A hot-gas engine in which the supply of fuel to the burner device is controlled by means of a control signal which is derived from a differential pressure signal which represents the volume flow of combustion air and which is corrected for variations in temperature and pressure of the ambient air.

The invention relates to a hot-gas engine, comprising at least one 
combustion chamber having connected to it at least one supply duct for 
combustion air, including a restriction, and at least one supply duct for 
fuel, the quantity of fuel to be supplied to the combustion chamber being 
controlled in proportion to the supplied quantity of combustion air by 
means of a control device which comprises a differential pressure sensor 
which communicates with the air supply duct upstream and downstream from 
the restriction. 
A hot-gas engine of the kind set forth is known from Netherlands patent 
application No. 7,308,176 to which U.S. Pat. No. 3,935,708 corresponds, 
laid open to public inspection, notably from FIG. 1. 
The pressure drop across the restriction detected by the sensor is a 
measure of the volume flow of air to the burner device. Variations in the 
temperature and the pressure of the ambient air, however, may 
substantially vary the air density. This means that, while the volume flow 
of air and the measured pressure drop remain constant, the mass flow 
(product of density and volume flow) of air to the burner device varies. 
As a result, the air/fuel mass flow ratio is undesirably disturbed. 
The present invention has for its object to provide a hot-gas engine of the 
kind set forth in which the air/fuel mass flow ratio is corrected for the 
effect of ambient temperature variations and ambient pressure variations 
in a structurally simple manner. 
In order to achieve this object, the hot-gas engine in accordance with the 
invention is characterized in that upstream and downstream from the 
restriction a branch duct is connected to the air supply duct, the branch 
duct comprising a first duct portion which includes a first flow 
resistance element and a second duct portion which includes a second flow 
resistance element, the differential pressure sensor being connected to 
the first duct portion upstream and downstream from the first flow 
resistance element, the second flow resistance element being subject to 
control means which, by controlling the flow resistance in response to 
variations of ambient air temperature and pressure, correct the 
differential pressure to be sensed by the sensor for the said variations. 
In a preferred embodiment of the hot-gas engine in accordance with the 
invention, the first flow resistance element is adjustable. 
This offers the advantage that the air fuel ratio which corresponds to the 
nominal operating conditions can be simply adjusted. 
A further preferred embodiment of the hot-gas engine in accordance with the 
invention is characterized in that a third flow resistance element which 
is connected to the first branch duct portion upstream and downstream from 
the first flow resistance element can be switched on. When the third 
element is suitably proportioned, it is readily possible, without 
modification of the adjustment of the first flow resistance element, to 
temporarily decrease the differential pressure signal applied to the 
sensor. The fuel flow then decreases and the air/fuel ratio increases, 
which is desirable for starting the engine.

The reference numeral 1 in the FIGURE denotes a hot-gas engine in which a 
working medium performs a thermodynamic cycle in a closed working space 
during operation. Heat originating from a burner device 3 is applied to 
this working medium from the outside through the walls of a heater 2. 
The burner device 3 comprises a burner 4, a combustion chamber 5, a supply 
duct 6 for combustion air and a supply duct 7 for fuel. Exhaust gases 
which have given off their heat to the heater 2 are discharged through the 
outlet 8. 
The combustion air supply duct 6 includes, on the suction inlet side of a 
fan 9, a restriction 10, for example, a valve as shown in FIG. 3 of U.S. 
Pat. No. 3,935,708 cited above. 
A fuel pump 12 supplies fuel from a fuel reservoir 11 to the combustion 
chamber 5. A relief valve 13 provides the desired pressure on the outlet 
of the fuel pump 12. The fuel supply duct 7 includes a restriction 14. 
The combustion air supply duct 6 has connected to it, on either side of the 
restriction 10, a branch duct 15 comprising a duct portion 15a, including 
a flow resistance element 16, and a duct portion 15b which includes a flow 
resistance element 18. 
On either side of the flow resistance element 16, the duct portion 15a has 
connected to it an auxiliary duct 19 which includes a third flow 
resistance element 20 and a valve 21. 
During normal operation the valve 21 is closed and the pressure differences 
prevailing across the flow resistance element 16 and the restriction 14 
are applied to a control device 22 which operates a control valve 23 in 
the fuel supply duct 7 in order to adapt the fuel flow to the combustion 
air flow in the duct 6. 
The control device 22 may be constructed, for example, as shown in FIG. 3 
of U.S. Pat. No. 3,780,528. 
If the pressure difference across the restriction 10 is .DELTA.P, the flow 
resistance of the element 16 is R.sub.1 and the flow resistance of the 
element 18 is R.sub.2, the pressure difference applied to the control 
device 22 amounts to 
EQU .DELTA.P.sub.1 = [R.sub.1 /(R.sub.1 + R.sub.2)] .DELTA.P. 
.DELTA.p.sub.1 can be varied by varying the resistance R.sub.2 of the 
element 18. This is effected by the control means 24 so that the signal 
derived from the restriction 10 and applied to the control device 22 
through the flow resistance element 16 is corrected for variations in the 
air density which are caused by temperature and pressure variations. 
The control means 24 comprise an assembly formed by a known pressure sensor 
and a known temperature sensor. The ambient pressure signals and ambient 
temperature signals measured are converted into electrical signals which 
control the element 18 which is constructed as a valve. 
When the ambient temperature increases, the valve 18 is closed further, so 
that R.sub.2 increases. The pressure drop .DELTA.P.sub.1 across the 
element 16 then decreases, with the result that the mass flow of fuel also 
decreases. This is desirable because the higher ambient temperature causes 
a decrease of the air density and hence of the mass flow of air through 
the restriction 10. When the ambient temperature decreases, the valve 18 
is opened further. 
When the ambient pressure increases, the valve 18 is opened further, so 
that R.sub.2 decreases and P.sub.1 increases. The mass flow of fuel then 
also increases. This is necessary because a higher air pressure implies a 
higher density of the air flowing through the restriction 10. A larger 
mass flow of air is then accompanied by a larger mass flow of fuel, so 
that the air/fuel ratio remains constant. Conversely, the valve 18 is 
closed further if the ambient pressure decreases. 
Obviously, a variety of alternatives are feasible. For example, the 
electrical control signal originating from the control means 24 can be 
used, for example, for controlling a heating element which heats the air 
flowing through the element 18, thus varying the resistance of the element 
18. 
The flow resistance element 16 is adjustable, so that the nominal desired 
air/fuel ratio can be adjusted. 
The flow resistance element 20 has a resistance which is substantially 
lower than the adjusted resistance of the flow resistance element 16. 
When the valve 21 is opened, the flow resistance element 16 is effectively 
short-circuited and the control device 22 receives a signal 
.DELTA.P'.sub.1 which is smaller than .DELTA.P.sub.1. As a result, the 
mass flow of fuel decreases. Thus, the air/fuel ratio can be temporarily 
increased, notably when the motor is started, without the nominally 
adjusted flow resistance element 16 being changed. 
If desired, the flow resistance element 20 and the valve 21 can be combined 
to form one element.