Gregory Hildstrom Projects Publications Resume Links Contact About Google+ Facebook Youtube Donate

Coaxial Flat-Twin (CF2/CFT) - Perfectly Balanced Boxer Engine/Pump

The page details a new vibration-free piston machine design from October 2008 applicable to engines and pumps. This page was last updated 11/25/2009.

Traditional BMW Boxer Twin Layout

BMW is famous for their horizontally-opposed boxer flat-twin (F2/FT) engines. These engines have excellent primary and secondary balance in the direction of stroke, but they suffer from rocking couple or torsional vibration on an axis normal to the axis of crankshaft rotation. It is easy to visualize this in the animation below when the pistons reach top dead center (TDC) and bottom dead center (BDC). In the overhead view just prior to TDC, the engine is decelerating the piston to a stop before reversing direction. This will have a tendency to torque the whole engine clockwise in the overhead view. The opposite happens just prior to BDC, but the torque is counterclockwise in the overhead view. This rocking couple vibration occurs at the first mode engine frequency. Many V2 engines suffer from this same imbalance because of the offset pistons, but some V2 engines use a knife-fork connecting rod arrangement to bring the pistons in line and eliminate the rocking couple. The piston offset should be clearly visible because of the separate crank pins and the center crankshaft main bearing journal space between. Some engines eliminate the center main bearing journal space to minimize pin offset, but this puts much higher tension and bending loads on the middle crank arm connecting the two pins together. This conventional two-bearing offset boxer design could be improved if the center crank arm were made into an oversize main bearing journal. The oversized center bearing journal design below has the advantages of the conventional two and three bearing designs. The additional stiffness this center bearing journal arm provides to the crankshaft might even enable the center crank arm to be made thinner to reduce the rocking couple. The oversized center bearing journal also converts many of the center crank arm loads into compression against the oversized bearing and limits the possible bending load. The lack of a center bearing certainly contributed to the broken crank in the photo below. However, a rocking couple still exists even with a very thin oversized center bearing journal.
3-bearing 4-arm boxer
2-bearing 3-arm boxer
fatigued 2-bearing 3-arm boxer crank
3-bearing 3-arm boxer with oversize center bearing journal

New Coaxial Flat-Twin (CF2/CFT) Layout

I call this design a coaxial flat-twin (CF2/CFT) because the the two opposed pistons move along the same linear axis, unlike traditional boxers. I came up with this idea in October 2008 after having a numb throttle hand thanks to the secondary imbalance of my 1994 FZR600R's I4 engine. So, how do we use a knife-fork arrangement with pistons that must be 180 degrees apart? Obviously the pistons must use separate crank pins to maintain primary balance. The key here is that the crank actually has three pins. Two outside pins for one cylinder are 180 degrees across from the other piston's crank pin just like a traditional boxer. The outside pins are for one piston's fork equivalent and the inside pin is for the other piston's knife equivalent. The crankshaft arms hold the pins away from the axis of rotation. If the loads in the crankshaft arms would be too high with only crankshaft end bearings (like some BMW boxers above), the crankshaft arms can be made circular and double as main bearing journals. In that case, the crankshaft would have four main bearing journals. These four journals can be narrow because they are so much larger diameter than conventional crankshaft bearings. The CF2 design is detailed in the animations and diagrams below. Having separate crank arms and main bearing journals would make the connecting rod fork and piston prohibitively wide. This narrow knife-fork triple-pin horizontally-opposed flat design brings the two cylinders and pistons perfectly in line with each other to completely eliminate the rocking couple. The CF2 design completely eliminates vibration from moving parts in a horizontally-opposed flat piston machine. Older BMW boxer engines did nothing to eliminate or reduce the rocking couple, but recent engines in the HP2 Sport and R 1200 families have incorporated a separate balance shaft that spins to reduce the rocking couple. However, I feel that adding a balance shaft is unnecessarily complex and not very elegant. Also, the balance shaft's sinusoidal motion cannot completely cancel the non-sinusoidal rocking couple. This balanced CF2 design is applicable to internal combustion engines using two, four, or six strokes, external combustion engines using single or double action, pneumatic engines, heat-transfer engines, hydraulic pumps, and pneumatic pumps. A motorcycle using this CF2 design would still need to overcome the drive line torsional vibrations resulting from the elapsed time between power strokes. Two-cylinder four-stroke engines have only one power stroke per revolution whereas six-cylinder four-stroke engines have three power strokes per revolution and twelve-cylinder four-stroke engines have 6 power strokes per revolution. BMW uses a torsion spring on the drive shaft, which smooths torque delivery to the rear wheel. Chain and pulley driven motorcycles typically use a set of rubber dampers mounted to the rear wheel or to the transmission output to smooth torque delivery to the rear wheel. The CF2 design could be easily extended to any multiple of two cylinders.

4-bearing 4-arm coaxial flat-twin (CF2 or CFT) or coaxial boxer

Triple-Pin Crankshaft Layout

Below is a close-up of the triple-pin crankshaft. Notice that the center pin has the same total length as the two outside pins combined. This is to equalize the rotating mass and bearing surface area. An additional advantage of the combination bearing journal crank arms is that small oil holes could be drilled in the bearing surface that connect with a hole in each crank pin to lubricate the connecting rod crank pin bearings. This way pressurized oil delivered to the main crankshaft bearings would be available at all of the bearing surfaces on the crankshaft.
4-bearing 4-arm triple-pin crankshaft