Hot exhaust gas recirculating system for a stirling engine

Abstract

An apparatus for a Stirling type engine is disclosed which is effective to aspirate spent exhaust gases into a downstream jet eminating from the outlet of a blower for the induction system of the engine.

Description

BACKGROUND OF THE INVENTION

To obtain good fuel economy, Stirling engines of recent design usually have a preheater for inducted air which is cumbersome and expensive. It is a device that extracts heat from the exhaust gases that otherwise would be wasted and exposes the incoming air to such heat content. One conventional mode of carrying out this function is to use a regenerative wheel which exposes approximately a half sector to the hot exhaust gases, then rotates to expose that same sector to the inducted air; the wheel, of course, is dynamically sealed so as to separate the exhaust gases from inducted air.

A problem which is being given considerable attention by engine designers is the ability to meet the emission requirements of the Federal government. In the Stirling type engine, it is necessary to recirculate spent exhaust gases by adding as a mix to the inducted air for further combustion of unburned hydrocarbons. However, exhaust gases in a Stirling engine are of a higher temperature level than other types of engines. If such exhaust gases were bled directly into the air intake for the induction system, air pump or blower parts may be detrimentally affected. Furthermore, the bleeding of exhaust gases to the inlet side of the induction system does not lead to adequate sequestering of exhaust gases so as to be tuned to the various modes of operation of the engine.

SUMMARY OF THE INVENTION

It is the primary object of this invention to provide an apparatus for a Stirling type engine which is effective to simultaneously reduce the size requirement for a preheater and to allow for a durable and controlled sequestering of exhaust gases into the induction system for the engine without affecting the mechanical capabilities of the engine components.

Another object of this invention is to provide an apparatus which is effective to recirculate spent exhaust gases from the heat source circuit of a Stirling type engine, utilizing the high exit velocities of the blower of the induction system to aspirate and mix low pressure high temperature exhaust gases with the high pressure lower temperature inducted air.

Yet still another object of this invention is to provide an anti-pollution device for a Stirling type engine which is simultaneously effective to lower the cost of construction of the engine while at the same time providing a low cost solution to exhaust gas recirculation control utilizing a simple shroud and nozzle arrangement.

SUMMARY OF THE DRAWINGS

FIG. 1 is a schematic perspective and exploded view of a Stirling type engine illustrating the induction system and the general placement of the elements of this invention; and

FIG. 2 is a slightly enlarged elevational schematic view of that shown in FIG. 1, showing that portion of the induction system broken away to illustrate a nozzle and shroud assembly for aspirating exhaust gases into the inducted air exiting from the air blower.

DETAILED DESCRIPTION

As shown in FIG. 1, a preferred mode of a Stirling type engine within which the environment for this improvement is adapted, comprises an open heat source circuit A, a closed working gas system B, a mechanical drive C and an aspirator means D which forms a principal part of this invention.

The open heat source system A is arranged to induct ambient air by way of a centrifugal blower 10 which is driven by suitable power take-off mechanism 11 connected to the mechanical drive C. The blower raises the pressure of the inducted air, and forces such air along a stream 12 passing through an induction passage 13 leading to the combustion zone or portion 14. The combustion zone preferably comprises a cylindrical chamber having a plurality opening 14a therein for admitting the pressurized inducted air; fuel is injected into said combustion chamber by way of a nozzle 16 drawing fuel by pump 50 from a reservoir 17. A fuel/air control 51 operates in conjunction with an inlet valve 52 to vary combustion.

Gases are combusted in zone 14 and travel about the exterior of tubing containing a part of the closed working gas system B; the gases enter a sector of the ceramic preheater wheel 18 which slowly rotates to expose said heated sector to the incoming air in passage 13. After passing through the sector of the preheater wheel 18, the exhaust gases pass outwardly through exhaust tube 19 and then eventually through terminal exhaust passages 20.

The closed working gas system B, though not particularly illustrated in FIGS. 1 or 2, may typically comprise a hot working space and a low temperature working space interconnected by a labyrinth of tubes, such as the heater tubes 22, which are effective to convey a high pressurized gas (such as hydrogen) reciprocally through a heat accumulator or heat regenerator interposed therein. The critical portion of the closed working gas sytem B, which is related directly to the open heat source system A, is that portion of the interconnecting tubes which lay to one side of the regenerators and are typically called heater tubes. In a double-acting type of Stirling engine, such interconnecting tubes connect the space disposed to one side of an operating piston in a first cylinder, to the space disposed to an opposite side of an operation piston in a second cylinder. In this fashion, several cylinders of the engine are cascaded. Heat is absorbed through walls of these interconnecting tubes and transferred to the closed working gas reciprocated therethrough.

