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.
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.