Follow the explanation
Back to the model ↑How the parts work together
Start with cylinder one
At 0° in this model, cylinder 1 begins its power stroke. The valves are closed and the spark cue appears near top dead center. Its paired piston, cylinder 6, is also at the top but is beginning intake.
Move 120° to the next firing event
Cylinder 5 fires next. Continue in the order 1–5–3–6–2–4: six events separated by 120° add up to the full 720° four-stroke cycle. The order is common in inline-six engines; the model is not a replica of a particular engine.
Inspect the common crankshaft
Cylinder 3 begins power at 240°. The rods keep a fixed length while their angle changes, so piston motion is not a simple sine wave. Seven main bearings support the shaft between the six throws.
Watch the intake valves
At 450° global crank angle, cylinder 1 is halfway through intake. Its two intake valves are open, its exhaust valves remain shut, and the camshafts have advanced only half the crank angle.
Close the cylinder for compression
At 630°, cylinder 1 is halfway through compression. All four valves are closed. After another 90° the model returns to its initial firing event. Actual engines use valve overlap and ignition advance; this view keeps the four stages distinct.
The main parts
- Cylinder 1 · piston & rod
- The piston slides in its bore while the rigid connecting rod pivots between its wrist pin and the crankpin. Cylinder 1 shares piston height with cylinder 6, but the paired cylinders fire one crank revolution apart.
- Cylinder 2 · piston & rod
- The piston slides in its bore while the rigid connecting rod pivots between its wrist pin and the crankpin. Cylinder 2 shares piston height with cylinder 5, but the paired cylinders fire one crank revolution apart.
- Cylinder 3 · piston & rod
- The piston slides in its bore while the rigid connecting rod pivots between its wrist pin and the crankpin. Cylinder 3 shares piston height with cylinder 4, but the paired cylinders fire one crank revolution apart.
- Cylinder 4 · piston & rod
- The piston slides in its bore while the rigid connecting rod pivots between its wrist pin and the crankpin. Cylinder 4 shares piston height with cylinder 3, but the paired cylinders fire one crank revolution apart.
- Cylinder 5 · piston & rod
- The piston slides in its bore while the rigid connecting rod pivots between its wrist pin and the crankpin. Cylinder 5 shares piston height with cylinder 2, but the paired cylinders fire one crank revolution apart.
- Cylinder 6 · piston & rod
- The piston slides in its bore while the rigid connecting rod pivots between its wrist pin and the crankpin. Cylinder 6 shares piston height with cylinder 1, but the paired cylinders fire one crank revolution apart.
- Crankshaft & counterweights
- Six offset crankpins share one shaft supported at seven main journals. The three pairs of crank throws are 120° apart. Counterweights are shown opposite the throws; this model does not calculate forces, balance mass or torsional vibration.
- Twin cams & 24 valves
- Two camshafts each carry twelve lobes. Every cylinder has two intake and two exhaust valves, with visible springs and tappets. The cams turn once while the crank turns twice. Valve lift here follows an ideal teaching schedule with no overlap.
- Timing drive · 16 : 32
- A 16-tooth crank pulley drives two 32-tooth cam pulleys. Their pitch radii give the 2:1 speed relationship. The belt path and moving teeth reveal the connection; belt flexibility and tooth engagement forces are omitted.
- Bored cylinder block
- The cast block locates the six cylinder bores and main bearings. The cutaway removes the near-side casting to expose the piston skirts, connecting rods and crankshaft. Bore locations agree with the animated assemblies.
- Head & cam cover
- The head contains the valve openings, plug bores and cam supports. The blue cover encloses the valvetrain in the exterior view. Internal coolant and oil passages are not reproduced.
- Oil sump
- The pan beneath the crankcase represents the oil reservoir, with a flange and drain fitting. Lubrication flow and oil quantity are not simulated.
- Intake runners
- Six curved runners connect the intake plenum to the head in the exterior view. Their paths are illustrative; air flow, mixture distribution and resonant tuning are not calculated.
- Exhaust runners
- Six exhaust runners collect spent gas on the opposite side of the head. Their proportions show the architecture without claiming a measured production exhaust design.
Questions worth exploring
What firing order does this inline-six use?
The sequence is 1–5–3–6–2–4, the order BMW M engineers describe for their inline-six engines. Each cylinder fires once over 720° of crank rotation. Dividing that cycle into six equal intervals gives 120° between events. This is an original teaching assembly using that sequence, not a replica of a particular BMW engine.
BMW M engineers · The 1–5–3–6–2–4 inline-six firing orderNASA Glenn · Four-stroke engine operation and cam timing
Why do the camshafts turn at half the crankshaft speed?
A four-stroke cylinder needs two crankshaft turns to complete one valve sequence. Each camshaft therefore makes one turn while the crankshaft makes two. This model shows that relationship with a 16-tooth crank pulley and 32-tooth cam pulleys. The valve movements follow an ideal teaching schedule; they are not a simulation of cam-to-follower contact forces.
NASA Glenn · Four-stroke engine operation and cam timingfab.cba.mit.edu
What happens during the four strokes?
Intake admits the fresh charge. Compression reduces its volume with the valves closed. Combustion then drives the power stroke, and exhaust clears spent gas for the next cycle. Follow one cylinder through all four stages in the cutaway. The colors identify stages; they do not report pressure or temperature, and the model does not predict engine power.