| Author | Message | ||
     Jlnance |
I'm trying to understand how a two stroke engine works, and I'm having a bit of a problem with some of the details. I know we have some really knowlegable engine people here. I was hoping you could help me out. Here is what I think I do know. A 2 stroke does not have an exhaust valve. Instead an exahust port is cut into the cylinder wall and the exhaust leaves through this port when the piston get far enough down the cylinder to uncover the port. I don't know about an intake valve. Does it have one, or is there another port? What is the timing relating the opening of the intake valve to the opening of the exhaust port? I know they are both open at the same time (scavenging), I assume intake has to open after exhaust, but I have no idea who closes first. And most confusing of all, I can't see how the mixture gets sucked into the cylinder. It seems it would always be running at a positive pressure. So where does the vacuum come from? | ||
| Iamike |
There is a reed valve at the intake that blocks the backpressure. The purpose of the expansion pipe is critical to performance. The bulge and length causes a backpulse that acts like an exhaust valve. What amazes me is that the air/fuel mixture is routed through the crankcase(hence no crank oil) before it is drawn into the cylinder. Short & sweet of it. | ||
| Oldog |
JL There are 3 basic arrangements of 2 stroke cycle engines and a number of varients. 1. piston port, the holes in the cylinder wall serve as the valves there are 3 of interest. Intake port / transfer port / exhaust port the transferport routs the fuel air from the bottom end to the top end. 2. Reed valve Iamike explained above a thin Fiberglass or Stainless reed seals off the intake passage so that the incomming charge is not blown out thru the carb 3 rotary valve [ Rotax & Kawasaki ] a disk attached to the crank is cut out matching a hole in to the bottom end the opening is "valved" by the spinning disk The "expansion pipe" and the entrainment effect of the moving exhaust cause the exhaust to pull the old spent charge out and pull the fresh charge in You are correct that the exhaust opens first the high pressure is vented the bottom end pressure is also rising (piston is moving down) the combined effects work together to scavenge the spent charge out of the engine. Timing is of course a function of location of the ports relitive to stroke, position and number of transfer ports also has an impact on scavenging, you may find the term "loop charged" or schneurle(SP) loop porting. used in the description of the motor, the transfer ports are usualy aimed at the side of the cylinder opposite of the exhaust some times a 3rd or even more transfer ports are added on the inlet side of the cylinder, the wildest thing is the crank shaft sealing of a 2 cycle multi I have never understood "laberenth seals" | ||
| Swampy |
Annnnnd don't forget about the two stroke diesel engine that uses intake ports that the piston covers and uncovers, and exhaust valves. | ||
| Mbsween |
JL, here's an animated pic that might help http://science.howstuffworks.com/two-stroke2.htm The thing to realize is on the upstroke you have compression (on top of the piston) and intake (vaccuum is from the upward movement of the piston) on the bottom of the piston. The intake charge is stored in the crankcase. On the downstroke you have power and exhaust as the piston passes the exhaust port. At the same time, the intake charge sitting in the crankcase is getting pushed into the cylinder from the downward motion of the piston and the now open intake port (when the piston uncovers it) And now you have the 2 strokes achilles heel. The exhaust port and intake port are now open creating a near straight path from intake to exhaust. Hence why two strokes pollute so much. This is why you don't see any more street going two strokes and why you're seeing fewer and fewer two stroke dirt bikes And since you're storing the intake charge in the crankcase you can't have oil in there. So you have to add oil to the gasoline ( or have some automated oil injection system) to accomplish the lubrication. Now you know why two strokes have that wonderful smell and why they smoke so much. So the big differences between 2 and 4 stroke, oil in fuel (premix), no oil in the crankcase (and no oil changes), no valvetrain, and power every revolution of the crank (vs every other) | ||
| Oldog |
in most cases 2 cycle engines do not have the operating life of a 4 stroke the ports are especialy hard on rings, lubrication is border line, and they are Quite sensitive to dirt (ask me how I know) me I love the smell of bean oil in the morning. | ||
| Bomber |
Dawg -- you gotta come to India -- a five block walk will getcha a lifetime's worth a 2-stroke smell ;-} | ||
| Kootenay |
I haven't looked at the illustration someone posted above, it's probably quite good. But, here's how a two-stroke engine works: 1) The piston moves up. This creates a negative pressure (suction) in the crankcase (which is sealed). Air is drawn through the carb into the crankcase by this suction, mixing with fuel as it passes. Oil in the fuel mists everything in the crankcase with a thin film of lubrication. 2) The piston moves down. This creates positive pressure in the crankcase; a valve (either reed or rotary) prevents air/fuel mix from blowing back through the carb. When the piston is near the bottom of it's stroke, it uncovers a port in the side of the cylinder which leads to the crankcase--air/fuel mixture pressurized by the descending piston blows into the cylinder. 