Problems revealed in the test runs of prototype
Test runs of kiha 391 revealed many problems.
The bad fuel economy was predicted from the
beginning. kiha 391 consumed the fuel nealy twice as much as series 181
DMU consumed, if run on the same speed. On the other hand, the tilt
system activated and higher speed cruising was allowed, it decreased to
about 1.7 times by some conditions.
Just as predicted, the high level noise at starting was serious problem. At high speed cruising, the noise level of the prototype was as same or less than other EMUs or DMUs due to its light weight. The revolution speed of the power turbine did not increase until the train speed increased with the single speed direct drive train. This was especially problematic at departing stations. To avoid this problem, the notch position 5 instead of 7 was to be used to start at stations.
Reabsorption of its own exhaust
The starting acceleration of kiha 391 was weak
due to its converterless single speed direct drive, just same as series
0 Shinkansen high speed EMUs or express EMUs for the flat route. This
caused the reabsorption of the exhaust gases when starting in tunnels
with steep grades. The low acceleration forced the train to stay on a
same place and exhaust gases accumulated there, causing the reabsorption.
Bad response time
Gas turbines are high speed rotating engines
and have a large rotational inertia. The torque of the power turbine
rises up after the gas generator spins up. So that the engine response
at a high speed is fairly good but at a low speed it is not. In
automotive applications it was serious problem but in rail or air
applications it was not the problem.
In converterless direct drive kiha 391 the clutch was used while switching to the neutral position and the powering position. This clutch often broke down and finally it was removed. This caused the extension of the braking distance due to the power turbine idling torque.
Bad braking capacity
The general poor braking capacity was pointed out at an early stage of testing The braking distance from 130 km/h shorter than 600 m was required even in rainy weather. To achieve this performance, anti-lock brake system was equipped on each axle. But unexpectedly this system didn't work well and resulting in the long braking distance. This was not improved during the test run.