Assuming the description is more or less correct I'm wondering what the implications would be for having an engine running in a hybrid configuration. I'm thinking that a slightly undersized oversquare engine 'straining' (with a driveline bias toward using higher revs) most of the time - with the e-motor making up for the shortage of torque - would be the more efficient configuration,

Undersized is pretty clearly the way to go: efficiency improves with higher load. This is the key reason for the existence of hybrids.

Otherwise, designing for high peak efficiency is the way to go. You will probably sacrifice engine flexibility and perhaps both low end torque and high end power to gain efficiency at an a particular peak (efficiency) rpm and load.

The Prius engine is undersquare and of higher efficiency that any other gasoline engine on the road. It has a geometric compression ratio of 13, and (other things being equal... which they rarely are) a small bore can help enable high compression without detonation because of a shorter flame front travel path. (Its actual effective compression ratio is not equal to the geometric compression ratio, because the Atkinson cycle has asymmetric compression and expansion.)

It might be interesting to experiment with a true Atkinson cycle engine, which could be slightly more efficient than the Prius engine, which simulates the Atkinson cycle by allowing some blowback into the intake manifold.

In general, high rpm does not promote high efficiency, partly because the engine must work too hard to pump air against drag, which goes up with the square of flow speed. But perhaps you have some other compensating effect in mind to favor high rpm. In general, any engine is most efficient at its peak torque speed (which tends to be at lowish rpm) for the same reasons that the torque peaks at that speed.

"In general, any engine is most efficient at its peak torque speed" - This kind of turns my primary assumption on it head, doesn't it?

I personally hope to put together a lightly hybrid-ed undersized engine to save on fuel for equivalent or slightly improved acceleration (esp. round town).

The engine I would be replacing is under a litre anyway, but although tech. (carbs & everything!) can turn out 45+ bhp as standard with 50+ mpg(imp) on a run (but is poor in town). For this reason I have been looking around at bike engines, but taking on-board the points made here that might seem the wrong (revvy/oversq) way to go.

Do the math. Undersquare=longer crank throw, but smaller piston area. Oversquare=shorter crank throw, but larger piston area. It's a wash, except short-strokes can go faster, giving more power strokes per minute, and thus more horsepower. If you want torque, you gear down horsepower. Duh.

REMARKABLE! Thus far all contributors seem to automatically assume 4-stroke petrol/lpg engines. No one even mentioned 2-stroke and/or diesel engines, although consequences of bore/stroke ratios are very different there. More interesting too...

There's a basic error here. An undersquare engine of a given capacity does not produce more torque because of leverage.

If P = cylinder pressure, L = stroke length, r = bore radius and V = swept volume per cylinder -

Torque T = P*pi*r²*L/2 and V = pi*r²*L so T = P*V/2. For given V the individual values of r and L do not come into it.

Cheers.........Codey

Shazam! Geeesshh and whussup...?!

Can't we keep ANY thread here IN-CONTEXT...?

Subject = Summary of Oversquare vs Undersquare engines. Do you agree?

The link in the OP takes you to (voila!) a motorcycle V-Twin forum...smak!

post2 goes right-after the Op with: "If you look at the engines produced by Honda, many are undersquare..." with the link dealing entirely with CAR engines. (ok, so a subsequent link gets into the correct context, later on)

post7 says "REMARKABLE! Thus far all contributors seem to automatically assume 4-stroke petrol/lpg engines. No one even mentioned 2-stroke and/or diesel engines..."

?duh? how many 2-stroke V-Twins have you ridden?! Or diesel bikes for that matter?

wth! ?

Please , please , please don't the next poster be talkin bout 9-ft diameter pistons on the oil tanker ships they build!!!

Sorry to be misleading - Although I did quote the article from a bike forum I originally found it quoted on another forum about cars (where they had, like me, quoted the source).

I have made an assumption here that the rules for bikes (in this case) are the same as for car engines (although obviously with different displacement and gearing).

As is pointed out, I did split the info about use in hybrids off to a reply, due to wanting a separate answer to 'is it a good description or not?'.

Obviously now, since the consensus is that it is a best an out-of-date description, I have to re-evaluate what is the 'right choice' where a DIY 'mild' hybrid is concerned.