Its not easy to think of something new and different when dealing with the internal workings of the two-stroke engine. The basic design has been about for years and, in spite of the inefficiency and emission problems, has been the choice of manufactures when looking for a cheaper and lighter alternative to the four stroke engine in most applications that call for a self contained power source. This is mainly due to the fact that a two stroke engine is cheaper to manufacture and will develop almost twice the power size for size than a four stroke unit. Many of the major automotive manufacturers have tried to solve these inherent problems with the two stroke engine because of the benefits of using a lighter, smaller, more powerful, cheaper to manufacture, easier to service engine would provide.

The main thing, especially with all the new emission regulations, that has held the two stroke engine back are the emission problems associated with the "total loss " lubrication system, if you can take the oil out of the petrol, and not use the crankcases as an air pump, and do away with the loop scavenge system to transfer the fuel air mixture to the combustion chamber while the exhaust port is open, your half way there.

For the last few years I, Julian @ Gadget Racing Products .com, have been working with this problem and have recently been granted a patent based on some ideas I had trying to solve these emission problems, and some efficiency issues, associated with the two stroke engine. In the following drawings I will try to illustrate how a two stroke engine works, for those who don't know, how my design is different, and what the problems and solutions are.

These explanations are in no way technical, and very simplified, so all those boffin's out there please give me a break :-).

TOP DEAD CENTRE
Lets start at the top, the compressed fuel/air & oil mixture is ignited buy the spark and leads to the rapid expansion of the burning mixture that forces the piston down to produce the power.The expansion or power phase.

EXPANSION OR POWER PHASE

EXHAUST PHASE

The next stage of the cycle is the exhaust phase, here's a small problem, as the piston moves down past the exhaust port opening, ending the power phase, the expanding gases flow into the exhaust or expansion chamber, through the silencer and out to atmosphere. Un burnt oil and fuel, and some fresh mixture,(Efficiency Problems), hits the hot exhaust and smokes. (Emission Problems)

TRANSFER PHASE
So far so good, the transfer phase, this is where the major problems start, as the piston moves passed the transfers ports, the fuel/air & oil mixture in the crankcases, that is being compressed by the downward movement of the piston, move into the cylinder and purge out the burnt fuel and provide a fresh mixture charge for the next cycle. By the design of the transfer ports, the mixture is directed backwards and away from the exhaust port, even so, some of the mixture escapes through the exhaust and out to atmosphere. (Emission & Efficiency Problems)

BOTTOM DEAD CENTRE

When the piston reaches the bottom of the stroke the exhaust and transfer ports are fully open and all transfer of burnt and fresh gases are about done. Here's another small problem, this is where the design of the expansion chamber, or exhaust, comes into play. The shape and length of the expansion chamber provides a pressure pulse, at certain speeds, that pushes some of the escaped mixture back into the cylinder before, as the piston moves up the stroke and closes the exhaust port, the compression phase of the cycle can begin. The requirement of a bulky expansion chamber that can only provide the correct pulse at certain engine speeds limits the spread of usable power of the engine. (Efficiency Problems)

COMPRESSION PHASE
Here's another problem, the compression phase. Compression cannot actually begin until the exhaust has closed which equals lost power. This is why the power valve came into being, it lowers the top of the exhaust port to increase the compression and improve the power out-put at lower engine speeds, and then rises to enable better gas flow at higher engine speeds. ( Efficiency Problems)

INTAKE PHASE
Here's another problem, intake phase. As the piston moves up the cylinder there is a partial vacuum created in the crankcases and the fuel/air & oil mixture is drawn through the carburetor via the reed valve and into the crankcases to await the transfer phase. The problem here is due to the connection of the crankcases to the cylinder, by the transfer passages, the intake phase cannot actually begin until the transfer ports are closed.( Efficiency Problems)
I hope that explains it OK, it's all done at zero R. P. M. just to make it easier to understand. If you start adding time scales to the explanation it gets very difficult, not only to understand, but to comprehend all that stuff happening every revolution of the engine.

Now this is my bit, it's the same thing but a little different, I will go through the cycle in the same way and explain the solutions, to some of the problems, as I go.

Again, try not to think about the time scales, your head will hurt !.

I will explain the different layout of the engine as I go because it will be easier to understand as the cycle takes place.

Starting at the top, fire starts in the usual way and the expansion phase begins.

TOP DEAD CENTRE

EXPANSION

EXHAUST
Same old story, exhaust port opens and the gases flow out through the exhaust and out to atmosphere. You will notice that the transfer ports are missing, the piston is at Bottom Dead Center, and there are no signs of any fresh charge in the cylinder. So no loss of fresh charge,(Better Efficiency), Oh! and I almost forgot, as the bottom end of this engine design is lubricated in the same way as a four stroke, no oil in the petrol. (Less Emissions)

TRANSFER
The transfer phase, this is when it starts to get a little bit different, the boffin's start to groan and the clever bit begins. The piston is still at Bottom Dead Center, as in the exhaust phase above, a valve opens to allow a charge of fresh air into the cylinder, fresh air at this stage, as I envisage fuel injection to be used with this layout. That's got to be better than a fuel & air mixture for economy and emissions. (Better Efficiency & Less Emissions)

BOTTOM DEAD CENTRE

COMPRESSION
Still at Bottom Dead Center, try not to think about the time scale it will be explained further down the page. As an external pumping device, not the crankcases, is used to provide the air charge, and the exhaust port is closed, extra volume can be introduced, turbo charging, as the compression phase begins to increase the "volumetric efficiency" of the engine. ( Better Efficiency) Also the need for a bulky "tuned" expansion chamber is removed.

