how is the diesel power transmitted to the traction wheels in a diesel locomotive
It's diesel electric.They have huge electric motors to drive the wheels.
In most Locomotives it is using a Generator driving Electric Motors. The post about the Western Regions Hydralics is correct. But the (Great) Western habits die hard. The Adelante high speed DMUs have a Voith Hydralic drive and the High Speed Train Power cars are getting MTU (Maybach) engines. Hydralic transmission is very common in muliple units as they are more efficient then electric transmission for the smaller engines (up to about 700hp). There is also Mechanical Transmission used in railcars, much like a car's gearbox, this is rare in the UK but more common in Europe
The diesel powered unit is coupled with a generator which in turn powers the traction motors that turn the wheels.
Diesel hydraulic transmission was used a great deal in Germany in the 50s and 60s and may still do (see link to Voith) Also the Italians are using them - Trenitalia took delivery of 25 in 2000. British Railways (Western Region), which still thought of itself as the Great Western Railway in the late 50s and early 60s ordered main line diesel-hydraulic locos from Germany, or using German transmission (Westerns, Warships, and Class 35) when other regions were ordering diesel-electric locos. The Railway Gazette reports that the diesel-electric v diesel-hydraulic 'battle' is to recommence. Hydraulic transmission is akin to, but very different in detail to, an automatic transmission on a car. The last link tells you all you need to know.
The most common form of transmission is electric; a locomotive using electric transmission is known as a diesel-electric locomotive. With this system, the diesel engine drives a generator or alternator; the electrical power produced then drives the wheels using electric motors. This is effectively an electric locomotive with its own generating station. For the first decades the motors were direct current. More recently alternating current has come to be preferred. In either case, a common option is the use of dynamic braking, in which the motors are switched to perform as generators, thus converting the motion of the locomotive into electrical energy, which is then dissipated through heating elements usually mounted on the top of the locomotive. Dynamic braking reduces brake usage in mountainous areas, though it is ineffective at low speeds. Electro-diesel locomotive These are special locomotives that can either operate as an electric locomotive or a diesel locomotive. Dual-mode diesel-electric/third-rail locomotives are operated by the Long Island Rail Road and Metro-North Railroad between non-electrified territory and New York City because of a local law banning diesel-powered locomotives in Manhattan tunnels. For the same reason, Amtrak operates a fleet of dual-mode locomotives in the New York area. British Rail operated dual diesel-electric/electric locomotives designed to run primarily as electric locomotives. This allowed railway yards to remain un-electrified as the third-rail power system is extremely hazardous in a yard area.
