Smoothest Engine

[brian eiland]

The in-line 6 is the most inherently balanced configuration for a 4cycle engine

Note that I did NOT say the inline 6 was the best engine, just the most inherently balanced engine
I repeat:

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 (but of course, 360° out of phase and on different strokes of the 4-stroke cycle). 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.

So the in-line 6 requires the least counterbalances on the crankshaft,

Are you referring to the balance of reciprocating and rotating parts only?

Primarily, and because there are less counterweights involved the wear and tear on the crankshaft bearings ultimately is less.

Of what significance is that when the torsional vibrations from power strokes (and perhaps other factors) make the 8 cyl engine much smoother?

I would challenge you to prove that statement....that a straight 8 is a better balanced configuration than a straight 6.

 

[brian eiland]

How about this !
Mercedes-Benz to Replace V6s With Inline Engines

 

[Nomad Willy]

There was a Wankel that OMC built as a OB prototype as I recall. I don't think the Mazda Wankel overcame the fuel efficiency issue as the new RX8 shows an MPG rating of 18. My 1995 3.8L Buick 4dr sedan does better than that. And in the 2013 Consumer Reports magazine the RX8 has a terrible mechanical maintenance record.

According to CR the engine is "exceptionally smooth providing you keep the engine at mid-revs, ..". At an idle my 03 Camry w variable valve timing is smoother than older sixes. It probably has balancers too.

I read about the Wankel motorcycle (Suzuki) and it was very complicated. Anti-backfire valves and lots of other stuff that was not on other engines. The induction and exhaust system had lots of gimmicks and sub systems to make the Wankel behave.

Looks like the Wankel has had it's chance but it's not totally dead yet as the RX8 is on the showroom floors and some people are buying.

 

[brian eiland]

I think the biggest problem with the Wankel engine is that it has a power stroke on every revolution,...that's just not fuel efficient.

 

[Nomad Willy]

Very interesting Brian. (the MB link)

The big problem w inline 6s is weight and length. The crankshaft is much longer. It flexes more and thus needs to be heavier yet. And then there is the automotive obsession w FWD and inline 6s just don't "fit in". So the V6 engine will probably prevail as long as FWD.

Fours are becoming even more popular and fewer cylinders usually means better fuel efficiency. A 2.5L four will almost always get better mileage than a 2.5L six. Mileage is increasingly important so I think fours will continue to outshine sixes in the marketplace and technology seems to be reducing the smoothness advantage.

Look at the JD fours that are about the same size as six cyl Luggers and Cummins and others. JD has the balancers and it seems to make them equal in smoothness but I'd guess the JD has better fuel efficiency just because it's a four. I suspect the 4 has less surface area cyl + combustion chamber and hence better thermo efficiency through less heat loss.

Brian wrote "I think the biggest problem with the Wankel engine is that it has a power stroke on every revolution,...that's just not fuel efficient"

I don't think that in itself is a problem and I can think of many advantages.

 

[RickB]

I think the biggest problem with the Wankel engine is that it has a power stroke on every revolution,...that's just not fuel efficient.

Why is that?

 

[OFB]

Define smooth

V24 Detroit Diesel - YouTube

 

[twiisted71]

Is it just me or does the 6 "cyl" rotary sound like one of the last big 2 stroke jet skis?

 

[healhustler]

Is it just me or does the 6 "cyl" rotary sound like one of the last big 2 stroke jet skis?

Yeah, that's it. Mix in the sound of a WWII buzz bomb and it would be perfect.

 

[capt jerry]

here is a smoothyBIGGEST ENGINE

 

[bobc]

I replaced a Lehman 6 with a JD 4. The JD is smoother. It was a pleasant surprise. The JD is a modern engine with higher pressure injectors and electronic controls.

I was worried about vibration when I installed the 4. I had no reason to be.

 

[Insequent]

Last boat show I was at the JD dealer was claiming that the Tier 3 4's were now smoother than the 6's. This came about after CAD work and balance shafts. And delivered 235HP, presumably M5 mode. The key is the power of CAD.

 

[psneeld]

here is a smoothyBIGGEST ENGINE

Thanks for the link Capt Jerry...interesting

This is what I got out of it....
A single unit and single screw design has also proved over time to have a longer life span than double or even quad screws. Longevity of this engine design is based on less moving parts equals less stuff to wear out, which makes sense when you think about it.

