diesel engine power stroke classifications

[cessna 310]

would someone explain the difference between a "two-stroke" and "four-stroke"

[Railway Man]

 2 stroke -- requires two complete strokes of the piston in the cylinder for every power stroke (the stroke that generates power)

4-stroke -- requires four complete strokes for every power stroke.

Rather than spend 10 minutes describing this, see these links, complete with animations:

http://auto.howstuffworks.com/diesel-two-stroke1.htm

http://auto.howstuffworks.com/diesel.htm

 

RWM

[BaltACD]

A two stroke engine has combustion every time the piston comes to the top of the stroke....a four stroke engine has combustion every 2nd time the piston comes to the top of the stroke.

[creepycrank]

cessna 310

would someone explain the difference between a "two-stroke" and "four-stroke"

The long form. First a diesel is a compression ignition engine in that the working fluid-air is compressed enough to heat it above the ignition temperature of the fuel that you are going to inject at the maximum compression,essentially top dead center. Then expand it by using a piston to turn a crankshaft. Second. The problem is to move the working fluid(air) through the system. The 4 stroke achieves these in easy to understand 4 strokes of the piston. starting art TDC the piston descends drawing air in through the intake valve, the second stroke is the compression stroke, the third stroke is the power stroke and the fourth stroke is to pump the combustion gases out of the exhaust valve. The first and fourth stroke are to move the charge through the engine or scavenging. The two stroke has an external scavenging blower that performs the same function as stroke one and four on a four stroke engine. The variation on the EMD engine is that it is a uniflow design in that the air flows in through ports in the cylinder when its at the bottom of the stroke and it has exhaust valves that start to open art about 114 degrees after top dead center before the ports are uncovered to start the flow. A four stroke diesel will run very nicely without a blower but any thing over 40 HP has a turbo as a means of getting decent power out of what would be a very heavy engine otherwise. The early two stroke diesel made use of a positive displacement blower as a much more efficient way to move air than piston pumps used on some direct reversing 2 stroke engines built by Fairbanks Morse in the 20's and 30's and applied to some LSM's in WW 2. Turbos really change things and applying one to 2 stroke engines can be tricky. EMD uses an overrunning clutch to get the air moving at low speeds and loads. Cleveland Diesel and Detroit Diesel still had a engine driven blower with lashed on free turning turbo that over-boosted the blower at higher loads. Sulzer and the 109,000 Wartsila engine all over the internet have motor driven blowers to pump air at low power. In general the four stroke which fires every other revolution to maintain the same power as a similar sized two stroke has a firing pressure twice as much. But that's a whole other story.

[Tugboat Tony]

for those of us who are a little slower  4 stroke has 4 distinct conditions during operation.  1. suck (intake) 2. Squeeze (compression) 3. Bang (ingition of fuel) 4.Blow. (Exhaust pushed out of cyl.)

 

a 2 stroke diesel essentially combines the first and last strokes, when the piston is at bottom of it;s stroke it opens a set of ports leading to a pressurized air box. when these ports are open it allows the clean, pressurized air to blow the dirty exhausted air out the open exhaust valves. leading to the squeeze and bang strokes.  Hope this helps