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The straight-4 or inline-4 engine (often abbreviated I4 or L4) is a four cylinder internal combustion engine with all four cylinders mounted in a straight line along the crankcase. more...

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The single bank of cylinders may be oriented in either a vertical or an inclined plane with all the pistons driving a common crankshaft. Where it is inclined, it is sometimes called a slant-4. The straight-4 layout confers a degree of mechanical simplicity which makes it popular for economy cars. This straight engine configuration is the most common in cars with a displacement up to 2.4 litres. The usual "practical" limit to the displacement of straight-4 engines in a car is around 2.7 litres. However, Porsche used a 3.0 L four in its 944 S2 and 968 sports cars; and larger four-cylinder engines used in industrial applications, such as in small trucks and tractors, are often found with displacements up to about 4.6 L.

Balance and smoothness

The straight-4 engine is much smoother than one, two, and three cylinder engines, and this has resulted in it becoming the engine of choice for most economy cars, although it can be found in some sports cars as well. However, the straight-4 is not a fully balanced configuration. While it is in primary balance because one pair of pistons is always moving up at the same time as the other pair is moving down, piston speed - as with all internal combustion engines - is higher through the top 180 degrees of the crankshaft rotation than the bottom 180 degrees. Since two pistons are always moving faster in one direction while two others are moving more slowly in the other, this leads to a secondary dynamic imbalance - an up-and-down vibration at twice crankshaft speed. This imbalance is tolerable in a small, low-displacement, low-power configuration, but the vibrations get worse with increasing size and power.

Most straight-4 engines below 2.0 L in displacement rely on the damping effect of their engine mounts to reduce the vibrations to acceptable levels. Above 2.0 L, most modern straight-4 engines now use balance shafts to eliminate the second-order harmonic vibrations. In a system invented by Dr. Frederick W. Lanchester in 1911 and popularized by Mitsubishi Motors in the 1970s, a straight-4 engine uses two balance shafts, rotating in opposite directions at twice crankshaft speed, to offset the differences in piston speed. However, in the past there were numerous examples of larger straight-4s without balance shafts, such as the Citroën DS 23 2347 cc engine that was a derivative of the Traction Avant engine, the 1948 Austin 2660 cc engine used in the Austin-Healey 100 and Austin Atlantic, the 3.3 L flathead engine used in the Ford Model A (1927), and the 2.5 L GM Iron Duke engine used in a number of American cars and trucks. Soviet/Russian GAZ Volga cars used aluminium big-bore straight-4 (2.5 L) with no balance shafts in 1950s-1990s. These engines were generally the result of a long incremental evolution process and their power was kept relatively low compared to their capacity.

Read more at Wikipedia.org