11 Fuel : air ratio compensation for fixed-choke carburettors

Fundamentals of carburation



361



air into the fuel contributes to atomisation, vaporisation and uniformity of

distribution. Manually actuated extra air valves, which have been mostly of

the slide-valve type, are no longer in use except on a few motor-cycles and

small industrial engines. The additional complication of the control system

entailed is one disadvantage, and it is doubtful whether many motor-cycle

drivers have the skill and experience to use them properly. In recent years,

pressures aimed at reducing emissions have, in any case, rendered such

devices unacceptable.



10.12



Compensation by compound and submerged jets



One of the earliest successful methods of compensation was the Baverey

compound jet illustrated in Fig. 10.4. The fuel is drawn through two jets, that

at A being the main supply and the other, at that at B, the compensating jet.

Originally only the compensating jet orifice was situated in the base of the

float chamber, but it soon became common practice to submerge both jets.

The advantage of submerged jets is that, because they are lower and therefore

subjected to a weaker depression than if they were in the venturi, they have

to be of larger bore. Consequently, they are easier to manufacture to close

tolerances and less likely to become clogged by debris or dirt in the fuel, and

the flow characteristics of two jets totally submerged at approximately the

same level are more favourable for mixture correction than if they were

discharging into the air. In the illustration, the main jet is shown discharging

fuel from the float chamber directly into the air, while the submerged

compensation jet discharges into the well D, whence the fuel is drawn on up

to the discharge nozzle C.

When the engine is started, the level of the fuel in the well is initially the

same as that in the float chamber, so there is no flow through the compensating

jet and discharge nozzle C: since atmospheric pressure, h2, acting on the

surfaces of the fuel, is transmitted to both ends of the jet, there is no pressure

differential across it. As the throttle is opened and the rate of flow of fuel

through both jets increases, the level of the fuel in the well falls and the

pressure differential, h2 – h1, across the compensating jet progressively

increases, h2 being the maximum head above the jet. Consequently, the flow

through the compensating jet similarly increases until, at the point at which

the well is totally empty, it remains constant despite increasing air flow and



C



h1



h2



Constant level



A



D

B



Fig. 10.4 A diagrammatic representation of a

main and compensating jet system



362



The Motor Vehicle



therefore a weaking mixture. In the illustration, h1 is the head when the level

of the fuel in the compensating jet well has fallen to an intermediate value.

Over the steeply rising portion of the air flow curve in Fig. 10.3, the

increasing depression progressively increases flow from both the compensating

well through the tube C, and that replenishing the well through the compensating

jet. This initially maintains the mixture at a constant value. Then, when the

well is empty and the flow from the compensating jet therefore remains

constant while the air flow continues to rise, the tendency towards increasing

richness of the mixture is countered, so it again remains constant. Over the

upper part of the range, when the basic enrichment curve is straighter and

less steep, some further compensation may be effected by the passage of air

from the well up through the discharge nozzle. This method of compensation

has been widely used in carburettors, though usually incorporating an additional

air bleed principle, to be described in Section 10.13.

In the meantime, Fig. 10.5 demonstrates the principle graphically and in

more detail. In this illustration, mass fuel flow curves for main jets No. 90

and 100, and compensating jets Nos 110 and 135 are plotted together with

the curve for the air flow through the largest choke from Fig. 10.3. It can be

seen that, as the head above the two compensating jets increases from zero

to h2 – h1, the latter being in this instance 3 cm of water (or 4 cm of petrol),

the rate of flow from them lies above that for the main jets. Beyond this

point, however, the fuel flow from these jets remains constant.

It will be found that, if the flow from the main jet 100 is added to that

from the compensating jet 110 and that from the main jet 90 to that from the

compensating jet 135, the curves produced by plotting the totals from both

will, at 20 cm depression, coincide with the air curve, indicating that, at this

27



1.8

Air flow large choke

Normal compensation

High compensation



1.6



24



Weight of petrol – g/s



1.2



18



100



1.0



Ma



in



j



ets



15



90



0.8



12



0.6



9



Weight of air – g/s



21



1.4



135

0.4



6



Compensating jets

110



0.2



0



5



10



15

20

25

30

35

40

Depression at choke – cm water



3

45



0

50



Fig. 10.5 Fuel flows through different sizes of main and compensating jet plotted

together with the air flow through the largest venturi plotted in Fig. 10.3



Fundamentals of carburation



363



depression, the mixture is chemically correct. Indeed, the jets were selected

to give this result. On the other hand, the larger main jet combined with the

smaller compensating jet gives reasonably good compensation throughout,

with only slight over-compensation.

