Tech Answer: Two-Stroke Terminology

If you're having a problem with your scoot, drop us a line!

Moderator: Tuna Cowboy

Tech Answer: Two-Stroke Terminology

Postby poopShotgun » Mon Apr 23, 2007 9:42 pm

Two-stroke engines are incredibly simple machines with complex internal dynamics. Pressure waves, scavenging, and expansion chambers are the meat and bread of vintage scooters and can often leave the novice with a spinning head (especially combined with too much beer!) I certainly do not claim to be more than a novice but I have dealt with some fundamentals of tuning and the terminology involved and hopefully can bring some of the wonderful world of two-strokes into light.

Two-stroke engines have a high power to volume ratio, as opposed to four-strokes which use twice as many strokes per cycle. They are also much simpler in terms of moving parts. The downside to two-stroke engines is that the engine lubricants are mixed into the fuel/air; Harmful emissions are greatly increased.

Why do I need to know this?
You don't, but you'll be more enlightened when a scooterist claims they have changed their port timing, advanced their spark timing, reduced the squish clearance, widened the exhaust and transfer ports, altered the expansion chamber, and over-bored the engine.

As well, a greater understanding of the internal workings of your bike not only increases your technical knowledge and helps diagnose problems, it provides a greater appreciation of the history and "whys" of the Vespa and Lambretta.

Technical Note:
Some of this may be wrong. Feel free to PM me if I have missed something or have it wrong. I feel I have done some good research but I'm not perfect.
Last edited by poopShotgun on Mon Apr 23, 2007 9:58 pm, edited 2 times in total.
Splash me on in the morning and wear the great smell of me all day long.
User avatar
Posts: 1601
Joined: Fri Apr 28, 2006 7:13 am
Location: Under the evil influence of Uranus

Postby poopShotgun » Mon Apr 23, 2007 9:42 pm

Basic Terminology

Piston ->
A cylindrical piece that compresses fuel and air to produce power within the engine. It is domed on a Vespa, thus all Vespas have Hemi engines (hemispherical). A piston has two (sometimes one) grooves within which the piston rings sit.

Top Dead Center (TDC) ->
The point at which the piston is closest to the cylinder head.

Bottom Dead Center (BDC) ->
The point where the piston is furthest from the cylinder head.

Cylinder (Barrel) ->
The sheath within which the piston moves. The cylinder is a crucial determinant of compression ratio, engine volume, and engine cooling.

Cylinder Head (Head) ->
The top of the cylinder, a seperate piece clamping itself and the cylinder to the cases via four (or more) studs. The cylinder head contains the compression chamber and is crucial in determining compression ratio.

Cooling Fins ->
Cooling fins increase the surface area of the cylinder and head in order to improve the efficiency of airflow cooling. The fins stick out from your cylinder and head.

Piston Rings (Rings) ->
Piston rings encircle the piston and provide a seal when the engine compresses fuel and air.

Crankshaft (Crank) ->
The crankshaft, more properly the entire crank assembly, is two round disks, each with an extending rod and joined at an offset by the crankpin. On a rotary-valve engine (which most vintage Vespas have) the crank comprises half of the intake valve.

Connecting Rod (Conrod) ->
The conrod connects to the crank via the crankpin and the piston via the wristpin. As the crank turns it's offset connection provides a means of transfering rotational vectors to linear vectors and vice-versa (it allows the piston to move up and down, turning the crankshaft).

Wristpin ->
The wristpin connects the conrod to the piston. The bearing that goes around the wristpin where it intersects the conrod is often called a small-end bearing.

Flywheel ->
The flywheel provides rotational mass to keep the crankshaft in motion. On a Vespa or Lambretta it also provides a convenient fan that, with the help of a cooling shroud, blows cool air over the cooling fins.

Fuel/Air Mixture ->
Atomized fuel mixed with outside air. On a two-stroke engine, lubricants are mixed in as well.

Carburetor ->
The unit that atomizes fuel and mixes it with air. Opening the carburetor valve allows more fuel/air mixture to enter the engine, increasing revolutions and thus power.

