air, it takes in the engine displacement of gas, then the valve closes and the engine goes to work.  With forced induction, gas is pushed by the turbo- or super-charger as well as being sucked by the piston, so you get more gas in that 450cc or whatever of space.  With a 2T, the piston pumps the engine displacement of gas into the cylinder, but some of it escapes out the exhaust.  The expansion chamber helps - but does not fix - this problem.  Additionally, before closing off the transfer ports, the piston starts pumping air back into the crank, further reducing the charge.  Thus, a classical 2T gets less air per power-stroke than a naturally-aspirated 4T, and a lot less than one with a supercharger.

In 2T diesel designs, even though the engine has a supercharger, it's referred to as a scavenging pump, because you're substituting the supercharger for that extra pair of strokes.  This doesn't mean that it's not an improvement, however.  If you go forced-induction, you can lower the exhaust port, giving you a longer power-stroke and less fresh charge lost in the scavenging process, resulting in higher compression and more power-per-stroke compared to a classical two-stroke.  The problem, of course, is the weight of a supercharger on a bike.  In an outboard motor or a car, it doesn't really matter as much, which is why I think Evinrude-style DI two-strokes should be the future of multi-cylinder internal-combustion.

Incidentally, this new configuration with the changed exhaust port placement may make the expansion chamber unnecessary, which would be quite convenient for a V12 2T.

Only highly developed 4 strokes achieve volumetric efficiency rates of 100% of better. Your average run of the mill 4 T is about 85% efficient. So to say they actually displace a full charge of displacement every cycle only applies to highly modified engines.

Gordon Jennings wrote a classic book called "Two Stroke Tuners Handbook". He covers all the aspects of 2 stroke including exhaust design. To say the piston is the only form of exhaust evacuation discounts blow down, transfer port efficiency and combustion chamber design as well as exhaust design. It's definitely worth the time to read it. Maybe a little outdated but still stands the test of time. Modern 2 stroke engines are far more efficient than they are given credit for.

BTW, that was what I was going to call my book (that I may write, but so far feedback has been sketchy at best) before I found out Mr. Jennings had used it.

[JETZcorp]

Wow, sounds like an amazing read.  Is it widely available or am I going to have to find some wild geese?

[JohnN]

See...... technology and Google are your friend!!

Free pdf copy of the Two Stroke Tuners Handbook at the link below;

http://www.google.com/#hl=en&source=hp&q=Two+Stroke+Tuners+Handbook&aq=f&aqi=&oq=&fp=cbc2f75bf9d43a8f

Imagine how long it would have taken you to find that book in the 1970's!!!

[JohnN]

Of course you could always buy one on Amazon dot com..... for the princely sum of $138.85 for a decent used copy or pay $271.57 for a used on in very good condition!! 

Just as a reminder it cost $5.00 when new!! :'(

http://www.amazon.com/Two-stroke-Tuners-Handbook-Gordon-Jennings/dp/0912656417

Think I'll read the pdf copy.......   

[AFG]

Man, got back and pulled up the site. I didn't notice before that anyone was being over the limit with the" I hate 4 stroke thing". If they were that's not cool. I had emailed John when this forum just started telling him how cool it was that people could share their thoughts without being hammered by some forum bully( that's why I didn't join in on any others ). I admit I enjoy joking around about how my KX500 is superior to my riding buddies CRF 450, but it's just that, joking around. I still enjoy riding with him! I believe the fastest way to make enemies, is to tell them they were dumb because they didn't buy what you did and stick out your tongue like a 5 yr (4?)old. I'm sure there are those even in the 2 stroke world that think I may not be right in the head because I ride a 500. I would like to think, if the dirtbike community as a whole see the people who use this site and ride 2 strokes as interesting, funny, and helpful the more likely they are to come join us on the 2 stroke bandwagon. Sorry if I took up to much space on this, I just wanted to share my opinions. Thanks, AFG

[JETZcorp]

Well, I was on my iPhone when I posted my question.  Glad there's a PDF!  Also, I like that it's from 1973.  It seems that's a good year for two-stroke literature.  It's the year that the Maico Shop Manual (which includes "How to Ride Motocross" by Preston Petty) was printed.  Unfortunately, I don't think I'll be getting as much homework done as I'd like, now that I know about this book.  Same thing happened with Monkey Butt.

And no, AFG, you didn't take up too much space.  "Taking too much space" is when you have a 1440 x 900 monitor, but need to scroll down more than twice just to be able to see the whole message.  I'm a firm believer in the policy of writing chapters, rather than posts, when it comes time to seriously tell it how it is.

