Diesels

Above: The Class 37 was one of the first diesels in the UK, still used for freight.

Diesel engines came about to replace steam. The only advantage they had over steam is that they were very much more efficient, and less polluting. It is wrong to think that diesels were more powerful and faster than steam counter parts. This become more apparent, the record for diesel is 148 mph, for steam 126 mph, and the diesel had an extra 50 years in between to develop.

At low speeds diesel engines have very little torque (turning force) and when stopped they have no turning force at all, engines have to be spinning to provide some traction. This presents a technical problem, how do you connect a diesel engine to wheels so that it can provide force to accelerate it from rest? Well cars and road vehicles get around this by a gear/clutch system. The clutch allows the engine to engage stationary wheels without having to slow down, and the gears allows the engine to keep the spinning at sufficient speed to keep the torque up.

Clutch/Gear systems were used for the very first diesel trains around, indeed I have travelled on one and its a very strange experience, just like being on a bus. However the forces involved are much greater on a train than on a road vehicle, and gearboxes couldn't really take it, and caused a lot of friction too. So that's where the electric component comes in. Electric motors have very high torque just when stationary. If you take two electric motors, wire them into each other, then if you turn one of them, the other one will turn. This principal is used in diesel engines, the engine turns one of the motors and the other is connected to the wheel axle. This is an excellent way of transferring the power. The to start the train the engines roar up, spinning the motor very fast. This puts a high potential difference across the axle motor bringing in enough torque to start the train moving off and accelerating.

Well that is not the end of the story, there are gearing systems which change the voltage of the electricity going to the axle motors so that the train can keep moving while the engine spins over at much lower revs saving fuel.

There are drawbacks to diesel engines. Speed and power are limited with these engines. The reason is that all diesel engines have a maximum speed at what they can rotate at. There are many moving parts, so friction increases with speed, and also they are limited by how much fuel they can burn.