Measuring a flow of gas through a combustion engine

In measuring the mass flow rate of the inlet air entering an internal combustion engine or other combustion device, or of the exhaust gas discharged therefrom, a constant metered flow of helium or other inert tracer gas is introduced through a temperature-controlled critical flow orifice into the air intake of the engine or device, and the exhaust gas is continuously sampled and the sample analyzed by means of a mass spectrometer. From the measurement of the concentration of the tracer gas in the exhaust gas sample provided by the mass spectrometer, the required mass flow rate is derived and displayed. The mass spectrometer may be employed to measure in sequence the concentrations of other constituents of the exhaust gas sample as well as that of the tracer gas, and from these measurements other parameters related to the mass flow rate of intake air may be derived by means of a microprocessor, and displayed.

This invention relates to methods and apparatus for measuring a flow of gas 
at a given point in a combustion engine. 
Traditionally measurements of gas flow quantities for internal combustion 
engines and other similar devices have been made by operating at a given 
steady state and using instruments requiring a definite time to register 
the flow quantity or quantities under investigation. Some of the flow are 
also pulsating ones which demand the use of special instruments or the 
provision of a smoothing capacity between the engine and the flow 
measuring device. By way of examples: (a) fuel consumptions are determined 
by measuring the time the engine takes to consume a known volume (or 
weight) of fuel whilst the engine is operated at the fixed test speed and 
load, and (b) air flow is measured by using an orifice plate after 
providing adequate smoothing capacity to reduce pulsation amplitudes or, 
better, using a viscous flow air (or gas) meter (British Pat. No. 473139). 
These devices require a finite time to obtain a steady pressure drop 
reading across the measurement element, also for a determination of the 
downstream pressure and temperature required for the calculation of the 
flow quantity referred to a standard temperature and pressure. 
Whilst such methods are satisfactory for steady state measurements, there 
is now a need for flow measuring devices having very fast response times 
to allow the virtually instantaneous variations of flow which occur when 
an automotive engine is driven on the road or on a roller dynamometer 
following one of the legally set driving cycles for the measurement of 
exhaust gas emissions. Such fast reading instruments are particularly 
required for experimental development in relation to the reduction of 
exhaust emission levels. 
In a typical spark-ignited, throttle-controlled, gasoline engine the 
full-speed full-power air consumption will be some 30 times the low-speed 
engine idling value. This raises problems with accuracy of measurement 
over the whole flow range quite apart from any question of instrument 
response times. 
The present invention is intended to meet these requirements. 
According to the present invention, a method of measuring the flow rate of 
gas at a given point in a combustion engine comprises introducing a 
constant metered flow of an inert gas, referred to as tracer gas, into the 
inlet gas entering the intake of the combustion engine during the 
operation of the engine, simultaneously withdrawing a continuous sample 
flow of the exhaust gas from the engine exhaust system and supplying a 
constant-rate flow of the sample gas to a mass spectrometer for analysis, 
and operating the mass spectrometer to provide a continuous or sequential 
measurement of the volumetric proportion of the tracer gas in the exhaust 
gas sample, and utilizing the said measurement to derive an indication of 
the transient value of the said flow rate of gas in the engine. 
The invention is applicable not only to I.C. engines of the 
reciprocating-piston type but also to I.C. engines of the rotary piston 
type and to gas turbines. It may also be applied to the external 
combustion chambers of engines operating on the Stirling cycle and to 
other combustion devices for use with automotive machines, and the term 
"combustion engine" is used herein to include all such devices as well as 
I.C. engines of all kinds. 
Conveniently the inert tracer gas comprises helium. Argon may also be used, 
but helium is preferred because its atomic weight is further apart from 
those of other possible substances present in the exhaust gas sample. 
Calibration may be effected by comparing the reading with one taken when 
supplying calibration gas flow containing a known proportion of helium to 
the spectrometer, for example a mixture of gases, such as carbon dioxide, 
nitrogen and oxygen, with helium in known proportions. 
