Flat-four Engine - Balance and Smoothness

Balance and Smoothness

Boxer engines are better balanced than other engine types in 4-cylinder configurations. The more common inline-4 configuration suffers from an engine balance problem caused by the fact that the pistons travel faster on the top half of the crankshaft rotation than the bottom half, which causes the engine to vibrate up and down twice per crankshaft revolution. This problem becomes worse with increased engine size and power so inline-4s larger than 2.0 L usually have balance shafts and ones over 3.0 L are seldom used in passenger cars. However, the flat-4 does have a less serious secondary imbalance that causes it to rotate back and forth around a vertical axis twice per crankshaft revolution. This is because the cylinders cannot be directly opposed, but must be offset somewhat so the piston connecting rods can be on separate crank pins, which results in the forces being slightly off-centre. The vibration is usually not serious enough to require balance shafts.

In addition, four-stroke cycle flat-4s have a problem common to all four-cylinder engines: the power strokes do not overlap. With a piston starting its power strokes every 180 degrees of crankshaft rotation, and the crank throws 180 degrees apart, all the pistons must come to complete stop and reverse before the next one can start its power stroke. This results in a gap between power strokes and a pulsating delivery of power to the flywheel. By contrast, in engines with more cylinders the power strokes overlap: the next piston starts its power stroke before the previous one has finished, and the delivery of power is much smoother.

As a result of the relatively high manufacturing costs of the flat-4 compared to the inline-4, most manufacturers now choose the inline-4 engine for economy models and have moved to inline-5 or V6 engines for models requiring more power. These engines also suffer from dynamic imbalance problems, but with modern computer-aided design techniques, the problems can be overcome with a variety of complex crankshaft, balance shaft, and engine mounting designs. Luxury performance car manufacturers prefer to use the inline-6, flat-6, or V8 configurations because these designs are in intrinsic primary and secondary balance and thus are much smoother than the flat-4, particularly at high power outputs.