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.

The main reason 4 bangers usually don't go higher than 2.5 liters is balance. Yes there are a few special snowflake engines (Porsche made a 3 liter for the 944) but it requires tons of engineering to keep it balanced. More cylinders means better balance and smoother operation. Inline 6 and v12 are both incredibly smooth. V8 is acceptable. V6 is trash.

by halving the cylinder volume and then doubling the number of cylinders to achieve the same total displacement, a 4cyl x 1L engine will not have the same power as a 8cyl x 0.5L engine.

I think it has something to do with compression ratio, surface to volume ratio, mean piston speed, etc.

you didn't say absolute shit so i guess you are the expert but refuse to give us the answer?..

>4L v8 making less power

What is a higher redline for 500

There's a lot of Toyota Diesel engines 4cyl with more than 4 liters and people like them a lot

ITT people literally making shit up in a half-assed attempt to explain things that they don't even understand

OP, go somewhere else if you want to actually learn

some configurations are rare in a given range of displacement
14 litre I4
to crude for the power put on it
1.5 litre v16
overly sophisticated compared to output and the demands put on it

the normal engines are something of a compromise between how hard they are to make vs how much power produced
like how a v8 below 3 litres is just not worth it

>1.5 litre v16
>overly sophisticated
pfft, pussy

>v8s are more expensive
There's more to engine costs than displacement and cylinders

From my understanding the reason you don't see very large low cylinder engines has to due with the propagation of the flame front in the combustion chamber. Gas can only ignight so quickly with very large combustion chambers that ignition time can be longer then the downward stroke of the pistion and if you still burning fuel when the pistion is on the way up your wasting a lot of energy. If you look to the 13b rotory engine this is why it had a pair of spark plugs in the combustion chamber because the flame front did not propagate fast enough.

Fiat did make a 28.4l 4 cylinder back in 1911 that was a very successful race car engine but that engine was as large as a truck.

Not by a long shot. There's nothing stopping you from maching parts out of higj strength steel or even Titanium, including a block.
Weak points are bearings, for starters, and the valvetrain. And lubrication/oil is still a black art to 99% of engine builders.

The limit, rather, is in your wallet. Cast iron is just so much cheaper. But don't be fooled. Cast iron now and 30 years ago are worlds apart.

1500+ horsepower engines are streetable nowadays. Well-behaved and reliable to a degree. Failures arw going to be bearings, valvetrain.

>flame propagation
Good point, i forgot to mention that in my last post. This is the reason you don't see gasoline engines in large ships. It just doesn't work at some point. Diesel compression ignites so even though it burns slower, it does so uniformly.

the block and crank might be the costly part
heads and exhaust you can take from any inline 4

inline 4 crank geometry has a second order vibration that is normally avoided by keeping the engine below that speed or in other cases above that speed

longer stroke makes the crank arms longer and puts the connecting rod closer to the piston bore
there more sideways thrust on the piston wall
and something not often talked about is how the crank will take longer to re set its moment of inertia

or put another way a larger movement of the piston is being translated into a smaller moment of the crank
like the point of effort being moved further away from the fulcrum and the point of load

at large in the public rotational force seems to be poorly understood even tho it can be explained in much the same way as a normal lever
given that both involve rotation
its just normal levers have less centripetal force

Block and crank are in fact not that different when it comes to cost. Material cost is a minor part of total cost and the work and machine time to make a V8 as opposed to an I4 doesn't scale linearly, but in favor of the V8 (it's more expensive, but not nearly twice, maybe 1.2-1.5x at most).
The fact you need 2 heads where you once needed only 1, and 8 pistons where you needed 4 is what adds up, although economy of scale helps a little bit here.
In the end, 1x V8 is cheaper than 2x I4, not to mention easier to package.

Not enough compression.

Bore = Torque

Stroke = Speed

>The fact you need 2 heads where you once needed only 1, and 8 pistons where you needed 4 is what adds up,
I was thinking more in terms of tooling
like how some brands are keeping the inline six about just because the heads can share development and tooling costs with the v12
rather than having to make a v6 that would share less parts and processes

on top of being able to fit in almost the same length as an inline engine (ford model a style)
the v has kind of neat packaging with symmetry that allows the crab to nest in the valley and gives the exhausts a place to sit
also the valve train can just be powered from a single hub just by taking and inline and rotating it 90 degrees and flipping it

>now on to the lesser types
Subaru DOHC boxer with its two heads and four cams
late bmw with over head cams operating an ohv type rocker system
or the Ariel square four square four/twin parallel twin
any number of multi bank x or h types

you daft cunt