v6 vs straight 6

[mobeyone]

Whats the difference?

I know mb uses v6 over the straight but BMW perfer the straight 6.

Anyone know why?

 

[minyak]

Straight configuration is inherently smoother than a V arrangement. Maybe thats why BMW like the I-6'es.

 

[Glideman]

I think you'll find the v is smoother.

 

[frog1520]

I think you'll find the v is smoother.

From my very limited understanding on this, the inline engine is inherently correctly balanced, whereas the V6 requires extra 'weights' (or whatever!) to give the engine the correct balance. If you're into engine theory, this might be a big deal, I dont know.

MB used to use the Inline 6, but switched to the V6 at the end of the 90's. The new engine gave supposedly better fuel consumption and was lighter and smaller. So it left more space under the bonnet for crumple zones. I think the equivalent V6 had slightly less BHP than the I6 it replaced.

I'm sure someone can correct me and fill us in on the details....!

 

[Ian B Walker]

In line 6's are easier to repair than V6 therefore Mb decided to V6's so that the Stealers could earn more revenue. But on a serious note V6's are smoother. Ford V4's used a balance shaft to stabalise the engine. I dont believe that the V6 needs balancing.

 

[GrahamC230K]

All I know is BMW's sound nicer!

 

[saorbust]

In line 6's are easier to repair than V6 therefore Mb decided to V6's so that the Stealers could earn more revenue. But on a serious note V6's are smoother. Ford V4's used a balance shaft to stabalise the engine. I dont believe that the V6 needs balancing.

The MB V6 320 Engine uses a counterbalance shaft. Not sure if its replacement does as well, but I think so.

 

[gary350]

All I know is BMW's sound nicer!

My SLK350 sounds much nicer than my BMW325 ever did. Not saying its a better engine just sounds better.

gary

 

[wallingd]

I'm thinking because a V6 has an odd number of cyls on each bank, you have a 'throbbing' effect result because you have 1 cyl @ top and 2 at bottom per bank (or vice-verse). Effectively like a 3-cylinder. Inlines have an equal number of cyls up/down all across the bank (very simplified model).

Make any sense to anyone?

 

[minyak]

I think the basic physics are underlined already, roughly by frog1520 and wallingd. I-6 are inherently smoother than V-6, but of course if you slap the V-6 with additional balancer shafts, u'll make them smooth anyway. But if you compare a standard I-6 vs V-6 wihtout all the additional smoothing gadgets, the I-6 still wins.

 

[minyak]

The I-4 engine fires every 180 degrees. For instance, #1 is firing when #4 is
filling. Likewise it suffers from free forces of the second order (inertial force
dependant on rotating mass,crank radius, rod/throw ratio and 2xcrank angle);
translation, the engine wants to shake up and down. Some manufacturers use
counterbalance shafts to help with these forces. But ultimately it is not a very
smooth engine.
I-5 engines fire every 144 degrees. That is the main reason why they are
smoother. However I-5's suffer from free moments of the first and second
order. This means the engine wants to rotate about the centerline of the car and
also rotate about the vertical axis. If large enough the free moments will rock
the body of the car from side-to-side, full size pick-up trucks do this alot when
you hit the gas in neutral. This is why many people (myself included) buy a
replacement torque bushing for the upper engine attachment. The free moments
rip the bushing apart as it ages and becomes brittle.

I-6 engines (flat 6 in Porsches too) fire every 120 degrees and do not suffer
from any inertial problems. It is an inherently balanced design.

V-6 engines come in two angles, 60 and 90 degrees. The 90 degree variant is
made on the same assembly line as V-8 engines. 90 degree engines fire at
150/90 degrees. This means 1 cylinder fires at 150 degrees of crank rotation
then the next 90 degrees later (repeat twice). Like the I-5 it suffers from first
and second order free moments. So it should run about as smoothly as an I-5.
Aside: I think Mercedes uses a counterbalance shaft to absorb/reduce the
effects of these free moments. The 60 degree v-6 fires every 120 degrees.
Based on this it should give power delivery similarly to the I-6. But this v-6
suffers from first and second order free moments (much bigger than all of the
other mentioned previously).

 

[Robby]

Using basic theory from bike engines and expanding it:

A straight 6 is a balanced engine. One cylinder fires every 120 degress, 120*6=720. Straightforward and balanced, and all of the pistons are doing the same movement reciprocated by another piston.