 

[FF]

>I think the biggest problem with the Wankel engine is that it has a power stroke on every revolution,...that's just not fuel efficient.<

Fuel economy was not the demise of the rotary , the huge volume of combustion chamber made too much polution for the Air Police.

Perhaps with a urine tank they will be able to pass todays specks.

 

[brian eiland]

>I think the biggest problem with the Wankel engine is that it has a power stroke on every revolution,...that's just not fuel efficient.<

Fuel economy was not the demise of the rotary , the huge volume of combustion chamber made too much polution for the Air Police.

Perhaps with a urine tank they will be able to pass todays specks.

Another interesting Wankel link. I for one always did like the ideas behind this motor.
The Last Mazda Wankel Engine Has Been Built

Yes it did run into pollution problems'
"The down side is that Wankels are always a bit more fuel-gluttonous than a piston engine, and almost always have dirtier exhaust. Poor fuel economy and more polluting are pretty much the only strikes you need against you in our modern age, so the mainstream Wankel is going away."

 

[Nomad Willy]

Very very much like a 2 stroke Brian. And I like them both a lot.

The inline 6 is a great engine for trawlers and many cars. But the long heavy crank, overall length, the fuel efficiency of fewer cylinders, the popularity of FWD and all the advances that makes all engines smoother means four cylinder engines will continue to be more popular.

 

[capt jerry]

hereWorld's smallest supercharged four-stroke V8 engine now in production is a few more? World smallest V12 engine - YouTube

 

[FF]

>and all the advances that makes all engines smoother means four cylinder engines will continue to be more popular.<

I would disagree, as modern diesel engines with electronic fuel injection can shut down as many cylinders as are not required.

A shut down to 3 cylinders would still be smooth , a shut down of a 4 might be much harder to get smooth.

Cost is always a factor , so for big mileage cruisers a 2 speed tranny , and the repairability of a mechanical injected engine would be a valid choice.

Other choice could be two 3 cyl engines mounted on LST style tranny , run one or both as desired.

The 3-71 would be a great engine for most 40ish cruisers , but there SO DAMN HEAVY!

But at least they are cheap.

 

[Nomad Willy]

Shutting down cylinders is not very effective as all the moving parts for the dead cylinders are still moving and causing a lot of lost power.

"Other choice could be two 3 cyl engines mounted on LST style tranny , run one or both as desired."

That would be much better.

 

[capt jerry]

yrs ago I was on a 58 Viking and it had mann engines 12cyl at below 1500 rpms it would run only on six cyls any thing higher the others would kick in. MANN GRANADES I know the spelling is wrong

 

[RickB]

Other choice could be two 3 cyl engines mounted on LST style tranny , run one or both as desired.

LST?

Do you mean a combining gearbox with CP wheels? Seems like a bit of overkill for a toy boat, doesn't it?

Or do you mean a honking great Falk reverse/reduction gear with air clutches?

 

[capt jerry]

the exp. I had with lst engines were not hooked to a tranny but 4 engines hooked up to a gear box and they had a varable picth props you good run 1 or 2 or 3 or 4 engines and only 2 engines had starters you started the other 2 buy throwing in the clutchs,that was a long tme ago. but those were the days my freind

 

[Codger2]

.............means four cylinder engines will continue to be more popular.

Huh?

 

[Nomad Willy]

What do you mean "Huh?"

I thought I pointed out some of the advantages of 4 cyl engines in the near future. Of course I'm talking about engines in general .. not boat engines.

I think you mean that you just disagree.

Do you think 6 cyl engines are going to become more popular than fours?

Why?

 

[FF]

<Shutting down cylinders is not very effective as all the moving parts for the dead cylinders are still moving and causing a lot of lost power.>

Dont let the folks with 2 engines that try to cruse cheap know that the second engine has internal HP costs!!!!

 

[Nomad Willy]

FF it would seem the second engine would have HP GAINS as the single running engine would be running much harder and in a more efficient mode because of it.

But then the second engine does cause efficiency losses as the weight of that engine and all associated equipment is dead weight to be carry across the water.

FF I have the feeling I may not be understanding your post.

But re the OP it looks to me like if one considers basic engines only w/o any high tech features that would make it smoother, engine vibration is dependent on how many cylinders it has. This of course would include 90 degree V type engines and exclude V type engines w other angles of cylinder inclination.