Bore and Stroke

Hey there Veeky Forums, i'm pretty much a retard with no understanding of engineering. Why is it that the bore/stroke of car engines always seems to just fall within a certain range, while more cylinders are added if more displacement is needed? For example, most 4-cylinders seem to be around 2-2.5 liters, i know Porsche made a 2.7l. But why are there no 4l+ 4 cylinders? (I guess the 4bt is an exception, being an industrial powerplant). Does balance become an issue? Wouldn't a shorter crank be less prone to damage from vibrations? And what is the theoretical limit on the max bore size of a cylinder/it's maximum stroke?

Short answer: Physics

Basically larger cylinder=lower top RPM, because larger pistons=more mass=harder to change inertia.
The larger the piston, the heavier the counterweights need to be, and again it accelerates/decellerates slower because it has more mass it needs to move. They exist in ships and stuff that don't need to accelerate quickly, but cars and bikes need it, so they don't have massive pistons.
Then again I'm not an engineer and this is just off the top of my head from my limited knowledge of physics.

Hey there op, i'm pretty much a retard with no udnerstanding of engineering myself, but fuck it, here is my answer.

Four cylinder engines have some advantages they are compact and lightweight, only have one cylinder head, in short, they are quite a simple engine with lower manufacturing costs.

In theory, a 4L I4 would have more power then lets say a 4L V8.
But, making the I4 overly complex would take away all the advantages it has in it's small and efficient form making it a top heavy, expensive and unreliable alternative to the V8.

>In theory, a 4L I4 would have more power then lets say a 4L V8.
I know you admit to being an amateur, but what theory are you going off with this logic?

Maybe he meant "could"? There's no reason I can think of that, most things being equal (materials, technologies, etc), the V8 wouldn't make more power than the I4. More torque in the I4 maybe, but much lower revs so less power overall.

Engine balance. Engines work best when the pistons are near set ratios for bore and stroke.
Also, having a larger stroke limits RPM, as an engine with a larger stroke has to move the piston a larger distance in the same amount of time, leading to higher piston speeds and internal stresses. This is why an LS engine and some of the 4 cylinder Hondas have near the same piston speed, but the Honda engine is at near double the RPM
A third Big reason is packaging. Bore size has a dramatic effect on overall engine size, and an engine with such high displacement and low cylinder count would be enormous.

Tl:DR The engine would be too large, Rev too low, and would be too unbalanced.

On a side note, in applications such as a ship where RPM doesn't matter and the engine is powering a generator, you can have ridiculously large engines. Like 7 cylinder inline engines that only move around 105 RPM max. These larger engines can have really high thermal efficiency, in some cases over 50%

Balance and weight, mostly. A huge inline 4 would shake itself to bits, and as others have said the inertia of a massive piston would prevent it from revving very high.
So what if you used a normal-sized bore and as a result a normal-sized/weight piston and just upped the stroke? Again, the engine wouldn't be able to rev because the huge stroke means the piston has to travel a huge distance, resulting in really high piston speeds and bent connecting rods.

You'd basically end up with an engine ONLY capable of making a crap ton of torque and no rev range at all.

Not too far off the mark, really.
There's a little more to it with regards to airflow and valve-area in relation to cylinder displacement. Bore limits valve size while more stroke demands more of it, hence bore increases with stroke, generally speaking. Alternatively, you can increase cam duration to compensate.
There's a further limit on per-cylinder displacement in the sense that larger valves are heavier and start limiting RPM (hence bikes rev faster). Adding cylinders instead avoids this problem, but again you can compensate. Hollow-stem or Titanium valves increase your redline for any given configuration. Stiffer springs will only go so far before they impact durability.

T. Actual engineer.

Yep. The truth is we can make engines way faster, but our materials don't handle the g-forces of the piston work, and your entire block will twist and throw itself apart if you rev it too hard.

We are at the upper limit of material strength and weight.