Modifications 

Motorcycle Home Home

Motorcycle Links


Combustion process Timing Fuel Management HP & Problems


In the next few pages I will try to raise your education level about how a 4 stroke engine works.  Hopefully, this will allow you to make educated choices as to the modifications that you make.  I will cover the basic 4 cycles, valve timing, ignition timing, fuel management, and finally, I will briefly discuss modifications that can be performed to get the most from your motor.

To understand the combustion process first lets talk about the basic operation of a 4 stroke motor.  For the purposes of this discussion we will talk about 1 cylinder through all 4 cycles. The 4 strokes of a 4 stroke motor are Intake, Compression, Power, and Exhaust strokes.

 

1. Intake

Start at Top Dead Center (TDC)  This is the highest position the piston can be in the cylinder.  The piston begins it's downward travel in the cylinder.  The camshaft (which is chain "Timed" to the crankshaft" opens the intake valve(s) to create a path between the cylinder and the airbox for air to flow through.  The downward motion of the piston creates a low pressure area where the piston used to be. This pressure above the piston is less than the airbox pressure thus air is drawn into the cylinder.  During this intake stroke, fuel is atomized and mixed with intake air.   Thus a mixture of Air and fuel is drawn into the cylinder

Insert Picture of Intake stroke here.

2. Compression The next stroke is the compression cycle.  When the piston reaches Bottom Dead Center (BDC) it is at the lowest point of piston travel in the cylinder.  All air that can be drawn in has been drawn in.  The camshaft closes the intake valve and creates an enclosed space full of air/fuel.   The piston begins upward movement compressing the air/fuel mixture that is trapped in the cylinder.   At some point in the compression stroke right around TDC the ignition timing system sends a pulse of electricity to the spark plug to cause a spark in the Air/Fuel mixture.  Voila,  you have ignition.

Insert Picture of Compression stroke here

3. Power When the air/fuel mixture ignites, it's kinetic energy is rapidly increased causing it to expand.  The expansion of the gas in the cylinder forces the piston back down towards BDC.  This is the stroke in which the engine is making power.

Insert Picture of Compression stroke here

4. Exhaust Once the piston reaches BDC from the Power Stroke, most or all of the fuel/air mixture has been burned.  The resultant combustion byproducts such as hydrocarbons and carbon monoxide are useless to the combustion process. 

They must be "Exhausted" from the cylinder to clear the way for a fresh charge of air and fuel.   The piston begins it's upward travel.  The camshaft opens the exhaust valve(s)  This aligns a path from the cylinder to the exhaust system of the engine.  The piston's upward travel forces the combustion byproducts out through the exhaust.   At some point around TDC the exhaust valves close and the intake valves open and the entire process starts all over. 

Insert Picture of Compression stroke here

Multi-cylinder Engine In a multi-cylinder engine.  Each cylinder or a pair of cylinders is at a different segment of the 4 strokes at any time.  In other words when 1 cylinder (or a pair) is making power, another cylinder (or a pair) is taking in air and fuel and preparing to make power.  

Depending on the design of the engine, sometimes cylinders operate in pairs, sometimes each cylinder is at a different phase of combustion.

Combustion process Timing Fuel Management HP and Problems

Modifications 

Motorcycle Home Home

Motorcycle Links

Feedback is always appreciated.

 E-mail me and let me know what you think