At low engine loads there is insufficient energy in the exhaust gases
to drive the turboblower at the speed required for the necessary airmass
flow. In addition, the small piston movement during the through
scavenge period does nothing to assist the flow of air, as in the fourstroke
engine. Accordingly, starting is made very difficult and off-load
running can be very inefficient; below certain loads it may even be
impossible. A solution was found by having mechanical scavenge pumps
driven from the engine arranged to operate in series with the
turboblowers. Standard on modern engines, however, are electrically
driven auxiliary blowers.