Economic, technical, environmental and ergonomic factors must be addressed when selecting traction power. However, parameters shift if tilt is to be used. Felix Schmid* examines the issues which engineers must assess
Travel time is a key element in the attractiveness of an inter-city or inter-regional rail service. Increasing maximum speed over part or all of the distance is not necessarily the best way to reduce journey times, which is what matters to the customer. In most situations it is more effective to narrow the spectrum of speeds; that is, to eliminate as many low speed stretches as possible.
Any reduction in the number of speed restrictions is perceived very positively by the customer. In the past, infrastructure improvement has been the orthodox way of achieving better performance, but this is not always possible or cost-effective. In many cases it is more appropriate to adapt the rolling stock to new needs, by using tilting trains for example. Selection of the right traction package for a tilting train depends on many factors ranging from availability of electric power to passenger comfort.
It might be argued that the decision to incorporate tilt technology has no bearing on the choice of diesel or electric power for a train. This is not so. Any route-specific comparison of diesel and electric options should embrace a host of economic, technical, environmental and ergonomic issues; tilt can affect the outcome significantly.
With tilt, a number of subsidiary technical questions must be answered. Should concentrated or distributed power be used? Will the tilt be passive or active, with hydraulic, electric or pneumatic actuation? Reliability and availability must be taken into account when making systems engineering decisions.
Diesel or electric?
On-train conversion of diesel fuel into mechanical power, followed in many cases by mechanical-electrical-mechanical conversion, offers substantial autonomy and reduces infrastructure cost. However, tractive effort at a given speed is restricted by the inefficiency of thermal power generation on a locomotive, typically around 30%. The overload capability of diesel engines is very limited because the heat resulting from this inefficiency must be dissipated.
Whilst these inadequacies can be partly overcome by good design, diesel power is still viewed with some apprehension in Europe because of past poor performance in three areas: power-to-weight ratio, reliability, and environmental impact. In fact, improvements in all these areas have been dramatic over the last few years.
With electric traction, there is no need to compromise between the weight and bulk of fuel carried and the operating range of the motive power unit. The overload capability of traction motors (the one-hour rating) is very high, especially for AC drives, and on-board computers will manage load optimisation even more effectively in future. Diesel traction is unable to exploit this capability fully.
Traditionally, the choice of electric power for new passenger services has been automatic where overhead electrification already existed, or a small amount of infill made a fully electric service feasible. So far, no tilting trains have been designed for third-rail operation, although this is clearly possible.
More recently, thanks to the development of quiet and reliable underfloor engines for DMUs like the Danish IC3, the economic and environmental benefits not only of new electrification, but also of replacing life-expired power supplies, have been questioned. This debate is now moving from the policy and strategy level to service-by-service decisions.
One example of such a debate was provided by Britain’s West Coast main line1. Although there has as yet been no case of a major established electrification being abandoned in favour of diesel traction in Europe, as has happened in America, the use of diesel power for services running mostly or wholly on electrified lines is no longer viewed as heresy.
High acceleration versus tilt
Key reasons for electrifying a main line in difficult terrain have been the high continuous power rating, excellent acceleration and good controllability afforded by electric traction. Other factors determine the choice on suburban and urban railways.
Electric traction permits regeneration of power when braking or on long gradients. Typical situations include the Schwarzwaldbahn in Southern Germany and the L
