Alternative engines

The engines described above are all engines with a reciprocating piston and where the combustion occurs within the cylinder of the engine, using conventional fuels. Neither have any thorough changes of these engines been discussed except the use of more electronic equipment in order to control the engine and auxiliary systems. For hybrid vehicles alternative engines (or energy transformers) are often discussed for used instead of the conventional internal combustion engines. A fuel cell package is also an alternative to be used in hybrid vehicles but it will be discussed separately in Section 6.4 since fuel cell engines are neither internal combustion engines or heat engines. Since the alternative engines, generally speaking, are not completely developed and are not regarded as commercial in this context, their potential for improved fuel economy and lowering emission levels is still unsure. Certain indications can, however, be seen for an estimation of the potential of some alternative system. There are for example such physical limitations in some of the alternative engines of energy transformers, which have to be taken into account when estimating whether their applicability is better than that of commercial engines. Seeing that it will take long time before alternatives to the present day commercial engines can economically compete, the alternative must be compared with further developed internal combustion engines.

The present day so-called alternative fuels can without any great difficulties be used in almost any type of internal combustion engines provided the engine is adapted to the fuel. Since the potential of the fuel is affected by the engine used, the engine and the fuel must, as already has been underlined, be regarded as an entirety. A new emission control technology for NOx

emissions when using an alcohol in a compression ignition engine can be mentioned as an example. In this case it has been shown that the function of the catalyst is most efficient within a limited temperature interval, when considering both regulated and non regulated (not limited by law) emissions. One possibility of avoiding or evening out a fluctuation of the exhaust temperature would be to use the engine in a hybrid system where the engine can be controlled so as to achieve the best conditions for the reduction of NOx. For the fuel economy it can generally be said that the fuel consumption, in terms of energy, for the same type and size of engine does not differ very much when either a commercial or alternative fuel is used.

The types of engines discussed in this section are the Stirling engine and the gas turbine. Of these engines the Stirling engine can be of interest for use in series hybrids for both light-duty and heavy-duty vehicles while the gas turbine or turbo generator may be used in series hybrids for heavier vehicles (buses and trucks). During the literature studies no hybrid vehicle with a Stirling engine have been found except for a hybrid system presented in a later paragraph in this section (Rajashekara et al., 1998). In the case of the gas turbine only two reports have been found and in one of these a hybrid system with a gas turbine developed for a bus was reported (Malmquist et al., 1998). In the other report the use of a hybrid system with a turbo generator was presented (Brown et al., 1999). For reasons of costs and/or effectiveness or other reason it is unsure whether a Stirling engine or gas turbine (or turbo generator) in a hybrid system will be able to compete with the future otto engine or diesel engine in a hybrid system.

Stirling engines are used today at least in one well-known application namely in submarines.

In the 1970’s and somewhat later a Swedish company, United Stirling, introduced the Stirling engine as a power unit for passenger cars. Later on the engine was tested in different applications in the US and plans have been discussed concerning the use of the engine for electric generation and in this case by using solar energy as a direct energy source.

The Stirling engine, which is a heat engine, differs basically from an internal combustion engine such as the otto engine and diesel engine in that it uses heated air or some other gas for

example helium as a working media. Since the working media is heated in a separate part of the engine any source producing heat can be used but in reality a liquid or a gaseous fuel is used. Trials have been carried out using solar energy as already mentioned. Nearly 200 years ago (1816) the Scottish priest and engineer Robert Stirling received a patent for his invention the heat engine named the Stirling engine. For reason of costs and also technical reasons the Stirling engine has not been accepted, so that it has not obtained a share of the market which its advantageous emission qualities in fact deserve.

The authors of the report ”Control System for a Stirling Engine Driven Induction Generator”

(Rajashekara et al., 1998) are of the opinion that the Stirling engine can be characterized by its high efficiency, low emission levels, that different fuels can be used and in passenger cars instead of otto engines.

The authors of the report also mean that Stirling engine which is coupled to an electric generator can be used as a power unit in a hybrid vehicle as an Auxiliary Power Unit (APU).

An APU is used in a series hybrid in order to load the batteries and to divide the power output from the engine and the batteries. The authors underline the fact that there is a difference in controlling the power from a Stirling engine compared to controlling the power from an internal combustion engine. In an Stirling engine the fuel flow does not have an impact on the mass of the working media (the gas) in the engine and therefore the flow of fuel cannot be used for controlling the power output. Another difference between a Stirling engine and an otto engine is that torque curve is rather flat in Stirling engine (see Figure 9) which can be seen as a special advantage.

0 20 40 60 80 100 120

10 20 30 40 50 60 70

Torque Power

x100 Speed

Figure 9. Typical torque curve and power curve respectively for a Stirling engine.

