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Problems revealed in the test runs of prototype


Test runs of kiha 391 revealed many problems.

Fuel consumption

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.
The idle fuel consumption was also the major issue. In train operations, coasting, braking and stopping at stations account for a large percentage and during these periods engines idle.  IM100-IR(CT58) turbine engine mounted on kiha 391 consumed 56 kg per one hour while idling. Yakumo consist of the day required 6 units of kiha 391 and this meant that 1 hour idling of a Yakumo train wasted 336 kg and this meant that more than one billion yen would be wasted in one year.
To reduce the waste, the idle-stop operation was planned actually while the train was descending long down grades or stopping at stations. Gas turbines can reach maximum output within only several dozens of seconds and make little effect on the train schedule. But such repeated stop-and-start operations would have an adverse effect on the life of the engine and might have caused the cost increas.
The bad fuel economy with low notch position was also the problem. Gas turbines are most efficient at its maximum output. When the engine load decreased from 100% to 25%, the fuel efficiency worsened more than 60% in a simple cycle gas turbine. In train operations, the time using the full notch position is very limited. Usually it is used while starting or ascending slopes and their percentage is small. The notch positions 3 or 4 is mostly used while operating on the comparably flat line like Hakubi. This also leads to the bad fuel economy.


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.
The reabsorption caused the abnormal rise in temperature of the inlet air resulted in the engine stall. If the notch position 6 was used instead of 7, this phenomenon ceased.

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.
But during the test run of kiha 391, the response time was abnormally long, 10 seconds or more. It was thought that the lag time might have affected the train schedule. But in the end the lag time was shortened to around 4 seconds, the normal level of a gas turbine. This was the same level of series 181 DMUs, which had the automatic transmission

Clutch failure

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.

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