Although the other combination (100 and 135) gives considerable overcompensation, this may give the result desired, since we need rich mixture

for starting and idling and then, for good fuel economy, a weak mixture

which can be enriched as and when necessary, for example for acceleration

and obtaining maximum power, by means of other devices to be described

later. It can be revealing to plot the curve for over-compensated mass fuel

flow from Fig. 10.5 together with the three mass air flow curves from Fig.

10.3, all against mass air flow, to show quality of mixture (both as a percentage

and a ratio). This has been done in Fig. 10.6.



10.13



Air bleed compensation



As has been previously stated, air bleed compensation can be incorporated in

the compensating jet well. However, to simplify the explanation, we shall

describe how it functions as the sole method of compensation in a main jet.

A good example, which was incorporated in some of the early Solex

carburettors, is illustrated in Fig. 10.7.

An advantage of using this device is that only one jet is needed. As can be

seen from the illustration, the jet D is drilled in the lower end of a thimbleshaped tube, where it is fully submerged. The open upper end of this tube is

subject to the depression in the venturi. Solex called this the jet tube, though

terms such as emulsion tube or diffuser tube are used by other manufacturers

to describe similar components.



30

20



High compensation



13



10



14

Chemically correct



0



15

16



% Weak



10

20

Large choke

Medium choke

Small choke



30



0



3



6



9



12

15 18

21

Weight of air flow – g/s



24



27



17

18

19

20

21



Ratio air/petrol by weight



% Rich



12



30



Fig. 10.6 The full line represents the total mass fuel flow through the small main and

large compensating jets in Fig. 10.5, while the other three lines are the corresponding

mass air flows depicted in Fig. 10.3 but all have been re-plotted against mass air flow

instead of depression in the venturi



364



The Motor Vehicle

Mixture flow



X



X



A

C

Air in



B

Air in



A

B

C

D

E

XX



Jet cap

Jet tube

Atmospheric vents

Jet orifice

Jet carrier

Static fuel level



C

D

E



Fig. 10.7 Illustrating the principle of the air bleed system



Drilled radially into the jet tube are as many air bleed holes as may be

necessary to perform the compensation function: in the illustration, two

diametrically opposed pairs of holes are shown. The open lower end of a

deep thimble-shaped cap A is screwed down over the jet carrier E, the lower

end of which is screwed into the carburettor body. In the otherwise closed

upper end of the cap, which seats firmly on the upper end of the jet tube to

retain it, there is a small hole.

Interposed concentrically between the jet tube and cap is a tubular upward

extension of the jet carrier, forming a well between it and the jet tube.

Between the upper end of this extension tube and the inner face of the upper

end of the cap is a clearance, so that air at atmospheric pressure passing in

through the two radial holes near the base of the cap can flow up and over

into the top of the well.

When the engine is started, the jet tube and the well are full of petrol up

to the level XX. As the throttle is opened and the well empties, the level of

the fuel in the well falls. Because this progressively uncovers the air bleed

holes C, the depression over the upper end of the jet tube is correspondingly

weakened, thus offsetting the tendency towards enrichment of the mixture.

At the same time, the air begins to bubble through the fuel in the jet tube and

emulsifies it, thus assisting evaporation. At high levels of depression, jets of

air squirt through the holes in the bleed tube into the rising column of petrol,

emulsifying it even more effectively.



Fundamentals of carburation



365



In some of the Weber downdraught carburettors air bleed systems are

used in conjunction with rotary valves. The latter are actuated by the throttle

controls, to reduce the air supply as full throttle is approached, thus enriching

the mixture for the development of maximum power.

Figures 10.8 to 10.11 are diagrammatic sections through an early Claudel–

Hobson and three later carburettors, showing variations on the air bleed

theme. In these, A is the air metering bleed plug, M the main jet, and V the

venturi, or choke. Also, a single air bleed orifice has supplanted the bleed

holes in the jet tube that was shown in Fig. 10.7, so the degree of compensation

can be adjusted simply by fitting an air bleed plug having an orifice of a

different size. As indicated in Section 10.12, an additional air bleed effect

can be obtained by virtue of the flow of air through the idling jet when the

well or feed passage becomes empty.