As for V engines, a 90 degree vee has perfect primary balance, a 60, or 45, or 72 etc, does not. This is why harleys with their narrow angle V engines (45 degrees I think) and no balance shafts vibrate a lot.

So you add a balance shaft to a V6, and it becomes smooth. However a balance shaft is weight, its a bit like adding a heavier flywheel. So if the engine has the same bore and stroke as the straigh 6, it will make a little less power but slightly better torque from the higher rotating mass.

I would also expect it to be slightly less effiecient - the engine has an extra shaft to turn. The Vee is a more modern engine than the straight though, so you can expect better tolerances there.

Of course, the straight 6 is also more vulnerable to the twisting forces along the length of its crankshaft, the chance of warping one of those long camshafts, and the simple fact that the merc head gaskets on the straight sixes always tended to start weeping oil after a few years and miles.

 

[PJH]

minyak's explanation of the different angle between cylinder banks says it all.
A shorter engine will also have fewer crankshaft bearings, 4 instead of 7.
As the V6 shares production facilities with the V8, it will reduce costs.
Twin bank engines can be programmed to shut down one side for economy, but these are rarely put into production.
I suspect costs and crumple zones are the main reasons for the change.
Perhaps BMW will change in the future.

 

[Aswall]

As most people have correctly stated, the I6 is the inherently smooth, perfect balanced engine, surprised so many car nuts didn't know that!

If you want an inherently smooth V configuration it has to be a V12

 

[Satch]

Sigh. The only true advantage of a V6 is that it is smaller.

A straight six is inherently going to be by far the smoothest because most of the troublesome vibrational forces are cancelled out. A V!2 is essentially two straight sixes but smoother still because there are simply more power strokes per revolution.

All here

http://www.e31.net/engines_e.html

Also explains why a flat plane V8 finds its way into TVR's

 

[peterchurch]

From my very limited understanding on this, the inline engine is inherently correctly balanced, whereas the V6 requires extra 'weights' (or whatever!) to give the engine the correct balance. If you're into engine theory, this might be a big deal, I dont know.

MB used to use the Inline 6, but switched to the V6 at the end of the 90's. The new engine gave supposedly better fuel consumption and was lighter and smaller. So it left more space under the bonnet for crumple zones. I think the equivalent V6 had slightly less BHP than the I6 it replaced.

I'm sure someone can correct me and fill us in on the details....!

The new 320 v6 engine was smoother,lighter, more economical than the previous engines. The figures are about the same as a the previous 280 engine I did have a link explaining it all but I can't find it... I will have a sniff round

 

[fuzzer]

The new 320 v6 engine was smoother,lighter, more economical than the previous engines. The figures are about the same as a the previous 280 engine I did have a link explaining it all but I can't find it... I will have a sniff round

My 240 V6 has twin balancer shafts to make up for the V.

 

[grober]

The inline 6 versus v6 arguement is probably a very good example of market forces and emission regulations dictating design rather than pure engineering principles. The present move to v aspect engines is probably down to engine dimensions more than anything else. The new v engines were probably designed with emissions in mind hence 3 valve twin sparkplug heads for example if my memory serves me correct. Easy to produce a v 6 and v8 on the same machine tool line also. The all aluminium design also will help weight distribution fuel consumption etc etc. Diesels were the last stronghold of the inline 6 but they appear to be on the way out also. The newer sophisticated designs should be more robust with the advantage of computerised design and the availability of new materials but perhaps less tolerant of poor maintenance proceedures. Bad news for the DIY enthusiast I guess without the increasingly complex diagnostic equipment required to keep these engines in spec.

 

[Aswall]

Sigh. The only true advantage of a V6 is that it is smaller.

A straight six is inherently going to be by far the smoothest because most of the troublesome vibrational forces are cancelled out. A V!2 is essentially two straight sixes but smoother still because there are simply more power strokes per revolution.

That's what I said

 

[wallingd]

"So why use V engines? Although twice as much camshafts are needed, the higher friction, the lack of smoothness and the higher production cost, the V6 uses much less space, which helps saving costs in other places and allows front wheel drive (which saves further costs)."

Just out of interest, given that the above actually says use of a V6 "allows" fwd, are there are I-6 fwd cars out there?