Flat-Plane Crankshaft Engines...a discussion!

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cblink.007

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For those who follow automotive developments, the talk of the town is the imminent unveiling of the 2023 Corvette Z06. What is most noteworthy is its Ferrari-esque high-pitched scream, as heard in the clip aired a week and a half back:


Certainly not the classic brute-force grumble typical of the traditional Chevrolet V8. This new sound was initially heard in the C8.R GT racer when it was spotted testing years ago, and when formally unveiled, Chevrolet told of the racer's powerplant- a naturally-aspirated 5.5L double overhead cam V8...but with a flat-plane crankshaft. Given that the current production C8 (to include the Z06 performance package) features the "pushrod" LT-2 engine, the subject of much speculation was the model of Corvette in which the engine would later be fitted, as IMSA & FIA regulations specifically state that the race cars not only have production-representative bodywork, but production-representative powerplants as well. The automotive world got its answer with the Corvette Z06, which will be seen on the showroom floor come 2023.

Don Sherman recently published a great piece on the SAE website about this new engine (attached below), and I thought you all might like to chime in about the unique characteristics of the FPC engine ( @wsimpso1 , you are the ultimate vibrations guru here; I would love to hear your take!) , and quite possibly, the role of FPC engines in the aviation world. Not 100% savvy on any that are currently out there flying, but would love to know!

GM puts its new Z06 Corvette V8 on a different plane
2021-08-03 DON SHERMAN
GM Propulsion hasn’t forgotten how to engineer compelling ICEs, and is readying an all-new, high-performance V8 with a race-proven flat-plane crankshaft.
Cadillac didn’t invent the V8 engine, but GM’s premium brand deserves credit for advancing that prime mover’s cause over the past century. One of the most significant strides came in 1923 in response to GM Research Laboratories head Charles ‘Boss’ Kettering’s challenge to his engineers: Rid Cadillac V8s of their inherent roughness. The result was Cadillac’s 90-hp Type V-63 V8 built with crankshaft connecting-rod journals spaced at 90 degrees instead of the more-conventional 180-degree intervals.

Although every V8 maker quickly followed suit to achieve similar gains in smoothness, that hasn’t discouraged a few OEMs – including GM – from reconsidering 180-degree cranks. The recently announced 2023 Chevrolet Corvette Z06 is the newest wave in this technological tide, with a teaser audio clip already available.

According to a highly-placed GM Propulsion source, the company’s V8 past is back. An all-new DOHC V8 earmarked for the upcoming Corvette Z06 will be equipped with a flat-plane crank to assure that the intake and exhaust events in each cylinder bank are spread 180 degrees apart. Expectations are that GM will shorten the current 6.2-L LT2 V8’s stroke to create a more powerful – and more exotically vocal – 5.5-L V8, internally coded LT6.

What’s old is new again
In 1921, GM mathematician Roland Hutchinson designed what he called a ‘quartered’ crankshaft at the company’s Dayton, Ohio, research lab. All production V8s at the time used flat, single-plane cranks that were the logical extension of four-cylinder practice. These cranks had four throws – each carrying a pair of connecting rods – lying in one plane. Such designs minimized both material and machining requirements.

Hutchinson was inspired by Archibald Sharp’s 1907 ‘Balancing of Engines’ reference book and subsequent technical papers. To visualize the alternative 90-degree – or cross-plane – design, imagine a compass attached to the end of the crankshaft. A flat-plane crank has two throws at the north position and two aimed south. The cross-plane design has one throw every 90 degrees at north, east, south, and west positions. Both designs attach two connecting rods to each throw and their blocks have 90 degrees between cylinder banks.

The ‘rough period’ Kettering called out was vibration at a frequency equivalent to twice crankshaft speed – second-order vibration. Despite Cadillac’s practice of carefully weighing and balancing pistons and connecting rods and counterweighting its crankshafts, the shakes common to four-cylinder engines persisted when the cylinder count was doubled in a flat-plane-crank V8. The only difference was that the vibration shifted from the vertical to the horizontal plane.

Hutchinson’s ‘quartered’ crank realigned unbalanced piston motions in such a way that shaking forces were completely cancelled. As a result, cross-plane-crank V8s have enjoyed broad acclaim over the last century of use.

Flat plane benefits
What’s inspiring some automakers to reconsider use of flat-plane-cranks are the compelling advantages exclusive to such designs. They’re notably lighter and have less rotational inertia. Surrounding crankcases can be lighter and smaller. Racing-engine designers appreciate every cubic centimeter of volume saved, every gram of weight trimmed, and a flat-plane crank’s inherent ability to spin more rapidly with each blip of the throttle. Ford’s 5.2-L DOHC flat-plane-crank V8 that powered the 2016-2020 Ford Shelby GT350 Mustang was known both for its willingness to rev – redline 8,250 rpm – and its captivating exhaust snarl.



