My hypothesis is based on the fact that the impeller-turbine shaft does not undergo any torsion, and within its suitable operating limits this should not be an issue. The turbocharger would require a third interfacing stage, so that the shaft drives an electrical generator.

Of course, the problem with this system is the added complexity and the fact that the impeller-turbine shaft will have to do work in turning the generator axle. Such a system does post an advantage though.

The research I have undertaken in this area suggests that most HEV designs operate around a singular energy store – from ultra capacitors to flywheels and the more “yet-to-be-viable” hydrogen fuel cells.

With regards to the operation of trains though, most of the ‘recharging’ if these energy sources is achieved during braking, and quite possibly, when running down slopes. Consider the use of the turbine attached generator above – in this scenario, it would be possible to deploy an independent energy store from those charged by the internal combustion engine itself. It would be possible to utilise this secondary energy store to provide extra horse power when needed, especially when the primary store depletes.

However, this idea may not be viable considering railway engines tend to utilise large displacement naturally aspirated engines running at low RPMs. Turbocharger efficiency is particularly poor at low RPMs as the exhaust gasses do not have sufficient energy to enough to overcome the inertia of the turbine to spool up. My idea requires the use of a turbocharger, as the diesel engine in HEVs tends to solely function as a means for driving the electric traction motors – the hot exhaust gasses are typically untapped as a secondary source of energy.

HEV design will most likely focus on smarter means of energy management, as there is a lot more room to make improvements in this aspect, whilst the usual advances in engineering and technology progresses.

The use of Solar powered trains is also quite limited considering there is much overhead obstruction in most cities, especially due to mountains and tunnels etc. Until we see “Terminator” type nuclear or hydrogen fuel cells in mass production, it might be a while till the HEV sector finds its silver bullet.