Why are there no 7, 9, or 11-cylinder engines?

"MegaSuper" (megasuper) 04/16/2015 at 03:38 • Filed to: None

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5-cylinders have shown up in a variety of sporty cars, and some are saying that 3-cylinders are going to be the new 4-cylinders of the future. Heck, even the BMW i8 has one.

So why not higher odd-numbered engines? Does the inherent imbalance of an odd-cylindered engine somehow become too much to handle at 7 and above? If so, why?

Flavien Vidal > MegaSuper 04/16/2015 at 03:50

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Complicated to balance and not really needed since we now master 6, 8 , 10 an 12 which produce WAY enough power and are a lot easier to balance.

E. Julius > MegaSuper 04/16/2015 at 03:51

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Flavien nailed it.

 V8 Rustler > MegaSuper 04/16/2015 at 03:55

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My guess is that because they're odd cylinder numbers, they'll have to be in an inline configuration, making them really long. An I6 is very long, and I think it is the limit for a passenger car.

The packaging issue could be solved with small engine bank angles, such as the VR6.

this is not matt farah's foxbodymiata > MegaSuper 04/16/2015 at 03:55

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That's a good question. I think your right in that it probably becomes unmanageably unbalanced in the type of engine you'd find in a car—inline, flat, or V.

On the other hand, radial and rotary engines are almost always built in odd-number configurations, to make an every-other-cylinder firing order possible, which makes for smooth running.

MegaSuper > this is not matt farah's foxbodymiata 04/16/2015 at 04:20

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To avoid having to make another topic: Why don't we see flat engines anymore, other than flat 4s? Why no flat 6, 8, 12?

samssun > MegaSuper 04/16/2015 at 04:44

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Check this link out: http://www.autozine.org/technical_scho…

I5s are on page 2, and can be balanced but have a rocking effect. They offer better power than a 4, package better than an I6, and don't need as many parts as a V6, so the compromise can be worth it.

Whether or not you could get close to balancing a 7 or 9, they don't offer anything you can't get in a 6 or 8. There really isn't much of an argument for a V10 (it's not as balanced or smooth as a V12, and you can build a V8 to whatever power level you want out of the V10), but you still see them because they're in pricier cars where you aren't fighting for every edge in mileage, performance, packaging, and cost.

Tareim - V8 powered > MegaSuper 04/16/2015 at 04:51

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there is still some flat 6s being produced in new subarus I believe it's the ez36

this is not matt farah's foxbodymiata > MegaSuper 04/16/2015 at 04:58

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My guess would be it's a packaging issue, but I have no idea. It seems everyone but Porsche and Subaru has given up on 'em. It's too bad, boxer engines are awesome.

desertdog5051 > MegaSuper 04/16/2015 at 05:03

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There are.

Simplify, then add beer > MegaSuper 04/16/2015 at 05:24

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Flat engines normally need more balancing.

Subaru's have flat 6s, they use them in the Outback and Liberty

Porsche had a flat 8 :

Ferrari had a Flat 12:

Porsche also briefly tested a Flat 16 in their 917 (but it never raced).

The only real exception was the Flat 10 - Chevrolet toyed with it but never brought it to fruition.

McMike > MegaSuper 04/16/2015 at 05:29

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Why no flat 6?

Subaru has been making a flat 6 for 10 years, and Porsche has been making them for over 40.

McMike > MegaSuper 04/16/2015 at 05:34

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I'm going to take a guess here.

There is no space in any modern car for a straight 7, 9, or 11 cylinder engine - so that's not going to happen.

You could make a V7, V9, or V11 engine, but it would take up as much room as a V8, V10, or V12 engine. It would be a challenge to create this new engine, so why bother when you already have an engine that fits. There is no advantage to invent these new configurations.

Cé hé sin > MegaSuper 04/16/2015 at 06:11

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I hear you want a 7? Have an Agco straight seven.

!!! UNKNOWN CONTENT TYPE !!!

You want more?

Have a Wartsila RTA 96C. They make this all the way from six to fourteen cylinders.

