- Design and Operation
- Four-cylinder engine with two banks of cylinders on opposite sides
- Each pair of opposed pistons moves inwards and outwards simultaneously
- Perfect primary and secondary balance, but more expensive than straight-four
- Secondary unbalanced torque pulse occurs when TDC and BDC pairs differ
- Applications
- Used in cars since 1897, especially by Volkswagen and Subaru
- Common in aircraft, especially Cessna 172 and Piper Cherokee
- Used in motorcycles, especially in shaft drive models
- Subaru retained flat-four engine for all-wheel drive applications
- Advantages and Disadvantages
- Advantages include perfect secondary vibration and low center of gravity
- Disadvantages include extra width and longer exhaust manifold
- Air-cooled versions are noisier and have lower power output
- Better suited for displacements above 2.0 L
- Historical Development
- First produced by Benz & Cie in 1900
- Volkswagen produced longest-running flat-four engine from 1938 to 2006
- Most manufacturers replaced flat-fours with inline-fours by 2000
- Subaru continues using flat-four engines in modern vehicles