The mechanical drive C for a Stirling type engine may typically be of the swash-plate type useful with a doubleacting type engine, whereby each of the connecting rods from the several cylinders of the engines are arranged to impart a working pulse to the swash plate at points 90.degree. indexed from the adjacent connecting rod. The drive plate is thereby rotated sinously to provide a smooth drive to a driven shaft 21.

To promote low emissions from a Stirling type engine, exhaust gas recirculation is typically utilized which comprises a passage 25 interconnecting an opening in the exhaust tube 19 with an opening in the induction passage 13. The aspirator means D of this invention, particularly useful in the sequestration of the exhaust gases for recirculation comprises a nozzle 46 and a shroud 30. The nozzle is disposed at the outlet opening 27 of the blower 10 and has a configuration effective to force the pressurized outlet air to form a jet 28 directed along the streamline 29. The shroud 30 is defined about the nozzle and is spaced radially from the nozzle outlet 46a; the shroud has opening 30a to receive the exhaust gas recirculation passage 25. A low pressure zone or suction is created outside and downstream of the nozzle outlet (46a) as a result of the nozzle restriction producing fast flowing air exiting from the nozzle. This low pressure zone draws the relatively higher pressure and higher temperature exhaust gases into the shroud for mixing with the stream 12 and follow along streamline 29. As a result, the high temperature exhaust gases are introduced to the relatively cool inducted air at a point which will not damage the working elements of the blower 10.

An exemplary temperature condition for the heat source system, utilizing exhaust gas recirculation, would have a temperature of about 648.degree.F existing at the point where the exhaust gases enter passage 25, such temperature being maintained up to the shroud. Such temperature would be for a mass flow of approximately 2,405 lbs. per hour when the engine is operating at approximately 4,000 r.p.m. When the engine is operating at part throttle, such as 1,131 r.p.m. (30 miles per hour), the maximum flow would be approximately 153 lbs. per hour and the temperature would drop to about 325.degree.F at such a point. At idle or 600 r.p.m., the mass flow would be about 118 lbs. per hour with the temperature at approximately 338.degree.F. In all these conditions, the temperature of the exhaust gas is high enough so that for sustained periods of time it would have a detrimental effect upon contacted blower parts; this is particularly true when nonmetallic blower components are utilized for reducing the weight of such blower assembly.

Without the use of this invention, exhaust gas recirculation would be typically introduced upstream from the blower. The regenerator or preheating wheel would be typically sized to transfer a certain quantum of heat from the high temperature exhaust gases to the incoming air taking into account the specific location of EGR. It has been found that by locating the EGR passage downstream from the blower and utilizing the jet concept herein, for proving adequate mixing of the two differential temperature gas mediums, the sizing of the preheater wheel can be reduced. For example, the inlet and outlet temperatures for the regenerative wheel at the sector where heat is introduced to the wheel, is approximately 1,820.degree.F and approximately 648.degree.F when exiting from such wheel; this is for a mass flow of 2,405 lbs. per hour when the engine is operating at 4,000 r.p.m. Such temperature does vary slightly depending upon the speed of the engine and may drop to as much as 1,389.degree.F entering the wheel with a mass flow of 306 lbs. per hour when engine is operating at part throttle 1,130 r.p.m. (30 m.p.h.).

With this invention, the rate of heat input and extraction from the wheel can be maintained with a smaller wheel mass. This is understandable in view of the fact that a lesser amount of the heat content is absorbed by surrounding parts, such as the blower and passages upstream from the nozzle. As a result, the mass size of the preheater wheel may be reduced as much as one-fifth or one-sixth and still accomplish the same heat transfer goals as that of the prior art.

Claims

I claim as my invention:

1. In a Stirling type engine, having an external heating system for operating a closed high pressurized working gas system, the combination comprising:

a. a heat source circuit having an induction portion, a combustion portion and an exhaust portion,

b. variable speed blower means in said circuit having an outlet, said blower means being effective to draw ambient air along a streamline into said induction portion,

c. a recirculation passage connecting said exhaust portion and induction portion, and

d. connection means having an aspirator, said means providing a joint between said recirculating passage, induction portion, and blower outlet, said aspirator being located at the immediate outlet of said blower to maximize flow velocity and provide a variable pressure differential between the pressurized low temperature blown air and the high temperature low pressure exhaust gases, said differential being varied in direct response to the outlet velocity of said blower means.

2. The combination as in claim 1, in which said aspirator means comprises a nozzle disposed about the outlet of said blower to create a jet of blown air and a low pressure zone surrounding said jet of blown air.

3. The combination as in claim 1, which further comprises a preheating wheel effective to exchange a substantial portion of the heat content of said spent gases to the streamline of inducted air at a location downstream from said connection means, said inducted air thus being heated by two paths, one including the preheater wheel and the other including the latent heat of said exhaust gases which are recirculated to said inducted air.