Time Scales

This is what happens during the transfer phase of the cycle in both engine designs, before and after Bottom Dead Center.

With this new design however, the transfer phase will be variable. I hope it helps to explain the time scale part of both engines.

TRANSFER ONE

TRANSFER TWO
Just before BDC the valve begins to open, and as it is electronically controlled can be adjusted to suit engine speed and permit variable valve timing, the fresh air is allowed into the cylinder. Buy correct design of the valve surface and piston crown, the introduced air can be directed under the burnt gas. As the piston moves up the stroke the valve moves ahead of the piston controlling the flow of air, at higher pressure, purging the burnt gases out of the cylinder and into the exhaust and then when the exhaust port is closed, fuel is introduced to the cylinder via the fuel injection and little, or none, is lost to atmosphere.(Better Emissions)

TRANSFER FOUR

TRANSFER THREE
That's the theory of it all now here's a bit of practical I did just to keep myself entertained and prove a couple of points to my-self. I purchased a 2004 Yz 125 from Ride MX in Stratford, at a very sensible price, and began to test a few theories.

Due to time restraints and having to keep up with my day job there hasn't been a lot of design work involved in this project, I picture what I want in my head, see how it's going to work, then make the bits rather than draw them, and only draw them if I have to. I started to modify the YZ 125 by dividing the exhaust port in half horizontally at the same level as the existing transfer ports and vertically between the cylinder wall and the end of the exhaust manifold, so the standard exhaust would still fit, and then blocked up all the transfer passages. I wanted to find out if you could "exhaust" the combustion chamber with only half a port size and still have the engine rev to high RPM. If you have ever crushed your exhaust pipe or had a blockage in your silencer you will have noticed a reduction in high RPM. I formed a new transfer passage up the front of the crankcases between the water jacket and into the bottom half of exhaust port. So now the exhaust port was divided into four apertures, the top two handling exhaust gas and the bottom two in charge of the transferring the fresh charge from the crankcases to the cylinder.

I was still using the crankcases as the "air pump" and mixing oil with the fuel at this stage to try and keep the job simple. When I had got that bit done, the first time, and the motor was revving as normal in the workshop I wanted to put the engine under load and see if it would do the same thing, not easy to run it up the road, so I took the engine apart modified a few bits, changed a bit of the layout and went in search of dyno facilities.
Through Business Link in Coventry, and due to where I was located, I qualified for a Government Sponsored assistance program and off I went to the University of Central England in Birmingham with my YZ 125 for 5 days of "Expert" assistance. This was the first motor bike that had been offered up for testing so the men were keen to get amongst it, the dynos there are not designed for motor bikes and usually had enormous Rover engines bolted to them as the Rover plant was just up the road (R.I.P). As you can see I had to manufacture a frame work to mount the bike on and a drive shaft attached to the front sprocket connected to the machine. It only took three days to sort out the software required to drive the dyno computer to get any numbers out, the best attempt used for the first day was to turn the computer off and back on again, I know some computer geeks who would chuckle at that.
I was beginning to lose the will to live when the software problem was sorted and I could start the bike up and do some tests, then the Health and Safety man showed up and wondered if the drive shaft ought to be covered incase it "let go" The drive shaft was made from 50mm by 50mm steel box section tubing with high impact tractor size knuckle joints at each end, I told him I didn't think the awesome power of a YZ 125 would be enough to rent the dyno from the floor, or something like that and we got away with it.

I did get to run the bike up a few times but bearing in mind it was a made up as you go along design with the engine modifications made from chewing gum, string, and devcon, I did not want to run the engine any longer that was necessary. As it was considered unsafe, Health and Safety, to be in the dyno room when the bike was running I had to start the bike, put it in gear and run it up to 10,000 RPM fix the throttle open with a hose clamp and then retire outside to the computer station.The expert then controlled the RPM, by varying the load with the dyno, I was just waiting for the engine to blow up.

As most of my engine experience comes from working with Motocross GP bikes which is pretty "hands on" and you need to be near a bike to get a feel of what the engine is doing and what's happening internally. I managed to sneak in a couple of times and rev the bike as I wanted and was able to tell, by the way the bike was moving on the mounting jig, it was making good power. You could feel the torque, the way the bike was twisting on the jig as it drove the dyno was good enough for me. When I managed to sneak the exhaust extraction hose off it wasn't producing anymore smoke than any normal YZ 125. That was all I wanted to know, then on the next run the motor overheated and she blew.
This is about where I'm up to at the moment, the engine and bike still live and are in good shape. I have the clever valve bit in my office pictured here, mounted on a jig with all the opening and closing rotary solenoids I need with the optical sensors, driven from a timing disc mounted on the fly wheel, waiting for the man I have found, to build the electronic control system for the valve timing . Then I can progress to the next stage.

I'm off now, to sort out what's new for 2007 as regards my day job, but I'll update this bit as soon as I get time.

If you would like more of an explaination as to how and why it works follow this link.

Julian @ Gadget