Two-stroke revival - Will TM keep them alive?

[Jones221]

that is sick! You know who else is keeping the 2 stroke alive??? Me and Jones! (It's the consumer that killed it in the first place)

e-high five!

[yzmxer608]

EFI on a two stroke would be pretty cool.

Next bike is going to be a two stroke, I don't think I would like a four stroke after riding an 85, plus I'm not all that big/strong. Plus the cost and maintenance is less (no valves ect.).

[Kawasakis]

that is sick! You know who else is keeping the 2 stroke alive??? Me and Jones! (It's the consumer that killed it in the first place)

e-high five!

What about me?

[gordy]

dont worry kawa, we'll start our own 2 smoke owners club, without them

[Kawasakis]

Wohoo,our club gonna be better hehe

[Sweendoggy]

aw, I wanna be part of your club? hahahaha

[JETZcorp]

I guess I missed this thread while it was going on, but I've just got to reply to this line here.

The real problem with two strokes is the narrow power curve. If you had a continuously variable transmission that wouldn't matter anymore. So my prediction is 2 strokes will come back when someone makes a CVT MX bike. Until then the wider powerband of the four stroke beats the lighter weight of the two stroke with the current displacement rules.

This is one of the big things that fuel injection solves in a two stroke. To understand why, we need to look at why the two-stroke is peaky to begin with. As I'm guessing you already know, the transfer ports and exhaust port in two-strokes are open at the same time, so some fresh and un-burned mix goes straight into the pipe, along with exhaust and a shock-wave. The expansion chamber is designed to turn that shock-wave back and ram the un-burned mix back into the cylinder to be burned, and incidentally acting a bit like a turbo. You may note that before expansion chambers started getting popular in the late '60s or so, motorcycles were almost all four-stroke, including MX. The problem is, the speed of the shock-wave is constant, meaning that it will always arrive back at the cylinder at the same time. RPM, though, isn't constant, and so they often don't match up. The shock-wave can come back early and push exhaust into the cylinder, or it can ram some fresh mix into a closed port, either way you lose power. Follow me?

Okay, well there's another reason, too. The air-fuel-oil mix goes from the carb into the crankcase before it goes into the cylinder. In the crankcase, it's compressed by the piston coming down until the transfer ports open and it flows into the cylinder. The volume of the crank-case is at its minimum at BDC. However, when the piston starts rising again, it increases the volume in the crank-case, and the precious air and gas gets sucked back OUT of the cylinder into the crank case, reducing compression and thus power. This is also part of what causes the two-stroke to have trouble when it's not on the meat of the powerband. Still following?

Okay, now let's look at a two-stroke with fuel injection. The concept that's being applied to snowmobiles and outboards, namely E-Tec, doesn't use the crank-case for scavenging, so there's no transfer ports to mess with, so the problem of air getting sucked back into the crank-case is gone (and it also means you don't have to mix gas and oil). The air intake and exhaust port are open at the same time, but now that it's just air and no gas, you don't have to worry about an expansion chamber, so that problem is gone as well. The exhaust port is fully closed by the time fuel enters the cylinder, so every bit of it gets burned - just like in a four-stroke. So, we've got a clean two-stroke that actually gets more power per unit of gasoline (because it's not "leaking" out the exhaust anymore) and all the problems that made it peaky before are eliminated. Injected two-strokes have remarkably fat and usable powerbands, and produce even more peak horsepower than the traditional two-stroke. As an example of this, there's a 500cc fuel-injected two-stroke in the works (for a street bike) and their goal is 200hp. That's a lot of power, considering a new 500cc Maico makes "only" 67hp.

Here's a link I happened to find a little while ago. It's a long read, but a very good one if you're interested in two-strokes. It's updated maybe once or twice a month, so bookmark it if you'd like to keep up with it. To make this post, I used information from the 28th August entry, the 08th September entry, and the 21st September entry.
http://www.dirt-bike-tips-and-pics.com/ ... rokes.html

[jlv]

You're probably correct in that using a supercharger instead of crankcase pressure will result in more torque. Since you will have more pressure to make the transfer happen you will be able to get away with a lower exhaust port and get more torque. It's the supercharger that would help though, not the injection. Also, I don't think a supercharger would be legal with many racing organizations.

The problem with a two stroke is you want a high exhaust port on the power/exhaust stroke to let the transfer happen quickly, and a low port on the compression stroke for more compression. An expansion chamber helps by using exhaust back pressure to effectively reduce the exhaust speed during the compression stroke. The problem is that with a constant speed of sound the chamber only works for a narrow RPM range. To get maximum torque throughout the power curve you need something like Boyesen's exhaust valve. Instead of an expansion chamber, this just closes the exhaust early. If you look back into the 70's, Puch did the same thing with their twingle. The exhaust side piston is ahead of the intake side, so the exhaust port opens and closes early.

