OMG Engines! - Rotation, Rotation, Rotation

The pistons push through connecting rods, acting on a "crank," or rotating lever arm that converts linear movement to rotation.	Now that we understand the processes that turn old dinosaurs into miniature explosions, there's got to be some way to link all those bouncing pistons together. Enter the rotating assembly. These chunks of metal have to take immense loads and all serve to turn fire into motion.

Four-stroke IC engines exist in a variety of configurations, from the single cylinder in that hipster's scooter to the 24-cylinder Allison engine that won the P-51D Mustang title of fastest prop plane in WWII. While the scale of these engines differ enormously, all of them operate under the same basic principles and all have a crankshaft, connecting rods, and pistons.  All of these rotating components produce considerable vibration, and much design effort is spend on minimizing this vibration to allow the engine to operate smoothly and have long service and operational life. (I will discuss these vibrational considerations in more detail in a later post.) The characteristics of an IC engine are highly dependent on the geometry of the components making up the rotating assembly. Torque output, volumetric efficency, mechanical efficiency, and other metrics are a result of the configuration. Other dimensions such as bore, combustion chamber size, and gasket thickness are used to calculate values such as compression ratio. These dimensions establish the displacement of the engine, or the total volume swept by all cylinders in the engine. For example, a Ford 302 V8 has a nominal bore of 4.00" and a stroke of 3", for a total single cylinder volume of 37.7 cubic inches and a total volume of 302 cubic inches (which amounts to 4.95L, just shy of Ford's 5.0L rating adopted in the 80's) They also directly impact the torque generated by the motor. By increasing the bore size of an engine at constant cylinder pressure, the pressure acts on a larger surface are and the force generated - and resultant torque - increases. By increasing the stroke of the engine, torque output can also be increased as the piston is acting on a longer lever arm. In gearhead terms, "there's no replacement for displacement." By increasing bore or stroke, you can increase torque output without increasing cylinder pressure, resulting in an engine that can more reliably output torque by stressing components less. Another consideration in design of engines is the stroke ratio. Most commonly defined as Bore/Stroke, the stroke ratio affects where an engine is most efficient. Holding total displacement constant, by increasing the bore and reducing the stroke, frictional losses and peak piston speed are reduced. An engine with a stroke ratio over 1:1 is known as oversquare. Because of the reduced piston speed, oversquare engines are more suited for higher revs than undersquare engines. While this property does not dictate the rev or power capabilities of an engine per se, oversquare engines are often tuned to make power at higher engine speeds. An undersquare engine is the converse: the stroke ratio is less than 1:1, increasing piston speeds and consequent friction. This tends to reduce typical operational engine speeds and encourages designers to tune the engine for power in lower speed ranges. A square engine is thus one with a bore/stroke ratio (SR) near 1:1. The W16 in the Bugatti Veyron is a square engine, with an identical bore and stroke. The Ford 289, which makes power at higher RPM to great effect, has a ratio of 2.1:1, and the 302  has a ratio of 1.33:1, making both of them oversquare. The Chrysler Slant-6 is a very undersquare engine (.82 SR,) and is known for incredible low-end torque. Many transverse mounted Honda, Mazda, and VW engines are undersquare as a smaller bore allows for a smaller overall width, aiding packaging. To illustrate my point that stroke ratio does not solely determine engine redline and torque peaks, the Honda Integra Type R engine is undersquare (.94 SR) and has an 8500 RPM redline. Rod length has an impact on both torque output and part life. A rod must be sufficiently long to accomodate the engine stroke and keep piston skirts from contacting the counterweights. Additionally, by increasing the overall rod length, you increase the torque delivered to the crank by maximizing the tangential force component imparted by the rod on the crank, and reduce the maximum piston acceleration and load. It impacts dwell time, or the amount of time the piston spends near TDC, which affects cylinder filling. Maximum rod length is also affected by packaging concerns, as the engine must fit under the hood or between the fenders. An even larger consideration is piston side-force - because the rod is a two-force member, the forces act parallel to the rod through the pin centers. this creates a lateral force on the piston skirt (the thrust face) and an overall rotational moment on the piston due to the frictional force acting about the piston center line. As a result, piston skirt length is another consideration of engine manufacturers. A shorter