Are gas velocities in a motored piston engine representative of the pre-ignition fluid motion in a fired engine?

Conclusions

The data we have presented comparing mean velocity and turbulence intensity measurements from scavenged and unscavenged engine cycles were obtained for two engine speeds, two intake flow configurations, and two locations of the spark plug relative to the LDV probe volume. The results show that in some circumstances the gas flow in the hotter unscavenged cycles is significantly different from the cooler scavenged cycles, but that for other cases the differences are within the uncertainty of the measurements. In addition, it is apparent that seemingly small changes in the geometry can strongly influence the local flow field. This sensitivity to detail leads us to conclude that it is not possible to make a comprehensive statement about the similarities of gas velocities in motored and fired engines. However, we do feel that some guidelines can be proposed: (1) It should never be presumed that the pre-ignition gas velocities in motored and fired engines are similar. Very large differences can occur. (2) Swirl that persists up to the time of ignition adds order and energy to the flow field, which reduces the likelihood for velocity differences between motored and fired engines; if the swirl motion is destroyed prior to ignition, to produce turbulence, this statement is far less likely to be valid. (3) Motored engine studies are sometimes dictated by the need to use the spark plug port for optical access; in this instance it is doubly important not to presume that the velocities measured are representative of the flow in the spark gap of a fired engine. (4) Skip-firing an engine is a valuable research technique for eliminating the complications of residual gases; however, the pre-ignition flow field of the fired cycles cannot be presumed to be applicable to a continuously fired engine.