Two-stroke engine dictionary, glossary of terms:
ATDC or After Top Dead Center: Any position of the piston in the cylinder bore after its highest point in the stroke, i.e. top dead center (TDC). ATDC is measured in degrees of crankshaft rotation after TDC. For example, the point at which the exhaust port opens as opens 80 degrees ATDC.
Blowdown – degrees between exhaust port opening and transfer port opening. If the exhaust port opens at 90 degrees ATDC, and the transfers open at 115 degrees ATDC, then the blowdown is 115-90=25 degrees.
Bore – is the diameter of a cylinder.
Compression Ratio – the ratio of the total volume enclosed in a cylinder when the piston is located at BDC compared to the volume enclosed when the piston is at TDC (volume at TDC is called the combustion space volume). The formula to calculate compression ratio is: (Swept Cylinder Volume + Combustion Space Volume)/Combustion Space Volume = Compression Ratio.
Detonation – the unplanned ignition (auto-ignition or preignition) in the engine of the air/fuel mixture in the combustion space before the spark plug fires. Early ignition of fuel creates a shock wave throughout the cylinder as the burning and expanding fuel-air mixture collides with the piston that is still traveling towards TDC. As a result knock or ping is the sound of the pistons slamming against the cylinder walls.
Exhaust port(s) – a hole or holes within the cylinder head that form the initial flow paths for the spent gases of combustion.
Ignition timing –
Port duration – period during which the port opens and closes. If exhaust port opens at 90 degrees ATDC then it closes at 270 degrees. The port duration is 180 degrees.
Reed valve – Reed valves are commonly used in high-performance versions of the two-stroke engine, where they control the fuel-air mixture admitted to the cylinder. As the piston rises in the cylinder a vacuum is created in the crankcase beneath the piston. The resulting pressure differential opens the valve and the fuel-air mixture flows into the crankcase. As the piston descends, it raises the crankcase pressure causing the valve to close to retain the mixture and pressurize it for its eventual transfer through to the combustion chamber.
RPM – Revolutions Per Minute (RPM) is a unit of measure for angular speed. As applied to the internal combustion engine, RPM indicates the instantaneous rotational speed of the crankshaft described as the number of crank revolutions that would occur every minute if that instantaneous speed was held constant throughout the measurement period. Typically the idle speed of a two-stroke engine is around 800-1000rpm and peak speeds on a two-stroke engines normally reach up to around 12-13000rpm.
Some interesting videos related to two-stroke rpm:
* Nitro Engine Visible Crankshaft Spinning at High Speed!:
Squish band – area of cylinder bore where the clearance between piston head and cylinder head are held to a minimum.
Squish band clearance – clearance between piston head and cylinder head’s squish band at the TDC. Should be kept to a minimum at the TDC to avoid contact between the piston head and the cylinder head, which is 0.015 inch (0,4mm) for 50-100cc engines – 100-250cc engines of 0.02inch (0,5mm) for high performance engines. Should not exceed 0.06 inch (1,5mm)
Squish type cylinder head – type of combustion chamber where mixture is trapped in a cylinder head’s socket under spark plug and the rest of the cylinder head surface is made so that it will fit closely against the piston head at the TDC
Stroke – distance between the top dead centre (TDC) and the bottom dead centre (BDC)
TDC – top dead centre (TDC) is the point in the two stroke cycle where the piston is at its top most point in the cylinder.
Two-stroke cycle – a cycle of an engine which completes a power cycle with two strokes (up and down movement) of the piston during one crankshaft revolution. During that time four events take place: intake, compression, power, and exhaust.