Engine Issue at Formula Bharat – Decompression Lever Damage



The team uses a KTM 390 as the heart of fireblade from the past 3 years. The fireblade 3.1 at
Formula Bharat 2018 was on the engine of make 2016 which was tested for around 550kms.

The team faced an engine issue at Formula Bharat 2018 during the Driver change in the endurance event. After the 1st half of the endurance the engine didn’t crank up, hence we had a DNF. But the engine did crank at the post TI, during the Intake check. The Engine system started off with the work to diagnose the issue. It was observed that the engine cranked when it is cold with CTS level indicating 3 bars or less and did not when heated. The engine also produced a significant/ noticeable sound when tried to crank. It sounded as if the starter motor was not able to move the  piston, the piston being constrained the motor just kept on drawing current with no hail. After certain attempts, the engine  completely failed to crank up. The fuel discharge, spark condition was all checked, as we got to know that there are no issues in them.

The engine was taken to a near KTM service centre and a complete overhauling of the engine was done. It was found that the decompression lever and its actuator did suffer from a small wear, due to which it became functionally inactive.

The decompression lever helps to leak out a bit of compression during the compression stroke by opening one of the exhaust valves. This compression leak in-turn decreases the load on the motor allowing it to reciprocate the piston with less effort. The wear was a part of wear that the engine undergoes during its life cycle and does not have any specific reason behind it.
As said earlier the engine did crank when it is cool and didn’t when hot was because the wear on the decompression lever started off earlier with a small pit. It actuated the decompression lever but not to its complete extent. At higher engine temperatures the load on the piston increases since the high-temperature gases inside the piston expand thermally leading to a higher overall load on the piston. The opposing load on the piston is the summation of inertial forces, frictional forces and gas pressure and the later increased at high engine temperatures.Hence due to comparatively lower compression leak and increased load on the piston the engine failed to crank up.
The life of the decompression lever cannot be determined on the basis of number of cranks or kilometers driven and hence has to be regularly checked for its pristine condition.
As we had this issue at the most crucial stage of our competition, we would like to bring this to all Formula Student Teams. Put up your issues and comments, we are open to discussions.