Another possible reason for a flat crank in a V-8 in racing cars is to reduce rotating inertia. A cross plane crank needs counterweights to cancel the piston’s inertia but with a flat crank the pistons balance each other and the crank can be made without counterweights. Not only does this minimize rotational inertia but the crankshaft is a significant percentage of the weight of most engines, and it is important for aircraft engines to be light.
Here’s the crank of a Curtis OX-5 V-8 aircraft engine.
The flywheel is to absorb the rotary motions caused by the firing pulses to smooth the power delivery. Counterweights are for internal balance to offset the weight of the connecting rods and pistons swinging side to side and up and down.
That huge propeller was enough flywheel for that engine.
On formula one engines, the crankshaft is the flywheel, and with the overlapping power strokes of 8 cylinder engines, any additional flywheel is simply more mass that the engine has to accelerate along with the car. Minimizing rotational inertia is one good reason for the flat crank.
A lot of four cylinder motorcycle engines use a four into two into one exhaust system. cylinders 1 and 4 into one collector and cylinders 2 and 3 into the other collector, then the output of those collectors into a single pipe.
My old Kawasaki ZRX1200R had such a system even though it looked like a four into one externally. This is also known as a 360 degree header, 1 and 4 is a 360 degree pair and 2 and 3 is another 360 degree pair.
In what appears to be a four into one collector, there is a divider plate between the 1 and 4 collectors and the 2 and 3 collectors which goes on for a few inches before it becomes a single pipe.
This is a 360 degree 4 into 2 into 1 header for a motorcycle.
This is a 180 degree 4 into one header, if you follow the 1-2-4-3 firing order of a four cylinder, you will see that the exhaust pulses go in a circle into the collector.
It is impossible to use either system on a cross plane crank V-8 without crossing over left and right bank cylinder exhaust tubes, although in a rear engine formula 1 style car, I suppose you could have reverse flow heads and have the exhaust system on top of the engine where the intake normally is.
In the car world that is known as a Tri-Y header. On the header you’ve shown, the first 2 pipes are 2x resonance pulse length and the length between the first and second collector is 1x. It makes a bunch of nice pulse resonances to build torque.
I had a set of 2x/1x tri-y headers on a Chevy race car I owned. I Y’d each bank together another 1x downstream. The total length was about 48 inches to the final Y. That combination was good for 50 Ft-lbs on a 350 CID engine over shorty (1x) headers Y’d together about 48 inches downstream.
Interesting photo. Pretty clear why that one would be called a “flat plane” crank. It lays flat as a pancake! This discussion is starting to make some sense to my limited-performance noggin’ now
I’m looking at the photo and can’t figure out how the firing order for that crank could be “12347856”. I mean if all the rod journals are all in the same plane, and the cylinders are 1-4 on one side and 5-7 on the other. Or are the cylinders numbered with the odds are on one side, and the evens on the other side?
The odd cylinders are on the left side and the even cylinders are on the right side as the engine sits in the airplane. Cylinders 1 and 2 are the ones closest to the pilot.
The nameplate shown in the first picture I posted shows the cylinder numbering as well as the firing order.
Each bank of four cylinders had the classic four cylinder 1-2-4-3 firing order except on the left bank they are numbered 1-3-5-7 and on the right bank they are numbered 2-4-6-8. The left bank pistons are at TDC 90 degrees before the corresponding right bank pistons and the right bank follows the left bank firing order 90 degrees later.
This results in a regular 90 degree firing interval just like the conventional cross plane crank V-8 engines have. Four power strokes per revolution, one every quarter turn of the crank.
Thanks, I can see how it works now that I understand the cylinders are numbered odd/even between the two banks. Each exhaust bank gets a puff of exhaust every 180 degrees, right?
Still don’t quite understand why this configuration would result in more vibrations though. It seems like it would run pretty smoothly, b/c the power stroke goes right/left/right/left down the crankshaft to the middle, then starting at the other end, the same right/left … sequence all the way back to the middle. Or does that jump from the middle to the ends cause a vibration?
The vibration comes from connecting rod geometry. Imagine a five foot ladder vertical against the wall, that ladder reaches 5 feet up the wall, now pull the base of the ladder three feet away from the wall so the ladder is not vertical, now the ladder only reaches 4 feet up the wall.
On a inline four, all the connecting rods are vertical when the pistons are at top and bottom dead center. The average distance of the pistons from the crank centerline is one connecting rod length. But when the crank is in the 90-270 degree position, all four connecting rods are at an angle like that ladder pulled away from the wall, and now the average distance of the pistons to the crank centerline is less than a connecting rod length. It’s as if the pistons moved up and down that small distance in unison at twice engine speed because the rods are all vertical twice per revolution and are all at maximum lean twice per revolution. This is why flat crank engines vibrate.
One a cross plane crank, the connecting rods in the center cylinders are vertical while the connecting rods in the outer cylinders are kicked out and vice versa, cancelling each other out. You never have all four connecting rods vertical at the same time and the average distance of the four pistons to the crank centerline is nearly constant.