10.14



Multiple venturis intensify air bleed compensation



In the Stromberg downdraught carburettor, Fig. 10.10, the exit from the



A

V

V



M



M



A



Fig. 10.8 Claudel–Hobson air bleed



Fig. 10.9 Solex ‘assembly 20’



A

W



V1

V



V2



E



C



M



Fig. 10.10 Stromberg air bleed



Fig. 10.11 Zenith air bleed



M



366



The Motor Vehicle



venturi V1 is in the throat of V2. Consequently, because the pressure difference

across V1 is higher than it would be if only a single venturi were used, the

velocity of flow through, and depression in it are also higher. Double venturis

not only increase the depression over the diffuser but also introduce a twostage mixing process: the primary stage occurs in V1 and the secondary one

in V2, where the rich mixture issuing from V1 is blended with the air emerging

from V2. Incidentally, a triple venturi can be seen in Fig. 11.22.



10.15



The Zenith V-type emulsion block



In Fig. 10.11, C is the compensating jet, which is installed in the bottom of

the float chamber, next to the main jet. The passage leading from it takes the

compensating fuel supply into an intermediate chamber in the emulsion

block E, instead of directly into the well W. In this chamber, the level of

depression is intermediate between those in the well and venturi, its actual

value depending on the sizes and characteristics of the communicating ducts

and the resistance to flow through the main passage in the emulsion block.

The general principle is discussed in detail in Section 10.18.



10.16



Secondary suction effects



In most carburettors, secondary suction effects are introduced to enhance

atomisation and mixing. These may arise from chamfering the ends of the

tubes, usually termed spray tubes, that deliver the fuel into the venturi, as in

Figs 10.8 and 10.11. Another device, which is shown as a black dot in Fig.

10.11, is the introduction of a spray bar, usually mounted diametrically

across the throat of the venturi. This has two effects: first, it increases the

depression over the end of the spray tube; and, secondly, the turbulence that

it generates downstream enhances mixing. The effects of these devices are

generally evaluated experimentally or by modelling.



10.17



Mixture requirements in more detail



So far, we have expressed mixture strength in terms of air: fuel ratio. However,

over the past 40 years, the symbol λ has been increasingly widely used,

mainly because λ values represent percentage mixture strength. Since λ = 1

is the stoichiometric, or chemically correct, mixture, the 10% weak and 10%

rich values are respectively λ = 1.1 and 0.9 which, expressed as percentages,

become respectively 110 and 90%.

Although a 10% weak mixture is the normal average for maximum economy

and 10% rich for maximum power, several factors complicate the situation.

At wide throttle openings, the cooling effect due to the evaporation of the

extra fuel increases the density of the charge and, at small throttle openings,

a slight enrichment is necessary to compensate for the effects of the residual

exhaust gas in relation to the small quantities of air getting past the throttle

and passing into the cylinders. Furthermore, to satisfy modern emissions

regulations, extremely accurate control over mixture strengths is needed.

The influence of mixture strength on engine performance is illustrated in

Fig. 10.12.

Electronic control of carburettors would appear to be necessary to meet

modern requirements. So far, however, it has unacceptably increased their



Fundamentals of carburation



367



g/kWh



Rich



Power (kW)



Specific fuel consumption (g/kW h)



kW



Correct

Weak

Mixture strength



Fig. 10.12 The influence of mixture

strength on engine performance



10%

0

10%

20%



Chemically correct mixture



80%–90% full load



20%



Weak



Mixture strength



Rich



overall compexity and, therefore, cost. Since fuel injection is easier to control

electronically and the product is ultimately less costly, the carburettor is

rapidly moving towards obsolescence.

Referring back to Fig. 10.1, it can be seen that the mass air flows through

the venturi under conditions (2) and (3) are identical, despite their widely

differing throttle openings. The mixture requirements also differ widely: for

(2) a rich mixture is required to obtain high mean effective pressure and

torque, to cater for acceleration; on the other hand, (3) must have a weak

mixture persisting up to 80 to 90% full throttle, for economy at moderate

loads. Condition (4) again calls for a rich mixture, for maximum power as

full throttle is approached. Mixture requirements in relation to load and to

engine speed at constant load respectively are illustrated in Figs 10.13 and

10.14.

