There is not much to say about the volumetric flow rate except if we are in presence of an engine with a known fixed displacement, i.e. we know how much air it can theoretically displace per crankshaft angular displacement ( V_{θ} ).
By definition, the measured volumetric flow rate ( Q_{m} ) is related to the theoretical volumetric flow rate ( Q_{th} ) and the volumetric efficiency ( VE ):
And the theoretical volumetric flow rate is simply defined with the volume of air displaced per angular displacement ( V_{θ} ) and the angular velocity of the output shaft ( ω ):
Displacement (more)
Detailing the volumetric flow rate
For a piston engine, the theoretical volumetric flow rate can be defined as:
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Where:
Note: For some engine cycles, such as the
Atkinson cycle or the
Miller cycle, one could argue that the theoretical volumetric flow rate is smaller because the effective intake stroke is smaller than the engine stroke. However, we will consider the engine stroke as the
potential volume of air that can be displaced and consider a lower
volumetric efficiency for those cycles.
One important thing to notice about equation (5):
The stroke is irrelevant for evaluating the air capacity of engine.
So the true potential of a piston engine depends only on its total bore area.
Wankel engine
The previous equation could be used with a Wankel engine as well. Although, mean piston speed is a rather meaningless value in that case. As a trial and to show that for any type of engine, the volumetric flow rate can be related to a characteristic speed and an «area», this site will consider the more meaningful mean rotor tip speed instead. So, for the Wankel engine:
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Q_{th} 
= 

N_{r}We v_{mrts} 

(6) 
If we assume that the mean rotor tip speed ( v_{mrts} ) is a characteristic of a Wankel engine, the rotor radius becomes irrelevant for evaluating the air capacity. The product N_{r}We becomes the basic dimension of the engine, which could be called the «Wankel area».