GhostZ > MegaSuper 04/16/2015 at 06:16

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Probably size and packaging constraints. Modern engines are often long-stroke narrow bore so that they can have the smallest, lightest pistons possible for fuel economy. A flat engine works best if you want short strokes and narrow bore, which goes against the trend.

Most constraint in an engine bay is longitudinal, so if you're going to add 2 more cylinders, might as well make it a V-shape so you can get longer strokes and reap the MPG benefit, while still keeping the bore the same.

There's also aerodynamics. A front, flat-engine motor is fairly wide, and since you can't really reduce the height of a car that much without eating into passenger area, this gives it a large frontal area, increasing the drag coefficient. Especially compared to a more common transverse V6, which only has the frontal area of an I3.

Flat engines also have good cooling characteristics because the cylinders are so far away and they have the most exposed surface area so that's why they make good rear/mid engined uses, a la Testarossa, Beetle, and 911, especially when air cooled.

So a Flat 6 or 8 Engine has the most advantage when you need good cooling, wide body, short stroke, narrow bore, and don't care about drag coefficients. This is becoming rarer and rarer today.

random001 > MegaSuper 04/16/2015 at 06:32

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It's all about crank angles. You have to either be able to divide the total angle of the crank rotation, 360 degrees, evenly by the number of cylinder you have, or be able to evenly split the pistons between two banks. This is true even if you don't want to use one lobe at every angle. It's a balance issue. You can have 5 because 360/5 = 72. You could technically have 9, as 360/9 = 40, but that would have to be inline, and it would be loooooong. Can't make a V9, either. When you get to a V9, it's just going to be so long. Look at the size of the V16 in the Veyron, and think even longer than that!

BrianGriffin thinks “reliable” is just a state of mind > this is not matt farah's foxbodymiata 04/16/2015 at 07:08

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And the word on the street is that Subaru (at least in the U.S.) is planning on discontinuing its H6 soon.

SidewaysOnDirt still misses Bowie > MegaSuper 04/16/2015 at 07:15

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Porsche makes flat 6s.

DoYouEvenShift > MegaSuper 04/16/2015 at 07:31

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This is the only one I can think of.

DoYouEvenShift > MegaSuper 04/16/2015 at 07:33

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Aircraft engine

RacinBob > this is not matt farah's foxbodymiata 04/16/2015 at 07:57

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Awesome until you try to keep oil in them for 100,000 miles.....

kanadanmajava1 > MegaSuper 04/16/2015 at 10:37

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It's not a car engine but AGCO Sisu Power is making a 7 cylinder engine. I think that it's mostly used in big harvesters (or is it combiner?).

Here's a video about it.

There's plenty of problems with 7 cylinder engines and with 9 or 11 it would a lot worse. Like 5 cylinder a 7 cylinder engine is badly out of balance. It vibrates a lot. You can correct this with a balance shaft or couple but besides that the torsional vibrations of the crankshaft are quite bad.

My friend designed the harmonic balancer for that Sisu engine and it is really big. I asked why didn't make it into a 8-cylinder engine (they made a V12 instead). They thought that the harmonic balancer would have been ridiculously large. The design parameters are a bit different in off road engines as they are designed so that they can be operated on full load for thousands of hours.

Really big marine engines can be 9 or 11 cylinder long. Their engine speeds are so low that the vibration problems aren't so big problem even though the masses are immense.

tromoly > MegaSuper 04/16/2015 at 12:03

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Inline-3 cylinder is inherently balanced l0ine an inline-6 engine, so a 9-cylinder could work, however an inline-9 is LOOOOOOOOONG and preposterous to package in anything that isn't 45-feet long.

Same reason a 7- or 11-cylinder engine hasn't been done, they can only really be in in Inline configurations (prime numbers, yo) and require a massive amount of space front-to-back compared to an even-numbered engine.

this is not matt farah's foxbodymiata > RacinBob 04/16/2015 at 14:26

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You can say that again. One of my stationary station wagons is a 98 Legacy GT.

RacinBob > this is not matt farah's foxbodymiata 04/16/2015 at 15:18

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Here's a topic. How do you get the oil to drain back from the valve cover in a race porsche. For thos flat motors with little sumps, it would seem like the oil would come out of the drain pan and end up over in the head..... Obviously they make it work, how?