Getting to grips with the smoky mysteries of the two-stroke engine can be the work of a lifetime. Let's try and get some of the basics. A two-stroke engine's combustion chamber receives its charge of fuel and air and evacuates its exhaust gasses via an arrangement of ports; holes in the crankcases, barrels and pistons designed to line up at different points in the combustion cycle to produce the right amount of gas flow at the desired point in the rev range.
Of course the size and shape of these apertures has great bearing on how a two-stroke engine will perform, in the same way port size and shape and valve diameter and lift affect the characteristics of a four-stroke engine. For the purpose of this discussion, however, we will stick to the basics of two-stroke port timing, although their size and shape can easily affect their behavior. And, of course, two-stroke being two-stroke, pretty much everything has an effect on everything else.
With a power stroke every cycle, the two-stroke engine has a lot to get through. From induction to exhaust, it all happens inside two-strokes, that is up and down movements of the piston and one revolution of the crank. For a crankcase-scavenged two-stroke, the process begins with the fresh fuel/air mix being drawn into the crankcase by the vacuum left by the rising piston. The charge is compressed (primary compression) by the descending piston and pushed into the cylinder via the transfer port exposed when the piston is about to begin its ascent and closed before it's completed. The piston further compresses the charge in the head (secondary compression), if fires and as the piston moves to bottom dead center, the exhaust port is the barrel are uncovered and the spent gasses exit. At the same time, the descending piston is compressing the fresh charge in the crankcases and pushing through the transfer port and the whole happy process continues.
There has to be some overlap in the opening and closing of the ports to allow the exhaust to exit and the fresh charge to enter and there is a point where they both help each other. The problem is in deciding how long the ports should be open for. If the exhaust is open too long, too much fresh charge disappears. Not long enough and not enough exhaust is evacuated.
Similarly if the transfer opens too early some fresh mix is wasted and too late there is insufficient time to transfer and compress it all. As little as one degree of timing out and engine performance can be dramatically affected. So the closer to optimum port timing you get, the stronger and more efficient the engine gets. The further out it is, the less efficient and rideable a two-stroke becomes.
So in order of what opens and closes when, first to open after top dead center is the exhaust port and most of the exhaust gases escape. The piston is now pushing down hard on the gas under it so that when the piston opens the transfers, it pushes the fresh gas into the cylinder and tries to evacuate the last of the exhaust gas. By the time the piston gets to bottom dead center it should have pushed all of the gas it's going to out of the crankcase through the transfers, through the exhaust port and into the cylinder and the exhaust itself.
The piston carries on up and the transfer port shut. The piston starts to draw the fresh gas in below it. The return wave of the exhaust pipe now pushes the fresh gas from the exhaust pipe back through the exhaust port and into the combustion chamber. At this point the piston shuts the exhaust port and traps the gas in the chamber, compressing it for the next bang.
Piston-ported engines have symmetrical openings and closing, rotary valve ones can have the opening and closing timed separately, while on a reed-valve engine, the valve opens and closes as required.
The time a port is open for can be measured by degrees of the crank and/or the distance between the piston at top dead center and the opening of the port. The period of duration from the exhaust opening and the transfer opening is known as the exhaust lead or blowdown period, when pressure in the cylinder is falling because of the open exhaust ports and before the transfer opens to allow the fresh charge to travel upwards.
The period or duration from the transfer ports closing to the exhaust closing is the cylinder charging period.
The port timing has a huge influence on the movement of gas through the cylinder. But it has to work in harmony with the exhaust pipe as this has the greatest effect on a two-stroke engine. Port timings have changed with the rapid evolution of the two-stroke engine prompted by Walter Kaaden's invention of the expansion chamber. As the engines evolved so have port shapes, sizes and timing to facilitate better running engines in terms of efficiency and performance.
The exhaust pipe always dictates at what part of the rev range the engine will work best. The exhaust port height has less influence than you might suppose. Giving more duration by making it higher is unlikely to allow the engine to make more revs than the exhaust pipe dictates.
The manufacturer has spent a lot of time and money on research and development getting the port timing right to get the engine to work efficiently. If we want anything more or different, our best recourse is the exhaust pipe.