Are you seated comfortably?

Wednesday, July 09, 2014

Tags :  induction-system

A race engine is effectively an air pump and, in theory at least, the greater amount of air it passes then the greater amount of power produced. However, the introduction of the inlet (or exhaust) valve to control the flow into (or out of) the combustion chamber brings with it a number of practical limitations, one of which is the potential for restricting or biasing the flow around the valve seat when at partial lift.

Engineers have grappled with this problem for many years, and while the currently favoured approach (given a healthy if not quite unlimited budget) would be to use computational fluid dynamics (CFD) and powerful computers, such facilities are rarely available to the average engine tuner or workshop. On the less virtual side of the industry, where funding is often hard to find, the task is downgraded to the physical: that of testing the airflow in the cylinder head. The CFD output of pictures of meshed intake ports and coloured streamlines flowing through the intake port may look ‘sexy’ (if sometimes a little confusing) but believe it or not similar results can be obtained using instrumented equipment measuring local flows on the airflow rig.

The most obvious way is to use a pitot tube inserted into the port. Handheld and used correctly, this can be moved around inside the port to give an idea of the distribution of the air flowing around the valve. However, introducing the tube to measure the velocity of the airflow in this way also has the effect of altering the local airflow around it, so what you think may be happening in the port and around the valve may not actually be the case when the tube is not there.

In the past of course, many engineers have grappled with introducing pressure tappings drilled into the port wall around the outside of the port throat, and while that can give an indication of the static pressure in this zone, it is difficult to do and destroys the cylinder head in the process (in drilling through water jackets an so on). Also, the data generated is relevant only to the static pressures around the wall of the port, as it makes no link with the bulk flow of the air away from the wall. Essentially, all we need is some kind of probe that extends into the critical airstream without altering the flow, and in a sense we already have that in every port – the valve!

By taking the pressure tapping off the seat of the valve and routing back through the valve stem, we can estimate the pressure of the air as it passes the restriction caused by the valve seat. Like the port wall tappings referred to above, since these seat tappings are normal to the direction of airflow then this measurement would be one of static pressure and not include the dynamic element of the flow, but by indexing the valve in a number of positions (different valve lifts and rotational position of the valve, for example) a good idea of how the air flows through the valve seat curtain area is obtained.  

This method may not impress your boss as much as CFD, or produce a pretty picture of how the air flows down and around the port, but in terms of speed and the fact that you will be testing the actual components to be used, the technique has much to recommend itself.

Fig. 1 - Airflow testing at the valve seat

Written by John Coxon

Leave your comment