Re: Advantages of square bore & stroke?
(Score: 1, Normal)
12-07-2005 20:41
bboule wrote: A square engine is going to be fairly "balanced" in that it should end up making similar torque & horsepower.
The reason a long stroke engine can't rev as well as a short stroke engine isn't the cylinder wall pressures it has to do with the acceleration of the piston & the forces on the connecting rod, crankpins, and crankshaft.
An engine with a 10cm stroke will have double the piston velocity of an engine with a 5cm stroke at the same engine speed.
At any given level of reliability & durability expected, there is a known piston speed which can be maintained with a given type of piston/rod/crank construction. As time goes on the maximum piston speed goes up as materials and design get better.
I am not sure why a tuner would prefer a square engine over an oversquare engine if they are just after a fast car. The RSX-S is square (86x86) for example, and the current S2000 is actually undersquare (87x90?), but take a look at the Ferrari F430... it is 92x81, take a look at sportbikes with their high rev capabilities.. they are extremely oversquare (e.x. CBR600RR - 67x42mm - revs to 15k or beyond) Take a look at an F1 car - if you could actually get an F1 team to tell you their bore & stroke it is going to be even more oversquare then the motorcycle.
The square engine should be more balanced for the street but make it oversquare for outright performance.
Note there are other issues to being able to rev high besides piston speed... such as valve issues.
think about the crankshaft on a long stroke motor - the throws are much longer (and thus the crank swings wider off the center of the cylinder bore), so the connecting rods swing out at much wider angles than oversquare motors. This introduces increased side loads on the cylinder walls and though you have more leverage on the crank (torque), there are higher frictional losses because of the increased sideload on the rings AND the fact that it has to travel that much further to complete a revolution, so you run into a point of diminishing returns. Also, since the piston has to move so far to complete a full revolution, you approach the limit of where combustion energy can spin the crank fast enough to make top end power.
Now an oversquare motor can pack many more explosions (revs) per minute because the piston stroke is shorter, but then it loses the leverage on the crankshaft, so generally torque production is reduced. But at the same time, frictional losses are reduced, so power can be equivalent or superior to an undersquare motor, albeit at higher RPMS (no problem really if you have gearing to deal with it) As others have mentioned, square motors obviously ride the fence. In the real world of road cars, it seems that valvetrains are generally insufficient to rev high enough to take full advantage of an oversquare design. Undersquare designs are popular in some applications because with their tendency to produce peak output at the lower end of the RPM range, they tend to feel more "relaxed" while operating (family cars, trucks, suvs, etc...).
Oh, with oversquare (short stroke motors) also you have a limit in terms of the speed of flamefront propagation. When bores start getting really large with comparably small strokes, the combustion event takes too too long to complete (not enough time during the power stroke) so you don't extract the same energy (power) from each explosion of mixture and in fact combustion could still be taking place after the piston has reached BDC. This can obviously be mitigated (F1, superbikes) but the compromises must be too extreme for typical road cars, because to my knowledge, oversquare motors aren't all that common.
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