Engine Layouts: Inline, V and Flat

In a four-stroke engine each cylinder fires once every 720° of crankshaft rotation, so evenly spaced firing gives an interval of 720° / n: 180° for four cylinders, 120° for six, 90° for eight. Shorter intervals mean overlapping power strokes and smoother torque delivery.

Balance is about the inertial forces of the reciprocating parts. Primary forces vary once per crank revolution; secondary forces vary at twice crank speed because the connecting rod's angularity makes the piston move faster in the top half of its stroke than in the bottom half. An inline-four cancels primary forces and couples, but its secondary forces add up, giving a vertical shake at twice engine speed that grows with displacement and the square of rpm; larger fours often use twin contra-rotating balance shafts. An inline-six is inherently balanced in both primary and secondary forces and couples. A flat-four cancels the primary forces between opposed pistons, but leaves small rocking couples because the cylinders are offset along the crank.

V engines are pairs of inline banks on a common crank. Bank angle and crank-pin arrangement set the firing intervals: a 90° V8 with a cross-plane crank fires evenly and can be fully balanced with counterweights, producing its characteristic uneven exhaust pulse per bank; a flat-plane V8 is lighter and revs more freely but has inline-four-like secondary shake. A 60° V6 suits even firing with offset crank pins.

Packaging is the other driver. Transverse inline-fours fit between the front wheels alongside the gearbox for front-wheel drive. Long inline-sixes are usually longitudinal. Flat engines lower the centre of gravity but are wide, which can constrain steering lock and exhaust routing.