Air fuel control system for Stirling engine

An air/fuel control system (including apparatus and method for a Stirling engine is disclosed. A signal generated by deviation of the temperature of the heater head gas temperature from a set-point is used to control an air flow throttle valve. Variations in the air flow of the external combustion circuit is sensed by way of a vortex-shedding device which delivers a D.C. electrical signal. The signal is shaped and amplified and used to control operation of one or more fuel injectors which feed into a common outlet manifold leading to the fuel nozzle serving the external combustion circuit. The fuel injectors are solenoid operated, one sized to provide a fuel flow rate of 0.4-2.0 g/sec., and at least two others are sized to provide a combined fuel flow rate of 2-15 g/sec., but 180.degree. out of phase with each other. The series of injectors provide an effective fuel control range of 0.4-15 grams/sec. to achieve an air/fuel ratio range of 37.5 to 1.

BACKGROUND OF THE INVENTION 
The Stirling engine derives energy from a continuous external combustion 
process. All of the heat supplied from the combustion process has to be 
transferred through metal walls (heater tubes) to a pressurized hydrogen 
working fluid. The pressure-volume (P-V) and temperature-entropy (T-S) 
diagrams of the ideal Stirling cycle help in understanding the derivation 
of power for the engine. These diagrams (see FIGS. 6 and 7) illustrate 
that heat is transferred to the working fluid during the constant-volume 
phase 2-3 and during the isothermal expansion phase 3-4. Heat is rejected 
during the constant-volume phase 4-1 and during the isothermal compression 
phase 1-2. During the isothermal expansion of phase 3-4, heat addition 
occurs at the same rate at which work is produced by the fluid expansion. 
Therefore, to maintain maximum possible power out of the engine, the 
temperature of the working fluid must be maintained at a constant level 
and as high as possible, taking into consideration the metallurgical heat 
limit of the materials. Typically, a Stirling engine designed for 
automotive use is optimized for a hydrogen temperature of 710.degree. C. 
or higher. 
An air/fuel control system is required to maintain such a constant hydrogen 
temperature. Such control system should also be capable of varying the 
ratio between air and fuel in response to a change in engine load, and 
also to provide a change in the air/fuel ratio as a function of fuel flow 
which may be varied as a result of exhaust gas recirculation. Air flow 
itself is a variable commodity since it is generated by a blower which is 
engine driven after the engine has been started. The air/fuel control 
system thus must respond to at least three superimposed parameters. 
Varying the air/fuel ratio is necessary, apart from the desire to seek a 
constant hydrogen temperature, to control exhaust emissions and to improve 
engine efficiency. Unburned hydrocarbons in the exhaust, due to a rich 
fuel mixture, represent an energy loss; however, an air rich mixture 
results in less efficient heat transfer, and, therefore, a less efficient 
heating system. Varying amounts of exhaust gas recirculation (EGR) is 
required for dilution and to reduce the generation of nitrogen oxide 
emissions. 
The prior art has attempted to provide an air/fuel control system for an 
automotive Stirling engine principally according to two concepts: (a) a 
closed loop system wherein the sensed hydrogen temperature was used to 
directly control a fuel metering device; or (b) an open loop system 
wherein a sensed change in the hydrogen temperature was utilized to 
control an air flow throttle valve which would modulate air flow, and then 
a fuel metering system was operated in response to a change in the air 
flow. The closed loop control system has proven deficient in spite of the 
fact that the fuel metering device employed dual pumps for improving the 
range of air/fuel ratios that could be administered. This resulted 
principally from low flow stability in the fuel injection rate range of 
0.4-0.9 grams per second. Such system also required a motor which would 
operate the fuel injection device while operating at a constant low rpm; 
this was difficult to devise. 
Open loop control systems have experienced comparable problems. One system 
employs a hydro-pneumatic fuel metering device responsive to an air flow 
measuring device consisting of a spring loaded flapper and a specially 
designed orifice. The flapper valve is located in the air inlet system 
between the air cleaner and the air throttle valve. The air flow signal is 
transmitted to a signal amplifier and it is designed so that the pressure 
drop in the device is proportional to the two-thirds power of air flow. 
This fuel metering assist is deficient because it is unable to compensate 
for the hysteresis of the open loop metering, and is not able to operate 
over a broad enough air/fuel range required of the engine. Another 
metering device typically used with the open loop system is a spool valve 
which in certain positions can bypass fuel. This latter device is not able 
to operate with a broad enough air/fuel ratio range. 
