Selection of number of cylinder | Engine design
After the overall engine displacement and compression ratio has been calculated, it is important to choose the number of cylinders into which the displacement is split. For any particular engine, expense and complexity definitely direct the engineer to the lowest feasible number of cylinders. The number of cylinders is directly multiplied by the amount of several components in the engine, so the fewer cylinders, the lower the cost of materials and assembly. Supercharging an engine would use less displacement and fewer cylinders on a cost-per-unit capacity basis, which decreases the cost of the base engine.
The addition of a compressor, intercooler, and extra intake plumbing to facilitate supercharging, however, increases the cost of the complete assembly of the engine. It is less costly to continue adding more cylinders for a given cylinder size and power output, until it tends to become cost neutral to introduce supercharging above roughly seven cylinders. In other words, building a four-cylinder, super-charged engine with a given power output can cost less than building a comparable eight-cylinder engine with the same power output.
First the engine designer must select the cylinder layout: in-line, V , W, horizontally opposed, radial, or Wankel. Packaging considerations include not only the engine’s range, width and height, and furthermore assembly specifications, such as those for intake and exhaust fittings and positions, and connections to the cooling system. For example, the need to repeat certain components in some engine specifications for cylinder heads or exhaust piping in ‘V’ engines must also be taken into consideration. The in-line engine, with a single cylinder bank provides the engine with the the easiest setup. The intake, exhaust, and cooling systems are all conveniently designed because only one bank of cylinders needs to be serviced. The engine height, however, poses packaging problems in some applications, and length can become prohibitive as the number of cylinders grows. The height is often handled by tipping the engine or mounting it sideways in a “slant” mount.
For alternate setups, the decreased height comes at the expense of greater width. The ‘V’ design allows, at the cost of thickness, a reduction in both height and volume, but now demands two cylinder banks to be assisted by cooling, intake and exhaust systems. The reduced crankshaft and connecting rod bearing area is a further theoretical drawback. Whereas the in-line engine required a main bearing to be mounted under each cylinder, between each pair of main bearings, the ‘V’ arrangement resulted in a pair of cylinders. A suitable bearing area for two rod bearings must be supplied with only marginally increased spacing between the main bearings. In most car engines, these difficulties are faced without effort, but they are very challenging to face in heavy-duty applications.
It is possible to classify the lightweight ‘W’ configuration as two ‘V’ engines located For a single crankshaft side-by-side. The ‘W’ motor also illustrates the trade-of between the width and length of the piston. Although there are now four cylindrical banks, their near proximity eliminates (but does not eliminate) the complexities of packaging for intake, exhaust, and cooling systems.
Minimum height but a very large set is given by the horizontally opposed engine. The engine length is identical to that of the ‘V’ engine and poses comparable problems in the bearing field. The greater physical gap between cylinder banks makes the support system problems larger.
Moving the design to a ‘V’ starting from an in-line engine greatly cuts the engine length around the crankshaft. A radial engine can be conceived of as just one cylinder long, with several cylinders arranged along the crankshaft to exaggerate the line of thought. This greatly lowers the engine length, but drastically complicates the packing of the connecting rod to the crankshaft. It needs a special form of articulated connecting rod. In air-cooled aircraft engines, this system was usually used, where each exhaust port had to share the same amount of cooling airflow for optimum power density.
The rotary piston engine, or Wankel engine after its maker, is another type of internal combustion engine. A triangular rotor travels around the crankshaft in this arrangement, generating volumes within its housing that are rising and diminishing. Many combustion events per rotation per working cylinder are possible, causing this engine to have a high power density. The working piston’s rotary motion creates much less vibration than a traditional reciprocating piston engine. Sadly, an inherently bad, poor human The shape of the combustion chamber and the complexity of sealing the combustion chamber add to issues with fuel consumption and exhaust emissions that have restricted the use of the Wankel engine.
