Quote:
Originally Posted by NHB
Why would an undersquare engine be any more capable to produce low end torque than an oversquare engine?
I checked what VW offers in Brasil. Their 3-cylinder engine is almost square, but VW's 4-cylinder 1.6 liter engine is much more undersquare.
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The undersquare engine has a higher mean piston velocity - and peak piston velocity - for a given rpm compared to the oversquare engine. This results in gas velocity through the inlet valves becoming choked - reaching sonic velocity - at a lower rpm compared to the oversquare engine. This holds true even if the valve size's are the same but is more pronounced if the bigger bore uses larger valves.
Peak torque occurs when the flow becomes substantially choked during the inlet phase for any engine.
Another reason is that there is less heat loss via the piston crown and cylinder head for the undersquare engine due to the smaller surface area's of the piston crown and cylinder head. As the peak combustion temperatures occur just after top dead centre, this also aids the sfc of an engine.
A 3 cylinder 1.0 lit engine has less total piston crown and cylinder head surface area compared to a 4 cylinder 1.0 lit engine, for the same bore/stroke ratio. It also has a longer stroke. The net result is an engine with better low speed pulling power plus better fuel economy.