Motor Trend on why the inline-6 might come back

[Kiwi_ME]

The straight six is certainly perfection as far as balance goes but engineering a complete contemporary passenger vehicle solution is a matter of compromise. But making 90 degree V6s sounds like the accountants had way too much say in the matter.

If I'm not mistaken liquid filled engine mounts were developed to filter out the 60-degree V6 buzz.

In the rest of the world BMW has sold four cylinders with twin balance shafts since roughly the mid-90s. Many fours over 2 liter today use balance shafts, an effective and compact solution to mitigating secondary forces.

I think that the VW VR6 failed to survive because of too much dead space between the top ring and piston crown which raises CO emissions. The piston crown was tilted 15 deg (or 7.5?) to the bore.
I don't think straight sixes are coming back at least to applications where they have already left. The remaining life of piston engines belongs to the turbocharger.

 

[Shannow]

Originally Posted By: artificialist
It didn't mention one of the major reasons inline 6 engines are uncommon. Having a longitudinal inline 6 reduces the amount of space for a crumple zone


I keep seeing that on BITOG, but Subaru and Audi have way more "stuff" ahead of the front axle centreline than an Aussie Ford Falcon...do Subarus and Audis have insufficent crumple zones due to their placement of the entire engine forward of the axle centreline ?

Besides, longitudinal engines in RWD have been designed to breakaway and travel down under the firewall for over 30 years.

 

[demarpaint]

The 4.9L Ford 6 one of the best engines made IMO. That and the Jeep 4.0L 6. I think both would sell if brought back and updated a bit.

 

[Shannow]

Two more reasons that straight 6 is better






Go to 2 min 17 on the second one.

 

[fdcg27]

Nothing better than a good inline six.
I love the old Jags and BMW always made a good inline six.
Unfortunately, everybody is now offering at least a few FWD models, for which V-6 designs simply package better.
I can't imagine any maker devoting the resources to building a new inline six that would only be used in the shrinking population of midsize and smaller RWD passenger vehicles. If a maker wants to build a special, low volume engine, it'll be something with a sexier cylinder count. Anyway, a turbo four can perform as well as any inline six.
I was very surprised that GM went to the trouble of developing (or having Izusu develop) a new inline six for the Trailblazer and its many badge-engineered sisters.

 

[Rick in PA]

+1 on the slant-six. We had a few of them when I was growing up. Just a nice solid little motor (although they did take a little while to warm up)

 

[Rick in PA]

You have to post the pic from the article:

And some music to go with it:


(don't take it too seriously)

 

[Hokiefyd]

I don't share the author's prediction that inline-6 engines will somehow stage a comeback. I certainly don't have anything against them, but there's no compelling reason to design one today. For all intents and purposes for most passenger vehicles, the 60-degree V-6 is a far better solution than a long-and-skinny inline. You can sit an I-5 sideways pretty well, and it is possible to cram an I-6 sideways, but you're going to have some unpleasant compromises as a result (like an unconventional transmission layout, hard-to-access serpentine belts and engine-driven accessories, etc. A V-6 just makes too much sense to NOT use if you want to have six cylinders.

The only market, really, for an inline-6 today (talking about road-going cars here) is a limited enthusiast RWD market and/or limited luxury RWD market. Most people cannot tell the difference between a good 60-degree V-6 and an inline-6, and a FWD layout pretty much precludes an I-6 architecture. To most people, a "good engine" is one that starts when they turn the key. That's the reality to which cars are designed and I don't see that changing in the future.

 

[mcrn]

Its good to hear. While I agree with the V6 packaging it does not hit on the fact that a manufacturer could use the same engine platform for the inlines. So that might have enough influence to bring them back!

 

[webfors]

Originally Posted By: d00df00d
Decent technical discussion. Also a good overview of why the inline-6 layout is great while the V-6 layout is a huge compromise.

http://www.roadandtrack.com/columns/the-enginerdy-dept-straight-six-revival


Yes!

Excellent article. Thanks for posting.

 

[Shannow]

Originally Posted By: fdcg27
I love the old Jags

 

[supton]

Originally Posted By: Hokiefyd
I don't share the author's prediction that inline-6 engines will somehow stage a comeback. I certainly don't have anything against them, but there's no compelling reason to design one today. For all intents and purposes for most passenger vehicles, the 60-degree V-6 is a far better solution than a long-and-skinny inline. You can sit an I-5 sideways pretty well, and it is possible to cram an I-6 sideways, but you're going to have some unpleasant compromises as a result (like an unconventional transmission layout, hard-to-access serpentine belts and engine-driven accessories, etc. A V-6 just makes too much sense to NOT use if you want to have six cylinders.

The only market, really, for an inline-6 today (talking about road-going cars here) is a limited enthusiast RWD market and/or limited luxury RWD market. Most people cannot tell the difference between a good 60-degree V-6 and an inline-6, and a FWD layout pretty much precludes an I-6 architecture. To most people, a "good engine" is one that starts when they turn the key. That's the reality to which cars are designed and I don't see that changing in the future.


I think I agree. Sharing one motor across FWD/RWD/AWD platforms is a big incentive. Sometimes I wonder why the makers insource engines and outsource everything else.

Seems like the 60 degree V6's are decently smooth and get the job done. For my FWD's I'm rather content with an I4, now that packages well. Slap a turbo on and I don't have a need for a V6. My RWD truck, ok, an I6 might be nice, but meh, its V8 gets the job done. Without a manual transmission to properly use low end torque it might as well rev a bit.

 

[d00df00d]

Originally Posted By: Darwin1138
Didn't all those problems were solved by VW with the VR6??

In terms of NVH and compactness, the VR6 is a middleground between an inline-6 and a V6. It's not a substitute for either.