piston skirt increases the overall wall force required to resist this moment and also allows the piston to rock more in its bore, but a longer piston skirt increases overall friction and mass and increases possibility of contact with the crankshaft. As an example, aftermarket manufacturers offer a "stroker" kit for the Ford 302 that increases engine displacement to 347 cubic inches. They achieve this by increasing the stroke of the crankshaft, the length of the connecting rod, and decreasing the piston skirt length and compression height to make up for this decreased room. As a result of this (as well as wrist pin interference with the oil control ring groove) cylinder wear is accelerated. Some engine builders recommend a 331 stroker kit, which has slightly longer skirts and no wrist pin/oil-ring interference. These values also impact the static compression ratio of the engine. The static compression ratio is exactly what it sounds like - the ratio of the volumes at BDC and TDC. Thus, an engine with a 10:1 compression ratio has a maximum volume that is ten times the volume at TDC. While the dynamic compression ratio varies based on volumetric efficiency of the engine resulting from valve timing and port flow, a higher static ratio will result in increased thermal efficiency and power all else being equal. (See Part One of this series for more information) Because all those dimensions are terrifically fascinating, I will discuss another - mass. When turning at full speed, a rotating assembly not only has to bear the input forces acting on the piston from combustion but also increasingly high centrifugal forces. In high speed engines, this stress can become quite large, even significantly higher than piston forces. To reduce the vibration generated by slinging the piston and rod back and forth, the crankshaft has counterweights (or possibly jackshafts with additional mass, though this is less common) that rotate 180 degrees out of phase with the piston and rod. Each connecting rod and piston weighs a slightly different amount due to machining, forging and casting tolerances. An engine must be balanced, where small amounts of metal are removed from the counterweights, rods, and pistons to balance all of the opposing forces and reduce vibration. Some engines are externally balanced which rely on masses in the flywheel and harmonic balancer to cancel the mass of the assembly, and others are internally balanced, when heavy metals are used to increase the mass of the counterweights in the crank that oppose each piston. Because internally balanced engines have the counterweight and the rods closer together, the internal moments generated in the crankshaft are reduced and the engine runs more smoothly overall. To make this easier to visualize, imagine spinning (1) between your hands. This represents an internally balanced engine. Now spin (2), and it will tend to rotate back and forth as it spins - the oscillating moment your hands are absorbing will be absorbed by the crankshaft main bearings in an externally balanced engine. The overall mass of the rotating assembly is also an important consideration with regard to power. The heavier the rotating components (or more specifically, the higher their moment of inertia) the more torque it requires to physically accelerate the components, and the higher the parasitic power loss. Shaving crankshafts, running lightweight pistons, rods, flywheels, dampers - all of these not only reduce the centrifugal forces generated but reduce the amount of power required to accelerate the components and thus increase power delivered to the ground. Between the considerations mentioned above, there also exist other properties. The size and width of main and rod bearings impacts bearing stress - the same force acting through a larger bearing area reduces bearing stress and promotes bearing longevity, but can increase friction from increased shear force in the oil.  Also, a larger bearing will rotate at a faster surface speed than a smaller bearing at a given RPM, which can increase friction and increase the necessary hydrostatic film pressure to keep the moving parts separated by oil. As an example, some circle track racers run a "Clevor" motor - a Ford Windsor block with Cleveland heads, because the Windsor block has smaller crank main bearings and the Cleveland heads have better flow, resulting in better power and longer bearing life with the prolonged high-RPM operation these engines see. All of these considerations, in conjunction with bearing clearances and designed required oil viscosity are balanced in any OEM design (some better than others!) and have an impact on everything from power to fuel economy to service and product lifetime. There are books written about subjects that I have covered in single sentences, so there will be some oversimplification in this short blog post, but I just want to put out some basic info for those that are interested. I apologize for the general appearance of the images and graphs, I don't have any 3D Modeling or graphical software on my current computer. Tune in next time for a look at induction and exhaust!