Potential of Reduced Fuel Consumption of Diesel-Electric APUs at Variable Speed in Mobile Applications 2011-24-0075

Auxiliary power units (APUs) are used in mobile applications to provide electrical power until approx. 10 kW. It is state of the art that these generators are driven by a diesel engine at constant speed and are selected according to the expected maximal power needed. These systems have a low efficiency and consequently a high fuel consumption particularly when driven at small loads. The system can yield a higher efficiency for partial load conditions by reducing the rotary speed of the driving diesel engine. The optimum in rotary speed of the diesel engine for different loads is pre-programmed (engine mapping) in the diesel control unit. A frequency converter allows a constant frequency of the electricity output at variable speed of the generator. These higher costs for frequency converter and diesel controller demand especially for mobile applications a proof of efficiency, i.e. a proof of economics, which is shown in this paper.

A diesel electric drive train has been mounted on the test rig consisting of a diesel engine, a permanent magnet synchronous generator, a frequency converter and an electric load. The components were analysed individually in a first step to determine the efficiency characteristics. With the individual efficiencies of the components, the total efficiency of the drive train can be calculated in respect to rotary speed and torque. Relevant load profiles where chosen which represent typical duty cycles in stationary and mobile applications. The consumption of the variable speed generator was tested and the control parameters for the diesel engine were optimized using a simulation model. The final control parameters were implemented into the diesel controller at the test rig. The recorded experimental results were compared to the simulation results. The test results proved the precise prediction of fuel consumption by the simulation model. Tests and simulations resulted in a reduction of fuel consumption of about 30% for all of the relevant load profiles compared to the state of the art power units operating at constant speed of revolution. Further fuel saving potential was achieved with a start/stop-function which stops the diesel engine after a determined time period when no power is needed in order to avoid idle operation periods.

An economic analysis concludes this study. It was proved that the higher investment costs will be amortized within approx. one year.