3) The piston rises. The air/fuel mix in the cylinder is compressed; at the proper time the spark plug ignites the mixture, and combustion occurs. (Of course, the underside of the rising piston is drawing in a new air/fuel charge to the crankcase at the same time.) 4) The combustion forces the piston down again--the power stroke (which also compresses the new air/fuel charge in the crankcase). As the piston nears the bottom of the stroke, it uncovers a port which leads to the exhaust, allowing the spent charge to escape. The exhaust port is generally opposite the intake port, and positioned slightly higher so it opens earlier--this allows the (still expanding) spent charge to begin moving into the exhaust. Some two-strokes have deflectors cast into the piston crowns to help deflect the incoming charge toward the top of the cylinder, the idea being it helps push the spent charge down the other side and into the exhaust port. Scavenging (creating suction in the exhaust port to help draw out the spent charge) is important in a two-stroke. Two-stroke engines are simpler than four-stroke ones, but the concept behind them is more complex, and they are a more recent invention. Their huge advantage is the fact that every stroke is a power stroke. Because they lack a valve train, they can be made lighter, and they generally have a better power/weight ratio (and so are found in applications where light weight is important, such as boat engines, hand-held tools like chainsaws, and off-road motorcycles). Their disadvantages include poor fuel efficiency, high emissions, and a generally narrow powerband. They usually employ ball bearings rather than the plain bearings usually used in four-stroke engines, because ball bearings can function with oil-mist lubrication. When I was a kid, I played with small, .049cc model airplane engines. They're dead simple, and are an excellent introduction to how a two-stroke engine works. | ||
| Djkaplan |
At one point, my entire motorcycling world revolved around 2-strokes and pre-mix. I kinda miss the smell of Castrol. | ||
| Newfie_buell |
Last year I found an old 1977 RD400 sitting in a shed for the past 18 years. I cleaned the carbs, new points and zingggggggggggggg. However, it was short lived as the sitting up for 18 years didn't do the crank and other seals any good. All the seals on my motor are now failing. The plus side is I now get to pull the motor, rebuild it and make it fresh again. I love the smell of the two-smoke and the power is like a switch and somewhat violent. The little RD is a blast to ride. | ||
| Brucelee |
I had a Yamaha twin back in the 60s. This two stroke was a screamer (305CC) that used to nearly throw me off the back of the bike once it spooled up. It had oil injection, a sep tank where you put the oil, so no pre-mix. Loud, smokey, and tons of fun. Felt like a Turbo kicking in. | ||
| Whodom |
Believe it or not, Chrysler was seriously researching two-strokes for automotive applications in the late 1990's. These engines would have been "blower-scavenged" engines (they'd have had a separate supercharger and a conventional 4-stroke-type crankcase with oil sump and pressure lubrication like the two-stroke diesels cited above) and would have used direct fuel injection similar to diesels. Read more here: http://www.allpar.com/neon/stroke.html From what this article says, they haven't completely given up on them yet. | ||
| Kootenay |
...and would have used direct fuel injection similar to diesels. I believe several outboard boat engine makers (including Yamaha) have been making direct injection, gasoline two-stroke engines for some time now (a decade? more?) because they can reduce emissions. I was a serious water skier a few decades ago, and I still love that sport when I get a chance, but from an environmental standpoint two-stroke outboards are a nightmare... | ||
| Whodom |
Kootenay, I believe you are correct on the direct injection on outboards. As I understand it, the emissions problems are a direct result of the oil-mist lubrication in which the oil is burned in the cylinders along with the fuel. The Chrysler design had a conventional wet sump lube system like most 4-stroke engines (and 2-stroke diesels) and so that part wouldn't have been a problem. | ||
| Curtyd |
My RD 400 is being overhauled as I write, I hope. Can't wait to be out driving it amongst the "big dogs", they simply hate it. But there "is nothing like the smell of napalm OR [pre-mix] in the morning." I thought I read somewhere years back about a "clean" two stroke technology down in Australia or somewhere, that was going to have a big impact on cars in the future because of the lighter weight engines. I also thought it said something about Harley purchasing a license agreement to use the technology, maybe I just have my wires crossed. Anyone else know anything concrete about that? | ||
| Curtyd |
http://www.motorcycledaily.com/02may01twostrokemystery.html Cut and paste or jump to this webpage in your web browser and go there, HONDA admits they have available technology to make clean two-strokes and could market a 300 lb., 130HP, motorcycle, YEEEKs, look out, never underestimate that damn HONDA Corp.  | ||
| Old_man |
My motorcycles with two stroke engines had neither reed valves nor rotary valves, the piston itself acted as a valve. I always thought that the rotary valve was the best. The open part of the valve could be made independent of the piston. | ||
| Rocketman |
My first 'real' bike was a 175 Bantam. Owned Yamaha RD250E, Kawasaki KH400, Suzuki GT 250L, GT250A, GT380, GT550, and just a couple of years ago, the best 2 stroke I've owned, the RGV250P. Yep I likes me 2 strokes. Rocket | ||
| Djkaplan |
I was looking at a friend of mine's outboard engine for his boat and it looked like the carbs were feeding from the bottom of the crankcase, not anywhere near the cylinders. There must have been reed valves or some kind of rotary valve between the carbs and engine. Seemed like novel packaging. | ||
| Diablobrian |
I really dug the tularis. The idea of putting a 2 stroke snow mobile engine into a modern chassis was awesome. You guys across the pond have been enjoying 2 strike street bikes for 2 decades while we have looked on with envy. The Bimota v-due was supposed to herald the return of the 2 strokes to America, but it sank the company instead. Wouldn't it be cool to marry a modern 750 2 stroke to a Buell chassis? or even make it a full liter since we're dreaming. | ||
| New12r |
Did you say Two Stroke Twin, 1000cc! | ||
| Old_man |
How about a 2 cycle 1000cc, V4 with variable rotary valves and interactive expansion chambers. My dream engine. | ||
| Diablobrian |
I take it I'm not alone then! | ||
| Buellnuts |
www.kenevey.com/twostroke.html Bob | ||
| Oldog |
Any one remember the crazy frog? ringgggg-da-ding-ding-dingggggg | ||
| Buellnuts |
www.kenevey.com/ really cool site  | ||
| Buellnuts |
Huh, That link doesn't work LOL! Bob | ||
| Buellnuts |
 Bob | ||
| Buellnuts |
Aw, Shoot, www.keveney.com/ Bob | ||
| Diablobrian |
That link is MUCH Much better. I'm not into Jazz. |
