Re: Compression check question

[frankr]

This subject has been discussed to death many times in the past. There is no mystery if one considers the basic principle of operation of the two types in engines.

On a 4-stroke engine, the cylinder draws air through the intake valve on the piston’s intake (down) stroke. That air passes through the carburetor on it’s way to the intake valve. A closed throttle plate creates a restriction and a partial vacuum in that incoming air flow. That partial vacuum prevents the cylinder from filling completely. An incompletely filled cylinder has less air to compress on the compression (up) stroke, thus lower compression pressure when it reaches the top of the stroke. Blocking the throttle plate open removes the restriction to incoming airflow, there is no partial vacuum, and the cylinder completely fills with air to compress on the up stroke (both valves closed).

A 2-stroke is a completely different animal. On the piston’s up stroke, it draws air through the carburetor, into the crankcase. On the down stroke, that air in the crankcase is compressed (within the crankcase). When the piston nears the end of the down stroke, the exhaust ports are uncovered first, allowing the previously admitted (now on fire) air to escape as exhaust and the cylinder is now mostly empty. Only once all this has happened, does the piston uncover the intake (bypass) ports. When the bypass ports are opened, that air which is compressed within the crankcase rushes into the cylinder, and some goes on out those open exhaust ports. The result is a near-atmospheric pressure situation within the cylinder as the piston begins its upstroke. There is no partial vacuum because the air is compressed (a positive pressure, not a partial vacuum). Very important: Note that a 2-stroke simply cannot run without this crankcase compression to fill the cylinder. Therefor, the throttle plate never completely blocks the airflow path. There must be enough air enter to accomplish a compressed-air crankcase situation, or the cylinder will not fill and the motor cannot run.

So, understanding all this, it is evident that a 4-stroke PULLS air into the cylinder (partial vacuum), and a 2-stroke PUMPS air into the cylinder under pressure. That pulled-in air of a 4-stroke can be restricted and not fill the cylinder. The pumped-in air of a 2-stroke must be adequate to accomplish a near-atmospheric pressure situation at the bottom of the stroke when all ports are open. So, the piston begins its upstroke at near-atmosphere pressure regardless of throttle position.

All this is speaking about cranking speeds to test compression, not actual running conditions. When running, stuff like exhaust back pressure and goodies like tuning and supercharging come into play. But that’s a whole ‘nother topic.

Everybody got it???

Final word: Go ahead and block the throttle open on your two-stroke if it makes you feel any better, I don’t care. Neither does the motor.