If we plot the requirements against mass of air flow, as was done in Fig.

10.6, we have the curves illustrated in Fig. 10.15. These are an elaboration

on the curve in Fig. 10.6 in that the ideal mixture requirements are plotted

for part and full load (for simplicity, emissions regulations have been ignored).

From this and the previous paragraph, it is clear that a carburettor in which

the mixture strength is determined only in relation to the depression in the

venturi could not meet these requirements.



Region of increasing quantity of mixture

Increase of throttle opening

Load



Fig. 10.13 Ideal carburettor characteristic



0

Region

of

increasing

strength



The Motor Vehicle

10

12

14



70



16



Air : fuel ratio



368



60



hp



50

40

500

hp



30



400

20

300



g/hp h



g/hp h



10

200

0

1000

25



2000

50



3000

75



4000

100



5000 6000 rev/min

125 150 km/h in 4



Fig. 10.14 Engine performance and mixture strength requirements at increasing

throttle opening increments in top gear

30



13

10



Fu



0



le

ll thrott



14

15



Chemically correct



% Weak



16

10



17

18

19

20



Part throttle

20



Air/petrol ratio



% Rich



12

20



30

0



0.25



0.50



0.75 1.00 1.25 1.50 1.75

Weight of air flow – kg/min



2.00



2.25



Fig. 10.15 Dual carburettor characteristic



10.18



Principle of the intermediate chamber



At this point it is appropriate to explain the principle of the intermediate

chamber, a device that is applied in various ways to carburettors. The fuel

discharge orifice is situated in the intermediate chamber instead of the venturi,



Fundamentals of carburation



369



and the areas of both the inlet and outlet through which the air passes are

controllable, for example by the air and throttle controls on a motor-cycle or

by a strangler valve, as described in Section 10.19.

As can be seen from Fig. 10.16, the value of the depression p2 in this

chamber, depending on the mass flow and relative areas of the inlet and

outlet, is intermediate between p1 and p3. At (a) the inlet is larger than the

outlet, the sizes being such as to cause the depression over the fuel discharge

orifice to be less than half that in the induction pipe. At (b) the relative sizes

of the openings are reversed but adjusted so as to maintain the same rate of

air flow. In the latter instance, the depression over the fuel discharge orifice

is more than half that in the induction pipe, so the mixture is richer. With

such a device, therefore, provided control over the adjustments is fine enough,

any change in mixture strength can be put into effect with constant air flow,

or the mixture strength can be held constant while the air flow is varied.



10.19



Starting and idling enrichment devices



When the vehicle is stationary and the engine ticking over, idling, or slow

running, the velocity of flow though the carburettor is such that the fuel

drawn from the jets is sufficient to develop only enough power to overcome

the resistance of its moving components. In this condition, the engine should

be running as slowly and quietly as practicable, but be capable of being

accelerated instantly, for instance for moving away from traffic lights.

However, in a fixed-choke carburettor, when the throttle is closed, for

either starting or idling, the flow through the venturi is too slow to create the

depression needed to lift the fuel from the jet, let alone to atomise and mix

it. To cater for these conditions, therefore, it is necessary to provide an

alternative, which takes the form of an additional jet and air orifice. Since, in

these circumstances, the highest velocity flow is that through the gap between

the edge of the butterfly valve and the throttle barrel, this is obviously the

best place for siting the idling system.

Because the depression at this point is strong and the velocities of flow of

p1



p3



p2



Induction pipe

Petrol



(a)



Atmospheric pressure

p1



p1



p2



d

p3



p2



p3

Induction pipe



Petrol



(b)



Atmospheric pressure

p1



p2



d



p3



Fig. 10.16 Intermediate

chamber



370



The Motor Vehicle



both air and fuel high, atomisation and mixing will be good. On the other

hand, air leaks, for example past gaskets downstream of the idling system,

will tend to represent a high proportion of the total flow. Therefore because they

would dramatically reduce the mixture strength, they must be carefully avoided.

Even when the engine is warm, the idling mixture must remain rich. If it

is not, it is impossible to be sure that an ignitable mixture will exist adjacent

to the points of the plug at the instant the spark is passed. This is because of

the low density of the charge and difficulty of obtaining homogeneity when

the jet is discharging down one side of the induction pipe. When the engine

is cold, only a small proportion of the fuel issuing from the idling jet will

evaporate, so the situation is even more difficult and therefore the mixture

has to be richer still.

