1l

FlO. 9.10. I lent balance for a single-cylinder, rhomliic-drivc Philips Stirling engine (a) as a function of engine Output at constant speed >i 1501» revolutions per minute, and Hi) ns a function of speed at constant maximum pressure = 13.7 MN/m (20110 Ib/sq in). iAfter

Meijer 1959b.)

ccnl of the total heat supplied and the balance was attributed to 'other losses', presumably due to incomplete fuel combustion and to convcctive and radiative heat transfer from the engine. This engine was tested on a dynamometer but without the auxiliary equipment necessary for independent operation.

Pari load performance

The relatively constant thermal efficiency of Stirling engines over a very wide range of power output or speed is a particularly advantageous characteristic for vehicle use. In most traction applications full power is rarely required and the engine operates for most of the time at between 0.2 and 0.4 full power.

A 'flat' part load characteristic is the prime advantage of the dicsel engine over gas turbines and gasoline motors. The fact thai the Stirling engine has a comparable part load characteristic is a prime reason for sanguinity when contemplating vehicle use.

Performance map

Meijer (1959a) gave the performance map of characteristics for Stirling engines, reproduced in Fig. 9.1 I. 'This is the most useful representation of performance data for a heat engine foi it shows the 'best' operating point for the machine.

The vertical axis is the mean effective pressure of the engine derived from the equation:

10 20 30 'lllbhp

10 20 30 'lllbhp

l io. 9.11. Performance map for single-cylinder, rhombic-drive Philips Stirling: engine. Curves of constant specific fuel consumption are drawn against the brake mean effective pressure (b.m.r.p.) pr aod engine speed n Curves of constant power output arc shown for 10. 20. and 30 hp (7.5. 15. and 22.5 kW). The upper and lower broken lines represent lines of constant maximum pressure of 140 and 50 kg/cnr (20011 and 735 Ib/sq in). (After

Meijer 1959a.)

l io. 9.11. Performance map for single-cylinder, rhombic-drive Philips Stirling: engine. Curves of constant specific fuel consumption are drawn against the brake mean effective pressure (b.m.r.p.) pr aod engine speed n Curves of constant power output arc shown for 10. 20. and 30 hp (7.5. 15. and 22.5 kW). The upper and lower broken lines represent lines of constant maximum pressure of 140 and 50 kg/cnr (20011 and 735 Ib/sq in). (After

Meijer 1959a.)

where P ^output of the engine pc = mean effective pressure

L = length of stroke of the piston

A = area of piston so that LA represents the volumetric displacement per cycle n = speed of the engine K — some constant. Consideration of this equation will reveal that the mean effective pressure is simply a measure of the engine work output per unit volume of piston displacement per cycle. As such it is of exceptional value for it permits the comparison of engines of radically different size, speed and net power output. Model aircraft dicscl engines and huge marine diesel engines both have a similar mean effective pressure.

The horizontal axis on Fig. 9.11 is the engine speed and the contours arc the lines of constant specific fuel consumption, grams of fuel per hour per horsepower. The lines for constant power output of 7, 15 and 22 kW (10, 20 and 30 hp) are also shown. The diagram is bounded at the top and bottom by the broken lines for the maximum (13.7 MN/m2) (1990 lb per sq in) and minimum (5 MN/m ) (725 lb per sq in) pressures at which dynamometer results were obtained.

The engine was unable to achieve its maximum efficiency (or minimum specific fuel consumption, the inverse of efficiency) because of the limitation of the maximum pressure. A more complete set of characteristics, but for a different engine, was published later by Meijer (1970b). These are shown in Fig. 9.12 and refer to an optimized four-cylinder Stirling

°0 200 ~400 600 80<r 1U0CI 1200 1400 1600 Engine speed,«(rpm)

Flti. 9.12. Performance map for 4-Cylinder, rhombic-drive Philips Stirling engine for traction applications. Characteristics arc calculated for optimized encine with nil auxiliaries.

(Aftci Meijer 1970b.)

engine for automotive use. These were calculated characteristics with all auxiliaries included.

emission characteristics of stirling fnuines

Stirling engines fuelled with diescl oil or gasoline have superior exhaust emission characteristics compared with internal combustion engines. This is a particular advantage for the automotive application given the present level of public interest in environmental issues. In the United States various federal and state government agencies have introduced exhaust emission standards. In general, the California vehicle standards are the most demanding.

The constituents of principal interest in an engine exhaust are:

(a) the unburned hydrocarbons (HC)

(b) carbon monoxide (CO)

(c) the oxides of nitrogen (NOx)

The reason for the favourable emission characteristics of Stirling engines is that combustion lakes place continuously at constant temperature and at low, near-atmospheric, pressure in a combustion chamber with hot internal walls. The How velocities are relatively low. A combustion process in such conditions is likely to result in the virtually complete combustion of the hydrocarbon fuel with minimal values of carbon monoxide.

Nitrogen, usually regarded as an inert gas, does react at high temperatures with oxygen to form nitrogen oxides. The reaction is strongly dependent on temperature anil also on the time of association at the high temperature. The quantity of nitrogen oxides formed in combustion is minor and in most cases can be neglected. However, nitrogen oxides have been identified as a contributory factor to the formation of atmospheric pollution. This happens, for example, in the very special circumstances of Los Angeles where high concentrations of automobile exhaust emissions are exposed to strong sunlight in a basin with limited air circulation.

The favourable combustion environs leading to low hydrocarbon and carbon monoxide levels invariably provide the proper circumstances Tor maximizing the levels of nitrogen oxides. The Stirling engine is no exception. Meijer (1971) gave the data reproduced in Fig. 9.13 for Hie concentration of nitrogen oxides in a Stirling engine as a function of the inlet air temperature.

Formation of the oxides of nitrogen can be suppressed with the provision of excess air which, of course, depresses the temperature of the combustion space and hence the rale of the nitrogen/oxygen reaction. Recirculation of the engine exhaust can achieve the same result. An indication of the dramatic reduction in nitrogen oxides concentration achieved with partial exhaust gas recirculation was given by Meijer

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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