The straight-6 or inline-6 engine (often abbreviated I6 or L6) is a six cylinder internal combustion engine with all six cylinders mounted in a straight line along the crankcase. 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-6. The straight-6 layout is the simplest engine layout that possesses both primary and secondary mechanical engine balance, resulting in relatively low manufacturing cost combined with much less vibration than engines with fewer cylinders.
Displacement range
Usually a straight-6 is used for engine displacements between about 2.0 and 5.0 litres (120-308 CID) in automobiles. It is also sometimes used for smaller engines but these, although very smooth running, tended to be rather expensive to manufacture and they are inevitably physically longer than alternative layouts. The smallest production straight-6 was found in the Benelli 750 Sei motorcycle, displacing 747.7 cubic centimetres (45.63 cu in) (0.75 L). However, because it is a fully balanced configuration, the straight-6 can be scaled up to very large sizes for industrial and marine use, such as the 16-litre Volvo diesel engine used in heavy vehicles.The largest are used to power ships. They use diesel fuel and have displacements as high as 1,820 litres (64 cu ft) per cylinder.
Modern trends
Historically, straight-6 engines were introduced much earlier than V6s, and while the first straight-6 was manufactured in 1904, it was 1950 before a production V6 was introduced. V6s (unlike crossplane V8s) had intrinsic vibration problems that were difficult to eliminate without modern computer aided design techniques. The length of the straight-6 was not a major concern in the older front-engine/rear-wheel drive vehicles, but the modern move to the more space-efficient front-engine/front-wheel drive and transverse engine ("east-west") configurations in smaller cars caused the much shorter length of the V6 to become a major advantage. As a result, in recent decades automobile manufacturers have replaced most of their straight-6 engines (and many of their V8s) with V6 engines.
Exceptions to the shift to V6 engines include BMW, which specializes in high-performance straight-6s, Volvo, which designed a compact straight-6 engine/transmission package to fit transversely in its larger cars, and the Australian Ford Falcon, which still uses a straight-6 configuration. Straight-6s also continue to be commonly used in medium to large trucks, and sport utility vehicles, where engine length is less of a concern. In 2002 General Motors introduced the Vortec 4200 as part of the modular straight-4, straight-5 and straight-6 GM Atlas engine line.
Balance and smoothness
An inline six engine is in perfect primary and secondary mechanical balance, which can be achieved without using a balance shaft. The engine is in primary balance because the front and rear trio of cylinders are mirror images and the pistons move in pairs. That is, piston #1 balances #6, #2 balances #5, and #3 balances #4, largely eliminating the polar rocking motion that would otherwise result. Secondary imbalance is avoided because an inline six cylinder crankshaft has six crank throws arranged in three planes displaced at 120 degrees. The result is that differences in piston speed at any given point in rotation are effectively canceled.
An inline four cylinder or V6 engine without a balance shaft will experience secondary dynamic imbalance, resulting in engine vibration. As a general rule, the forces arising from any dynamic imbalance increase as the square of the engine speed—that is, if the speed doubles, vibration will increase by a factor of four. In contrast, inline six engines have no primary or secondary imbalances, and with carefully designed crankshaft vibration dampers to absorb torsional vibration, will run more smoothly at the same crankshaft speed (RPM). This characteristic has made the inline six popular in some European sports-luxury cars, where smooth high-speed performance and good fuel economy are desirable. As engine reciprocating forces increase with the cube of piston mass, inline six is a preferred configuration for large truck engines.
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