Engines and Motors

The idea

An engine is a converter of one kind of motion into another. The piston can only go up and down; the wheels need something turning. The slider-crank does the conversion: the piston drives a connecting rod, the rod drives a crank throw offset from the crankshaft axis, and reciprocation comes out as rotation. Everything else in the cutaway is in service of that one linkage.

The four-stroke cycle spreads the work over two full turns of the crank. Intake: the piston descends with the intake valve open, drawing in air and fuel. Compression: both valves shut and the piston rises, squeezing the charge into a fraction of its volume, which raises its temperature and its pressure. Power: the spark fires near the top and the burning charge pushes the piston down — this is the only stroke that produces work. Exhaust: the piston rises again with the exhaust valve open and clears the burnt gas. Four strokes, two revolutions, one push.

That is why the flywheel matters. Three of the four strokes take energy out of the crankshaft and only one puts energy in, so the crank has to coast through the rest on stored rotational inertia. The valves are not blown open by pressure either; they are lifted by cam profiles turning at half crank speed, which is the only way a four-stroke's timing can repeat every two revolutions.

The electric motors on the last two tabs reach the same output by a different route entirely. There is no linkage and nothing reciprocates: a current in a magnetic field feels a force, F = I L x B, and if the geometry puts that force as a couple around an axis, the thing turns.

What to look for

• Watch one point on the crank and count: the cycle takes two full revolutions, not one. Pick the spark as your marker and it fires every other time the crank passes the top.
• The piston does not move sinusoidally. It dwells at top and bottom dead centre — momentarily stopped while the crank sweeps on through — and moves fastest around mid-stroke. That asymmetry is the connecting rod's finite length, and it is why the crank needs momentum to get past the dead centres.
• Both valves sit shut through compression and power. Open valves mark the two breathing strokes, and each opens only on its own.
• On the inline-four tab, the four pistons cascade rather than move together: the throws sit at 0, 180, 180, 0 degrees and fire 1-3-4-2, so a power stroke lands every half revolution. That is the whole reason for adding cylinders — the torque stops arriving in lumps.
• On the steam-engine tab the same slider-crank runs horizontally with no valves and no spark. Strip the combustion away and the linkage is unchanged, which is the point.
• On the dc-motor tab, watch the split-ring commutator at the moment the coil passes the poles: the current through the coil reverses, and the torque keeps its sign instead of flipping.
• On the ac-motor tab, compare the rotating field with the cage rotor chasing it. The rotor never catches up, and the gap does not close.

Getting it right

• The power stroke is a burn, not an explosion. The spark lights a flame front that travels across the chamber in a controlled few milliseconds. Actual detonation — the end gas igniting on its own ahead of the flame — is knock, and it damages engines.
• Compression is not free. The piston does work on the gas going up, and the cycle's useful output is the power stroke minus what compression and pumping cost. A four-stroke gives back roughly a quarter to a third of the fuel's energy as shaft work, and the ceiling on that is thermodynamic, not a matter of better bearings.
• The valves are driven, not pushed. A cam opens them on a schedule; cylinder pressure has no vote. Timing is a design choice, which is why cam profiles are something engineers argue about.
• Adding cylinders does not make each cylinder stronger. It makes the delivery smoother — more, smaller pushes per revolution — which is a different benefit from more power per push.
• A commutator does not reverse the motor's rotation; it reverses the current in the coil so the force couple keeps pointing the same way around the axis. Without it the coil would stall after a half turn.
• An induction motor can never run at synchronous speed. At zero slip the rotor sees no changing field, so no current is induced, so there is no torque. The lag is not a defect — it is the mechanism.

Turn the knobs

• scenario picks the branch: 'combustion' is the four-stroke cutaway here, 'inline-four' the phased engine, 'steam-engine' the bare slider-crank, 'dc-motor' and 'ac-motor' the two electrical machines.
• crankRadius is the crank throw, so the piston's stroke is twice it. Raise it and the same engine becomes longer-stroked.
• rodLength is the connecting rod, and it must exceed the crank radius. Shorten it toward that limit and the piston's motion gets visibly more asymmetric between the up and down halves.
• flywheelR sizes the flywheel that carries the crank through the three unpowered strokes.
• rpm sets the running speed on the readout; playback is clamped to a rate you can actually watch.
• scale resizes the whole assembly without touching the kinematics.