The invention from another aspect comprises apparatus for measuring the 
flow rate of gas at a given point in a combustion engine, which apparatus 
includes means for introducing a metered flow of inert tracer gas into the 
flow of inlet gas entering the intake during the running of the engine, 
means for simultaneously and continuously withdrawing a sample flow of the 
exhaust gas from the engine exhaust system, a mass spectrometer and means 
for supplying a constant rate flow of the exhaust gas sample for analysis 
to determine the volumetric proportion of the tracer gas in the sample. 
In one form of the invention the metered flow of tracer gas is provided by 
a temperature-controlled critical orifice connected to a supply of inert 
tracer gas under pressure. 
In one simple form the invention is employed for determining the exhaust or 
inlet gas mass flow rate. In this case the mass spectrometer is tuned to 
the single mass number of the tracer gas e.g. 4 for helium. 
The invention may also be employed for measuring other parameters related 
to the engine inlet gas charge flow. Thus in another form of the invention 
the method also includes catalytically reacting the sample of exhaust gas 
with oxygen, to oxidize any carbon monoxide or unburnt hydrocarbons 
present, and comparing the concentration of the tracer gas with that of 
one or more other components of the oxidized sample. For this purpose a 
mass spectrometer of Quadrupole type is preferably employed and may be 
arranged to measure the concentrations of CO.sub.2, N.sub.2 and O.sub.2 as 
well as that of the tracer gas. The output from the mass spectrometer may 
be fed to a microprocessor to derive measurements of one or more of inlet 
gas mass flow, fuel flow, inlet gas mass flow plus fuel flow, and inlet 
gas mass flow divided by fuel flow.

A simple embodiment of the invention set up to measure the exhaust or inlet 
gas mass flow rate of an i.c. engine 10 is shown in FIG. 1. 
The general principle is to inject a small accurately-measured flow of 
inert tracer gas into the engine air intake system by a small pipe 11 
which is readily clipped into the intake pipe 12, and then determine the 
proportion of the tracer gas present in the engine exhaust gases 
discharged through the exhaust system 13. Helium is suggested as the 
tracer gas since it will not interfere with signals due to other gases 
involved at the subsequent measuring stage. 
Thus, helium gas is injected at a fixed rate into the engine air intake 
pipe 12 to have a concentration of around 30 parts per million (p.p.m.) in 
the resultant air-helium mixture when the engine 10 is operating at full 
speed and wide open throttle. With a constant quantity of helium per unit 
time continuously injected, and if the idling air flow is one-thirtieth 
(1/30) that at full power conditions, the helium concentration under 
idling conditions in the intake mixture will be 900 p.p.m. or 0.09%. Pure 
helium is supplied from a compressed gas cylinder 14 via a reducing valve 
15 and to a temperature controlled "critical flow" orifice 16 to the 
injection pipe 11. The orifice 16 is surrounded by a temperature-control 
coil 16A connected to a control unit 16B. 
A "critical flow" orifice is one through which the gas flows at the speed 
of sound, so that the downstream pressure has no influence on the flow 
rate. To achieve this condition a certain ratio of upstream to downstream 
gas pressures must be imposed. However when the critical flow condition is 
satisfied, the flow rate varies directly as the absolute upstream pressure 
and inversely as the square root of the upstream absolute temperature. 
Thus, if the upstream pressure and temperature are both accurately 
maintained, as is proposed, the helium flow rate will remain constant 
regardless of the discharge conditions in injection pipe 11 and engine 
intake pipe 12. 
It is, of course, possible to use other means for ensuring a constant-rate 
mass flow of injected helium. These can be based on a number of principles 
but require the taking of further measurements and their utilisation 
through a servo feedback circuit to control the flow of helium. 
The engine intake air plus its small helium addition now passes into the 
engine 10, combustion occurs, and the combustion products discharge into 
the tail pipe 13 of the exhaust system as usual. At any convenient point 
18 a sample of the exhaust gases is continuously taken by a pump 19 
through a line 20, filtered to remove particles larger than 1 micron by a 
filter 21, and then returned at 22 to the exhaust discharge pipe 13. 