[Recovered]

Man, got back and pulled up the site. I didn't notice before that anyone was being over the limit with the" I hate 4 stroke thing". If they were that's not cool. I had emailed John when this forum just started telling him how cool it was that people could share their thoughts without being hammered by some forum bully( that's why I didn't join in on any others ). I admit I enjoy joking around about how my KX500 is superior to my riding buddies CRF 450, but it's just that, joking around. I still enjoy riding with him! I believe the fastest way to make enemies, is to tell them they were dumb because they didn't buy what you did and stick out your tongue like a 5 yr (4?)old. I'm sure there are those even in the 2 stroke world that think I may not be right in the head because I ride a 500. I would like to think, if the dirtbike community as a whole see the people who use this site and ride 2 strokes as interesting, funny, and helpful the more likely they are to come join us on the 2 stroke bandwagon. Sorry if I took up to much space on this, I just wanted to share my opinions. Thanks, AFG

If "they" think you are not right in the head for riding a KX 500, what do you think they would say about riding a 2004 500 MAICO? You'd be in a rubber room!

[AFG]

Probably right. Sorry, gotta go the guy's with white coats say it's time for atr and crafts. he he  ha ha ho ho

[JETZcorp]

I've been reading that .pdf (and not getting my homework done as a result) and I have to say, it's pretty gnarly.  Here's an excerpt.

That is at the same time one of the least complicated and most
depressing calculations you can perform. Let us consider the Yamaha DT-1, which in
fully developed configuration had an intake duration of 160-degrees, a transfer duration
of 123-degrees, and an exhaust duration of 172-degrees. Yamaha claims a power peak at
7000 rpm. Let's have a look at the actual time, in fractions of a second, available for the
completion of these functions. To arrive at these times, use the following formula:

T = (60/N) x (#/360)
Where T is time, in seconds
N is crankshaft speed, in revolutions per minute
# is port open duration, in degrees
(This formula can be abbreviated to T = #/6N)
Thus, to find T for the 160-degree intake duration,
T = (60/7,000) x (160/380) = 0.0038 sec.

With application of the same formula to the transfer and exhaust periods, we find
that the former is open 0.0029-second, and the latter open 0.0041-second. Even the
longest of these, the exhaust-open duration, is only 41/10,000-second, and that is not very
much time in which to empty exhaust gases out of the cylinder. Actually, that particular
process is substantially finished in the 29-degrees, or 0.0007-second, between exhaustand
transfer-opening.

Those are some scary-ass numbers.  It's incredible that our beloved two-strokes even work at all.

[Recovered]

Jennings work may be somewhat dated in a very few repspects but it is loaded with great information. They guy learned the hard way.

It's a good read and worth your time if you're into 2T's. I'm due to read it again!!

[juliend]

If we could go back to the bit about forced induction, someone said that the two-strokes de-facto forced induction helps it against four-strokes.  Not really.

When a 4T takes in air, it takes in the engine displacement of gas, then the valve closes and the engine goes to work.  With forced induction, gas is pushed by the turbo- or super-charger as well as being sucked by the piston, so you get more gas in that 450cc or whatever of space.  With a 2T, the piston pumps the engine displacement of gas into the cylinder, but some of it escapes out the exhaust.  The expansion chamber helps - but does not fix - this problem.  Additionally, before closing off the transfer ports, the piston starts pumping air back into the crank, further reducing the charge.  Thus, a classical 2T gets less air per power-stroke than a naturally-aspirated 4T, and a lot less than one with a supercharger.

In 2T diesel designs, even though the engine has a supercharger, it's referred to as a scavenging pump, because you're substituting the supercharger for that extra pair of strokes.  This doesn't mean that it's not an improvement, however.  If you go forced-induction, you can lower the exhaust port, giving you a longer power-stroke and less fresh charge lost in the scavenging process, resulting in higher compression and more power-per-stroke compared to a classical two-stroke.  The problem, of course, is the weight of a supercharger on a bike.  In an outboard motor or a car, it doesn't really matter as much, which is why I think Evinrude-style DI two-strokes should be the future of multi-cylinder internal-combustion.

Incidentally, this new configuration with the changed exhaust port placement may make the expansion chamber unnecessary, which would be quite convenient for a V12 2T.

Thanks for that post!

[westyzkx]

There's a lot of child like "hater" foofoo going on around here wasting bandwidth.

Really? I've not seen any of that - and anyway if someone has an opinion that they hate 4 strokes (which I do by the way) then so long as they put up a reasoned post explaining the whys and what fors then I don't see a problem - there will prob be quite a few members who agree with them. This is twostrokemotocross.com after all, not 4 stroke motocross...