in-line 6 Cylinder Engines, 'straight-six'

For years now I was aware that the in-line 6 cylinder engine was likely the BEST configuration for a 4-cycle internal combustion engine (the most naturally balanced configuration). I had been told this long ago by some car racing buddies, but I don't know that I ever looked it up in the library or whatever,...at least I may have, but I had forgotten the specific’s.

Today I decide to see what I might find via google, and this site was the first to pop up:
http://en.wikipedia.org/wiki/Straight-6

Wow, this is pretty inclusive. I'm going to excerpt a few of the passages from that site for posting here on this subject thread that might ask a few more questions of more specificity as related to 6-cylinder Marine engines.

The straight-six engine or inline-six engine is an internal combustion engine with the cylinders- mounted in a straight line along the crankcase with all the pistons driving a common crankshaft. The bank of cylinders may be oriented at any angle, and where the bank is inclined to the vertical, the engine is sometimes called a slant-six. The straight-six layout is the simplest engine layout that possesses both primary and secondary mechanical engine balance, resulting in much less vibration than engines with fewer cylinders.

Balance and Smoothness
An inline six engine is in perfect primary and secondary mechanical balance, without the use of a balance shaft. The engine is in primary couple balance because the front and rear trio of cylinders are mirror images, and the pistons move in pairs. That is, piston #1 mirrors #6, #2 mirrors #5, and #3 mirrors #4, largely eliminating the polar rocking motion that would otherwise result. Secondary imbalance is avoided because the crankshaft has six crank throws arranged in three planes offset at 120°. The result is that the secondary forces that are caused by differences from purely sinusoidal motion sum to zero.

An inline four cylinder or V6 engine without a balance shaft will experience secondary dynamic imbalance, resulting in engine vibration. As a general rule, the forces arising from any dynamic imbalance increase as the square of the engine speed — for example, if the speed doubles, vibration will increase by a factor of four. In contrast, inline six engines have no primary or secondary imbalances, and with carefully designed crankshaft vibration dampers to absorb torsional vibration, will run more smoothly at the same crankshaft speed (rpm). This characteristic has made the straight-six popular in some European sports-luxury cars, where smooth high-speed performance is very desirable. As engine reciprocating forces increase with the cube of piston bore, straight-six is a preferred configuration for large truck engines.

Straight-Six Diesel Engines
The straight-six in diesel engine form with a much larger displacement is commonly used for industrial applications. These include various types of heavy equipment, power generation, as well as transit buses or coaches. Virtually every heavy duty over-the-road truck employs an inline-six diesel engine, as well as most medium duty and many light duty diesel trucks. Its virtues are superior low-end torque, very long service life, smooth operation and dependability. On-highway vehicle operators look for straight-six diesels, which are smooth-operating and quiet. Likewise, off-highway applications such as tractors, marine engines, and electric generators need a motor that is rugged and powerful. In these applications, compactness is not as big a factor as in passenger cars. Reliability and maintainability are much more important concerns.

Most of the engine components and accessories can be located along both sides, rather than on top of or underneath the cylinder banks, meaning that access and maintenance is easier than on a V engine in a truck or industrial configuration. In addition, a straight-six engine is mechanically simpler than a V6 or V8 since it has only one cylinder head and the overhead camshaft configuration has half as many camshafts

 

6 cylinder straights have more harmonic vibration than any other engine. They are also not the best at breathing. Using exhaust pulses and intake pulses the straight 5 cylinder is better.

A slant 6 or any slant is to use gravity to combat side wall wear on the cylinder opposing thrust side

 

Sounds plausible if there is no consideration of the relative forces involved. The transverse force on a piston due to gravity of a "slant" engine, even one with a slant angle of 90deg, ie horizontal cylinders, is miniscule compared to the transverse force due to the acceleration of the piston and combustion gas pressure on the top of the piston being reacted by the connecting rod at an angle as the crankshaft goes round.

 

Are first and second harmonic vibrations the only criteria for the "BEST" engine for any application?

 

The whole reason for "slant" and "v" engines was was to reduce the length and height of the cowl in automobiles for a given Hp. A secondary effect was the increase in stroke for a given height. Vessel designers have used the low height "v" configurations in sport boats to enable the engines to be placed further aft under the working deck. Almost all LSD's are "straight" configurations. For absolute vibration reduction, you want to use opposed pistion, deltic, or radial pancake engines.

