Engine principles and four-stroke / two-stroke operation
Engine principles and four-stroke / two-stroke operation covers the events that convert fuel energy into crankshaft rotation, plus the arithmetic of capacity, compression ratio and firing order.
Four-stroke cycle
Induction draws air–fuel mixture (petrol) or air (diesel); compression raises pressure and temperature; power follows ignition or injection; exhaust expels gases. The crankshaft turns twice per cycle (720°) while the camshaft turns once (360°), giving a 2:1 cam drive. Inline four-cylinder firing orders are commonly 1-3-4-2 or 1-2-4-3; the order balances loads and avoids consecutive firing of adjacent cylinders.
Two-stroke cycle
One crankshaft revolution per power stroke. Piston movement opens and closes transfer, exhaust and inlet ports; crankcase compression pushes mixture through the transfer port, and fresh charge scavenges burnt gas. Two-strokes give high power-to-weight but poor scavenging and need oil mixed with fuel or a metered oil pump.
Worked example. Swept volume of one cylinder = (π/4) × bore² × stroke. For 80 mm bore and 75 mm stroke: 0.7854 × 0.080² × 0.075 = 0.000377 m³ = 377 cm³. Four cylinders give 4 × 377 = 1,508 cm³, quoted as about 1.5 litres.
Compression ratio = (swept volume + clearance volume) ÷ clearance volume. With 377 cm³ swept and 42 cm³ clearance, CR = 419 ÷ 42 = 9.98:1.
Traps that cost marks
Saying four strokes need four crankshaft revolutions. Omitting clearance volume from CR. Assuming a two-stroke has poppet valves. Confusing valve overlap with valve lead.