Source: Rajashekara et al., 1998. .

In order to control the power output from a Stirling engine different methods have been developed. These control methods usually control different parameters of the engine, such as temperature, piston stroke (swash plate^* control), pressure, phase angle, speed, load or dead volume. Each scheme has its advantages and disadvantages; however, the temperature control and swash plate control methods are most commonly used. These affect the fuel flow and its effect on the temperature of the engine. Another method of power control is by changing the depth of the piston stroke at constant gas mass. This method is less complex more reliable and faster than the control method using the working gas mass.

* The expression ”swash plate” is used for a rotating disc mounted with a certain angle and is attached with the piston (see Figure 10).

In the developed system the Stirling engine drives an inductive generator and the electric current from the generator is converted to a variable direct current by the use of a special transistor. The control variables for the generator are easy to manipulate and by this the control of the piston stroke, as a method for varying the power output from the generator, does not need to be used. The control method which has been developed uses a field oriented technique in order to control the generator system and vary the power output from the Stirling engine. By this the system for control of the power output is designed so as to control the generator instead of the Stirling engine.

The Stirling engine – induction-generator-system was extensively tested in the laboratory and at present is running in a mini-van. For the operating power range of 8 – 40 kW and speed range of 3000 –12,000 rpm, the system efficiencies ranged from 85% to 92%. While the Stirling engine runs at approximately 40% efficiency, the predicted system efficiency was about 34% at most operating points. At low speeds, below 3000 rpm, the total APU efficiency was below 34% due to the nature of the operation of an induction machine.

General Motors (GM) in the US has also been looking at the possibility of using a Stirling engine in a hybrid vehicle. Information from USCAR Media Center says that GM has been interested in using a Stirling engine in a series hybrid (USCAR, 1999). According to later information from other sources the conclusion can be drawn that this interest has not lead to any action concerning the use of Stirling engines.

A model of the Stirling engine is shown in Figure 10.

Figure 10. Stirling engine^*. Source: USCAR, 1999.

Gas turbines are commonly used in airplanes but also in certain types of ships and for the generation of electricity. The gas turbine has also been of interest for passenger cars at, among others, Volvo. In its most simple execution the gas turbine system consists of turbine and one of the turbine-operated compressors and a combustion chamber. A simple small type of turbine is a part of a super charger primarily for diesel engines but during recent years also for passenger cars equipped with otto engines.

In the report (Malmquist et al., 1998) a new generation of a gas turbine for a hybrid bus is described and this has been developed in co-operation between Volvo and ABB (Asea Brown Boweri). The development of the new hybrid bus is based on an earlier development of a hybrid bus with a gas turbine. The hybrid system in the bus is briefly presented in Section 8.

* A "swash plate^* is shown in the figure.

In this section the Volvo hybrid system is briefly discussed, based on the report referred to above about the system for control of the power flow.

Three different algorithms were determined for the generation of the power reference for the power module controller. The three algorithms were for line related control, energy balance control and average power control and they were all implemented for evaluation purposes.

The sum of the three signal paths forms a reference signal which is then directed to the power module controller. The structure of the signal paths is chosen for the purpose of easy system evaluation and optimization.

In another project for the evaluation of a hybrid bus, presented by NASA’s John H. Glenn Research Center a type of gas turbine fueled with natural gas is used but in this case it is named ”Turbogenerator”, see Figure 11. The reason for this may be that it was originally a jet engine from an airplane. The project is carried out in co-operation between the government, industry and scientists (Brown et al., 1999).

The goal for the project is to improve the fuel consumption by 50 % (“double the fuel economy”) for buses in city traffic and to reduce the emissions by a tenth of the EPA (USA) emission standards. What is unique about the hybrid system for this bus is its system for storage of energy. For a buss with a maximum weight it may be advantageous to use ultra-capacitors for energy storage since ultra-capacitors seem to be superior to batteries concerning accelerating for regenerative braking and low-temperature characteristics.

Figure 11. GM’s gas turbine. Source: USCAR, 1999.

The car industry seems to have lost its interest in gas turbines for use in hybrid systems.

However, according to an early information from USCAR Media Center, GM has expressed its interest in a gas turbine to be used in a hybrid vehicle. GM points out that a gas turbine is easy to fuel, “just burns anything that burns”, and that an increase in the energy efficiency of up to 50 % can be expected. The gas turbine is a strong candidate for use in order to meet the goal for the PNGV program. With regard to there being no great interest in gas turbines within the car industry this efficiency seems to be somewhat optimistic. The conclusion to be drawn here is that there must be a lack of clear information about the advantages and disadvantages of the gas turbine. A gas turbine is shown in Figure 11.