Two Stroke Primer
Welcome to my primer on two strokes. The Vespa P200 engine is a good example of a by the book rotary valve two stroke. There are only 3 moving parts in the engine's top end: no pushrods, no camshafts, no lifters, nothing you would expect to see in a 4 stroke automobile engine. In this document, I hope to enlighten even the mechanically inept on how these beasts work.
If you have ever gotten that feeling that people around you are speaking an alien language when it comes to their bikes, they're probably talking about the engine. :) Never fear, however, I have the cure. Let's have a look at the parts in the engine top-end:
First thing on the roster is the barrel, or bore, or jug - however you may have come to know it. This is the container that holds back the huge forces behind the combusting gas and directs them to the piston. This part is centered with cylinder studs and is highly polished (or honed) to allow the best seal possible. If the polished area is scratched or burnt from overheating, it can be fixed by oversizing. Oversizing can be done at specialty shops and involves machining a larger hole through the bore and buying a larger piston to fit.
Secondly, we have the piston. The piston is made of aluminum alloy and moves up and down inside the cylinder barrel, channelling the force of the combustion into the rod beneath it: the connecting rod. The rings that skirt the piston head are called piston rings. The piston rings form a tight seal to prevent gas from escaping and center the piston head in the barrel. The gudgeon pin, or wrist pin, connects the piston head to the connecting rod.
Speaking of the connecting rod, we will look at that for a minute. The connecting rod is also made of a light metal and transfers the power from the piston and pushes on the crank. There are two bearings on the connecting rod, or con-rod, the small end bearing, or the wrist bearing; and the big end bearing, or the crank pin bearing. These bearings allow a nice circular motion that occurs as your piston moves up and down inside the cylinder.
The crankshaft, or crank is another moving part within the engine. This component has an offset point, called the crank pin , inside it which turns the up-and-down motion of the piston into rotational motion necessary for gears and wheels. The crank is usually made of a really heavy metal, like steel. The crankshaft also plays a part in injecting the gas into the expansion chamber (the place where the piston lives).
The combustion chamber is the place where the compression and combustion of the gas mixture occurs. This part is in the head of the cylinder and looks like a hemisphere (see below). This is the part that your spark plug threads into.
The expansion chamber is the length of the exhaust pipe. The exhaust pipe is approximately 12 times the volume of the displacement of your engine and is 'tuned' to give you the largest power bands. The reason for this phenomenon is that the expansion the combusting of gas is turned into force on the piston, but the gases from the combustion have to escape before the engine can take in new gas. The onus is on the exhaust to facilitate quick removal of the expended gas. The exhaust itself will be tuned such that it will contain exactly the right amount of waste vapours. This creates a pressure difference, that lets some of the exhaust gases leave the pipe and the some get pulled back into the cylinder with the new charge. The strength of this back pressure will result in better or worse performance from your motor. This is why better (tuned) exhausts are soughtafter: the stronger the back pressure, the more efficiently the engine works.
When people talk about 'good compression', they are indicating that the engine is probably in good health. Good compression means that the piston rings are well compressed and sealing properly, the cylinder head and bore are sealed well, the spark plug is tight, the engine seals are tight and the exhaust is providing good back pressure. You can test compression with a compression tester. this device measures the change in pressure when the piston reaches the top of the cylinder. A stock vespa engine typically has a compression of about 120psi. It takes 90psi minimum to start the bike. You can change the compression ratio of the motor by using different cylinder heads and maintaining the piston rings. This higher the compression ratio, the more pressure you will get, but heat will build up faster.
The engine on the P200 uses a 'disc valve' carburettion system and a 'rotary valve' induction system. What the hell am I talking about? I will explain. Disc valve induction from the carburettor is the slide moving in and out of the venturi letting more or less gas get through the hole in the bottom of the carb. Conversely, Rotary valve induction is the cut-out in the crank (you can see this if you turn your flywheel with the carb disassembled) that pulls in the gas being let through the disc valve at exactly the right time.
With the gas collected, the piston continues it's travel up the bore. The heavy flywheel turns the crank and pushes the con-rod, and subsequently the piston, to the top of it's travel. As the piston reaches the top, the gas becomes compressed against the hemispherical area called the combustion chamber. Now we get into timing. When we talk about timing, we are observing the firing point of the spark plug. Ideally, the firing point should occur as the piston reaches the top dead center (TDC) of its travel. The high voltage electronics that control this firing point a take a few milliseconds to develop a spark across the spark plug. Because of this delay, we have to offset the timing to make up for it. This offset is usually measured in degrees and sets off the electronics before the piston actually gets to TDC. The spark plug develops a huge voltage and arcs through the gas, positively combusting all the gas and putting a large pressure on the piston head. This gives rise to the Power stroke.
The con rod, connected to the piston and crank, transfers energy of the blast from the piston head to the crankshaft. The crank coverts the strong linear forces into equally strong rotational forces. This is where the engine develops all of its power. By the time the piston opens to the exhaust port again, the pressure in the pipe is now low and discharges the high pressure combustion exhaust into the pipe. Most of the gases escape through the tailpipe, but some are reflected to assert a strong reigon of high pressure on the exhaust port so new charge can enter the cylinder. The piston travels down to the transfer ports once again and restarts the induction process. This cycle will continue until you run out of gas or you stop the spark plug from sparking (kill the engine or turn the ignition switch).
All Material Copyright 2001-2021 by Richard Hoar. Use at your own risk.