The straight-six engine or
inline-six engine (often abbreviated I6 or L6) is an internal
combustion engine with the cylinders mounted in a straight line along the crankcase
with all the pistons driving a common crankshaft (straight engine).
The bank of cylinders may be oriented at any angle, and where the bank is inclined to the vertical, the engine is sometimes called a slant-six. The straight-six layout is the simplest engine layout that possesses both primary and secondary mechanical engine balance, resulting in much less vibration than engines with fewer cylinders.
Straight-six engines were introduced much earlier than V6 engines. While the first straight-six was manufactured in 1903 by Spyker, it was not until 1950 that a production V6 was introduced. V6s (unlike crossplane V8 engines) had intrinsic vibration problems not present in the straight-six.
The length of the straight-six 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 (left-to-right versus front-to-back) configurations in smaller cars made the length of the V6 (one half the length of an L6 with the same bore size, plus the width of one rod) a major advantage. As a result, in recent decades automobile manufacturers have replaced most of their straight-six engines (and many of their V8s) with V6 engines; Nissan have replaced their earlier turbocharged inline-6 models with larger-displacement naturally aspirated V6 engines while maintaining the FR setup and Mercedes-Benz now uses V6 engines, despite also retaining the FR layout. Increasingly straight-six engines are also being replaced by turbocharged in-line four cylinder engines (e.g. in the current BMW range), which offer comparable top-end power output and reduced low-end torque but with better fuel efficiency, due to smaller displacements and lower friction from the reduced number of cylinders. The poor secondary harmonic balance of four-cylinder engines is largely addressed with the use of balance shafts although it can never match the in-line six.
An inline six engine is in perfect primary and secondary mechanical balance, without the use of a balance shaft. The engine is in primary couple balance because the front and rear trio of cylinders are mirror images, and the pistons move in pairs (but of course, 360° out of phase and on different strokes of the 4-stroke cycle). That is, piston #1 mirrors #6, #2 mirrors #5, and #3 mirrors #4, largely eliminating the polar rocking motion that would otherwise result. Secondary imbalance is avoided because the crankshaft has six crank throws arranged in three planes offset at 120°. The result is that the secondary forces that are caused by differences from purely sinusoidal motion sum to zero.
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 — for example, 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 straight-six popular in some European sports-luxury cars, where smooth high-speed performance is very desirable. As engine reciprocating forces increase with the cube of piston bore, straight-six is a preferred configuration for large truck engines.
The bank of cylinders may be oriented at any angle, and where the bank is inclined to the vertical, the engine is sometimes called a slant-six. The straight-six layout is the simplest engine layout that possesses both primary and secondary mechanical engine balance, resulting in much less vibration than engines with fewer cylinders.
Because it is a
fully balanced configuration, the straight-six can be scaled up to very large
sizes for heavy truck, industrial and marine use, such as the 16 L
(980 cu in) Volvo diesel engine and the 15 L Cummins ISX used in
heavy vehicles. The largest are used to power ships, and use fuel oil. The
straight-six can also be viewed as a scalable modular component of larger
motors which stack several straight-sixes together, e.g. flat- or V-12s, W-18s, etc.
Modern Trends
Straight-six engines were introduced much earlier than V6 engines. While the first straight-six was manufactured in 1903 by Spyker, it was not until 1950 that a production V6 was introduced. V6s (unlike crossplane V8 engines) had intrinsic vibration problems not present in the straight-six.
The length of the straight-six 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 (left-to-right versus front-to-back) configurations in smaller cars made the length of the V6 (one half the length of an L6 with the same bore size, plus the width of one rod) a major advantage. As a result, in recent decades automobile manufacturers have replaced most of their straight-six engines (and many of their V8s) with V6 engines; Nissan have replaced their earlier turbocharged inline-6 models with larger-displacement naturally aspirated V6 engines while maintaining the FR setup and Mercedes-Benz now uses V6 engines, despite also retaining the FR layout. Increasingly straight-six engines are also being replaced by turbocharged in-line four cylinder engines (e.g. in the current BMW range), which offer comparable top-end power output and reduced low-end torque but with better fuel efficiency, due to smaller displacements and lower friction from the reduced number of cylinders. The poor secondary harmonic balance of four-cylinder engines is largely addressed with the use of balance shafts although it can never match the in-line six.
Cars
Exceptions to the
shift to V engines include BMW, which specializes in high-performance
straight-sixes used in a lineup of front-engine/rear-wheel-drive vehicles, Volvo,
which designed a compact straight-six engine/transmission package to fit
transversely in its larger cars, and the Australian Ford Falcon, which still
uses a straight-six configuration. TVR used a straight-six configuration
exclusively in their final cars before their demise.
Trucks
Straight-sixes
continue to be used in medium to large trucks. Ford is one notable exception
using a V8 in medium duty trucks. Ram pickup trucks still option the
straight-six(diesel only) Ram pickups had petrol slant sixes until 1988 when
they were replaced by the 3.9 LA V6. GM pickup trucks abandoned the
straight-six in 1984 for the 4.3 V6, Ford dropped the straight-six in favour of
the Essex V6 in 1996. In 2002, General Motors introduced the Vortec 4200 as
part of the modular straight-four, straight-five and straight-six GM Atlas
engine line. It was used in their small sport utility vehicles. Jeep abandoned the straight-six in 2006 with
the 2006 Jeep Wrangler being the last vehicle.
Balance and Smoothness
An inline six engine is in perfect primary and secondary mechanical balance, without the use of a balance shaft. The engine is in primary couple balance because the front and rear trio of cylinders are mirror images, and the pistons move in pairs (but of course, 360° out of phase and on different strokes of the 4-stroke cycle). That is, piston #1 mirrors #6, #2 mirrors #5, and #3 mirrors #4, largely eliminating the polar rocking motion that would otherwise result. Secondary imbalance is avoided because the crankshaft has six crank throws arranged in three planes offset at 120°. The result is that the secondary forces that are caused by differences from purely sinusoidal motion sum to zero.
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 — for example, 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 straight-six popular in some European sports-luxury cars, where smooth high-speed performance is very desirable. As engine reciprocating forces increase with the cube of piston bore, straight-six is a preferred configuration for large truck engines.
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