Port Speed Calculations

[rpmag]

I would like to calculate the port speed in an engine at different rpms. I am aware of the equation (bore*bore/port dia*port dia)*mean piston speed. However when I use this I get port speeds of 162m/s @ 7000rpm which seems way too high. The results from the equation seem incorrect, but is there a better equation or should i reduce the above by a % as per expected VE?

 

[EngJW]

It does sound high by possibly a factor of 2. Is the piston speed correct (and reasonable)? If it is a 4 valve head, did you count both intake ports? An order of magnitude would be 250 ft/sec port velocity and 4000 ft/min piston speed.

 

[rpmag]

the engine in question is single In valve ie a 2 valve head. I realise that the equations does not allow for a variable pressure fluid.

 

[EngJW]

The equation is just a rule of thumb to put you in the right ballpark before doing any design and testing, or for comparing different engines. It is, as you show, just the ratio of areas times the mean piston speed. The mean piston speed is 2x the stroke times rpm. If you did all that and were consistent in your units, then you must have done it right.

A better equation is the inlet mach index, but that is also just a rule of thumb.

 

[rpmag]

It has been suggested that below is a better equation for calculating port speed based on peak piston velocities. Does this not also have the same issues of not taking into account residual gases, VE and compressable fluids?
Runner velocity = (piston velocity x piston area) / inlet runner area

 

[EngJW]

RP,

Whether you use mean piston velocity or peak, the equation is the same. In either case, you have to know what a reasonable value is. I think more information is available in the literature that is based on the mean velocity. Several magazines like Circle Track have suggested typical port velocities, and at least one SAE paper has covered the inlet mach index.

What you use also depends on what you are trying to accomplish. Do you want to rough in the valve and port sizes for a new design, or compare the ports in different engines? If so, the simple rule of thumb equations will work. However, if you are looking for results that are accurate to within a few percent, then you need to use a simulation that takes into account wave mechanics and all the other variables.

 

[rpmag]

I would like to compare peak vs mean piston speeds for a couple of engine designs and also what the mean/peak flow difference will be and Re involved.

 

[EngJW]

You should have all the information you need then. Bore area, port area, rpm, and stroke for mean piston speed. You would need the rod length for peak speed. I'm not up on the equations for Reynolds Number but they are in the textbooks.

You would have to decide what port area you want to use- such as port area minus stem area, area through the seat, area under the seat, or just valve head area- and then do it the same for each engine.

 

[rpmag]

I agree, however I have just tried Engine Analyser Pro, it may be able to do most of what I want...I will check

 

[rpmag]

Port speeds at the peak of the flow at 2000 rpm resulted in a Re of approx 50,000, thus so far above the laminar levels of 4-7000 Re.