Smaller cylinders can achieve higher specific output for the same stress level. Smaller cylinders tend to be less efficient due to proportionately greater heat loss from the combustion chamber, and proportionately greater friction.
All dimensions in this hypothetical engine are reduced to 0.7937... of their previous values. Call it 80% for round numbers.
If you want to maintain comparable stress levels that implies comparable piston acceleration and piston speed. This means it will be permissible to rev about 25% higher (1 / 0.7937...).
If cylinder filling is kept in the same range at this nominally higher speed - which ought to be at least approximately the case, if all cylinder port and intake runner dimensions are reduced by the same factor, which will maintain all flow velocities the same and since the speed of sound is the same, the pressure waves are travelling a shorter distance through the intake runners - then the engine will make approximately 62% of the original power output. (50% of the original torque but at 25% higher revs.)
Efficiency should stay in the same range. There are certain effects that don't fully scale in direct proportion. which will tend towards the smaller engine having ever so slightly poorer thermal efficiency under comparable (dimensionally scaled) load conditions.
Natural question arises - "But if this is the case - why does a car with a smaller engine normally get better fuel economy?" - and the answer is that normal automotive engines are considerably oversized for their actual applications. The smaller engine will be less oversized, and if it's under the same actual load (only a fraction of the actual maximum power output) it will generally be running at a more favorable point on the BSFC map.
Further to your very well thought out answer, the smaller engine is also lighter and has less torque so can use a lighter transmission and even maybe lighter sub-frame in the engine/transmission mounting areas.
It might even be more compact and allow for a slightly lower bonnet (hood) line and better aerodynamics.
Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
and my point is that the tax was proportional to cylinder area, not displacement, which encouraged long stroke, small bore engines to minimize the ratio of tax to actual horsepower (or at least torque!).
So the output is 63%. Still, where about is the efficiency? It shouldn't be exactly the same, or else we'd have engines n times smaller/lighter of the same output and efficiency (with the mentioned benefits) just by employing downsized cylinders...
Nobody has refuted my statements in post #3 above, and if they do, I'll be glad to take up the argument. A standing deterrent to increased cylinder count (of downsized cylinders) for a constant output is the increased nmanufacturing & assembly cost due to higher parts count (even though the net mass decreases).
Cylinder volume, count, bore, & stroke are all compromises intended to achieve the best commercial outcome for the application. It's not surprising that engines competing against each other tend to be very similar in these and other parameters. Even though their designs are sometimes perverted due to arbitrary rules/restrictions and other external factors, this is also true of race engines, e.g. F1.
The general rule of thumb is that larger engines are more efficient.
Think of the power output to friction ratio as the displacement increases (frictional losses)
the surface area to volume ratio (heat loss)
also with larger engines, you generally operate at a lower RPMs, which generally means more time for the atomized fuel to evaporate and fully mix, resulting in a more homogenous fuel/air mixture. There is also a much lower volume ratio of "low turbulence areas" in nooks and crannies of the combustion chamber in larger engines than smaller, which further increases combustion efficiency.
Some of the largest diesel engines in the world are also some of the most thermally efficient (somewhere around 50%) as where smaller, lower compression ratio gasoline engines run in the 20% range (I think, I'm going off of what I remember). So in general, smaller engines of similar design will be slightly less efficient due to a combination of different things all working together.
Provided, of course, that you have a use for all the extra power that a bigger engine is going to provide.
An engine that is oversized for its application - and that includes almost all current auto engines - will end up running in a less-favourable, light-load part of the BSFC map, and this effect by far overwhelms the effects described above.
Define "oversized" :-D j/k... I fully understand & concur. Downsized turbocharged engines and cylinder cutout are (initial) cost adders that are mainly driven by CAFE (in USA) and by motor fuel taxes, in other markets where these solutions are being deployed. Of course these "have your cake and eat it" solutions are not as efficient as a "right-sized" engine in the first place. Which gets us back to defining "oversized"...
Agreed, I actually own a turbocharged diesel vehicle for commuting to work (a compact car, and im in the US, bet you can't guess what the make is ^_^) While smaller engines are technically less efficient than larger engines, in an automotive application smaller engines (to a point) are a better option as explained by BrianPetersen.
