In the second half of the 20th century diesel-electric and diesel-hydraulic stock has widely replaced older diesel stock with mechanical transmission. With advances in gear technology, some manufacturers and operators are recently starting to rediscover diesel-mechanic propulsion for reasons such as low price, low weight and high efficiency.

Manufacturer

• Ansaldo Breda (IC4 for DSB)
• Siemens (Desiro)
• Bombardier (Talent)
• etc.

Diesel-mechanic stock plays a minor role in many fleets. However, some operators do successfully use modern diesel-mechanic vehicles. For example Danish DSB has relied on diesel-mechanic transmission in their IC3 DMUs for more than ten years and is currently ordering the IC4, the next generation of diesel-mechanic MUs. The Deutsche Bahn AG has purchased a large number of diesel-mechanic trains (of the TALENT and DESIRO type) for regional service in recent years .

• Low price
• Low weight
• Fuel economy

Low price

Diesel-mechanic transmission is cheap for two reasons:

• Simplicity (compared to hydromechanic and electric transmission)
• "Off-the-shelf" solutions from mass markets (busses and trucks) can often be used.

Low weight

Mass of diesel-mechanic gear is low, especially compared to diesel-electric solutions, which involve additional traction motors and inverter equipment.

Acceptance

Diesel-mechanic transmission meets strong scepticism since it is often seen as an old technology with low reliability and high maintenance.

Vehicle design

In contrast to diesel-electric power packs (cf. Alstom Lirex), diesel-mechanic modules cannot be located on the roof but have to be installed completely underfloor. This is a barrier for low-floor vehicle design (which is sometimes preferred to facilitate access for elderly and disabled passengers).

Maintenance

Diesel-mechanic transmission requires more maintenance than hydro-mechanic transmission. However, these additional costs are usually compensated by lower initial and energy costs yielding equal or lower LCC.

Regenerative braking

Compared to diesel-electric stock, in diesel-mechanic propulsion the use of braking energy is much more limited (cf. Environmental criteria - Energy efficiency potential ). This comparison is however somewhat theoretical because brake energy recovery is an exception in diesel-electric stock as well.

In modern stock diesel-mechanic transmission is limited to DMUs. The power range of locomotives is difficult to realize with state-of-the-art diesel-mechanic gears.

When comparing energy efficiency of different diesel transmission technologies, the following issues have to be considered:

Transmission efficiency

The main energy saving effect of diesel-mechanic compared to diesel-hydromechanic and diesel-electric propulsion comes from better efficiency of transmission. According to DSB, mechanic transmission has an efficiency of ~ 95 % compared to ~85 % for the other two systems. This alone yields energy savings of about 10 %.

Operation of engine at optimum load

For modern diesel-mechanic stock with 16 speeds, most of the time the engine can be operated close to its point of best efficiency. This is an advantage over diesel-hydraulic transmission.

Recovery of braking energy

Compared to future solutions of diesel-electric vehicles equipped with energy storage systems, the energy recovery potential of diesel-mechanic stock is very poor. However, in contrast to what could be expected it is not zero. For example, the DSB IC3 train set has a mechanical transmission that is not disengaged during normal braking but keeps the engine motoring. This way the auxiliaries (air conditions, generator, air compressor) can be operated during braking without any consumption of fuel. This feature virtually requires no additional investment. The system could be further optimised by ensuring that the pneumatic and thermal buffers in the system are fully exploited. However, the corresponding energy savings are obviously small in comparison to braking energy recovery in electric stock.

Weight

There is a small additional energy saving effect for diesel-mechanic transmission by a relative weight advantage.

Overview

The following table gives an overview over efficiencies of different transmission technologies in diesel propulsion:

	Diesel –mechanic	Diesel-electric	Diesel-hydraulic (Voith gear)
Engine efficiency	equal	equal	equal
Transmission efficiency	~95%	~85%	~85%
Possibility for optimum engine load	high	high	low

Source: DSB

Overall effect

Reduced transmission losses will clearly be the dominant factor for the energy advantage of diesel-mechanic stock. Compared to diesel-hydraulic stock, there are additional effects through optimised load management of the engine.

The overall energy efficiency advantage of diesel-mechanic stock over electric and hydraulic transmission is between 10 and 15 %.

In mid-term, energy storage systems will not become a widespread application in diesel-electric stock. In case this changes in long-term, the efficiency advantage of diesel-mechanic stock is compensated by the recovery advantage of diesel-electric stock.