The purpose of this article is to explain the basic operation of a four stroke dirt bike engine. In reality, the dirt bike four stroke engine is just like any other four stroke engine, in principle. Of course, the type of engine being discussed here is a gasoline engine, but from a practical sense, the type of fuel is irrelevant when theory of operation is the focus. To provide a good understanding of how the engine works, it is not necessary to indulge the variation of possible fuel types. The operating principle remains the same. Just realize that specific engine design criteria are based upon the intended fuel. As most of us know, dirt bikes are traditionally gasoline engines whose transmissions are integral to the engine assembly.
The engine draws in a fuel and air through its intake. This fuel-air mixture is then compressed inside the engine's combustion chamber. At the right time after compression, the fuel-air mixture is ignited. The result is a powerful explosion inside the engine's combustion chamber. This explosion, what we refer to as combustion, provides the needed energy to cause the engine to rotate. As the engine is set into motion, waste gases are expelled through the exhaust. So how does this really work? Well, the process occurs through four basic steps, called strokes, during the engine's rotation. These strokes are: intake, compression, combustion and exhaust. Before proceeding with an explanation of how this all works, let us examine the components of the engine.
Engine structure: cylinder
The cylinder is the space within the engine assembly where the combustive work takes place. Within this cylindrically shaped space, a moving piston is fitted. The piston is free to move up and down within the cylinder. Cooling passages surround the cylinder to facilitate removal of heat so that the engine can operate at a constant desirable temperature.
Engine structure: cylinder head
The top of the cylinder is capped off by a cylinder head. The space underneath the head, above the piston, is called the combustion chamber. When the piston is as high as it can go within the cylinder, there still remains space. This space is carefully designed to maintain proper pressure characteristics within the cylinder. The cylinder head also provides control of fuel-air into the cylinder, means of controlling exhaust flow out of the cylinder and also a fixing point for a fuel ignition device (spark plug). The cylinder head contains the intake tract, exhaust tract, fuel injectors (unless the engine is carbureted), spark plug, intake valves, exhaust valves, compression release, cam and/or rocker arm(s).
Engine structure: piston
The piston is accurately fitted into the cylinder and freely moves up and down within it. When the fuel-air mixture is ignited within the combustion chamber, the expanding hot gases drive the piston downward. The moving piston, via a connecting rod, spins an attached crankshaft. There is sufficient energy in motion from a single combustion event that the piston is able to return to the top of the cylinder, down and back up again.
Engine structure: piston rings
Rings are fitted to the upper portion of the piston. The purpose of this is to seal the piston diameter to the cylinder wall to prevent gases from escaping down through the minute clearances between the piston and cylinder walls. Additionally, rings are designed for specific purposes. There are designs for dealing primarily with combustion while others are designed for distributing lubrication onto the cylinder walls. While fitting tightly in the cylinder bore, the rings must be allowed some movement into and out of the grooves in the piston. This allows the rings to form to the cylinder, even as wear occurs.
Engine structure: crankshaft (crank)
The crankshaft is the primary rotating mechanism in the engine, located in the lower section of the engine case. When we here the term "RPM" (revolutions per minute) of an engine, this is what it refers to: the speed of the crankshaft. The crankshaft center portion is offset so the attached connecting rod can push on it like a lever, which results in crank rotation. From the crankshaft, gears are used to transmit rotation to the final output shaft, where the counter (drive) sprocket is attached.
Engine structure: connecting rod (rod)
The connecting rod couples the piston to the crankshaft. Because the crankshaft has an offset, the connecting rod, from the force of the piston, is able to push on it as one would push on a lever. The pivots at each end of the rod enable the lower portion of the rod to swing around the crank as it rotates. The upper pivot wobbles back and forth while the lower one makes full rotations.
Engine structure: intake valve(s)
Intake valves are housed in the cylinder head and are fitted with springs to keep them closed. Depending on the engine, there may be one or more valves. A closed intake valve prevents air from entering the cylinder through the intake tract. A closed intake valve also acts to prevent gases from escaping the cylinder via the intake tract when compression, combustion or exhaust are occurring. When intake air is needed, the valve is opened by action of the cam, either directly or by a rocker arm.
