The four-stroke cycle

Nearly every car engine on the road runs the same choreography: four piston strokes — intake, compression, power, exhaust — spread over two full turns of the crankshaft. Everything else in this course (cycles, kinematics, balance) hangs off this one schedule, so we pin it down precisely first.

The four strokes

A stroke is one full sweep of the piston between its two dead centres: TDC (top dead centre, minimum cylinder volume) and BDC (bottom dead centre, maximum volume). Each stroke is 180° of crank rotation, and the four of them do four different jobs:

• Intake (0–180°). The piston descends with the intake valve open, pulling in a fresh charge — air + petrol vapour in a spark-ignition engine, pure air in a diesel. The growing volume drops the cylinder pressure slightly below atmospheric, and the manifold pushes the charge in.
• Compression (180–360°). Both valves shut; the piston rises and squeezes the charge into the small clearance volume above TDC. Pressure and temperature climb steeply — this is where the compression ratio earns its keep, as the next two articles show.
• Power (360–540°). Ignition near TDC sends pressure far higher still, and the hot gas drives the piston down. This is the only stroke that puts work into the crank; the other three all consume a little.
• Exhaust (540–720°). The piston rises with the exhaust valve open and pushes the spent gas out. At 720° the crank is back where it started and the cycle repeats.

Valves don’t open on the textbook boundaries

The tidy 0°/180°/360°/540° boundaries describe the piston, not the valves. Gas has inertia: a column of air rushing down the intake port at high engine speed cannot stop dead the instant the piston does. Real valve timing exploits that:

$\text{IVO}$

intake valve opens ~15° before TDC — the leaving exhaust helps start the intake flow [705°]

$\text{IVC}$

intake closes ~50° after BDC — the moving column keeps ramming charge in [230°]

$\text{EVO}$

exhaust opens ~50° before BDC — pressure blows down early, saving pumping work [490°]

$\text{EVC}$

exhaust closes ~15° after TDC [15°]

Around the 0°/720° TDC both valves are briefly open at once — valve overlap (here 30°). At speed, the departing exhaust column helps pull fresh charge in; at idle it lets a little exhaust sneak back, which is why cammy engines lope.

Why the spark comes early

The plug fires before TDC of the power stroke — typically 10–40° early (ignition advance). Flame takes real time to cross the chamber (a few milliseconds), and at 6 000 rpm the crank sweeps 36° per millisecond. Firing early lets the pressure peak arrive just after TDC, around 10–20° ATDC, where it pushes on a crank that has leverage. Fire too late and the peak is wasted on an expanding cylinder; too early and the burn fights the still-rising piston.

Watch it run

The Four-Stroke Engine demo animates exactly this schedule: scrub the crank slowly through 720° and watch the valve lift follow the timing bands of Fig. 2, the spark fire at 335°, and the PV trace sweep out the loop we dissect in the next article.