Railperf
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- 30 Oct 2017
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Thanks for the very detailed maths.At low speeds, power is not the factor that limits acceleration; it's the friction between the wheels and the rail that matters. If you try to use more power, the wheels will start to spin; wheelslip protection will activate; and this will limit the power that is actually applied to the wheels.
If all the axles of a train are driven, then the maximum starting acceleration is given by [acceleration due to gravity(g)] x [coefficient of friction(f)].
- "g" is close 10 m/s^2: i.e. every second a falling object increases speed downwards by 10m/s.
- With steel wheels on steel rails, "f" is usually between 10% and 20%, depending on how good the wheelship protection is and the condition of the rails.
A 5-car class 222 has 10 out of 20 axles driven, so only half its weight is contributing to friction on the driving wheels. Its
quoted acceleration of 0.8m/s^2 implies that it the coefficient of friction (f) for a 222 is 16%. [ g x 50% x 16% = 0.8 m/s^2]
A 5-car class 810 has only 8 out of 20 axles driven, so only 40% of its weight is contributing to driving friction. Assuming the same 16% coefficient of friction gives a slower starting acceleration for a 810 as: g x 40% x 16% = 0.64m/s^2.
I don't know about the 805, but a 5-car class 800 has a UIC classification of: 2′2′+Bo′Bo′+Bo′Bo′+Bo′Bo′+2′2′, so it has 12 out of 20 axles driven. With 60% of the weight of the train on driven axles, starting acceleration will be g x 60% x 16% = 0.96 m/s^2. That's half as fast again as a class 810 at starting speeds.
== == == ==
Note these limits only apply at starting speeds. When it is moving at 1m/s, the power required to accelerate a 250t class 810 is given by Power(P) = mass x acceleration x velocity. = 250,000 kg x 0.64m/s^2 x 1m/s = 160kW. This is a tiny fraction of the 3MW available. But at 10m/s (22mph) you need 1.6MW to achieve the same acceleration; at 20m/s (44mph) you would need 3.2MW, which is more than the 4 x 735kW diesel engines can provide.
So above around 40mph, it's the power/weight ratio that matters. A class 810 on diesel with 4x735kW = 2.94MW will outperform both a class 222 (5x559kW = 2.895MW), and a class 800 on diesel (3 x 700kW = 2.1MW).
Unfortunately it doesn't explain Hitachi / MTU Generator Unit (GU) setup on the 810's and whether drivers are being specifically instructed to hold power to lower levels when starting from a stand in diesel mode as opposed to electric mode.
The 810's are QUICKER than a Class 222 on electric power despite having fewer motored axles.
The issue is what happens in diesel mode.
Despite being endowed with four hugely powered diesel engines - with enough power to feed the traction motors, the acceleration rate is a LOT slower than the same train on electric.
This is due to: 1. the traction system programmed to feed power in a slower more progressive way.
Or
2. Drivers being instructed to apply power more slowly and progressively in diesel mode.
One driver aboard a very slow 810011 told me that he starts the train with 100% power and lets the electronics take care of the rest.
Which implies the traction control system is programmed to deliver less power and a slower build up of power from a station start on diesel.
It seems to have nothing to do with friction coefficients or adhesion.