Video: Why Cross-Plane Crankshafts Are Rarely Used In Four-Cylinders

One of the most prolific engines globally is the venerable inline-four-cylinder engine. This engine is so common you can find them on just about every street, anywhere you are in the world. What isn’t common is finding four-cylinder engines with cross-plane crankshafts.

With all the hoopla over flat-plane crankshafts in V8 engines lately, what is interesting about the inline-four engine configuration is that more than 99-percent of them share not only four cylinders in a straight line but that the crankshafts are almost always of a flat-plane design, unlike the majority of V8 engines.

The YouTube channel Driving 4 Answers explains that there are only a few exceptions to the flat-plane inline-four crankshaft because the irregular firing order of the cross-plane creates more disadvantages than benefits in most cases. We say MOST cases because there are times when the cross-plane design is worth the extra effort.

Take, for example, the Yamaha R1 inline four-cylinder engine. This engine is used in the company’s 1,000cc sportbike motorcycle and is also heavily used in motorcycle racing.

Every single flat-plane inline-four engine has the same crankshaft, explains driving4answers:

“The key feature [of a flat-plane crankshaft] is that two crankpins point up, and two crankpins point down. In other words, two crankpins are 180-degrees apart from the other two crankpins.

But Yamaha took a radical step away from this convention, which is more than a century old, in 2009 when it introduced its production model R1 (note: it had already made versions of the cross-plane in racing models).

Instead of having the crankpins separated 180-degrees from each other, all the crankpins point in their own direction. In this configuration, crankpin 1 points up, and crankpin 2 is 90 degrees away from crankpin 1. In other words, one points up, one left, one right, and one down.

One of the questions we had about the Driving 4 Answers video was why cross-plane four-cylinder crankshafts are not used in cars? The reason is that it would cost too much to produce and not offer enough advantage in real-world use. Still, we have to give props to Yamaha for even attempting this since it was such an engineering challenge for so little gain. The cross-plane creates a “rocking couple,” which means that the two ends fire 180-degrees apart. This makes the one end push up while the other is pushing down, and they must be balanced out to reduce vibration.

httpv://youtu.be/P2sn8TWm60M

But if you break down the details of why it works for motorcycles – especially in a track/racing environment where the bike is being pushed to 10/10ths – it makes more sense. Plus, it just sounds cool. The engine note sounds more like a V4 than an inline engine. And when you get up into the upper RPM ranges, the combustion impulses are far more manageable than on a flat-plane crank.

Why do combustion impulses even matter? Again, Driving 4 Answers explains how the combustion impulses (or bangs) create torque and a little bit of traction control coming out of the corner. Riders can get better throttle response out of the corner and get to full throttle sooner than they would on a flat-plane engine design. Even the legendary MotoGP champion Valentino Rossi concurs that the cross-plane bike was one of his favorites because of the rideability.

While the cross-plane is unique for inline-fours, it is widely used in V8s due to the balanced firing it offers, which is just the opposite for four-cylinder engines. Of course, there’s more to it than this, but the idea gets the imagination going.