[JETZcorp]

You don't necessarily have to use a supercharger. While that is one solution, and the I presume the E-Tec uses, there is still a way to make the transfer port method work. Tim Hickox, the guy behind that like I gave earlier, has what he calls AST, or "Assisted Scavenging Technology." What you have to do, apparently, is add something called a "plenum chamber" that's at least as big as the piston displacement. I'm still not exactly clear on how this concept works, or at least not enough to explain it, but it's outlined in the link. You can start reading at the 5th October section. Here's an excerpt below.

To our traditional two-stroke - with DFI - we add a plenum chamber. This is nothing more than an empty space in the crankcase/gear casing. The only requirements are that it be air-tight and that the volume be at least equal to the piston displacement. (There are also what we call 'packaging' considerations.) This chamber is connected to the crankcase through a reed valve. If the pressure is higher in the crankcase, air will flow into and charge the plenum. When the pressure in the crankcase falls, the higher pressure remains in the plenum. There are other changes required, but these are mostly modifications to existing parts (like cast-in ports that permit transportation of the air), and not additional parts. It sounds too simple, but let's run through it.

As the piston compresses the air in the crankcase, the plenum chamber is also charged. The exhaust port opens, then the transfer ports. The pressure quickly drops in the crankcase until it is equal to the pressure in the cylinder. So far, everything is normal, except that the crankcase pressure, when the ports open, is lower; this is because we have lowered the primary compression ratio through the additional volume of the plenum. This is not a problem. This lower pressure, at this point in the cycle, is consistent with Sevilla's engine. At low speeds - where we want to pick things up - lower pressure means lower velocities through the ports, less mixing of fresh charge with exhaust residuals, and less charge loss out the exhaust. In technical terms, the 'trapping efficiency' is improved. Of course, if we only lowered the primary compression ratio, we would see a power loss at high speeds.

Now, as the piston nears BDC, a valve opens and the compressed air in the plenum chamber discharges into the cylinder. The pressure at this time is actually higher than when the transfers first open, because the pressure in the cylinder is lower. The only 'pressure' that means anything is the differential.

[Phathry25]

4-Strokes are not more expensive to maintain than 2-strokes! I wish people would just give up on this already. I can gurantee that in the two year's I've owned my MX bike I've spent more money on chains and sprockets than I have on engine maintenance. This summer I put in a new piston/rings and two new intake valves after I sucked some dirt through it. I would venture a guess that I could have gotten away with just a new piston this winter if I didn't mess it up. Even at that it cost me under $300. In that same time I would have put at least two new top ends into a 2-stroke. A piston kit from Wiseco is $254. Enough said.

Now if you don't maintain your bike and blow it to pieces then yes, owning a 4-stroke is more expensive. Checking you're valves and paying attention to the maintenance schedule is free however.

[jlv]

I can see how that (the plenum chamber) would help a little, but it won't give you the extra compression that the Boyesen exhaust valve will. A high exhaust port really limits the compression.

[JETZcorp]

Cost. For the time being let’s leave out the cost of rebuilds and maintenance. In order to be competitive in the A class or at the National level, you’re required to spend, spend, spend. It costs enormous amounts of money to race a 250F machine competitively.

At the National level the 250F class is ruled by money. If you have enough you can be competitive, if not you will be left behind. This is the reason that so many privateers chose to race the 450 class, less money is required to improve the engine.

Recently MXA magazine featured a KX450F set up for the Glen Helen National. The cost of the upgrades? $12,500! The problem is this does NOT include the cost of the machine itself.

The newest four-stroke machines are technologically advanced. They are fantastic machines and fun to ride/race. But in no way are they “better” or more “technologically advanced” than a two-stroke. They are just bigger.

http://twostrokemotocross.com/2009/09/f ... ting-ploy/

[Phathry25]

Pointless post Jetz. What is the cost of setting up a 250 2t?

[JETZcorp]

Who knows, nobody does it anymore because they'd have to race against a bike with twice as much engine and it would be hopeless. There is not one other motorsport you can point to that has such a crazy handicap in place. Imagine if, in 1969 and 1970 when the 426 Hemi won NASCAR, that Chevrolet went to the rule-makers for help. (This is just an example and it could go either way for our purposes, so Chevy people don't kill me.) In a board meeting, in the presence of executives from Chevrolet, NASCAR decides that in order to race against the 427 Chevy, Chrysler needs to make a 213 (or less) Hemi. Who do you think is going to win next year? In our example, the 427 engine won the championship not by being a better engine than the 426, but by complaining about the 426. Imagine the outrage!

This is just what's happened to motocross. A new type of engine (two stroke) came along and swept the competition, then companies who wanted to make four-strokes got the rules tweaked so they could replace the new with the old again. But two-strokes are coming back. Outboard motors and snowmobiles are quickly adopting new two-stroke technology, and every major auto manufacturer with the exception of Honda have two-stroke designs on the drawing boards for future cars. The Dodge Neon, for example, was originally going to come with a two-stroke but they didn't have it ready in time.

[Phathry25]

So you're blaming the current "unfair situation" on two strokes coming into the sport way back when? Interesting... I can definitely use this in other places when I hear the two stroke whiners raising their voices again.