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Flo. 9.1-1. Elfect of cxlmusl |ws »ecirculniion on the concentration of oxides of nitrogen in the exhaust of a Philips Stirling engine (after Meijer 1970b).

could be comfortably maintained below the then contemporary regulatory proposals for 1975/76. The comparative values are summarized below:

Emissions Michels (1.972) 1975/76 Regulatory proposals tng/m (g/mile) mg/m (g/mile)

Oxides of Nitrogen 0.109(0.175) 0.249(0.4)

Similar data confirming the favourable emission characteristics of Stirling engines with specific data for the United Stirling and MAN/MWM engines was given by Aim et al. (1973). Earlier Uenische and Wade (1968) reported on the Stirling engine emission studies carried out at General Motors.


Stirling engines make little noise when running. This is perhaps the most advantageous characteristic compared with internal combustion engines. Silent power units have obvious attraction for the armed forces, but low noise is equally attractive in civil applications at all power levels from lawnmower and motorbike engines to large locomotive or heavy traction motors.

The reason why a Stirling engine runs, literally, like a sewing machine is that there arc no valves or periodic explosions in the cylinder or the combustion space. Combustion proceeds as a steady continuous process at atmospheric pressure in a well-insulated and, therefore, muffled chamber. The rates of pressure change of working fluid in the cylinder are very low and there are no metal parts in intermittent or violent contact. The motion of the sliding and rotating parts is moderate with no sudden acceleration or deceleration. Furthermore, many designs of engine can be dynamically balanced, either perfectly, or partially, to eliminate or minimize vibration.

Frequently, where noise in a Stirling engine installation is a problem, the source can be identified as some auxiliary or ancillary unit. In many cases the principal noise comes from the fan used in the radiator cooling system. Other cases where noise was considered excessive have been traced to gears.

Post m a ct a I. (1973) mention noise tesls with the Philips bus which returned sound levels of 68 db, some 10 to 15 db lower than current passenger cars. Aim et at. (1973) have given other data equally favourable about the German and Swedish engines from the aspect of noise. Test results given by Schab (1964) in U.S. navy tests of the large 265 kW (360 hp) four-cylinder Philips Stirling engine confirmed their extremely favourable noise characteristics.


Free-piston Stirling engines are self-starting, a unique characteristic among heat engines. This alone makes them excellent candidates for those applications where automatic starting is necessary or advantageous. They are. for example, well suited for solar applications with a fixed concentrator. When the sun is at the appropriate level in the morning, the concentrator will heat the engine cylinder enough to start operation which then continues until the late afternoon. Another possibility for automatic starting is iu furnace applications where a thermally activated system is required to provide enough power to operate a fan or water pump to circulate the heated fluid.

Stirling engines with crank, rhombic, or swash-plate drive systems are not self-starting. Indeed engine starting is one of the less attractive aspects of these Stirling machines compared with internal combustion engines.

The heater head assembly of a Stirling engine must be hot for the engine to operate. Therefore the hot parts must be preheated before I he engine can be started. In an automotive engine using gasoline or diesel fuel this requires a rather complicated starting system, perhaps similar to that shown in Fig. 9.15 (after Meijer 1970b). To start the engine a

Exhaust Atomizer with ♦ ignitor

Exhaust Atomizer with ♦ ignitor

- Fuel control valve

Starter Overhaul Burner- Atomizing Fuel Elcctromotor/gcnerator motor clutch air blower air compressor pump

- Fuel control valve

Starter Overhaul Burner- Atomizing Fuel Elcctromotor/gcnerator motor clutch air blower air compressor pump

Fto. 9.15. Starting system for a Stirling engine (after Metjer 1970b).

battery-driven motor/generator first drives the fuel pump, an air compressor for fuel atomization, anil a fan to provide the burner air to heat the cylinder head. After a suitable interval the engine is turned by a conventional starter motor and, providing the engine is hot. it cannot fail to start.

The preheat interval is a difficulty that may deter consumer acceptance of the engine in automotive applications. Postma et al. (1973) have projected that the warm-up time to drive-away conditions will be 15 seconds at ambient temperatures of 2l°C (70 °F) and longer at lower temperatures.

Stirling engines, of course, are not necessarily fuelled by combustion systems using fossil fuels. In the 'electric economy* that lies ahead, automotive Stirling engines may be driven from thermal batteries, charged overnight by electric heating. In this situation no preheat period will be required and instant starting of the Stirling engine will be possible.


Stirling engines can be made to respond rapidly to sudden changes in load and speed. 1'his is necessary for engines in automotive applications where very substantial changes in load and speed occur continuously except for long-haul, motorway traction and locomotives. On the other hand, the ability lo maintain constant speed with sudden load change, on or oir. is an important characteristic of stationary engines used for power generation, total energy systems, and heal pumps.

An increase or decrease in load on the engine will, naturally, result in a concomitant adjustment of the fuel supply to maintain the heater lube temperatures al the specified temperature. Due to the large thermal mass of the engine hot parts the response of the fuel system to sudden changes in the load would not he adequate with this single control. Instead, an additional power control system is used which can ellect a virtually instantaneous response of the engine to sudden load changes. Control systems for Stirling engines are discussed in more detail in Chapter 10.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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