Auxiliary Power Unit: MobilGen™ G70

On-board power supply, auxiliary heating and auxiliary air conditioning rethoughtThe invention of the alphagamma® technology provides the basis for a completely new generation of small-power heat engines in the range from 500 W to approx. 20 kW.

Qualified for a maintenance-free operation, the engines work quietly, with little vibration and can be produced economically due to their simplicity. They use elements of oil-free running piston engines (compressors) that have been tried and tested for decades and, together with a patented structure, hold a worldwide unique selling point in the field of Stirling engines.

The latest developments reveal a gratifying performance for the use as auxiliary power units on vehicles such as transit trucks, motor homes and yachts. Based on the measured performance characteristics, the basic requirements for an integration of the unit in the on-board power supply of various vehicles should be collected.

Prototype MobilGen™ G70 (Picture: Erwin Berghammer)

40 Ampere of charging current

In August 2021, the research team of frauscher thermal motors celebrated a successful milestone: the prototype of an auxiliary power unit with an alphagamma® engine with an expansion displacement of 70 cm³ achieved a stable charging current of 40 amps on a truck battery set with a nominal voltage of 24 volts. This means a doubling since the first runs on the test stand.

With this value it must be taken into account that the losses of the generator and the rectification are at around 20%, so that the shaft power of 1.25 kW leads to a respectable mechanical efficiency of over 30%. The team can be proud of this achievement, especially since it is becoming increasingly difficult to reach the efficiency levels of larger units with such small engines.

Numerous optimization steps led to a remarkable improvement of the thermodynamic process. However, mechanical parts such as piston rings, bearings and guides could also be significantly improved in terms of minimizing friction.

From the beginning, special attention was paid to cost-effective manufacturing processes. Almost all parts can be manufactured using CNC technology, the manual work is reduced to the minimum.

Quiet and slim power pack

The low weight is also impressive: the first prototype had a weight of 48 kg. Since this was only a first functional model, no weight optimization has been made.

As a result of the positive performance values, the designers immediately started with a weight-optimized design. The result is a saving of another 15 kg without compromising performance.

One can be fascinated by the quiet operation of the unit. The low noise emissions already lead to the conclusion that, with optimal installation conditions, the operating noise is barely as high as that of a refrigerator.

MobilGen™ G70: Design of the weight-optimized engine unit

Clean combustion and sustainable fuels

With Stirling engines, the heat is supplied externally, that means under atmospheric conditions. Due to the nature of the system, this burner technology produces extremely low levels of exhaust gas pollutants.

The emissions of NOx and CO are below 20% of the EU’s Stage V maximum values for the operation with liquid gas in this area of application (diagrams 1 and 2).

The external heat supply has another advantage that will be important for the future: the flexibility in the type of fuel.

While the first test bench units are running on liquid gas, Frauscher engineers are already collecting data for other fuels and burner solutions. Operation with diesel is the next goal. After the appropriate burner adjustment, the unit is also suitable for various fuels such as bio oil, natural gas or biomethane in the form of CNG or LNG, e-fuels or hydrogen.

NOx emission limits according to EU Regulation 2016/1628

CO emission limits according to EU Regulation 2016/1628

Endurance runner

The unit’s simple structure and the operation without lubricating oil is absolutely revolutionary. As a result of the low piston forces due to the alphagamma® configuration, it it possible to use dry run technology, which has proven itself in oil-free compressors.

This technology is the basis for a long service interval. frauscher thermal motors can rely on many years of experience with engines based on the alphagamma® principle. Accordingly, with careful forecasts, the maintenance interval can most likely be as long as that of the vehicle itself.

Hybrid use

When the undemanding operation is brought in line with the low noise emissions, it makes sense to use the unit as an auxiliary heater in addition to the on-board power supply. This not only re-duces system effort but also saves a lot of energy that can be used sensibly for cabin heating in the cold season.

In addition to the heat energy of the unit, the electrical output can also be te-porarily integrated into the heating circuit, so that a heating power of up to 4 kW is available. The preheating of the vehicle’s engine is another possible feature.

In the summer, however, the electrical power of approximately 1 kW can supply parking air conditioners or an electric air conditioning compressor. There is still something left for other consumers of elictricity such as refrigerators, coffee machines or electro-hydraulic drives such as tail lifts.

Ultimately, the on-board batteries only cover the peak loads, for example when the main engine is started. Since they are always fully charged, they will reward the owner with a long service life.

Space and weight issues

While US long-haul trucks have enough space available for diesel-powered APU units, the space available for European trucks is much more limited. However, there is always enough space for the on-board batteries, which typically are two standardized 12 Volt batteries (according to DIN EN-50342-4) connected in series.

If you consider the space required for two 225 Ah batteries, a volume of at least LxWxH 558 x 518 x 250 mm must be provided. Below’s picture shows that a MobilGen™ G70 can easily be placed in a battery box and that there is also sufficient space available for installing two 100 Ah starter batteries.

This combination indeed represents a promising perspective for new types of on-board power supplies, especially since weight can also be reduced.

Draft of a MobilGen™ G70 in a battery box