Mar. 23rd 2010
MECHANICS 5.0: "WASTED SPARK" EXPLAINED
"Wasted Spark" Explained
While the following discussion of "wasted spark" assumes prior knowledge of electromagentic induction and battery-coil-point ignition, the reader should keep in mind that: (1) both electric and magnetic field strength are measured in lines of force per square inch which is termed flux (2) the electric potential, measured in volts, generated in a coil is proportional to the rate of change of flux, that is, the speed at which the fields are building or collapsing (3) an ignition coil consisting of primary and secondary windings is, in reality, a step-up transformer (4) electrons, the charge carriers in metals, make progress through a conductive lattice very slowly, typically at a mere 1 millimeter per second; however, it is the electric field traveling near light speed that carries a signal, a disturbance which sets the electons in motion before it (5) the current reaching the spark plug is direct and thus the center electrode can be positive or negative depending on the circuit; ideally, the center electrode should be negative when igniting the fuel charge--a positive electrode requires 40% higher voltage to make it spark; hence it will erode faster and have an increased chance of misfiring.
When the ignition is switched on to start a motorcycle engine, the initial current comes from the battery and flows to the primary winding while the points are closed. This induces a magnetic field to emanate from the soft iron core within the coil. During the dwell the current flows through the closed points and into ground. The opening of the points by the rotating timer cam breaks the circuit and the magnetic field cuts through the primary and secondary windings as it collapses to the core. The collapse is too slow to induce a sufficient voltage in the secondary to arc the air gap of the spark plugs. However, it does induce an electric potential and current in the primary of approxitmately 200 volts and 0.002 amperes respectively, which charges the capacitor (condenser) connected in parallel with the points. This has two effects: (1) it prevents severe arcing at the points (2) the condenser then discharges a reverse current through the primary winding and the battery, thus equalizing the voltage on both the positive and negative sides of the condenser.
To be clear, while the points are open, there is no current flowing from the battery. Hence there is little resistance prohibiting the current from leaving the condenser, passing back through the primary (magnetic field builds) to ground at the battery, and then through the frame to the condenser ground for circuit completion. The discharge from the condenser reverses the usual flow of current through the primary windings and though not sufficient to flip the polarity of the magnet, does neutralize the magnet which rapidly collapses its field and causes the magnetic flux to cut through the secondary winding. The more lines of force that cut the wires on the secondary per second, the greater the induced voltage. With the reversal provided by the condenser, an electric potential of 20 kilovolts is induced which passes down the high-tension leads and causes a spark to jump the air gap of the plugs. The flow of current, occuring only when there is a spark, runs from one plug lead, down the center electrode of its spark plug across the air gap to the ground (side) electrode. Then through the engine to the ground electrode of the other plug, across its air gap and up the center electrode, through the plug lead and back to the coil. The spark that arcs from the side to the center electode is relatively weak and occurs during the exhaust stroke of its cylinder and hence is "wasted". The hotter spark from the negative center to positive side electrode is timed to occur optimally before the piston reaches top dead center on that cylinder?s compression stroke. Complete ignition of the fuel charge creates maximum gas pressure just after the piston passes top dead center to initiate the power stroke.
In lieu of a distributor or miniature coil-on-plug electronics, most motorcycles use conventional coil systems in which one coil services two spark plug leads. Taking a modern in-line four as a typical example, the plugs for the outboard cylinders fire at the same time, with the spark for cylinder #1 occurring before top dead center of the compression stroke and the other spark during the exhaust stroke of #4 . Therefore, in-line fours usually have two coils--one for the outboard and one for the inboard cylinders. The entire preceeding discussion invites the interesting topic of the relationship betwen timer cam-lobe-length and vee angle in twin cylinder machines, as well as many other electromechanical phenomena.