Downstream of the filter 21 a mass spectrometer 23 is attached to the line 
20 via a small pipe 24 containing a restriction in the form of a tiny 
`leak` orifice 25. To avoid condensation of the water vapour contained in 
the engine exhaust gases the sampling line 20 and pipe 24 must be kept at 
a minimum temperature of 80.degree. C. 
The mass spectrometer 23 can conveniently be of the Quadrupole type which 
is a known commercial type of instrument. Suppliers include Edwards High 
Vacuum International Ltd., Manor Royal, Crawley, Sussex, England. 
Since such an instrument is continuously pumped with its self-contained 
vacuum pump down to a pressure of less than 10.sup.-5 Torr (1 Torr = 1 mm 
Hg = 1.33 millibar), a continuous small leakage (approximately 1 cm.sup.3 
/minute) of exhaust gas products will flow at a constant rate through the 
restriction 25 into the mass spectrometer 23. 
FIG. 1A shows diagrammatically the general arrangement of a Quadrupole mass 
spectrometer, which operates by ionizing the gas sample and separating the 
ionized gas particles in accordance with their mass-to-charge ratios. The 
instrument is housed in a high vacuum enclosure 231 provided with a vacuum 
pump (not shown), and comprises an ionisation chamber 232 provided with a 
heated filament ioniser 233 in the enclosure. The ionisation chamber 237 
is connected to the sampling pipe 24 so that a flow of gas to be analysed 
is drawn through the leakage restrictor 25 into the ionisation chamber 
where it becomes ionised. Some of the ions are electrically extracted, 
indiscriminately, from the chamber and are focussed by means of 
electrostatic lenses indicated diagrammatically at 234 into a narrow beam 
235 which is directed into a corridor extending between four 
precisely-positioned elongate parallel cylindrical electrodes 236 in the 
form of metal rods, constituting the so-called Quadrupole mass filter or 
ion separator. The rods 236 are electrically connected individually to an 
r.f. potential and in addition opposite pairs of rods have respective 
positive and negative d.c. potentials applied to them. The r.f. and d.c. 
potentials can be adjusted, and can be programmed in any required manner. 
Depending upon the electrostatic fields thus created at any given time, 
ions of a certain ratio of mass to charge will be subject to balanced 
attractions to oppositely-situated electrodes 236, and will continue 
substantially undeflected along the path of the beam 235, whilst the ions 
of all the other mass-to-charge ratios will be excessively attracted 
towards electrodes on one side or the other and will be deflected towards 
and collected on those electrodes, where they are neutralised, in effect 
being filtered out. The ions of the particular mass-to-charge ratio which 
pass through the ion separator 236 impinge on the collector plate 237 of a 
digital electrometer 238 whose output signal, representing the ion current 
to which the filter is "tuned," is amplified and digitalised in amplifier 
26 and supplied to a numerical read-out device 27. The instrument is tuned 
by adjusting the r.f. and d.c. potentials to select the particular ion 
mass-to-charge ratio which is to be passed through the ion separator 236, 
and hence the particular gaseous constituent of the gas sample whose 
composition is to be measured. In this way a direct, virtually continuous 
reading of the transient value of the volumetric proportion of the 
selected constituent of the gas sample to which the instrument is tuned 
can be read off on the read-out device 27. 
In the arrangement of FIG. 1, the mass spectrometer 23 is set up and tuned 
to the single mass-to-charge ratio of 4 (helium). Calibration is obtained 
from the observation of the mass spectrometer reading when supplied from a 
gas bottle 28 having a known concentration of helium in a mixture of 
carbon dioxide, nitrogen and oxygen. This calibration gas mixture is 
supplied through a reduction valve 29, a zeroing change-over valve 30 and 
a pipe 31 to a two-position calibration change-over valve 32 by means of 
which either the exhaust gas sample flow from the exhaust system or the 
calibration gas flow from the cylinder 28 can be switched into the pump 
circuit and hence to the mass spectrometer for analysis. The zeroing valve 
30 has an air inlet 30A which enables atmospheric air to be switched into 
the pipe 31 for zeroing purposes. The change-over valves 30 and 32 may be 
operated electromagnetically or otherwise. 
Since the atmosphere contains about 4 p.p.m. of helium a periodic 
calibration check should be made since this value of helium content 
requires to be subtracted from the values measured as a result of the 
helium injection process. 
