Measurement of air mass flow into an internal combustion engine

An air mass flow measuring system for measuring flow into an internal combustion engine comprises a throttle angle transducer and further transducers measuring ambient air temperature and pressure as well as the pressure drop across the throttle valve. The signals from these transducers are supplied to calculating circuit which carries out the calculation M=K.sqroot.P.multidot..DELTA.P/T. The values of the coefficient K used in this calculation are derived from an empirically programmed read only memory incorporated in the calculating circuit, such memory storing empirically determined values of K for each combination of a plurality of values of throttle angle and a plurality of values of .sqroot. P.multidot..DELTA.P/T.

This invention relates to the measurement of air mass flow into an internal 
combustion engine, particularly for the purpose of controlling engine 
fuelling and ignition timing. 
In the past it has been proposed (see for example UK Pat. No. 1,321,989) to 
utilize a digital memory to store data relating to the engine fuelling 
and/or ignition timing for a plurality of different combinations of two 
different engine parameters, such as engine speed and throttle position or 
inlet manifold pressure, and memory being empirically programmed to 
achieve satisfactory engine running in all engine conditions. Such a 
system is dependent on the provision of a memory containing data matching 
the engine type with which it is to be used. Any minor changes in engine 
design are likely to require a new memory. 
More recently, various attempts have been made to measure air mass flow 
directly, either by aerodynamic methods (e.g. by using a spring-loaded 
hinged flap in the air intake) or by more sophisticated methods (e.g. by 
hot wire anemmometry or corona discharge type mass-flow meters). These 
methods have not proved entirely successful as it is found that parameters 
other than air mass flow--e.g. pressure or humidity could affect their 
accuracy, giving rise to the need for complex compensating circuitry. 
Other flow measuring systems, such as those described in U.S. Pat. Nos. 
3,817,099; 3,818,933; 3,888,458 and 3,949,717 overcome some of the above 
mentioned disadvantages, by utilizing a valve in series with the throttle 
flap and maintaining a constant pressure differential across this valve by 
servo-control of the valve in response to the pressure differential signal 
across the valve. This system, however, is not only very complex and 
costly as a result of the need for a second valve of complex shaping and a 
servo-mechanism, but the servo-mechanism can also introduce hysteresis 
problems. 
The present invention has for its object to provide a relatively simple air 
mass flow measurement system for an internal combustion engine. 
An air mass flow measurement system in accordance with the invention 
comprises an air intake throttle position transducer and further 
transducers sensitive to ambient air pressure, ambient air temperature and 
the pressure downstream of the throttle and a calculating circuit 
connected to said transducers and arranged to execute the calculation 
##EQU1## 
where M is the air mass flow, 
K is an orifice coefficient of discharge, 
P is the ambient air pressure, 
T is the ambient air temperature, and 
.DELTA.P is the pressure drop through the throttle, 
said calculating circuit including a digital memory containing empirically 
determined data representing the value of K for each combination of a 
plurality of different values of the throttle position and a plurality of 
different values of one or a combination of P,.DELTA.P and T. 
Preferably, the memory data represents a value of K for each combination of 
a plurality of different values of the throttle position and a plurality 
of different values of the expression .sqroot.P.multidot..DELTA.P/T, which 
is related to the Reynolds Number of flow through the throttle. 
With this system, provided a standard throttle arrangement (i.e. a standard 
combination of throttle flap and surrounding tube) is used, the system 
will provide an output accurately representing air mass flow irrespective 
of the design, tune or wear of the engine it is used with. 
The air intake throttle position transducer may be non-linearly calibrated 
to produce an output signal directly related to the coefficient K.sub.0 
(i.e. the value of K at a standard P,.DELTA.P and T). 
Alternatively the air intake throttle position transducer may be a linear 
transducer.

Referring firstly to FIG. 1, the air intake includes a five inch length of 
tube 10 of diameter 2 inches provided at its ends with conventional means 
11, 12 for mounting it between the air cleaner and the induction manifold 
of an engine. Four transducers, 13 to 16 are associated with the intake. 
Transducer 13 is a throttle butterfly angle transducer which may simply be 
a potentiometer on the butterfly shaft. The potentiometer 13 may be a 
linear potentiometer, in which case it provides a signal relating to 
butterfly angle, or it may be calibrated to give a direct indication of 
the coefficient K.sub.0 referred to above. The transducer 14 is a 
commercially available differential presure strain gauge type transducer 
measuring the difference in pressure between tappings 2 inches upstream 
and 1 inch downstream of the butterfly spindle. The transducer 15 is 
another commercially available strain gauge-type transducer which is 
connected to measure the absolute air pressure at the upstream tapping. 
Transducer 16 is a commercially available NTC thermistor. 
Turning now to FIG. 2 the block diagram shows the four transducers 13, 14, 
15 and 16 (in the latter case by an amplifier) connected to the four 
inputs of a signal selector circuit 17 (an integrated circuit type 4052) 
controlled by two bits of the output word from one port 18 of a 
microcomputer. The output of the selector 17 is connected via an amplifier 
with a gain of 2 to the analog input of an analog-to-digital converter 
module 19 (an integrated circuit type AD571) also having control 
connections to one bit respectively of ports 18 and 24. The resultant 
digital signal is input to the microprocessor via ports 24 and 22, and the 
mass flow signal is output in digital form from ports 18 and 23. The 
micro-processor unit also includes RAM 20, ROM 21 and a central processor 
unit 22 (e.g. an INTEL 8051 micro-processor) but the remaining elements of 
the micro-computer are commonplace and will not be listed herein. 
Turning now to FIG. 3 the programme provides for periodic sampling of the 
signals produced by the four transducers 13 to 16. Since ambient pressure 
and ambient temperature are variables which change relatively slowly as 
compared with the throttle angle and pressure drop the former signals are 
sampled less frequently than the latter. To this end the RAM includes a 
sampling cycle counter which determines the relative sampling frequencies. 
Initially, when the sampling cycle counter content is zero (C=0), a full 
sampling cycle, including successive inputting of T,P,.DELTA.P and K.sub.0 
is undertaken. The value of K for the existing flow conditions is then 
derived by looking up stored values in the ROM and the sampling cycle 
counter is then incremented. The value of M is now calculated and 
outputted following which the content of the sampling cycle counter is 
examined. If this content is not at a value C.sub.max, the program returns 
to the input .DELTA.P stage. If C=C.sub.max then the program returns to 
the input T stage. The effect of this is that .DELTA.P and throttle 
position are sampled more frequently than P and T and, being the most 
likely to change quickly, are kept updated. 
Where (not shown) the transducer 13 merely reads the angle of the 
butterfly, this angle data is inputted and, as an additional program step 
preceding the looking up of K(P) in the ROM, the value of K.sub.0 is 
looked up in another ROM look-up table. 
A detailed description of the program in structured English is annexed as 
an appendix hereto. 
FIG. 4 shows a typical K characteristic for a throttle valve. To derive 
this characteristic the tube 10 was connected in series with an accurate 
mass flow meter and a valve between the inlet of a vacuum pump and 
atmosphere. The throttle butterfly was set in turn to different angles of 
opening and, at each angle the actual mass flow and the corresponding 
values of P,.DELTA.P and T were noted for different settings of the value. 
The value of K for each such setting was then calculated and plotted. 
It will be noted that the values of the parameter shown as ROOT in the 
graph, which represents the value of .sqroot.P.multidot..DELTA.P/T are 
limited to a progressively more limited range as the throttle butterfly is 
opened. This is taken into account in the program described in the apendix 
hereto. 
##SPC1##