Flat-four engine

The difference in TDC vs BDC inertial forces is explained in the Engine balance section.

In a flat-four engine, each pair of opposing pistons successively moves inwards together and outwards together.

Boxer-four engines are therefore better suited to displacements above 2.0 L (122 cu in), since they do not require balance shafts to reduce the secondary vibration.

In a boxer engine, each cylinder is slightly offset from its opposing pair due to the distance between the crankpin journals.

This offset gives rise to a slight rocking couple,[5]: 27  but any resulting vibration is normally insufficient to require balance shafts.

As with all four-stroke engines of four cylinders or fewer, the lack of overlap in the power strokes results in a pulsating delivery of torque to the flywheel, causing a torsional vibration along the crankshaft axis.

[citation needed] This engine was used in Benz racing cars, produced 20 hp (15 kW), had a displacement of 5.4 L (330 cu in) and was designed by Georg Diehl.

This engine was mounted longitudinally in the chassis, water-cooled, produced 9 hp (7 kW) and had a displacement of 2.4 L (146 cu in).

The 1936 Tatra T97 pioneered the rear-engined, air-cooled flat-four, backbone chassis layout (later used by the Volkswagen Beetle), and at the same time, though unrelated, came the Steyr 50 from Austria, sporting a front boxer 4 engine with rear wheel drive.

[13] This engine is water-cooled, has gasoline direct injection, produces 147 kW (197 hp) and has a displacement of 2.0 L (122 cu in).

[20] Although the side-valve format has long been abandoned for most automotive applications because its combustion chamber is a bar to high engine rpm, the massively over-square (1.295:1) D-Motor is a very simple, low-revving, compact, reliable lightweight aero-engine (without the heavy complication of ohv valve-gear)[21]