How a motorcycle gets running: the starter, the charging system and the spark

Originally published on MotoSutra in January 2021, and re-verified against current reference material during this recovery. This is what the article is actually about, which its original title did not say: a kickstarter’s helical spline, a roller one-way clutch, a stator being swept by permanent magnets, and a capacitor waiting to be told when to let go.

About the name. The page used to be called How Motorcycles Engine Works – The Basics, but it never discusses the four-stroke cycle at all — it follows the electricity needed to get one running. The title now matches the content, and the cycle explanation people expect from that older title is not being smuggled in here from the later version of the same URL.

The 2021 text also assumed a cut-away model on the page beside it, and that artwork is gone. Where it said the model shown here, the description now has to stand on its own; the wedge-shaped pocket between the two races, the raised tooth on the flywheel’s edge and the densely wound exciter coil are all spelled out in prose, and no replacement drawing has been introduced to fill the gap. Two voltages the original attributed to an unnamed “particular model” have been struck rather than repeated.

How a motorcycle engine starts

Every motorcycle has either a kickstarter, a self-starter, or both. Both exist because turning an engine by hand or by motor is the same problem, and it has to be solved twice over: you have to apply a lot of force briefly, and then you have to get out of the way immediately.

The kickstarter. As the lever is pushed down, a helical spline forces a gear to slide along until it engages a freewheeling gear on the main shaft. The spiral cut of the spline is what converts the lever’s arc into the axial movement that meshes the two parts.

Push the lever further and the gear turns on its axis, transferring that effort to the clutch hub; the clutch hub carries it to the crankshaft, and the engine turns over.

The freewheel is the important half of the design. When the kick ends — or when the engine fires halfway through one — the crankshaft is suddenly spinning far faster than the lever ever moved. Without an overrunning clutch in the path, that speed would be driven straight back through the spline into the pedal and into the rider’s leg. The freewheel simply stops transmitting, which is why a kick-start that catches early is a thump rather than a broken ankle.

The self-starter. A battery-powered motor provides the initial spin instead. Once the engine is running, the crankshaft is turning faster than any starter motor is designed to be turned, so the starter has to be disengaged — otherwise the running engine would drive the starter armature beyond what it can survive.

This is achieved with a one-way clutch, and the classic form is the roller type. It has an outer race, an inner race, and a set of small rollers caught in a wedge-shaped space between them. When the starter motor spins the outer race in the driving direction, the rollers are wedged between the two surfaces and the friction locks the assembly, so power passes to the inner race and from there to the crankshaft.

Once the engine fires, the inner race overtakes the outer one. The rollers are released from the wedge, the inner race spins freely, and the connection disappears without anything having to be commanded to disconnect it. That is the whole trick: a clutch with no control, no spring to fail and no timing to get wrong, which is why the arrangement has survived more than half a century of motorcycles.

Where the spark’s voltage comes from

Like any petrol engine, a motorcycle engine needs a source of ignition to set the air-fuel mixture burning, and it uses a spark plug. The plug needs a voltage far above anything the battery supplies, because it is firing across a gap into a cylinder full of compressed mixture.

Corrected. The original gave specific figures — “above 20,000 volts to fire properly in this particular model” and an ignition coil that “increases voltage up to 200 times”. Those numbers named a particular model that the article did not identify, and nothing in the archive establishes their source, so they have been removed. What remains is the mechanism, which is not in doubt.

That voltage is generated by the machine itself. A stator is a set of stationary coils of wire; the flywheel — the magnet rotor, the heavy wheel that also keeps the crankshaft turning smoothly — carries a set of permanent magnets that revolve past those coils. A magnet sweeping past a conductor induces an alternating current in it, and that is what a motorcycle’s charging system actually is: a generator whose output rises with engine speed.

The stator’s outputs are not all the same, and different coils feed different jobs — running the lights and ignition electronics, and keeping the battery charged. The one that matters here is the exciter coil, wound with a denser set of turns of thinner wire than the others. Many turns of fine wire produce a higher voltage from the same passing magnet, and the ignition wants a higher voltage than the battery does.

The CDI unit: storing a charge and choosing when to spend it

That exciter output is fed to a component called the capacitive discharge ignition unit, or CDI. Its primary function is exactly what its name says: store an electric charge, and dump the stored charge when required.

Inside it, a capacitor holds the charge built up from the exciter coil. What decides the moment of release is a second small sensor called the pickup coil — a permanent magnet with a coil of wire wound around it. On the edge of the flywheel there is a small extrusion, a raised tooth that the pickup coil passes once per relevant interval.

As that tooth approaches and passes the pickup coil, the changing magnetic field induces a small voltage in the coil. That pulse goes to the CDI unit, and its effect is to close an electronic switch called a silicon controlled rectifier, or SCR. An SCR, once triggered, conducts and does not stop until the current through it falls — so the trigger does not have to hold anything open. The capacitor now has a path, and its entire stored charge leaves through the CDI’s output in one very short, very sharp pulse.

This is why the system is built around a capacitor rather than simply feeding the coil from the stator. A spark’s usefulness comes from how fast the energy arrives, and a slow source cannot empty itself quickly. Storing the energy first, then releasing it through a switch with no moving parts, decouples how fast the engine is turning from how sharp the spark is.

From that pulse to the plug, and onward to the wheels

The pulse arrives at the ignition coil, which is a step-up transformer: many turns of thin wire on its secondary for every turn on its primary, so a modest voltage in becomes a very large voltage out. That is what finally crosses the plug gap.

Keeping the engine running is then a matter of timing the rest of the cycle to the crankshaft’s position. The intake and exhaust valves are driven from the crank by a timing chain, so that they open against the piston’s motion rather than at random moments.

Power now exists, and the remaining components are there to make it usable. As long as the engine runs, the crankshaft turns and delivers that rotation onward. A single-cylinder engine generally uses a counterbalance shaft, because one piston’s mass accelerating up and down has nothing to cancel it, and the result would otherwise be vibration transmitted into the frame. The clutch takes the crankshaft’s rotation and lets it be engaged and disengaged smoothly, so the machine can stop without stalling and change gears without breaking anything. The transmission then does the job the engine cannot, altering speed and torque as the road demands — which is covered in its own article, how a manual transmission works.