 

There you go and I thought it was the 3 cyl that was the best balanced engine. Rolls Royce first engine was 3 cyl then he put two together to make a 6 ...and I thought it was on its side to get it under the van floor!!

 

Straight 6 engines have mainly historic value. Harmonic vibration, long manifolds, high engine weight and unequal breathing has made them obsolete. Except for a few exotic cars, all Asian and European engines, built in large numbers, have 3, 4 and 5 cylinders.
The increased power to displacement ratio has also reduced the need for 6 cylinders.

 

Seems to be a diversity of views here, and naively or otherwise I always thought the slant engines were 100% about low bonnet height.

 

I like inline 6. The balance than 4 or 5. Less parts than v engine, more room to work around.
I have run a few large sixes, they are best inboard engines. V engines advantage is length, most diesels are supercharged or turboed so lenght of intake runners not as critical as NA gas engines.

 

i take it you haven't seen australian cars. the ford barra 6 cyl is an excellent engine. holden always had straight 6's until they went to v6 to fit the global car body in 1988. chrysler had fantastic 6 cylinders in the old days, the 265 r/t put out over 330 hp in 1972. getting back to the current ford barra, it is a really good engine with a lot of torque and power. euro cars have their place but where the roads are rough and the conditions are tough you can't beat a good 6.

 

That appears to be in direct conflict with this quote from that weblink??...and I'm sure I could find many other praises for the natural balance of an inline 6.
"An inline six engine is in perfect primary and secondary mechanical balance, without the use of a balance shaft. The engine is in primary couple balance because the front and rear trio of cylinders are mirror images, and the pistons move in pairs. That is, piston #1 mirrors #6, #2 mirrors #5, and #3 mirrors #4, largely eliminating the polar rocking motion that would otherwise result. Secondary imbalance is avoided because the crankshaft has six crank throws arranged in three planes offset at 120°. The result is that the secondary forces that are caused by differences from purely sinusoidal motion sum to zero."

I would ask you to find some verifiable references to that statement.

I do remember that Chrysler did some experimenting on their slant-6 and achieved some success in racing when engineers utilized the slant of the engine for very long intake ports to boost horsepower by tuning the intake system.

Any breathing problems for a marine 6 would be overcome by turbo boosting which is a natural fit for a good strong 6 engine not worried about turbo-lag as in quick acceleration in a vehicle.

I'm not quite sure what you are referring to here?....more wear or less as a result of the slant?

I seem to recall that the Plymouth Valiants and Dodge Darts had quite a good record for durability....exceptional I believe?

 

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This guy has a rather large amount of Aussie experience to back him up. The old Chrysler slant-six was heavily used in Australia, as well as variants of the vertically-oriented 265 family.

There was a boat company back in the 60's and early 70's that also used the slant (or two of them) as a powerplant

( Slantsix.org )

Roger

 

...from another forum...

 

...two other quotes from another forum...they seem to be in agreement about that 6-71 engine

 

None of this has made them obsolete...go talk to BMW!

The reason Asian an European cars have few 6 cylinder in Europe is the price of fuel there and almost all Asian cars sold in Europe and Asia are small cars.
Few 4 cylinder European cars are sold in N. America..IIRC the 4 cylinder take rate is 10% or so.

Packaging an inline 6 is for RWD/AWD and is hard to do a FWD ..l. Volvo did a transverse 6,and Daewoo/GM.

Misc,too many things to cut and paste so....:

-DD 6-71 are 2 strokes..so they have the same power pulses as an inline 12.

- 5 cylinders are not better than 6. More displacement than a 4 where a 6 won't fit into the body designer's ideas.
They were/are used solely for packaging purposes. They need a couple balance shafts,with the extra complexity of drive chains,shafts,bearing for such.Go find one,and watch it idle and note how soft the engine mounts are.
IIRC the recent DOHC Audi 5 has 2 balance shafts.

-There's lots of Cat and other 6 cylinders which are not so good as well.

-RE Slant sixes-Long intake runners are good for low end torque.

-cylinder wear on the bottom of the bore on a slant engine due to gravity is a myth.

If you want smooth,touch a Mazda rotary/wankel when it's running...
A friend's dad has a 45 degree Cadillac v16...can balance a coin on the block and rev it up.
Check out any 60 degree V12.

And..for the ultimate in slant six diesel etc...Detroit Diesel made a lay down 6-71s and other engines (3-53?) IIRC it's about 30 degrees off the horizontal,a friend has one still in the crate.