Engine Configuration and Balance
Once the fuel type, engine operating cycle, total displacement, and supercharging decisions have been made for a new engine, the next tasks will be to decide upon the number of cylinders over which the displacement will be divided, and the orientation of
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Engine Configuration and Balance
6.1 Determining the Number and Layout of Cylinders Once the fuel type, engine operating cycle, total displacement, and supercharging decisions have been made for a new engine, the next tasks will be to decide upon the number of cylinders over which the displacement will be divided, and the orientation of the cylinders. The factors that must be considered include cost and complexity, reciprocating mass and required engine speed, surface-to-volume ratio, pumping losses, packaging, and the balancing of mechanical forces. The majority of this chapter will address the mechanical forces and engine balancing as this is a key factor explaining why particular numbers and orientations of cylinders are repeatedly chosen.
6.2 Determining the Number of Cylinders Having determined the total engine displacement, the number of cylinders into which that displacement is divided must be selected. Table 6.1 shows common cylinder volumes for different applications. These volumes vary depending on engine speed, duty cycle, and fuel choice. Cost and complexity certainly lead the engineer to the lowest practical number of cylinders for any given engine. The quantities of many parts in the engine are directly multiplied by the number of cylinders, so the fewer the cylinders, the lower the cost of materials and assembly. On a cost-per-unit power basis, supercharging an engine will require less displacement and fewer cylinders, which reduces base engine cost. However, adding a compressor, intercooler, and additional intake plumbing to enable supercharging increases the cost of the total engine assembly. For a given cylinder size and power output, it is less expensive to continue to add more cylinders, until it starts to become cost neutral to add © Springer Vienna 2016 K. Hoag, B. Dondlinger, Vehicular Engine Design, Powertrain, DOI 10.1007/978-3-7091-1859-7_6
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70 Table 6.1 Typical ranges for cylinder volume Engine type Small utility Motorcycle (sport) Motorcycle (cruiser) Automotive gasoline Automotive turbo-diesel Over-the-highway truck turbo-diesel
6 Engine Configuration and Balance
Cylinder volume (cc) 140–500 100–350 375–1000 350–850 475–840 840–2660
supercharging above approximately seven cylinders. In other words, it may cost less to build a four-cylinder, super-charged engine of a given power output than a comparable eight-cylinder engine of the same power output. Conflicting with the requirement to reduce cost and complexity, reciprocating mass is reduced by increasing the number of cylinders for a given engine displacement. As cylinder diameter is reduced the mass of the piston, rings, piston pin, and connecting rod diminish as well. At any instant in time the force transmitted through a connecting rod, and seen by the rod and main bearings, crankshaft, cylinder block and main bearing caps is the product of the mass of the reciprocating components and the instantaneous rate of acceleration or deceleration. Higher engine speed requirements result in greater acceleration rates, and in o
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