Intake Valve ->
Either rotary or reed on a vintage bike, the intake port is where the engine gets it's fresh fuel/air. This valve determines the timing of when fuel/air enters the engine.

Transfer Ports ->
Transfer ports bring fresh fuel/air into the cylinder from the crankcase. The T5 engine had five transfer ports (which I do believe is where it got the name). Most vintage bikes have two or three (The third being the boost port. See below.)

Exhaust Port ->
The exhaust port moves spent fuel/air from the cylinder to the expansion chamber.

Expansion Chamber (Exhaust, Pipe, Muffler) ->
The expansion chamber allows heated exhaust gasses time to expand, which facilitates the formation of a pressure wave crucial to the two-stroke process, It also allows exhaust gasses to exit the engine.

Case Chamber (Crankcase) ->
The area within which the crankshaft spins. It provides a means of intake for fresh fuel/air mixture, which is then moved into the compression chamber to produce power. The crankcase has it's own seperate compression ratio which is determined by the movement of the piston. Think of it as a reverse compression chamber; As the piston moves up, it compresses fuel/air for power while at the same time creaating a vacuum to move fresh fuel/air into the crankcase. As the piston moves down, it expands with the buring fuel/air while at the same time compressing the crankcase to push fresh fuel/air into the compression chamber.

Rotary Valve ->
This is an intake valve formed by the crankshaft and rotary pad. If you have a rotary-valve engine, you'll notice that your crankshaft has sections cut out from the left-side disk. This cutting determines when the intake valve opens and closes. It is important not to confuse the rotary-valve type engine with a rotary-disc-valve engine. Although both work on similar principles, they have an entirely different engine setup.

Rotary Pad ->
The rotary pad is the other half of the rotary valve. It is an upraised section of the case chamber that helps form a seal with the crankshaft.

Reed Valve ->
A reed valve works via vacuum within the case chamber, opening when a vacuum is formed and closing when the case chamber compresses. A reed valve replaces or is used in lieu of a rotary valve.

Two-Stroke Process ->
The two-stroke process performs compression, exhaust, intake, and expansion within two strokes. A four-stroke engine uses one stroke for each process. A stroke is one complete movement of the piston either up or down, so a two-stroke does one up and one down for each cycle (whereas a four stoke does two ups and two downs per cycle). In a nutshell, the two-stroke process is:

Up stroke -> Intake/Compression
Down stroke -> Exhaust/Power

But if you look at the process closely, you'll notice that intake and exhaust happen during both strokes, and that there are two working chambers. So the process more closely resembles:

Up stroke -> Intake/Exhaust/Compression
Down Stroke -> Intake/Exhaust/Power

Following is a detailed explanation of what is going on in your engine.

Cylinder Chamber - The fuel/air mixture has been compressed to maximum density, all ports are closed. The explosion caused by spark and fuel/air has reached the top of the piston.
Exhaust - With the exhaust port closed, the pressure wave has bounced back and is finished pushing spent fuel/air out of the muffler.
Intake/Case Chamber - The piston has reached apex and the Intake valve is fully open.

(After TDC)
Cylinder Chamber - The expanding gasses ignited by the spark plug have moved the piston down towards BDC, providing the power portion of the stroke.
Exhaust - The exhaust port is closed and the expansion chamber is empty.
Intake/Case Chamber - The intake valve has closed and compression of the case chamber has begun.

(Between TDC and BDC)
Cylinder Chamber - The exhaust and transfer ports begin to open. Spent fuel/air moves out the exhaust port and fresh fuel/air moves in from the transfer ports.
Exhaust - Exhaust gasses begin to move into the expansion chamber, forming a pressure wave.
Intake/Case Chamber - As the case chamber compresses, fresh fuel/air moves through the transfer ports into the cylinder chamber.

(Before BDC)
Cylinder Chamber - The ports are mostly open and the fuel/air mixture is moving into the chamber.
Exhaust - The pressure wave formed by exhaust is reaching the end of the eøpansion chamber.
Intake/Case Chamber - The case chamber is almost fully compressed and fresh fuel/air is moving into the cylinder chamber nicely.