In terms of performance, it's certainly a good way to get 6 cylinders into a small package, but there are some problems. The fact that all of the cylinders are crammed together like that really limits how the ports and combustion chambers can be designed. It also makes the intake and exhaust tracts run past each other, which heats the intake charge. And of course cooling can be an issue when you have so much heat in so little space.

 

[d00df00d]

Originally Posted By: Hokiefyd
there's no compelling reason to design one today.

...except everything listed in the article?

No one (competent) will argue against the packaging advantages of a V6. The point of the article is that an inline-6 can offer efficiency advantages over a V8 (fewer cylinders) or a V6 (less rotating mass for a given level of NVH).

Don't forget that there are tangential benefits to a smooth engine. A smooth engine doesn't need as much cushioning between the engine and the driver (engine mounts, steering column bushings, etc.). That means the car feels much more direct and "together," without an NVH penalty. An inline-6 is inherently smooth enough to achieve that effect without balance shafts or weights, which also means it will be more efficient and responsive.

 

[Hokiefyd]

Originally Posted By: supton
Sometimes I wonder why the makers insource engines and outsource everything else.


Slightly off-topic, but I've pondered that as well. We're seeing an increasing trend to out-sourcing transmissions. We haven't yet seen that with engines. The engine seems to be the "heart and soul" of the car, and consumers care...or at least they used to. You know the old story where General Motors briefly used Chevrolet 350s in some Buicks or Oldsmobiles in the '70s during an engine shortage, and customers got pretty upset. They bought an Oldsmobile; they want an Oldsmobile engine.

The engine also is subject to a great amount of marketing focus. Honda advertises its VTEC. Subaru advertises its boxer layout. Fiat markets its MultiAIR system. Each manufacturer seems to differentiate itself from the others with engine technology. That's not necessarily mutually exclusive with engine out-sourcing, but I think that subbing engines out to someone else would muddy that marketing water some.

 

[IndyIan]

Suzuki and Chevy sold a Daewoo with a transverse inline 6 in the mid 2000's here. I kind of wanted to drive one just to see what its like, but never tracked one down.
2.5L and ~160hp, so functionally, its not really any better than a bigger 4 cyl. I suppose they could've made it rev a bit higher than a big 4 for more hp, but that's not what the car was about.

 

[A_Harman]

I sort of agree with the basis of the article; that the I6 has some characteristics that may lead to a comeback. But the author really hurt the credibility of his argument with this statement:

"Engineers don't fantasize about V-6s—accountants do. The V-6 layout has proliferated because it "packages well," a sexless industry term that's code for "easy to cram into a variety of engine compartments.""

This shows that he doesn't know much of anything about designing cars to meet crash standards, and is too dismissive of the importance of properly packaging an engine into a vehicle. After all, engines aren't designed to just run on dynamometers.

The I6 is the simplest engine that is perfectly balanced in primary and secondary shaking forces and rocking couples. I4's require balance shafts to eliminate secondary shaking forces, and V6's require balance shafts to eliminate rocking couples. Flat 6's on the other hand are also perfectly balanced. V8's require proper counterweight design to eliminate primary rocking couples.

The most interesting thing in the article for me was:

"At the moment, BMW remains the sole champion of the straight-six. Munich engineers admit that they regularly develop and test prototype V-6s, per internal policy, but say the results don't come close to meeting company noise and harshness standards."

BMW's dedication to the I6 is legendary, and I always said that "[censored] will freeze over before BMW builds a V6". Could the Apocalypse be nigh?

 

[Hokiefyd]

Originally Posted By: d00df00d
No one (competent) will argue against the packaging advantages of a V6.


Right. Just like no one will argue against the mechanical benefits of an inline-6 (there are many, as the article and others in this thread have pointed out). But the packaging compromises are huge unless you're looking at a longitudinal layout; for that reason, I believe that RWD is really the only market where an inline-6 makes sense. So it stands to reason that the only manufacturer still building an inline-6 is BMW...a company that produces only longitudinal powertrains (except perhaps with that new iBMW thing, or whatever its called).

I continue believe that an automaker doesn't have a compelling reason to design an inline-6 today. I suppose I'm not the only one; nobody but BMW has one. They just wouldn't see a return on their investment to design an engine that can be used in only one or two vehicles. GM, for example, can spend more money on the chassis and interior and other bits of its new Colorado because it can engine-share that 3.6L with other vehicles. The money saved from designing a purpose-built inline-6 can be spent on other areas of the vehicle.

You'll never read an argument from me that V-6s are inherently better than I-6s. That said, I just don't see this happening (an I-6 resurgence).

 

[d00df00d]

Originally Posted By: A_Harman
I sort of agree with the basis of the article; that the I6 has some characteristics that may lead to a comeback. But the author really hurt the credibility of his argument with this statement:

"Engineers don't fantasize about V-6s—accountants do. The V-6 layout has proliferated because it "packages well," a sexless industry term that's code for "easy to cram into a variety of engine compartments.""

This shows that he doesn't know much of anything about designing cars to meet crash standards, and is too dismissive of the importance of properly packaging an engine into a vehicle. After all, engines aren't designed to just run on dynamometers.

...or doesn't care. Either way, good point.

To add, I'm pretty sure Nissan picked a V6 for the GT-R largely because its compactness helped the weight distribution. I also remember hearing they had great success with V6s with the later versions of the R34 Skyline GT-R in racing, for the same reason.

 

[BMWTurboDzl]

Some manufacturers are moving to a modular engine design where switching between an I4 and I6 isn't that expensive from a manufacturing perspective.

I also think we're going to continue to see the turbo I6 replace the V8.

Aside from a brief stint with an I4, I've always had an I6. Smooth and simple..