Examples of contemporary cross-plane (GM LT4 on left) and flat-plane (Ford 5.2L) V8 crankshafts showing arrangements of crank throws. (GM; Ford)

Road-car engineers, especially those at Ferrari, relish transferring these technical advancements from the track to the street. Before turbocharging and hybridization were adopted at Italy’s renowned sportscar maker, their smallish V8s employed flat-plane crankshafts to produce both exemplary horsepower and a rousing exhaust shriek. With their modest displacements, second-order shaking was less an issue.

A Ferrari V8 aural “soundtrack” is more exhilarating than those of American V8s because their smaller piston displacement allows them to rev faster to higher rpm. Also, the exhaust pulses leaving each bank of cylinders are evenly spaced.

That last attribute is key to optimizing volumetric efficiency and maximizing horsepower. In the cross-plane crankshaft 6.2-L V8 powering the 2022 Corvette Stingray, the firing order is compromised with the third cylinder in the left bank firing immediately after the first cylinder in that bank. That means there is only 90 degrees of crank rotation instead of a Ferrari V8’s 180 degrees between those two exhaust pulses. This diminishes the scavenging benefit resulting from the flow exiting one cylinder helping to draw exhaust out of the next. It’s also easier to optimize intake airflow when each cylinder bank draws a breath every 180 degrees of crank rotation.

Flat-plane LT6 only the start
With dual overhead cams, four valves per cylinder and variable intake and exhaust timing, SAE-rated output for the upcoming flat-plane-crank LT6 should top 600 hp (447 kW) at an estimated 8000 rpm and 450 lb-ft (610 Nm) at 6000 rpm. Experience gained racing the Corvette C8R powered by a flat-crank 5.5-L DOHC V8 for two seasons in the IMSA WeatherTech SportsCar series has expedited development, confirms a senior engineering executive at GM Propulsion who was not permitted to speak on the record.

There’s more. Informed speculators insist there’s an LT7 engine next in line to power the 2024 or 2025 Corvette ZR-1. This twin-turbocharged variant of the flat-crank 5.5-L V8 should up the ante to at least 850 hp (634 kW) and boost torque to around 800 lb-ft (1085 Nm). And following Ferrari’s astute lead toward hybridization, two Corvette hybrids also are on the way. Second-order vibration be damned.

Original Article: GM puts upcoming Z06 Corvette V8 on a different plane
 

Toobuilder

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Flat plane cranks have been in Drag Racing V-8's for decades. Plenty of info out there
 

TFF

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Flat plane cranks are not that special. Ferrari’s to me were special because it was like production hot rods. Now they make too many and I have lost my chance to ever get a real one. People have gone crazy. I think Corvette and Mustang have done it because computers are so good, they can fool the engine into being docile. I also think we are seeing the last hurrah. Once this rear engine Vette runs its course, there won’t be any pure ones. No matter how fast an electric is, it’s not the same. If it was purely about speed , my Triumph Spitfire and Alfa’s would not be entertaining to me; any new Honda Accord will smoke them, but yet not in the same league of cool.
 

jac

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The Ford Flatplane crank shown above has an awful lot of counterweight compared to its crossplane cousin. One engine I always thought might appeal to the aviation community was/is the Chrysler/Dodge all alloy V8 that had
twin plugs per cyl, guess it has enough other faults that put it in the too hard basket. Stuff like valve seat inserts dropping out are not exactly cool!
 

Pops

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Never cared for the Corvette's until now with the mid engine. But just to pricey for me. Rather take that amount of money and buy land.
 

cblink.007

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Cool as these may be, the present GM LS V-8 engines are fine for powering aircraft where you aren't screaming them for high hp anyway. Direct injection and VVT are of limited use here too so the Gen V LTs are even less desirable than the LS in my view.
True. I think that even of this engine became available for purchase today, I highly doubt anyone would do an aircraft conversion on it. Fascinating article either way. I am far from a vibrations engineer; I am still in awe watching an O-360 startup and move around the way it does without a cowling around it. However, I know the Rotax 582 is pretty notorious for its vibration/rocking coupling...and that is a flat plane crank as well if I recall.

Also, do you know if anyone has ever tried magnetorheological mounts on an aircraft engine?
 