SUMMARY OF THE INVENTION 
A primary object of this invention is to provide an improved air/fuel 
control system for a Stirling engine adapted for automotive use, the 
system being characterized by the ability to operate accurately over a 
considerably wider range of air/fuel ratios. 
A detailed object of this invention is to provide an air/fuel control 
system which is effective to vary the air flow in response to temperature 
changes in the heater head of a Stirling engine, and then to vary fuel 
introduction in response to a change in the air flow, the variation in 
fuel introduction being able to meet a fuel range as broad as 0.4-15 grams 
per second. 
Another object of this invention is to provide an air/fuel control system 
of the open-loop type which eliminates hysteresis of the fuel metering 
function in response to a change in air flow. 
Yet another object of this invention is to provide an air/fuel control 
system for a Stirling engine which employs an air sensing system linearly 
proportional to variations in air flow normally experienced by Stirling 
engines. 
Yet, still another object of this invention is to provide an air/fuel 
control system which additionally provides for the shutting off of fuel 
introduction during deceleration of the engine and at the same time 
provides for shutting off of fuel during an excessively high hydrogen 
temperature condition which normally would occur during part of the engine 
deceleration. 
Features pursuant to the above objects comprise: 
(a) the use of an air sensing device which operates on a vortex shedding 
principal wherein the cooling effect upon a sensing rod stimulates an 
electrical signal responsive to the amount of vortex flow present in the 
fluid engaging said rod; (b) the use of a fuel metering device which 
employs at least three fuel injectors placed in parallel and deriving fuel 
from a common fuel manifold, each injector functioning to inject fuel by 
way of a fuel nozzle into a common exit manifold, and (3) an electronic 
shaping circuitry which is effective to take the pulse output signal of 
the air flow sensor and process it to provide a signal strong enough and 
in proper form to control the fuel injectors.

DETAILED DESCRIPTION 
Turning first to FIG. 1, the Stirling engine has a thermodynamic cycling 
mechanism from which work energy is extracted, such cycling apparatus 
requiring the input of a continuous supply of heat. To this end, a 
combustor or burner unit 10 located at the end of a heater head 11 is 
supplied with both air (from passage 12) and fuel (from nozzle 13) which 
is ignited in the combustor. The air supply system 14 is comprised of an 
air blower 15 which sucks air through a delivery unit 16 having an air 
filter and silencer 17 at its far end. The driven air is delivered to a 
recuperator device 18 which transfers heat to the incoming air before it 
enters the burner unit. A fuel supply system 19 is provided which has a 
fuel pump 20 drawing a suitable quantity of fuel from a fuel tank 21 which 
is thence metered by an apparatus 22 (requiring pressure regulation by 
unit 23) and delivered to the atomizing nozzle where it is mixed with air 
from supply 24 and the burner unit containing air supplied from passage 
12. Additionally, a fuel and air supply may be further introduced by way 
of exhaust gas recirculation employing a passage 25 interconnecting the 
exhaust system 26 and the air supply system 14 (immediately upstream from 
blower 15). The exhaust gases which, if containing a sufficient amount of 
oxygen and unburned fuel, are recirculated to a portion of the suction 
side of the air blower and permitted to mix with the incoming air. 
The combusted hot gases within the heater head 11 transfer heat units to a 
pressurized hydrogen working fluid operating in a closed system 27 in a 
known manner of the Stirling cycle to power pistons in cylinder 28. 
A control system for the air fuel sypply system requires measurement of 
hydrogen temperature in the heater head, measurement of air flow, and 
control of fuel, air and EGR in response thereto. The heater head hydrogen 
temperature is measured by a thermocouple 30 inserted into the heater 
tubes of system 27. This measurement signal is processed electronically in 
unit 31 where, after amplification, it is compared to a reference voltage, 
representing the desired hydrogen temperature; the difference in voltage 
is used to operate a positioning motor 32 which in turn operates an air 
control valve 33. 
Air flow is selected because it is dependent on blower speed, which in turn 
is dependent on engine speed, the latter responding more slowly to a 
demanded change than fuel flow. This assists in reducing the hysteresis of 
the control cycle. Fuel flow is needed to follow in the desired air/fuel 
ratio in response to a change in air flow. 