If the mass flow rate of injected helium is M.sub.He, that for the exhaust 
gases is M.sub.EX, and the molecular weight of the exhaust gases is 
N.sub.EX, then following Dalton's and Avrogadro's Laws the helium 
concentration in the exhaust gas sample is given by: 
##EQU1## 
M.sub.He is known from the flow setting of the critical orifice controlling 
the injection of helium into the engine's intake system, and ConcHe is 
read off from the mass spectrometer read-out device 27. 
So the exhaust mass flow rate can be determined if its average molecular 
weight is known. The average molecular weight of the exhaust gases is 
dependent on the air/fuel ratio. Assuming water vapour is not removed from 
the sample gas, it can be shown that the average molecular weight for the 
exhaust gases is constant for mixtures leaner than 15.5 but varies for 
mixtures richer than this. 
Provided the air/fuel ratio is greater than 15.5 the above expression (2) 
permits the exhaust gas mass flow rate M.sub.EX to be determined. The 
read-out device 27 of the spectrometer may be provided with a 
microprocessor operating a display calibrated directly in terms of 
M.sub.EX. 
If it is desired to determine the mass flow rate of inlet air into the 
engine intake this can be found from the value of the exhaust gas 
molecular weight by use of the expression 
##EQU2## 
and M.sub.a = mass flow rate of inlet air 
V.sub.h = volumetric injection rate of helium 
C = concHe 
.lambda. = the equivalence ratio defined as the ratio of the operational 
air/fuel ratio to that for stoichiometric mixture. 
The quantity K in practice has substantially constant values for either dry 
or wet mixture measurements for equivalence ratios greater than about 
1.05. At smaller values of equivalence ratio K falls significantly in 
value with .lambda.. Subject to the limitations mentioned, K may be taken 
as 28.6 for dry gaseous mixture measurements and 27.5 for wet mixture 
measurements. 
The read-out device 27 will include a microprocessor with a display 
calibrated directly in terms of Ma. 
If the mass spectrometer 23 is switched on when the engine is not 
operating, it will not detect any helium and the read-out instrument 27 
will indicate an infinite flow rate. Similarly, after the engine stops the 
flow measurement system will continue to sample the remaining gas in the 
exhaust system. This can be avoided by interconnecting an engine oil 
pressure sensing switch or, in the case of a spark ignition engine, the 
ignition switch, to control the mass spectrometer. Alternatively, as 
indicated a vibration sensor switch 33, which is a simple inertia switch 
fitted to the engine or its exhaust system, can be used. Whichever 
switching device is employed it can be used to switch the valves 30 and 32 
into their positions for measuring exhaust samples when the engine is 
operational and to connect the mass spectrometer to the standard 
calibration gas bottle 28 when the engine is switched off. Thus in FIG. 1, 
the repeated on/off signal from the vibration sensor switch 33 is 
amplified at 34 and is used to operate a control unit 35 for the valves 30 
and 32. The same sensing switch can also be used to turn on and off the 
supply of helium to the critical orifice. 
The basic idea system described with reference to FIG. 1 can be expanded to 
permit the rapid determination of the following related parameters 
provided that the carbon/hydrogen ratio of the fuel is known: 
(i) Instantaneous total mass flow of exhaust products, M.sub.EX. 
(ii) Instantaneous inlet air mass flow rate into engine intake M.sub.a. 
(iii) Instantaneous fuel mass flow rate into engine M.sub.f. 
(iv) Air/fuel ratio. 
(v) The sum of the air and fuel flow rates. 
To do this requires two modifications to the system of FIG. 1, and a 
microprocessor programmed to process the output signals to provide and 
display the required values, and FIG. 2 shows such a modified system. 
The first modification shown in FIG. 2 is the provision of, and insertion 
of, a small oven 40 and oxidation catalyst 41 in matrix or granule form 
maintained at 600.degree. C, together with a tube 42 adding oxygen, in the 
sampling line 20 between the sampling point 43 from the exhaust system 13 
and the mass spectrometer leak tap-off point 44. The tube 42 is connected 
via a regulating valve 45 to a cylinder 46 of compressed oxygen. 