Cylinder Chamber - The ports are fully open.
Exhaust - The pressure wave formed by the exhaust is on it's way back to the cylinder. Fresh fuel/air is moving out the exhaust port.
Intake/Case Chamber - The case chamber is fully compressed and fuel/air has finished it's journey into the cylinder chamber.

(After BDC)
Cylinder Chamber - The ports are closing.
Exhaust - The pressure wave is pushing fresh fuel/air back into the cylinder from where it escaped into the exhaust port.
Intake/Case Chamber - The intake port opens and the exhaust pressure wave blows some of the fresh fuel/air out of the port. This is called carbuerator blowback.

(Between BDC and TDC)
Cylinder Chamber - The exhaust port has almost closed and (hopefully) all of the fresh fuel/air that escaped through the exhaust port has been pushed back into the chamber via pressure.
Exhaust - The pressure wave is almost spent and most is moving back towards the end of the pipe.
Intake/Case Chamber - As the piston moves upward it creates a vacuum that sucks in fresh fuel/air from the carbuerator.

(Before TDC)
Cylinder Chamber - Compression truly begins. The sparkplug will fire before TDC is reached, forming an expanding explosion that will reach the top of the piston right at TDC.
Exhaust - The exhaust gasses finish leaving the expansion chamber.
Intake/Case Chamber - The intake valve has completely opened allowing fresh fuel/air to fill the case chamber.
Last edited by poopShotgun on Wed Aug 15, 2007 8:54 pm, edited 6 times in total.
Splash me on in the morning and wear the great smell of me all day long.
User avatar
Posts: 1601
Joined: Fri Apr 28, 2006 7:13 am
Location: Under the evil influence of Uranus

Postby poopShotgun » Mon Apr 23, 2007 10:46 pm

Advanced Terminology

Locating Pin ->
These are found within the ring grooves on a piston. Because a piston ring is split, the locating pin determines where the ends sit and keep the ring in a relatively constant position in the groove.

Combustion Chamber ->
The combustion chamber is located within the piston head. Since the inside of a head is hemispherically concave, it is best described as a "more-concave" bubble in the head, with the spark plug hole usually located so that the plug gap is roughly at the center of the compressed gasses. If the engine is not a squish-type, the combustion chamber usually comprises the entire head volume.

Offset Combustion Chamber ->
Most combustion chambers are directly over the center of the piston, but some are offset to the side opposite the exhaust port. The idea is that, at higher compressions, the heat from exhaust and ignition will be spread more evenly over the surface of the piston. As heated exhaust leaves the engine, it passes over one part of the piston as the other side is cooled by intake gasses. An offset sparkplug heats the opposite side of the piston instead of the center, spreading the heat more evenly. The downside to this approach is that the piston rings may scrape lubricating oil from the cylinder walls and splash it onto the sparkplug, resulting in fouling (I have experienced this on my Rally, where the combustion chamber nearest the side of the cylinder is darker than the opposite side).

Cubic Capacity (cc) ->
Cubic capacity is calculated by using the formula for volume of a cylinder (pi * r^2 * h), where h=Stroke Length and r=Piston Radius, both expressed in centimeters. A Rally 200 has a piston radius of 3.33cm and a stroke of 5.7cm, so it's cubic capacity is (3.1416 * 3.33^2 * 5.7) or 198.57cc. "cc" Is also the abbreviation for cubic centimeters, which is what you get using the above formula.

Compression Ratio ->
The ratio between chamber volume at BDC and TDC (the extreme ends of the stroke). It is calculated by taking the volume of the cylinder head and adding it to both the BDC and TDC volume of the cylinder. Dividing the larger volume by the smaller provides you with a ratio in terms of Result:1. Some tuning guides will also use a term called "True Compression Ratio", where the calculation is determined not from BDC, but the moment the exhaust port closes. The reasoning there being that fuel/air can move freely until all the ports are closed at which point compression actually begins (since the mixture is now essentially trapped). Manufacturers will not use the "True Compression Ratio" calculation because it is not nearly as impressive. For instance, s stock Rally 200 has a compression ratio of 8.2:1, whereas it's "True Compression Ratio" is closer to 5:1.