Engine structure: exhaust valve(s)
Exhaust valves (one or more) are housed in the cylinder similarly to the intake valves. These valves are also fitted with springs to keep them closed. Their function is exactly the same as that of the intake valves, except that they control opening and closing of the exhaust tract.
Engine structure: camshaft (cam)
The camshaft (typically referred to as cam) is a rotating shaft mounted to the cylinder head. The cam has shaped lobes the act as levers as the cam rotates around. Depending on the angle of rotation, a cam lobe may or may not push on a valve stem (or rocker arm). The shape of a lobe is designed so that a valve will open and close at the exact intended engine rotation angle. Separate lobes are used for exhaust and intake. In some cases, there may be two separate cams. The rotation of the cam is accomplished by means of the crankshaft. Typically, a timing chain gears the two shafts together. The crankshaft rotates twice for every single cam rotation.
Engine structure: Compression release
The compression release is a mechanical device used to relieve pressure in the combustion chamber to ease starting of the engine. For a four stroke engine with a kick starter, getting the engine to rotate when there is compression in the cylinder can be extremely difficult if not impossible. The release may be manual or automatic. In either case, the device operates by lightly opening an exhaust valve to expel compressed gas, making it easier to kick start. Most (if not all) of the newest four strokes utilize automatic compression release. It works on the principle of centrifugal force. When the engine is rotated slowly (as when being kicked over), an exhaust valve is slightly opened (more correctly, prevented from fully closing). When the engine fires and begins rotating at idle speed, a centrifugal mechanism allows the exhaust valve to fully close during its closing period.
Engine structure: Spark plug and ignition coil
A spark plug is used to ignite the fuel mixture that has entered the combustion chamber and has been compressed. At a precise moment, the spark plug produces a high energy spark across an electrode gap. For electrical current (in the form of an arc, or spark) to flow across the gap, a significantly high voltage is required (many thousands of volts). This high voltage is produced and supplied by the ignition coil. The ignition coil itself is a transformer so to speak. A low voltage signal is applied to the primary coil of the ignition coil. A field is generated as a result which then induces a very high voltage in the secondary coil.
Engine structure: Fuel injector(s)
For fuel injected engines, a fuel injector (or injectors) is used to deliver fuel directly to the combustion chamber. The injector is typically an electrically operated valve. The fuel outlet ports of the injector are very small and precise. The ports are designed to produce a desired spray pattern in a controlled manner. Opening time of an injector varies depending on demand for fuel. Due to the small size of the fuel openings, fuel cleanliness is critical.
Engine structure: Throttle body
A throttle body is part of the intake tract of a fuel injected engine. It is located between the air filter and engine cylinder head. Its function is to vary the flow of air coming into the engine based on throttle position. Designs vary, but they are basically throttle controlled air valves. When the throttle is turned back, the throttle body opens according, typically by attached cabling.
Engine structure: Carburetor
Carburetion is much different than fuel injection. In a carbureted engine, a carburetor is part of the intake tract and is essentially an air valve that is opened and closed by the throttle, just like a throttle body. However, a carburetor is designed to deliver fuel to the incoming air stream. This is achieved by venturi effect. As air moves through the venturi (from the intake filter to the engine), fuel is pulled up from a float bowl into the air stream through finely made passages called jets. The result is a fine mist of fuel in air. Jet sizes are very specific because fuel mixtures must be precise for correct engine firing.
Engine structure: Engine control system
As modern dirt bike engines evolve into more efficient, more powerful engines, control systems do also. Modern fuel injected engines have computer systems whereas older carbureted engines have controls without computers. In both cases, the fundamentals of basic engine control is the same. An older carbureted system would consist of a magneto (which produces electrical energy at a time where spark is needed), ignition box (CDI box, aka Capacitive Discharge Ignition), ignition coil and a spark plug.
A computer controlled system is much more advantageous because variables that affect performance can now be more controlled through the use of various sensors. For example, a modern fuel injected engine would consist of an engine control unit (computer), ignition coil, spark plug, throttle position sensor, crankshaft position sensor, air intake temperature sensor and other possible sensors depending on a specific bike model. Tuning in a modern system is an advantage as well. Now, the control system can automatically make adjustments for such things as "changes in air temperature", eliminating the need for cumbersome mechanical tuning.
