Editing Straight-six engine (section)

== Characteristics ==
In terms of packaging, straight-six engines are almost always narrower than a [[V6 engine]] or [[V8 engine]], but longer than [[straight-four engine]]s, V6s, and most V8s.<ref>{{cite web |title=V6 Vs Straight-Six: The Pros And Cons |url= https://www.carthrottle.com/post/v6-vs-straight-six-the-pros-and-cons/ |website=Car Throttle |access-date=12 September 2022}}</ref><ref>{{cite web |title=Everything you need to know about inline-six engines |url= https://www.whichcar.com.au/car-advice/inline-6-versus-v6-why-are-straight-sixes-making-a-comeback |website=WhichCar |access-date=12 September 2022}}</ref><ref>{{cite web |title=The 9 best straight-six engines |url= https://www.hagerty.com/media/archived/best-inline-six-engines/ |work=Hagerty |access-date=12 September 2022 |date=2 January 2018}}</ref> Compared to V-configuration engines with similar power and displacement, the straight configuration has fewer injectors, a single head, and a single exhaust manifold, all contributing to better reliability and performance.

Straight-six engines are typically produced in [[engine displacement|displacements]] ranging from {{convert|2|to|4|L|cuin|0|abbr=on}},<ref name="Cheetham">{{cite book|last=Cheetham|first=Craig|title=Ultimate American Cars|publisher=MotorBooks/MBI|year= 2006|url= https://books.google.com/books?id=y6DUvUpGmn0C&q=largest+six-cylinder+engine+automobile&pg=PA209 |isbn=978-0-7603-2570-4|access-date= 2012-10-11}}</ref>{{refpage|p209}} however engines range in size from the {{convert|0.7|L|cuin|0|abbr=on}} [[Benelli 750 Sei]] motorcycle engine<ref>{{cite web|last=Siegal |first=Margie |title=The Benelli Sei 750 |publisher=Motorcycle Classics |url= https://www.motorcycleclassics.com/classic-italian-motorcycles/benelli-sei-750/?PageId=1#axzz3MI6weXK5 |date=11 July 2007 |access-date=26 May 2024}}</ref> to the {{convert|10972.2|L|cuin|0|abbr=on}} [[Wärtsilä-Sulzer RTA96-C]] two-stroke marine diesel engine.  Due to its well-balanced configuration, the straight-six can be scaled up to substantial sizes for heavy trucks, locomotives, industrial and marine use.<ref name="Nunney"/>{{refpage|p4}}. When two straight-six engines are mated with a common crankshaft it forms a [[V12 engine]].

=== Engine balance and vibration ===
[[File:Inline 6 Cylinder with firing order 1-5-3-6-2-4.gif|thumb|Typical firing order of 1-5-3-6-2-4]]
[[File:marinestraight6.jpg|thumb|upright|right|3890-litre ''MAN B&W 6S60MC'' marine diesel engine]]

If an appropriate [[firing order]] is used, a straight-six engine has perfect primary and secondary [[engine balance]]. The primary balance is due to the front and rear trio of cylinders moving in pairs (albeit 360° out of phase), thus canceling out the rocking motion present in a [[straight-three engine]]. The secondary balance is due to the [[crank throw]]s being arranged in three planes offset at 120°, resulting in the non-sinusoidal forces summing to zero for all free forces until the sixth order.

The engine balance characteristics of a straight-six engine compare favorably with the more common [[straight-four engine]]s, [[V6 engine]]s, and [[V8 engine]]s which experience significant secondary dynamic imbalance, resulting in engine vibration.<ref name="Nunney">{{cite book|last=Nunney|first=M J|title=Light and Heavy Vehicle Technology |edition=Fourth |publisher=Butterworth-Heinemann |year=2006 |isbn=0-7506-8037-7}}</ref>{{refpage|pp12-13}} As engine reciprocating forces increase with the cube of piston bore, the straight-six is a preferred configuration for large truck and industrial engines.<ref name="Nunney"/>{{refpage|pp40-47}}

===Two-stroke engines ===
An even-firing six-cylinder [[two-stroke]] engine requires ignitions at 60° intervals, or else it would run with simultaneous ignitions and be no smoother than a [[straight three engine|triple]] in power delivery. As such, it also requires crank throws at 60°{{Snd}}. Such designs appear to have been limited to diesel engines such as the Detroit [[Detroit Diesel Series 71|71 series]], marine engines, and outboard motors.

Some of the 120 possible crankshaft configurations have useful properties. Still, all of them have a rocking imbalance that may or may not require a balance shaft, depending on the application. The six pistons with six unique phases cannot be "paired" as in the four-stroke case. The Detroit engines used a configuration that, once the primary [[rocking couple]] was balanced out, was also perfectly balanced at all other rocking couples until the 6th order.<ref>{{cite web|url= http://dieselpro.com/detroit-diesel-crankshafts/671-crankshafts/detroit-diesel-r-5107033-crankshaft-671-standard-rebuilt.html#.Vntg9epsghU |title=Crankshaft For Detroit Diesel 6-71{{Snd}} DP R5 107033{{Snd}} |website=dieselpro.com}}</ref> [[Mercury Marine|Mercury]] came to use a configuration that canceled only the primary rocking couple and was run without a balancer.<ref>{{cite web|url= http://forums.iboats.com/forum/engine-repair-and-maintenance/mercury-mariner-outboards/394245-reed-valves|title=Reed valves Page: 1{{Snd}} iboats Boating Forums{{Snd}} 394245 |website=forums.iboats.com}}</ref>

The reciprocating masses of all configurations are still imbalanced at only 6th-order and up in their plane of motion. Still, the balance of kinetic energy exchange between pistons has improved to a residual 6th-order-and-up inertial torque oscillation compared with the four-stroke design being imbalanced at 3rd-order and up.

=== Crankshaft ===
[[File:I6crankandblock.jpg|thumb|Crankshaft with four main bearings]]
[[Crankshaft]]s for straight-six engines usually have either four [[main bearing]]s (i.e., a bearing in between each pair of [[crankpin]]s and one at each end) or seven main bearings (i.e., a bearing between every crankpin):
* Large displacement and diesel engines typically use seven bearings to minimize crankshaft flex. When an engine is subject to high loads at low speeds, the greater distance between main bearings causes increased crankshaft flex. Modern high-compression engines subject the crankshaft to greater bending loads from higher peak gas pressures, requiring the crank throws to have more significant support from adjacent bearings. So, it is common for straight-six engines to use seven main bearings.<ref name="Nunney"/>{{refpage|p38}}
* Smaller and high-performance engines{{when|date=September 2022}} typically use four bearings since having fewer main journals decreases friction and increases the torsional stiffness of the crankshaft. The lack of torsional stiffness can make the seven main bearing designs susceptible to torsional flex and potential breakage at high engine speeds. Any torsional flex in the crankshaft is compounded by the torsional flex of the [[camshaft]]s for the rear cylinders since the camshafts are rather long and subject to torsional flex. At high engine speeds, the combination of camshaft and crankshaft flex results in inaccurate timing of the valve openings, which, in the worst case, can cause the valves and pistons to collide with catastrophic results.