PMD

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Flat plane cranks go back well before 90 degree cranks but they certainly DO have a second order balance issue. I once thought of a billet flat plane to add a few cubes and the right sound to my 215 BOP with 300 heads (meant for car, not plane), but the crank maker said to stay the heck away from 7,000 RPM - thus maybe why many such engines seem to be redlined at 6,500? The RV8 based engine used by TVR with flat plane also has no fewer than TWO balance shafts - hanging a real anchor around an otherwise delightfully light engine. Curious to see what GM has done to address such issues.
 

wsimpso1

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Flat plane cranks go back well before 90 degree cranks but they certainly DO have a second order balance issue. I once thought of a billet flat plane to add a few cubes and the right sound to my 215 BOP with 300 heads (meant for car, not plane), but the crank maker said to stay the heck away from 7,000 RPM - thus maybe why many such engines seem to be redlined at 6,500? The RV8 based engine used by TVR with flat plane also has no fewer than TWO balance shafts - hanging a real anchor around an otherwise delightfully light engine. Curious to see what GM has done to address such issues.
I only have one thing to say about rev limits - resonance determines how high you can rev and keep the engine together. If it was designed with crank and accessory resonance when you exceed 6500 rpm, you will blow it up if you run faster ... Engines intended for production cars and trucks are designed to put 2x firing (8x rotation in a V8) at redline safely below resonance, and not much more. They weigh more and cost more when you put in more stiffness than you need, so the design guys set a redline and run to it, even while turbocharging. Want more revs? You might have to beef some stuff up.

This where the beauty of boost comes in. Instead of spinning the engine faster to move more air (to increase power), you just pump in more air within your existing rev limits. Resonance is kept at bay. Then you find out how much torque (boost) the bottom end can stand.

Those balance shafts serve the purpose of making the engine run with more apparent smoothness for the customer. All the vibe action is still there, but balance shafts prevent the engine from transmitting it through the mounts...
 

jac

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One thought, most if not all the production steel forged Small Block Chev crank forgings start off as single plane shape before at a point in the forging process they twist the still red hot forging 1/4 of a turn to place the two ends in the right position for cross plane use.
 

Pops

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I only have one thing to say about rev limits - resonance determines how high you can rev and keep the engine together. If it was designed with crank and accessory resonance when you exceed 6500 rpm, you will blow it up if you run faster ... Engines intended for production cars and trucks are designed to put 2x firing (8x rotation in a V8) at redline safely below resonance, and not much more. They weigh more and cost more when you put in more stiffness than you need, so the design guys set a redline and run to it, even while turbocharging. Want more revs? You might have to beef some stuff up.

This where the beauty of boost comes in. Instead of spinning the engine faster to move more air (to increase power), you just pump in more air within your existing rev limits. Resonance is kept at bay. Then you find out how much torque (boost) the bottom end can stand.

Those balance shafts serve the purpose of making the engine run with more apparent smoothness for the customer. All the vibe action is still there, but balance shafts prevent the engine from transmitting it through the mounts...
Back when I was drag racing a long time ago, I was shifting at 7500 rpm. Running at the national record in my class at the time. Engine built by race engine company that built Indy cars and engines. Loved the competition.
 

cblink.007

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Back when I was drag racing a long time ago, I was shifting at 7500 rpm. Running at the national record in my class at the time. Engine built by race engine company that built Indy cars and engines. Loved the competition.
I remember reading something years ago of 500cid Top Fuel engines hitting 10,000rpm at the top end of a 330mph quarter mile run...but those nitro-fed beasts are something else. Supercharger airflows so extreme (with that crazy 13-71 ratio) that it requires nearly 500bph to turn at full power, fuel flows so extreme that they are near hydraulic lock before TDC, sparks plugs burning out before the 1/8-mile point...meaning the engines are literally dieseling after that, etc. Awesome mayhem going on in other high end motorsport powerplants as well...

My hat is off to every powerplant/drivetrain engineer out there!
 

PMD

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One thought, most if not all the production steel forged Small Block Chev crank forgings start off as single plane shape before at a point in the forging process they twist the still red hot forging 1/4 of a turn to place the two ends in the right position for cross plane use.
I watched forged V8 crank blanks being made at Welland Forge about 10 years ago when we were installing some other equipment nearby. The steelworker would grab the billet with a long pair of tongs and throw it under the press - that hammered down 3 different dies at once. IIRC, 2 hits in the first die, one in the second and the third is the one that twisted the blank by 90 degrees. Each time the forge would strike the billet would bounce and the operator would flip the blank 180 degrees and land precisely in the second die and once more to the third and then pull the raw forging out on the fourth hit/bounce. The level of skill required was awesome to say the least.
 
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