Let us consider a basic 4-stroke single cylinder, direct port fuel injected naturally aspirated engine which is given a constant throttle. Now, why does it produce very low torque at low rpms, meaning 'why does the torque curve rise to a peak ?' Literally no one seems to be curious about this, and there is no question at all in any forum regarding this. What everyone seems to ask is why the torque falls off beyond the peak, at high rpms. This is understandable, as the piston moves too fast to suck in the required mass of air from the intake manifold due to inertia of air. But this should not occur at low rpms. The more slower the piston moves, more closer is the mass of air sucked in, to the mass of air corresponding to cylinder displacement. Why then does the volumetric efficiency low at rpms less than the peak torque rpm ? If the reason is that slow moving air sucks in very less fuel, thus producing less torque, this doesn't apply to fuel injected engines. Those can literally inject the same amount of fuel for wide range of rpms. Any other reasons like heat loss during after compression stroke or swirl etc (someone told me this , but I didn't quite understand it) ?
The valve timing in an ICE typically contains overlap, in which the intake valve begins opening before the exhaust valve is finished closing. This lets the engine "breathe" better at high speeds and produce more power there, but it also allows some of the exhaust gas to blow backwards into the intake before any more work can be extracted from it when running at low speeds. This upsets the inlet flow, wastes power, and makes for a rough-running engine at low speeds.
It also makes it possible for hot exhaust to ignite the incoming charge while it is still in the intake manifold, giving rise to backfire and setting the carburetor on fire, which is a bad thing.
As the engine speeds up, there's less time for the blowback to happen and the engine smooths out and it makes better use of the energy available in the expanding gas in the cylinder. This causes torque rise.
Engines can be cammed to be low or high torque-rise. A low torque rise engine has low overlap and produces an almost flat torque curve vs. RPM. A high torque-rise engine has overlap and produces peak torque at higher RPM's. Since power = torque x RPM, the engine with a torque peak at high RPM will produce more power for the same displacement.
Low torque rise engines are used as prime movers in things like tugboats, locomotives and some diesel trucks. High torque rise engines are used in race cars and motorcycles, and some trucks. In fact, when buying a fancy diesel truck (like a Peterbilt) new, one can specify either a high or low torque-rise engine for installation.