[peppercorn (OP)]

This is (paraphrased) from what I think is a very good description of the differences:
http://www.v-twinforum.com/forums/twin-cam-engine-mods/117640-bore-vs-stroke-yeilds-what.html#post1289498

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A piston engine is undersquare or longstroke if its cylinders have a smaller bore (width, diameter) than stroke (length of piston travel). Oversquare is the opposite.

Undersquare engines

These produce strong torque at low to mid range rpm's because of the "leverage" advantage of a longer stroke.  But, undersquare can be a negative trait, since a longer stroke usually means greater friction, a weaker crankshaft and a smaller bore means smaller valves which restricts gaseous exchange; however, modern technology has lessened these problems (explanation?).  An undersquare engine usually has a lower redline, but should generate more low-end torque. In addition, a longer stroke engine can have a higher compression ratio with the same octane fuel compared to a similar displacement engine with a much shorter stroke ratio. This also equals better fuel economy and somewhat better emissions. Going undersquare can cause pistons to wear more quickly (greater side-loads on the cylinder walls) and can cause ring seal problems and lubrication problems; with increased loads on the crankshaft, pistons, the piston pins, connecting rods, and rod bearings (due to  piston speed). In general, a longer stroke leads to higher thermal efficiency through faster burning and lower overall chamber heat loss.  A longer stroke will have greater port velocity at a given RPM, more torque due to more leverage on the crank, will achieve it's greatest efficiency at a lower RPM.  Smaller combustion chambers are also more efficient, with the flame front having a shorter distance to travel- this leads to being more detonation resistant, and having an advantage for emissions.


Oversquare engines

These are generally more reliable, wears less, and can be run at a higher speed. In oversquare engines power does not suffer, but low-end torque does - it being relative to crank throw (distance from the crank center to the crankpin). An oversquare engine cannot have as high a compression ratio as a similar engine with a much higher stroke ratio, and using the same octane fuel. This causes the oversquare engine to have poorer fuel economy, and somewhat poorer exhaust emissions. Breathing is an important advantage for oversquare engines, as the edges of the valves are less obstructed by the cylinder wall (called "unshrouded"). The big bore can fit larger (or more) valves into the head and give them more breathing room.

With shorter crankshaft stroke (and therefore piston travel) parasitic losses are reduced. Ring drag is the major source of internal frictionand the crankshaft assembly also rotates in a smaller arc, so the windage is reduced. Oil-pressure problems caused by windage and oil aeration are lessened.

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It also states that 'engines used at sustained high rpm are usually better suited to running with less stroke and more bore (oversquare).'
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If building a 'mild' hybrid (or more meaningfully [in the case of my idea] a 'bolt-on' e-motor hybrid) my gut feeling is an oversquare engine with the electric motor giving back the 'lost' torque is the more efficient solution.  But (if correct) the reduced thermal efficiency from going oversquare would offset some of this, although improved breathing will 'win' some of that back.


Thoughts please....

[SteveFish]

Peppercorn:

Here is an alternate idea. Diesel engines are seriously under-square and are very reliable and efficient, especially when they are run at a constant speed and temperature. A serial hybrid, where the diesel engine runs a generator to charge batteries and/or to run the electric drive motor might be very efficient. Further, some clever version of the Toyota (Prius) "synergy" drive, whereby a diesel generator and the batteries can be seamlessly adjusted (depending upon battery charge) to drive the wheels might even work better.

If you want to get even more radical with diesel, there is a well developed technology for 2-cycle diesel engines that can provide 2X the power for a little more than 1/2 the weight and space. Unlike 2-cycle gas engines, the diesel version does not loose efficiency.

Steve

[Geezer]

'tis all about the dreaded Thermodynamics. The squareness has some impact on flame fronts and combustion effects which have an effect on an engine's torque curve, but modern transmissions (including hybrid systems) can accommodate all sorts of different torque characteristics (note that we have gone from three speeds to six speeds quite rapidly)

As hydrocarbon fuels become more expensive, the emphasis on extracting the greatest amount of mechanical energy per unit of fuel consumed increases. One way to do this is to maximize the expansion ratio. This works well with steam engines and Atkinson cycle IC engines.

The "ideal" compression ratio is not necessarily the same as the "ideal" expansion ratio, so, you can finagle the valve timing to meet both objectives. That's what the Prius does.

[peppercorn (OP)]

Diesel engines are seriously under-square and are very reliable and efficient, especially when they are run at a constant speed and temperature. A serial hybrid ... might be very efficient.

Your post raises the good question of why there are no commercial diesel-hybrids around. One key factor in making a fully-fledged parallel hybrid efficient with today's crop of cars seems to be using a partial Atkinson cycle (as Geezer mentions) - I'm guessing no similar cycle is possible for diesel, plus the diesel cycle is has very different power and torque curves that don't suit e-motor assist as well (again, my guess).

Also, if staying as lightweight as possible (esp. after adding genny, batteries and motors) a diesel block is going to be too heavy - due to peak cylinder pressures and larger displacement for equivalent output.