Putting it all together
It is difficult to briefly explain every aspect of the engine. My intent here is to provide as much understanding of how it works without authoring an entire book. Each component of the engine has a story of its own and can be lengthily discussed. But, now that there has been a basic explanation of most of the engine components, let us see how it all comes together.
Getting the engine going
I have already mentioned that a four stroke engine cycle consists of four strokes, hence the term four stroke. To recap, the strokes are intake, compression, combustion and exhaust, and they occur in that order. When the engine is kicked over or electrically started, these four strokes must complete for the engine to fire and run. Remember, when the engine runs, there is sufficient energy from a single combustion event that the engine can rotate through each of the cycles so that it can continue to fire on its own.
The intake stroke is when the piston moves from its top position (called top dead center) to its most bottom position. The cam rotates with the crank such that the intake valve opens and closes at the precisely required rotation angles. We'll say, as a reference, the crankshaft rotates from 0 degrees to 180 degrees, while the camshaft rotates from 0 degrees to 90 degrees.
The motion of the downward moving piston causes vacuum to occur in the cylinder. When the intake valve opens, air rushes into the cylinder through the valve opening via the intake tract. As the piston nears the bottom of the stroke, the intake valve closes so that the received air charge is now trapped in the cylinder. During this stroke, for fuel injected engines, fuel is injected as commanded by the ECU (engine control unit). The injector is pulsed for varied lengths of time depending on engine load. For a carbureted engine, the fuel is atomized in the air stream as it rushes inward.
It is also important to note that the amount of air entering into the cylinder is dependent on throttle opening. This enlightens us to the need for a throttle position sensor on injected engines because throttle position is what dictates the amount of fuel needed (basically). No sensor is needed for carburetion because the fuel is always metered proportionately to the incoming air stream by venturi effect.
The compression stroke occurs when the piston moves from the bottom most position (180 degrees) to the top most position (to 360 degrees, or, back to 0). During this stroke, the intake and exhaust valves remain fully closed so that the fuel air mixture is compressed. The cam rotation thus has rotated from 90 degrees to 180 degrees.
The combustion stroke occurs next. When the piston is at top dead center, after compression, spark is triggered which ignites the fuel air mixture. A tremendous amount of energy is therefore released by the rapidly expanding hot gases inside the cylinder, driving the piston downward to the 180 degree (bottom of stroke) position.
Now to be clear, spark does not truly occur at 0 degrees (top dead center). Rather, it occurs a bit sooner, a few degrees before top dead center. This spark angle varies though, depending on engine speed. The reason has to do with the time it takes for fuel ignition. Though ignition of the fuel is very fast, it still takes time. Therefore, spark must occur before top dead center so that the fuel combusts at the right time, just as the piston begins its journey back down. The faster the engine rotates, the sooner spark needs to occur. During the combustion stroke, the intake and exhaust valves remain closed as the camshaft rotates from 180 degrees to 270 degrees.
Lastly is the exhaust stroke. Here, the piston continues upward from 180 degrees back to 0 degrees. During this time, the exhaust valve transitions to open and then closed again. The camshaft completes a full rotation from 270 degrees to 360 degrees(back to 0). The exhaust gases are expelled into the exhaust pipe during the valve open period such that little to no pressure resides in the cylinder as the next intake stroke proceeds.
The four strokes described complete a full engine cycle. The moving piston, pushes and pulls on the crankshaft offset like a lever, via a connecting rod. The crankshaft therefore spins as long as the piston is moving up and down.The exhaust, intake, and compression strokes complete due to the energy produced by combustion. By means of transmission gears, rotation of the crankshaft translates to rotation of the final drive gear. Within the transmission gear set, different gear ratios are selected by the rider operating the gear shifter, each of which changes the speed and torque of the final drive relative to the crank. Energy, ideally, remains constant from the crank to the final drive. However, low gear selections (for example first gear) produce slow transmission output RPM compared to high engine RPM. High gear selections (for example gear 5) produce high transmission output RPM compared to low engine RPM. To explain this further, rear wheel torque is high in low gear but speed is low. Conversely, in high gear, rear wheel torque is low but speed is high.
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