Electric Motors Drive Wheels on Diesel Trains

201 Class locomotive No. 228 at Belfast Central in Enterprise livery. Milepost98, CC BY-SA 4.0, via Wikimedia Commons.

At least on some of them. In the trade, these are called traction motors. All of the locomotives on the Irish rail network are diesel-electric, with engines that run on diesel. However, the engines don't drive the wheels directly. Instead, they power a generator and the wheels are driven by electric motors. This makes it easier to control speed using electronics/electrics, rather than controlling injectors on a diesel engine to vary power. Also, electric motors have full torque at zero RPM, unlike an engine which has to spin up to suck in fuel to produce power, and has zero torque at zero RPM. (Interestingly, steam engines also have full torque at zero RPM, because steam pressure is instantaneously available to power pistons). Both the 071 and 201 Class locomotives, used for InterCity passenger and freight transport, use this system. 071 Class locomotives are mainly used for freight operations. The DART is, of course, electric, and is supplied with power from overhead lines that operate at 1500 V DC. Electricity is "collected" using pantographs attached to the roofs of carriages. All the locomotives on the network are diesel-electric, pulling unpowered coaches. Commuter trains are diesel multiple units (DMUs), as are some InterCity trains. A DMU consists of two or more railcars coupled together to form a trainset, with some or all of the cars powered by their own diesel engines and driving cabs at the ends. Multiple trainsets can be coupled together to form a longer train. While DMUs can be diesel-mechanical (i.e. direct-drive), diesel-electric or diesel-hydraulic, the 22000 Class units, widely used by Iarnród Éireann for commuter and InterCity use, are diesel-hydraulic. Torque is transmitted through a hydraulic fluid in the transmission or gearboxes to drive the wheels on bogies. The 29000 Class units are also diesel-hydraulic. The new DART+ fleet will use battery-electric carriages, in advance of an extension of the electrification of lines beyond the Greater Dublin area. These are known as BEMUs (Battery Electric Multiple Units). On the Dublin-Cork line, electrification will extend to Hazelhatch & Celbridge. 155 of the 185 carriages on order are battery-electric, and a five-carriage train has a battery capacity of 840 kWh and a range of up to 80 km. These batteries are charged by regenerative braking and also "replenished via fast charging stations at chosen terminus locations". According to Iarnród Éireann, charging infrastructure is currently under construction. More info on the DART+ Programme here.

Table 1. Locomotive and DMU classes and their uses on the Irish rail network

Class	Type	Description	Propulsion	Power
2600	DMU	Two-car commuter train	Diesel-hydraulic	~700 hp/car
2800	DMU	Two-car commuter train	Diesel-hydraulic	~600 hp/car
29000	DMU	Four-car commuter train	Diesel-hydraulic	~700 hp/car
22000	DMU	InterCity and commuter train	Diesel-hydraulic	~750 hp/car
071	Locomotive	Diesel locomotive	Diesel-electric	~3,300 hp
201	Locomotive	Diesel locomotive	Diesel-electric	~3,200 hp
New BEMU	BEMU	Five-car battery-electric train (840 kWh battery)	Battery-electric	Not published
New EMU	EMU	Five-car electric train	Electric	Not published

Credits

Thanks to the members of the Facebook group, Irish Railways Present and Past and other Interesting Railways for reading and fact checking this article. 

Further Reading