In the early days, a spring-loaded plunger was provided on top of the float

chamber, so that the driver could, by depressing it and thus pushing the float

down momentarily, raise the fuel level in that chamber to enrich the mixture

for cold starting. Subsequently, manually actuated stranglers were installed

to perform the enrichment function. They comprised a cable-controlled valve,

usually of the butterfly or flap valve type, upstream of the venturi. Partially

closed, they increased the depression over the jets, and therefore enriched

the mixture. However, if the drivers forgot to open them again, the engine

could run for long periods with rich mixtures, causing sparking plug points

to become fouled with soot. Even worse, in very cold weather, neat fuel

could be drawn into the cylinders, wetting the sparking plug points and

draining down into the sump, thus diluting the lubrication oil.

Subsequently, a demand arose for automatic stranglers, which in the first

instance were actuated by bimetal strips or other thermostatic devices. Some

drivers, however, because they were unlikely to be aware whether the device

was malfunctioning, did not entirely approve of automatic chokes. Another

method of overcoming the problem was to use combined cold-start and

warm-up devices which, although independent of the carburettor, were in

most instances mounted on it. Some of these will be described in the following

sections. More recently, to satisfy the exhaust emissions regulations,

electronically controlled cold- and warm-start enrichment devices have become

virtually mandatory. These are mostly used with fuel injection systems and

therefore will be described in Chapter 12.



10.20



Separate starting and warm-up enrichment devices



The separate starter devices in use after the Second World War were brought

into operation manually, but semi- or fully-automatic adjustment followed

progressively as the engine warmed up. In many instances, however, when

the engine was warm, the device had to be disengaged manually. The enrichment

fuel was delivered either through the idling jet system or independently

through ducts that were opened and closed by the manual control.

Progressive weakening of the mixture as the engine becomes warm can be

effected by regulating the temperature, depression or velocity of flow of the

ingoing air. It should be borne in mind, however, that the velocity can be

increased only by increasing the depression. Since this can be simply a result

of increasing engine speed, it is not strictly an independent means of control.

Lowering the pressure or raising the temperature increases the rate of

evaporation, so both tend automatically to enrich the mixture.



Fundamentals of carburation



371



Interconnection of the strangler and throttle controls is widely used for

increasing the idling speed to prevent the engine from stalling after it has

been started from cold. Such devices, actuated automatically by movement

of the strangler control, usually comprise a mechanism for moving the throttle

stop also. To this end, links are employed to actuate either a swinging arm on

which the throttle stop is mounted or to rotate a cam, in both instances to

open the throttle.

The closing of the strangler and opening of the throttle increases the

depression over the main jet, on the principle illustrated in Fig. 10.16. This

causes fuel to be drawn from the main in addition to the idling jet, to enrich

the mixture. On the Zenith VIG and VIM models, mounted on the strangler

valve was a spring-loaded flap valve which would be opened by increasing

depression. From the comments in Section 10.18, it can be deduced that such

a valve could be spring loaded to admit an increasing volume of air to

maintain either a constant or an increasing depression in the venturi.



10.21



Zenith VE starter carburettor



A good example of a separate semi-automatic starting device is the Zenith

VE unit, Fig. 10.17, production of which ceased several decades ago. It is

simply a fixed choke, single-jet, miniature carburettor drawing its fuel from

the float chamber of the main carburettor, the output of which is delivered

directly into the induction pipe. It is calibrated to provide the appropriate

rich mixture for cold starting and has an automatic air bleed valve actuated

by the depression in its own venturi.

In the illustration, the cone valve V is opened by a cable connected to the

pull-out control on the dash to bring the device into operation. The depression

in the venturi C causes fuel to flow from the enrichment jet J in the base of

the main float chamber, to mix with the air passing through the venturi and

the open valve C, out through a discharge orifice in the wall of the induction

pipe just downstream of the throttle valve, shown dotted in the illustration.

As the engine speed increases, so also does the depression in the space S,

until it is high enough to unseat the air bleed valve B. This allows air to bleed

V

C



S



B



J



Fig. 10.17 Zenith starting device