Engines are complex pieces of machinery that are built entirely on a “best decision for the supposed use” basis. Notably, one such choice is the design of the engine’s crankshafts, or more precisely, the crankshaft’s weights.
A flat-plane crank engine is basically an engine where the crankshaft’s counterweights are alternating each other in a 180-degree angle. Further, the crank has multiple journal bearings: ones where the connecting rod sits, others that sit on the engine’s block. The counterweights are set between the connecting rod bearings, on each side of said bearing. These counterweights will alternate at a certain angle from each other based on the engineering and the engine’s purpose.
Why does this angular alternation even matter? Well, a crankshaft obviously spins. That’s the component that links to the transmission, and as such the wheels. If the crankshaft isn’t balanced, that is, it’s center of mass doesn’t sit on the axis of rotation, the crankshaft will wobble.
Specifically, wobbling means vibration. If you feel the wobble by revolving such item slowly, imagine how it will feel when the shaft spins at 8000 rpm with a cluster of force. Consequently, the car will tremble like mad. Large components which will raise the center of mass quite remarkably are the connecting rod and piston. Therefore, by having those joined to the crankshafts utilizing journal bearings, if the crankshaft spins even at 500 rpm, the engine will shake severely due to it having a notably high center of mass.
Therefore, to counteract this and balance said crankshaft, engineers add counterweights to the pistons and connecting rods, which move the center of mass back down on the rotation axis. To illustrate, is it easier for you to walk with weights on your shoulders, on your feet, or around your waist? Probably the waist as it’s easier for you to keep balanced.
This alternation matters. If they are alternating at a 180-degree angle, you are contributing an agglomeration of mass vertically which must spin with the engine. Thus, you are only balancing weight vertically, when the engine starts spinning, the weight shifts sideways while spinning, vibrating horizontally.
Importantly, to counteract this, we shift weights at 90-degree increments, as an example. By doing this, the weights and forces also start overlapping horizontally on the same bank, making the entire engine vibrate less. This configuration is called a cross-plane crank.
Flat-Plane Crank: Advantages and Disadvantages
Certainly, the flat-plane crank engine has several advantages and disadvantages. For instance, the biggest advantage mechanically is the fact that the overall engine is lighter. As a result, weights can be smaller. This means that the explosion produced by the air-fuel mixture pushes a smaller weight, so there are fewer inertial losses there. (Bigger mass means bigger inertia, a bigger negative force to overcome). Consequently, this gives the engine the capability of high RPMs. Moreover, being that this engine design is light, a flat-plane crank also has an exemplary throttle response. Lastly, the engines are more cost-effective, both to manufacture and design.
The design allows the engineers to produce a stronger and muscular engine while keeping the overall size down, due to material usage. Another big advantage appears when it’s used in V-style engines. Consequently, the design allows engineers to pick a very specific firing order. You could call it consecutive firing, considering it makes a great deal of sense. It fires a piston on the left bank, followed by the symmetrical piston on the right bank.
This allows the engine to push the crankshaft continuously with each firing stroke, allowing the engine to reach high RPMs fast. Moreover, it permits the engineers to develop a simple exhaust manifold with ample flow for each bank. Also, this manifold dictates a very distinct exhaust sound, with a sharp note and sounding very much like a “racing engine.” On the other hand, the engine vibrates quite a bit, especially at high engine volumes and RPMs.
Flat-plane crank engines are rather niche and specific, since as far as V engines go, an overwhelming majority of manufacturers go for cross-plane due to quietness and smoothness. Manufacturers use flat-plane crank engines on sports cars, race cars, and surprisingly on cheap four-cylinder engines.
For the four-cylinder engines, this layout is preferred since they are cheap to design, produce, and manufacture. On the other side, when talking sports cars and race cars, chiefly European manufacturers produce them.
American manufacturers prefer smooth and powerful V6s, V8s, and V10s. Conversely, Europeans prize fast screamer cars with V8s and larger. Ferrari, McLaren, Lotus, and Koenigsegg are some that come to mind. For example, think of the new Koenigsegg Jesko, which has a 12.5 kg crankshaft with a redline of 8500 RPM. The McLaren P1 is also a worthwhile example as is the Ferrari F430.
Certainly, European manufacturers are not the only ones who have delved into the world of flat-plane crank engines. For instance, the Ford Mustang Shelby GT350 utilizes a flat-plane crank on their 5.2L V8 “Voodoo” engine, which outputs 526 horsepower and has a redline of 8250 RPM. In addition, the new Chevrolet Corvette ZR1 will reportedly have a 5.5-liter V8 with a flat-plane crank. With twin turbos included, the ZR1 should top 800 horsepower.