The second modification is to programme the voltages applied to the four 
parallel cylindrical electrodes 236 in the mass spectrometer 23 so that 
they sequentially change at short but regular time intervals to permit the 
collection in sequence of ions having molecular weights of 44 (carbon 
dioxide), 28 (nitrogen) 32 (oxygen) and 4 (helium). Such a programming is 
known practice within the field of mass spectrometer technology. 
The four sequential signals from the mass spectrometer are supplied as 
inputs to a microprocessor 50, being digitalized in a form which is 
compatible with the microprocessor used. The microprocessor 50 is 
programmed with the necessary mathematical formulae and provided in 
addition with three thumbwheel switches for the manual insertion of inputs 
representing: 
(a) The hydrogen/carbon ratio of the fuel used. 
(b) The concentration of helium in the ambient air. 
(c) The helium flow rate into the engine intake 12 controlled by the 
conditions of pressure and temperature at the critical flow orifice 16. 
The values for air flow M.sub.a, fuel flow M.sub.f, the sum of the air and 
fuel flows (M.sub.a + M.sub.f), and the air/fuel ratio M.sub.a /M.sub.f 
can be read off in sequence or from separate displays of the output 
display unit 27. As changes are made to the engine operating regime new 
values will be automatically determined and displayed. In addition, when 
required, the microprocessor can be arranged to provide integrating 
functions, to totalise the air and fuel consumptions during any period of 
engine testing from the rapidly repeated redetermination of the 
instantaneous values at short but discrete time intervals. 
Now the combustion of the fuel CH.sub.n, and the subsequent mixing with 
oxygen and catalyst oxidation can be expressed as: 
##EQU3## 
where n is the hydrogen/carbon ratio of the fuel. 
This assumes that a sufficient number (Y) of molecules of oxygen is added 
to complete the combustion. 
Let the number of molecules in the exhaust gas be B. 
##STR1## 
Exhaust concentration of helium = conc He = 
##EQU4## 
average molecular weight of exhaust gas. 
From (1) and (2), the concentration of the exhaust components can be 
expressed as: 
##EQU5## 
Average molecular weight of exhaust gas = 
##EQU6## 
From these is obtained 
##EQU7## 
The expressions are programmed into the microprocessor to enable it to 
provide output readings of M.sub.a, M.sub.f, M.sub.a + M.sub.f and M.sub.a 
/M.sub.f, in response to the input signals from the mass spectrometer 
representing the transient values of Conc CO.sub.2, Conc N.sub.2, Conc 
O.sub.2 and Conc H.sub.2 in the exhaust gas sample. 
As regards the oxygen supply 46, 42 and the oxidation catalyst 41, only 
enough oxygen is required to ensure that at the entry to the matrix or 
pellet box of the catalyst 41 the mixture strength is leaner than 
stoichiometric, i.e. that an excess of oxygen over that required for 
complete oxidation of any carbon monoxide, unburnt hydrocarbon or hydrogen 
is present. The inflow of oxygen through the pipe 42 need not be metered, 
but a typical flow rate would be about 60 cm.sup.3 /min for every 
liter/min of exhaust sample gas flow, or 6%, permitting the recording of 
air/fuel ratios down to 11 to 1. The exhaust gas sample plus the oxygen is 
heated in the oven 40 and passed through the catalyst bed 41 maintained at 
600.degree. C. 
In this case the calibration gas bottle 28 will be the same as for the 
simple case of FIG. 1, being a known mixture of carbon dioxide, nitrogen, 
oxygen and helium. The separate supply of oxygen from cylinder 46 is 
turned off during calibration of the mass spectrometer. 
Although the exhaust gas sample is shown extracted from the tailpipe of the 
engine in FIG. 1, and in FIG. 2, there is no reason why it cannot be taken 
close up to the engine with a consequent reduction in time lags. If 
required, the helium can be injected into any one of the engine's intake 
ports close to the inlet valve, and the mass spectrometer sample taken 
from the exhaust branch of the corresponding cylinder, permitting the 
charge flow into that individual cylinder to be determined.