Octane ->
Octane is the measure of a fuel's resistance to detonation, whether by spark or compression. Higher octane fuels ignite at higher temperatures and pressures, while lower octane fuel ignite more easily. On a low compression engine, like that found on a scooter, lower octane fuel will give more overall power because it ignites easier and more of the fuel will be burnt. I'm not certain where you will want to start using higher octane fuel, but in my experience, you'll want to have a compression ratio of at least 10:1. My Rally 200 has a compression ratio of about 8.9:1 and regular gasoline (straight unleaded) gives more power than premium gasoline. However, I have experienced predetonation at higher revs using lower octane fuel, so do be careful and listen to your engine.

Predetonation (Pinging, Pinking, Knockng) ->
High compression can cause the fuel/air mixture to spontaneously detonate before the spark plug fires, causing loss of power. It can also occur if the combusiton chamber is too large, where the advancing flame-front from the spark-plug heats and ignites the unburnt fuel before the flame reaches it. Predetonation can be alleviated to some extent by using a higher octane fuel. As you can tell from the other descriptive words for the phenomenon, you will be able to tell by sound when this is happening.

Squish Band ->
A squish band is used to reduce predetonation by placing as much of the fuel/air as possible near the sparkplug. The mixture that is in the squish band is left unburnt and emissions in a squish-type engine are higher, but power gains and engine reliability (in terms of predetonation) are increased. The squish band surrounds the combustion chamber and conforms to the piston shape. It is placed roughly 1mm to 1.5mm from the piston at TDC in a 150cc to 200cc engine.

Chamber Pressure ->
Chamber pressure is produced by the compression of the piston and the back-pressure wave from the expansion chamber. Other factors determining chamber pressure include crankcase compression (from the intake valve), port area and port timing. Increasing compression does not automatically increase pressure, there is other work to be done as well. Most Vespa engines will have chamber pressures around 120psi or less. You can take an air-cooled two-stroke up to about 160psi before heat becomes an issue; 135psi is probably a good ideal to shoot for on vintage bike tuning.

Power Band ->
This is when the engine is working most efficiently (when you are getting the most power) and is expressed as a range of revolutions. Most Vespas and Lambrettas have a fairly wide powerband. As you tune an engine, the powerband tends to shorten. Most tuned engines have a powerband in the higher revolutions and can be slow off the line, yet others are tuned lower (we call these wheelie machines). Some tuning can keep the powerband nice and wide, but you will lose high or low end potential power. Tuning is always a matter of trade-offs.

Scavenging ->
Scavenging is the process of removing waste gas from the engine by replacing it's volume with fresh gasses.

Boost Port ->
An extra transfer port (or two) added across the cylinder from the exhaust port to improve scavenging by increasing the turbulence of incoming fuel/air.

Porting ->
Porting is a process whereby the transfer, exhaust, and/or intake ports are altered to produce more power or alter the powerband. There are two "flavors" of porting: Port timing and port shaping. Port shaping is most often what is refered to as porting.

Port Timing ->
Port timing is the shaping of a port to alter the timing of when it opens or closes. This is accomplished by raising or lowering the cylinder or by raising the top and/or lowering the bottom of the port. Port timing mainly affects where the powerband is, but can also increase power.

Port Shaping ->
Changing the overall area of a port, widening it, polishing the air pathway, and adding material for smoother transitions encompass port shaping. This is usually what is refered to as "Porting". Porting allows more air to enter and exit the compression chamber, resulting in higher pressure, more fuel mixture, and greater power.
Splash me on in the morning and wear the great smell of me all day long.
User avatar
Posts: 1601
Joined: Fri Apr 28, 2006 7:13 am
Location: Under the evil influence of Uranus

Return to Scooter Workshop

Who is online

Users browsing this forum: No registered users and 2 guests