Equal performance V4 vs V6 engine length

[murpia]

Just speculating on the likely shortest engine for a given performance (based on equal swept volume & maximum revs):

V4 would have 2x larger bore diameter vs. 3x smaller bore diameter for V6
V4 would need shorter stroke to achieve similar revs to V6, so slightly larger bore again
'lost' length (bank stagger, FEAD length, Flywheel length) likely to be similar

Feel free to challenge any of the above / provide real data. Also, which might have lower friction, thus skewing the 'equal' swept volume / revs comparison.

Thanks, Ian

 

[BrianPetersen]

Absolute shortest regardless of any other considerations?

How about a radial-5?

 

[pwildfire]

As a rotary guy I have to throw out the Wankel option...
Otherwise, why a V4 and not H4? Subaru would be a good reference for H4 lengths.

 

[gruntguru]

"V4 would need shorter stroke to achieve similar revs to V6, so slightly larger bore again"

Why?

je suis charlie

 

[PJGD]

If you are prepared to go with a 2-bank engine, then the shortest length can be had with a disk-webbed crank with roller bearings since the cylinder-to-cylinder offset is significantly less than normal. This subject matter has been discussed elsewhere on this forum, but you can also research Tatra V12, Maybach/Bristol Siddeley, and also Gleniffer V8 engines. Then, if you go with a horizontally opposed flat four engine, you can go with a 4-main bearing [disk web] crank since throws 2 and 3 can be shared while 1 and 4 will be conventionally non-shared which will make it shorter still.

PJGD

 

[JackAction]

Just speculating on the likely shortest engine for a given performance (based on equal swept volume & maximum revs)

If you want to compare engines for a given performance, you should look at the total bore area and at the mean piston speed rather than swept volume and maximum revs.

V4 would need shorter stroke to achieve similar revs to V6, so slightly larger bore again

That is absolutely false. To achieve similar revs for a given mean piston speed you need to have the same stroke.

The stroke will not affect the performance level, only the rpm range of the engine.

So a 4-cyl with a 1.225x piston bore [= sqrt(6/4)] would compare with a 6-cyl with a 1x piston bore, performance-wise. If they have the same stroke, they will compare rev-wise. So a V6 would be about 22% longer than a V4 [= (3*1) / (2*1.225) - 1]. In the end, with this scenario, they would have the same swept volume.

 

[murpia]

Hi Jack,

If you want to compare engines for a given performance, you should look at the total bore area and at the mean piston speed rather than swept volume and maximum revs.

Agreed, Volumetric Flow Rate seems to be a better comparison.

Here's my original reasoning on the stroke:

The V4 will have larger diameter, therefore heavier, pistons. So to keep the crank / rod / piston stresses at similar levels the mean piston speed (and therefore stroke) needs to go down relative to the V6.

Comments? I guess my original query now morphs to "the likely shortest engine for a given Volumetric Flow Rate". I still think the 'lost' length of bank stagger / FEAD / Flywheel will be a significant factor. Does anyone have any good references?

Regards, Ian

 

[dicer]

W8 Volkswaggon engine as short as a normal V6.

 

[JackAction]

Well, in the end, this is the reduction in length when going from a V6 to an equivalent V4:

3 * V6-bore - 2 * V4-bore
= 3 * V6-bore - 2 * 1.225 * V6-bore
= 0.55 V6-bore

So, if you have a V6 engine with a 3.75" bore, going to an equivalent V4 would lead to a free space of 2" in front of the engine.

How significant is this 2"? I guess it depends how badly you need it!

 

[murpia]

Thanks!

How significant is this 2"? I guess it depends how badly you need it!

This is where I was coming from, you'd have to really want it, to make it worthwhile.

Regards, Ian

 

[BrianPetersen]

Besides which, the V4 is either going to be an irregular-firing engine or require balance shafts. Or both.

Most common place V4 is seen is in motorcycle engines, where the irregular firing can be a feature rather than a bug. V6 are nonexistent in bike engines.