Two Cycle Engine Diagram


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A Look at a 2 Stroke (2 Cycle) Engine Diagram

This is a look at how a 2 cycle engine operates.  Because of ever increasing emissions regulations and controls, the 2 cycle engine is becoming less and less common.  What used to be a mainstay in the powersports industry (motorcycles, ATVs, snowmobiles, watercraft), the 2 stroke is now being replaced by the 4 stroke engine.  Shown below is a 2 stroke cycle engine diagram.  This particular diagram shows a hobby style RC nitro type engine.  The fundamentals are basically the same apart from the glow plug being replaced by a spark plug.  On a 2 cycle engine, the piston in conjunction with ports in the cylinder wall act as the intake and exhaust "valves".  As the piston moves up and down, it covers/uncovers these ports at specific times to all the air/fuel mixture to enter and the exhaust gases to exit.  On a 4 stroke engine, there are separate valves that are actuated off a camshaft that control the intake and exhaust processes. 

2 Cycle Engine Diagram

Below is another diagram showing some of the common parts of a 2 stroke engine.  Again, this is a hobby type engine, but the fundamental principles and components are the same.         

2 Stroke Engine Diagram

1. Carburetor Fuel Needle Valve.

2. Carburetor Throttle Barrel.

3. Carburetor Assembly.

4. Crankshaft End.

5. Crankshaft Inlet Port.

6. Crankcase

7. Crankshaft Counterbalance.

8. Crankshaft Pin.

9. Glow Plug (or Spark Plug).

10. Cylinder Head.

11. Head gasket.

12. Piston Crown.

13. Piston Pin.

14. Piston.

15. Connecting Rod.

16. Pull Start.

By design, two stroke engines have twice as many power strokes (explosions) for a given engine speed.  This means that the 2 stroke engine can usually make more power for a given engine size.  As a result, it has been popular with applications where a high power-weight ratio is needed.  For many years, motocross dirt bikes, snowmobiles, and high performance ATV's have utilized 2 cycle engines extensively.  This because it is possible to get extremely high peak power output from a relatively lightweight 2 cycle engine if it is designed properly.  Even though higher peak power output is possible, sometimes this peak power comes on in a narrow RPM range.  This is what is sometimes referred to as a "narrow powerband" or a "peaky engine".  The power delivery of a 2 cycle engine can be much more abrupt (all or nothing) than a 4 cycle engine.  Four stroke engines typically have a much more linear powerband that is easier to control.

On some high powered 2 stroke engines, especially older designs, it was like the power came on all at once in a huge rush.  When you hit the powerband, you better hold on!  It was like a rocket booster kicked in.  This can be fun, but it can also make it harder for the average person to ride a motorcycle or ATV with this kind of powerband.  For an example of how this 2 cycle powerband behaves, watch this short helmet cam video clip of a Yamaha RD350 2 stroke motorcycle.  Keep an eye on the tach (gauge on the right) at around 0:13 seconds into the video.  At this point, the engine reaches around 9,000 RPM and it appears to hit the powerband.  The engine sound becomes more intense and the front wheel lifts off the ground.

Later, advanced in 2 stroke engine design including power valves and other improvements helped to widen the useable powerband.  Another of the disadvantages of the 2 stroke engine is that they can be more finicky to operate.  In the past, the problem of fouling spark plugs plagued some 2 cycle engines depending on their carb tuning.  A 2 stroke engine can be less forgiving of the air/fuel mixture being off of ideal.  Too rich and you foul plugs and are dead.  Too lean of a mixture, and you can get detonation and a hole in your piston!  Now, some of these problems were more with the older engines, and more modern designs were less prone to having these issues.  Another disadvantage of the 2 cycle engine is that they require the lubrication oil to be mixed with gas.  Since there is no oil in the crankcase like in a 4 cycle engine, the engine receives it's lubrication separately.  Either the oil is mixed with the fuel or the oil is kept in a separate reservoir and injected into the engine.  In both cases, the oil is mixing with the air/fuel mixture.  Since the engine is constantly consuming oil, you get more visual smoke and exhaust emissions.  In addition, long term reliability and longevity are typically not as good on a 2 cycle engine.  Rings and cylinder bores often wear quicker and it's not uncommon to require multiple rebuilds before a 4 cycle engine ever has to be cracked open for repair.  Then again, the 2 stroke engine is a much simpler design and it is often cheaper to rebuild one compared to a 4 stroke engine.  So, there are both advantages and disadvantages to the 2 stroke engine design.  The biggest advantage was the superior power to weight ratio.  Modern advances in 4 cycle engine technology has bridged the gap some, which has made the 2 stroke engine a little less advantageous.  Most of all, the pressure for cleaner burning engines has pushed many 2 cycle engines to the wayside.  Even so, it is useful to examine a 2 stroke cycle engine diagram to better understand this important engine design in the history of the internal combustion engine!