Camshaft Design & Profile Generation Software
ENGINE MODELING & CAMSHAFT DESIGN SOFTWARE
INCREASING VOLUMETRIC EFFICIENCY
The Volumetric Efficiency (VE) of a naturally aspirated engine can be defined as the ratio of the mass of the air-fuel mixture drawn into the cylinder, to the mass of the cylinder volume at atmospheric pressure.
The first step in being able to increase the VE at any RPM will be determined by the port’s velocity capability (mean port velocity). To increase the VE, the rate of volume drawn into the cylinder must increase, therefore the ports velocity must also increase to create that increase in rate of volume.
While the mean port velocity is critical to our end result of increasing VE, we also have another parameter that needs to be met. That is the CFM required at the peak piston demand point.
Assuming we have a port capable of the mean velocity required for 115% VE, we must determine if the port can also supply the peak piston demand CFM. Using the peak piston velocity and the piston area, we can calculate the cylinder’s maximum rate of volume demand in CFM. We can then convert that value for reference against CFM values measured on a flow bench.
Knowing the crank angle at which this peak demand occurs, as well as the valve opening lift curve, lets us know the valve lift at which maximum piston velocity occurs, and that the port must flow this peak CFM demand at or before that lift point to achieve the required VE.
Assuming the port does meet the flow requirements for 115% VE, we can calculate the cylinder’s mean rate of volume required to fill the cylinder for 115% VE. Knowing the mean rate of volume and the mean port velocity required, we can calculate the average port area (average inlet charge column) required to create the mean port velocity for 115% VE
To summarise, the required VE creates a rate of volume demanded by the cylinder at the peak piston demand point and that rate of volume demands a flow capability of the port to supply it. This will be at a certain valve lift coinciding with the peak demand point. The mean rate of volume demanded by the cylinder for 115% VE and the mean port velocity required to create the 115% VE, determine the requirement for the average port area (average inlet charge column).
Now we use the mean port velocity required for our target VE, to calculate the camshaft profile which will give us that correct mean port velocity. With everything designed with the capability of supplying 115% VE to the cylinder, we use the resonant tuning pressure waves to assist in sustaining the maximum port velocity past bottom center long enough to increase the cylinder’s VE to 115%.