2 Stroke Engines


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Operation of 2 Cycle Engines

This is a brief overview of how a 2 stroke engine operates, and it will be primarily focused on the typical design for RC nitro engines.  There are variations in 2 cycle engines found in other applications like snowmobiles, chainsaws, weedeaters, etc...  However, the fundamental principles are still the same.  For starters, below is a cutaway illustration showing a typical 2 cycle RC nitro engine design. 

2 cycle engine cutaway


1. Carb fuel needle valve

2. Carb throttle barrel

3. Carburetor assembly

4. Crankshaft end

5. Crankshaft inlet port

6. Crankcase

7. Crank counterbalance

8. Crank pin

9. Glow plug

10. Cylinder head

11. Cylinder head button

12. Piston crown

13. Piston pin

14. Piston

15. Connecting rod

16. Recoil pull start

As the piston rises upward, this creates a vacuum/suction in the engine crankcase area.  Air & fuel are mixed in the carburetor and then drawn into the 2 stroke engine through the crankshaft port inlet (see #5 above).  The air/fuel mixture then travels through a passage within the crankshaft.  The crankshaft is hollow on these type of RC nitro engines, and this acts as a transfer passage/tunnel between the carburetor and crankcase.  As the crankshaft spins around, the crankshaft inlet port is covered and then uncovered.  While spinning, this crankshaft inlet port acts as a rotary valve for this type of 2 stroke engine. 

As the piston goes down, the crankcase is pressurized as the underside of the piston goes downward and compresses the air/fuel mixture in the crankcase below.  As the piston moves further down, the inlet port in the cylinder wall is uncovered and the pressurized air/fuel mixture is forced into the combustion chamber.  As the crankshaft continues to spin and the piston changes direction and begins to move upwards, more air & fuel are drawn into the crankcase.  At the same time, the air/fuel mixture that is trapped above the piston in the combustion chamber is compressed.  Then the spark plug fires and ignites the air/fuel mixture.  The hot expanding gases from the combustion process drive the piston downward.  As the piston moves downward further, an exhaust port in the cylinder wall is uncovered and exhaust gases are allowed to escape out the exhaust port and out into the exhaust system.  As the pistons continues to move downward after the power stroke, more air/fuel mixture is pressurized and pushed up into the combustion through the inlet port.  Then the cycle begins all over again.

In the cutaway view above, it's hard to see all the details clearly.  One of the things that is not clear in the cutaway picture above is the inlet and exhaust ports in the cylinder wall.  Below is another picture showing some additional details of 2 stroke engines: the different components, and a better view of the inlet and exhaust ports.  The 2 cycle engine is like a pump that pumps in fresh air/fuel mixture for combustion, and afterwards pumps out the exhaust gases.         

Components of 2 stroke engines

A 2 stroke engine produces 1 power cycle for every 2 engine strokes, and that is how this engine gets its name.  There is a power stroke for every revolution of the crankshaft.  That is 2X as many power cycles as a 4 stroke engine.  As a result, the additional combustion gives the 2 stroke engine a potential for producing more power in a unit that is more compact in size than a typical 4 stroke engine.  Good power, compact size, and light weight makes these type of 2 stroke engines a good choice for RC nitro scale models and other applications where this high power to weight ratio is useful.