6 Mechanical Arrangements

introduction

Tun. elements of a Stirling engine include two spaces at different temperatures having volumes lhat can he varied cyclically and which arc connected through a regenerative heal exchanger and auxiliary heat exchangers. These simple elements can be combined in a surprisingly wide range of mechanical arrangements. Some have been identified (Finkelstein 1.959) by the name of the inventor or original user. Many variations were used in the nineteenth century and have been adopted or re-invented for application to modern engines. In other cases novel mechanisms or embodiments previously unknown are used. New arrangements are still being devised, some are good, some bad; only time will tell which will attain commercial application.

The key identifying feature of any regenerative system is the manner in which the flow of working fluid is controlled. There are two possibilities; llow is controlled either by valves or by volume changes. In many respects the two types of machines are similar but in details of construction, operation, and fields of application, they are quite dillercnt.

The use of valves for llow regulation has the advantage of increased flexibility in Mow control and timing with the possibility of pressure ratios (iK^Jpu,i..) that are virtually unrestricted. On the other hand the presence of valves adds to the mechanical complexity of the system, provides sources of noise, and additional points of wear, so the possibility for long life with low maintenance is likely lo be reduced.

In this work the name Stirling engine is limited to regenerative engntes where the flow is controlled by volume changes. Machines where the llow is controlled by valves are called F.riesson engines. These names are chosen somewhat arbitrarily in an attempt to rationalize a very confused situation. No agreed distinctions have been established in the general literature and 'Stirling engine* is used as a generic title for all types of regenerative engines.

The names Stirling and Ericsson are themselves not entirely satisfactory for they suggest operation on ideal cycles having isothermal compression and expansion with regenerative transfer processes that are either constant pressure or constant volume. Practical engines operate with conditions far removed from those represented by the ideal cycle.

So far as is known, all Ihc engines devised by Robert Stirling were of the closed-cycle type where the flow is controlled by volume changes.

Design variants of Stirling engines

Stirling engines can be broadly classified into two distinct families which may be idcntilied as: (a) single-acting or (b) double-acting engines.

Single-acting engines arc an ensemble of expansion space, compression space and associated heat exchangers in one or two cylinders with two reciprocating elements, one of which must be a piston. The other may be a piston or a displacer (see Chapter 5 for the distinction). I vvery ensemble constitutes a complete system which operates independently of any other single-acting systems that may be coupled on a common crankshaft or other kinematic mechanism.

The celebrated Philips rhombic-drive engine, invented by R. Mcijcr, is an example of a single-acting Stirling engine. It may be more completely described as a single-acting, single-cylinder, piston-and-displacer Stirling engine with rhombic-drive mechanism. Rhombic engines have been made in sizes ranging from miniature engines having output powers of a few watts in large engines of 100 horsepower per cylinder. They may operate as single-cylinder engines or in multiple units arranged on a common craukcase and connected to a common crankshaft. A well known multiple-cylinder, rhombic-drive engine was the Philips Type 4-235. four-cylinder, in-line engine, of about 100 horsepower installed as the propulsion motor in a DAF bus (see Chapter 12) and another was installed in a motor launch.

DI-SUiN VARIANTS OI DOU HIT - ACTING I N CUNES

Double-acting engines, shown in Fig. 6.1, are ensembles of multiple cylinders arranged such that the expansion space of one cylinder is connected through the associated heat exchangers to the compression space located in an adjacent cylinder. There is one reciprocating clement per cylinder, a piston-displacer. The number ol Stirling-engine systems is equal to the number of cylinders. The great advantage of double-acting engines is that the number of reciprocating elements is half the number required in multiple arrangements of single-acting machines. This can lead to major simplification in the kinematic drive arrangements and so to reduced cost. The principal disadvantage is the limited flexibility in design and, to a lesser extent, operating conditions. Furthermore, in prototype development it is necessary to proceed with the whole multiple-cylinder engine rather than an experimental single-cylinder unit which can then be reproduced in multiple versions. Nevertheless the elimination of half the moving parts is so compelling that all significant development of large emiines (over 20 horscnower) is c-iirn*nllv limitrd ir» rlnnl-il«>.íu-iíno

Double acting piston engines

1 wo cylinder twin system Compression in one cylinder and expansion in the other for both systems, i—i

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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