The control aspects of the fuel supply system is comprised of an air flow 
measuring device 34, an electronic control module 35, fuel metering 
apparatus 22 having three fuel injectors (36, 37 and 38) mounted between 
common inlet manifold 39 and common outlet manifold 40, and the fuel pump 
20 and a pressure regulator 23. A fuel safety shutoff valve 41 is used 
downstream of apparatus 22. The air flow measuring device senses the 
volumetric flow, the temperature, and the pressure of the incoming air. 
The output signal of the air flow measuring device has a frequency 
proportional to mass air flow. It is sent to the electronic control box 
where the pulse is converted to a direct current signal used to control 
both the pulse width and the switching points of the three fuel injectors. 
Turning now, in particular to FIGS. 2-3, the air flow measuring device has 
a sensor constructed to utilize a vortex shedding phenomenon. A two-rod 
system (42 and 43) is used, with the upstream rod 42 generating vortices 
45 in the air stream, as shown in FIG. 3. The vortex frequency is 
proportional to air velocity (and volume), and is detected by a 
thermosensor 44 (comprised of strips 44a and 44b) placed on the second rod 
43 downstream from the shedder rod 42. The two nickel sensing strips (44a 
and 44b) are placed on the second or glass sensing rod 43 in a position 
such as to react to the vortices which are generated alternately on each 
side of the upstream rod. The second downstream rod reacts to the cooling 
effect of these vortices on its two self-heated nickel-on-glass elements. 
These elements are connected in an electrical bridge arrangement to 
eliminate common mode factors. The output of thermosensor 44 is a vortex 
frequency resistance variation created by the heating and cooling of the 
nickel elements. A D.C. current through the nickel elements maintains the 
heat, and an amplifier boosts the millivolt signal generated at the bridge 
output. The amplified sinusoidal frequency then goes through a circuit in 
the electronic control unit 35 that gives a square wave pulse output 
signal (see modification in FIG. 4). The ambient temperature signal (from 
Sensor 8) and ambient pressure signal (from sensor 7) are applied by 
circuit 9 in the control box to the volumetric signal to produce a 
frequency proportional to mass flow. 
FIG. 5 presents a block diagram showing how the pulse output signal from 
the air flow sensors is processed for control of the drive signal to the 
series of three fuel injectors (36, 37, 38). The pulse is changed to a 
D.C. level signal by a frequency to voltage converter 50. This D.C. level 
signal proportional to air flow, is applied to width modulators 51, 52, 53 
associated with each injector. Each pulse modulator has a clock signal 
input from a clock oscillator 54 which sets the repetition rate. This 
determines how many times per second the injectors will be turned on. The 
air flow signal determines the pulse width through the action of the pulse 
width modulators. This determines how long the injectors are turned on. 
The output of a level detector 55 is fed to a switching logic unit 56 
along with the output of the pulse width modulators (51, 52, 53) and the 
logic circuit therein determines which of the injectors is to be turned on 
at any one moment. Driver amplifiers 57, 58, 59 boost the low level output 
of the switching logic to a high level current pulse for operating the 
solenois injectors (36, 37, 38). 
The metering apparatus 22 has three electrically actuated solenois fuel 
injector valves. Each consists of essentially a tapered pin 6 and tapered 
orifice 5; the pin being normally biased to close the orifice. A solenoid 
winding 4 is energized to withdraw the pin and permit fuel flow through 
the orifice. Fuel floods an inlet manifold 39 in communication with the 
inlet ports of each injector. The pressure of the fuel in the inlet 
manifold is maintained at a constant pressure of about 39 p.s.i. One or 
more of the injector valves are turned on during engine operation, since 
the Stirling cycle requires a continuous external combustion circuit. 
Thus, the fuel flow in the output manifold 40 will vary, but still have a 
pressure of 39 p.s.i. One of the three injectors has a smaller flow 
orifice and covers the flow rage of 0.4-2 grams per second. The other two, 
operating together but 180.degree. out of phase, cover the flow range of 
2-15 grams per second. preferably, fuel injector 36 is sized to provide a 
fuel flow of 0.4 to 2.0 g/sec., injector 37 of 1.0 to 7.5 g/sec. and 
injector 38 of 1.0 to 7.5 g/sec. The injectors have combined metering and 
atomizing orifices, which serve only as a metering valve. In this manner, 
a cascaded addition or subtraction of their combined fuel flows will give 
the required fuel metering range. 
The time constant of the fuel metering system is electronically 
controllable allowing the matching of the air flow measurement device time 
constant to provide an accurate predetermined air/fuel ratio in the 
combustor during transient operation.