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.