THE 4 - 95 STIRLING ENGINE FOR UNDERWATER APPLICATION

Christer Bratt Kockums Marine AB, 205 55 MALM6, Sweden

Abstract

The 4 - 95 Stirling engine was originally developed by Uni- ted Stirling in the 1970s and more than 50 engines have been manufactured and used in different applications.

The engine was used as a baseline engine in the Automo- tive Stirling Engine program managed by NASA and in solar programs together with JPL, Advanco and McDon- ne1 Douglas.

In 1988 United Stirling was acquired by Kockums Marine AB, which is one of the world leading submarine builder, and the Stirling engine development was concentrated on engines with air-independent combustion systems for underwater application.

Parallel to the development of the 4 - 275 engine, which is in operation in two submarines, Kockums Marine is also developing a prototype energy system based on the 4 - 95 engine for use in an Unmanned Underwater Vehicle ( U ~ ) .

This paper describes the conversion of the 4 - 95 engine to an underwater engine with a combustor that burns diesel fuel with pure oxygen. Measured performance and experi- ence from an endurance test will also be reported.

Introduction

The development of the Stirling engine for different appli- cations has been going on in Sweden for more than twenty years. Between 1968 and 1988 the development work was performed by United Stirling AB and in the beginning of 1988 Kockums Marine AB acquired United Stirling and decided to continue the work with the underwater appli- cation of Stirling engines.

The Stirling engine V4 - 275R is a 75 kW engine specifical- ly built for submarine installation. It is designed for low noise and vibration and the high pressure combustor is

easily integrated with the basic engine. Thisengine type has been installed in the Swedish submarine Nacken and in the French submarine SAGA. Both submarines are in operation and the results of the Stirling systems in the sea- trials have been very good.

In 1987 another project was initiated to develop a power system for an unmanned underwater vehicle. Based on available market information it has been concluded that the most interesting power levels would be in the range of 5 to 15 kW. In some applications the power demand could be increased to 20 kW.

The engine used in the project is the 4 - 95 engine, which originally was designed as a 40 kW automotive engine in 1975 and has been used also in generator sets and in solar applications. A total of 50 engines of this type has been built and more than 150,000 running hours have been accumulated and the engine itself has proved to be very re- liable.

The project is co-sponsored by the Swedish Defense Ma- terial Administration and by the Swedish National Indus- trial Board. The first phase of the project which includes construction and testing of a prototype system in the labo- ratory is now concluded.

A second phase where the engine system is integrated with liquid oxygen tank and fuel storage in a hull section has commenced. The dimension of the hull section coincides with the dimensions used for the first generation DARPA unmanned underwater vehicles for the US Navy.

Prototype system

A prototype system comprising engine with generator and high pressure oxygen combustor has been built and tested in the engine laboratory.

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Before the complete system was tested separate tests were performed on critical subsystems. The new transmission for driving the generator was developed and tested on an engine with atmospheric combustor for more than 1,500 hours. The transmission utilizes a toothed belt drive with the dual purpose of syncronizing the two crank shafts and transferring the torque to the generator. This design (see fig. 1) creates a very compact package and the whole unit can be installed into a circular pod with a diameter of less than 900 mm. The noise level is also low compared with the geartrain transmission that is used on the standard engine.

The high pressure combustor was separately tested on an air cooled combustor test rig (see fig. 2) before it was inte- grated on the Stirling engine. Very good flame stability and an even temperature distribution was achieved over the whole load spectra from full load down to 30 percent load. The combustor was also tested with different com- bustion pressures corresponding to maximum diving depths 'from 50 meter to 150 meter.

Test results

The prototype system (see fig. 3) was installed in a test cell and gaseous oxygen and fuel was supplied via tubes from outdoor installations. The generator is a standard 15 kW induction motor connected to the grid and acting as a generator when running at an oversyncronous speed, e.g. over 1,500 rpm.

Net electrical power is measured with a wattmeter of high accuracy. Oxygen flow is measured with a mass flow trans- ducer. Fuel flow is measured continuously with a positive displacement meter and for the performance measure- ments a high precision fuel balance is used. In order to further control the oxygen and fuel flow measurement is taken on the oxygen excess ratio in the exhausts down- stream the back pressure valve.

The following specific oxygen conxumption was meas- ured at differnt loads;

Electric power (kw) Oxygen consumption (g/kWh) 5 10 15

1,300 1,100 1,050

The oxygen excess ratio during all the measurement was 10 percent.

Preliminary noise and vibration .measurements were per- formed and the results indicate that both noise and vibra- tion in the higher frequencies are about 20 dB below those of a diesel engine.

An endurance test has also been performed. The engine has been running for 1,000 hours at a conqant load of 10 kW and with a combustion pressure of 10 bar. No major failure has occurred during the endurance test but minor errors that occurred in the auxiliary system limit the mean time between stop to about 100 hours. It is, however, anti- cipated that further development would increase the mean time between stop to more than 500 hours.

The energy hull section

An energy hull section with LOX tank, fuel tank and control systems are now being designed with the 4 - 95 en- gine as the power conversion unit.

A diameter of 1.1 meter (44 inches) was selected in order to be able to make direct comparison with the battery sec- tion of the vehicles in the DARPA program (see fig. 4).

The length of the energy section is governed by the total energy content. The energy objective for this design study is 600 kWh which corresponds to 60 hours of operation at 10 kW power output. With the use of commercial avail- able oxygen storage technology the energy requirement can be achieved within a total hull section length of 3.5 meter.

The section will have its own weight compensating tank and the whole section will be neutral boyant. The dis- placement of the section is 3.4 ton which corresponds to an energy density of 175 kWh/ton. As only half of the length of the hull section is occupied by the LOX tank and the other half by engine and controls, it is obvious that an increase in energy content also will increase the energy density.

According to figures that have been published on the DARPA UUV, a 104 inches long silver zink battery sec- tion has an energy content of 300 kWh which corresponds to an energy density of 116 kWh/ton.

Conclusions

An air-independent power system based on the Stirling engine and LOX storage can be used to provide power to UUVs. For vehicles with a total dis lacement of more

the Stirling system has higher energy density than the sil- ver zink battery system.

than 5 tons and an energy content o P 600 kWh or more,

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Figure 1. 4-95 Underwater Stirling Engine,

Figure 2. Combustor test rig. 532

Figure 3. Prototype system in test cell.

Figure 4. Energy hull section.

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