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motors:boxer [2018/06/28 16:12] (current)
ellenor2000 created
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 +===== Boxer Engine Whitepaper =====
 +Modern wisdom on small conveyance prime movers is that an overhead camshaft is the only workable solution for acceptable power while evading tough puissance fiscale regulations in places like Europe and Japan.
 +This wisdom has wrought havoc on the packaging of the largest displacement diagonally opposed engines; the Ford Coyote 5.0L V8 is physically wider & taller than a GM EcoTec3 6.2L V8 - an engine that still combines yesteryear'​s camshaft-in-block compactness with today'​s best flowing 2-valve cylinder head, and may also be lighter than a comparable Coyote.
 +The way this relates to small-displacement,​ 4 and 6 cylinder horizontally opposed engines is not obvious at first glance; as well as the combination of the fact that camshaft-in-block is unstable at higher engine speeds due to increased risk of valve float with the fact that a 4 cylinder horizontally opposed engine has severe imbalances at lower engine speeds, horizontally opposed engines just aren't generally used in automobiles today; Subaru (formerly Fuji Heavy Industries, but always branded Subaru) and Porsche form notable exceptions, and both use overhead camshaft designs, requiring long chains to either end of the engine to drive the valvetrains.
 +Installed transversely,​ a horizontally opposed engine with overhead camshafts is unusually long, raising the cargo floor for an unacceptable portion of the length of the vehicle, if installed in the rear, or requiring an unusually long bonnet, if installed in the front. Installed longitudinally,​ a horizontally opposed engine with overhead camshafts may exceed the maximum legal width for some types of vehicle, and creates a packaging problem few automobile manufacturers are both willing and able to overcome; for longitudinal front-engine,​ rear wheel drive, the engine is either placed ahead of the front axle, creating unacceptable handling characteristics,​ or behind it, creating a debacle for steering that, again, few manufacturers want to overcome, or over it, raising the centre of gravity unacceptably high, or restricting displacement to an unacceptably small figure if a reasonably long stroke (for low-speed performance) is to be retained.
 +The solution is simple. Using the outdated camshaft-in-block,​ pushrod-and-rocker-arm valvetrain technology that continues to be used in some, but not all, diagonally opposed 8 cylinder and some 6 cylinder engines, a reasonably high performance 6 cylinder horizontally opposed engine, practically limited in longitudinal applications to two and one half litres engine displacement,​ and in transverse applications (optimally "​forward control"​ to maximise cabin space - the rear bank of the engine is under the cabin floor, essentially mandating water-cooling) to four litres displacement,​ may be composed. The single camshaft may be driven from a gear off the crankshaft, at the required gear ratio for the cycle type (for a four stroke, two turns of the crankshaft for one of the camshaft), rather than using a chain and sprockets at that gear ratio - the lack of a chain or belt will improve refinement and durability. 4-valve cylinder heads with pushrod control have been devised; however, 2 or 3 valves, larger valve count favoring intake for the odd count, is entirely sufficient for applications the ordinary driver will place the engine in.
 +While the camshaft-in-block,​ overhead-valve layout presents a packaging advantage for transverse and longitudinal front engine layouts, as it reduces the width/​length of the engine for the same stroke length, placing a camshaft-in-block horizontally opposed engine underfloor in the rear of a vehicle, either transverse or longitudinal,​ has a drawback that the overhead-camshaft horizontally opposed engine does not have - the block is taller due to the valvetrain gear. To compensate, lubrication could be supplied from outside of the engine, rather than in the block, allowing the engine to be placed lower without reducing the vehicle'​s ground clearance, and conferring additional advantages in instances of lateral or longitudinal acceleration that the engine was not designed to cope with. However, in the situation of longitudinal mounting, the author considers a mildly raised cargo floor to be a small price to pay for an engine mounting that allows for more complex suspension at all wheels to improve comfort and handling.
  • motors/boxer.txt
  • Last modified: 2018/06/28 16:12
  • by ellenor2000