Hello Friend:
	I'm a old Detroit Diesel man. But new here.
First let us define trems.
Four-cycle means the piston strokes ( moves up or down ) four times
	1. Intake, piston going down 
	2. compression, piston coming up [injection at near top dead center] 
	3. Power, piston going down
	4. exhaust. piston coming up
So the piston pushes and sucks the air in and out, stokes #1 and #4.
Two-cycle Detroit Diesels. Are not model air plane or lawn mower 
engines. They do have engine oil and exhaust valves.
	Let us start with the piston down at bottom dead center. The exhaust 
valves, up in the head, are open. The piston has uncovered air holes in the 
cylinder walls. And a blower is pushing fresh air in and exhaust gasses out 
(this is called scavning).
	The piston starts moving up, covers the air holes, and the valves 
close, this is compression stoke.
	At top dead center injection is happening, the piston is moving down. 
This is the power stroke. Now as the piston is about 90 Degrees ATDC, the 
exhaust valves start opening. At about 132 degrees ATDC, the piston starts 
uncovering the air holes, to scavning again.
	The best all round diesel tech. book is " DIESEL MECHANICS " 
by ERICH J. SCHULZ      #ISBN 0-07-055664-4.
	I hope this has been helpful.  Jerry Force
 
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