|The SU Carburettor|
The carburettor. One of the most importants parts on your engine. (Unless you've got injection). Ok Americans might call it a carburetor.....but hey we are talking about a British car here!!...so no arguing please carburettor it is, although the V-8 engine is (was) a tiny bit American.
It may sound simple, mixing air and fuel. But reality is a bit more complicated than theory. So let's have a closer look at this little device.
The purpose of the carburettor is to mix air and fuel, but what ratio should be used? A lot of air and little fuel? or vice versa? Well theoretically for 1 kg of petrol about 14.6 kg of air has to be be burned. So very roughly the fuel/air ratio is theoretically 1:14,6. When you supply more air the ratio gets higher than 1:14,6 i.e. a leaner mixture, there still will be some free oxygen left in the exhaust gases. If you supply more fuel than air the ratio gets below 1:14 i.e. a richer mixture,there will be no free oxygen left but some fuel will leave the engine unused.
For optimum economy we want the engine to burn a mixture as lean as possible. For the best power the ratio has to be richer to make sure all the available oxygen gets burned. The optimum mixture range for poweris therefore different from the economy range, as you can see in the diagram.
For the best power the ratio has to be 1:10 to 1 :14 for optimum economy it has to be 1:16 to 1:19. The standard setup from the factory mostly tends to go towards the fuel consumption optimum rather than power.
Emission control systems on modern cars with catalysators and a labda sonde are trying to achieve a labda=1 situation. This means the injection or carburations system tries to keep the mixture at a ratio of about 14:1 At that point the catalysator works at its best...But not the engine as you can see. There is some power to be gained....or fuel consumption could be better...Defenitely not a win/win situation but a compromise. Fortunately european SD1's were never equipped with catalysator systems, they were however in America.
So the task for a carburettor is to mix fuel and air with a near constant ratio. Because we want don't want to travel at one single speed only, the amount of fuel and air fed to the engine also varies a lot. From idle to full speed. The carburettor has to maintain a near constant fuel/air ratio over the entire rpm range of the engine....defenitely not a simple task.
Right from the start engineers have been fiddling and tweaking around with various systems to mix
air and fuel under different loads and engine speeds. Basically there are two types of
The static carburettor
When air flows through a carburettor the velocity will be at its highest at its narrowest part, the Venturi. As speed increases the pressure decreases. So in the venturi the pressure is also at its lowest. If we let fuel in at the venturi, because of the low pressure, the fuel would be drawn in. To prevent flooding the fuel level is maintained at a constant height relative to the outlet by the float. Now all that is left is to incorporate a throttle in the air passage to control the amount of air we admit to the engine and our carburettor is ready.
The problem with this carburettor is that it will work fine at only one speed!, because air is compressible and fuel is not. When we open the throttle more air will go through the carb. and the pressure at the jet decreases. Being compressible the density of the air decreases. This means that the weight for a given amount of air decreases. But the carburettor still sucks in the same amount of fuel!. Thus with increasing airspeed we get a richer air/fuel ratio! We already have seen that we want a steady air/fuel ratio so it is necessary to reduce the amount of supplied fuel with the rising of the air speed.
To compensate for this effect, the static carburettor is equipped with an emulsion tube. This premixes some air and fuel before it enters the main port. In this way the correct ratio can be obtained. The size of the fixed choke itself is only a compromise. A small choke will deliver a correct and stable mixture at light throttle conditions, but the flow of air at full throttle will be restricted. A large diameter choke, can be quite satisfactory for full throttle conditions but gives poor fuel metering at low speeds because of the relatively low air velocity passing the fuel jet.
This is why Double throat type carburettors are used. They almost eliminate the problem of venturi size in fixed choke designs. At light throttle, low speed conditions the mixture is supplied by one small choke (approx. 22mm). At high air speeds a second larger choke (approx. 26mm) automatically comes into operation and in conjunction with the small primary choke allows a sufficient volume of fuel-air mixture to enter the engine.
At low speeds the pressure is often not low enough to get enough fuel out of the jet, certainly not if there is an emulsion tube. Therefore the carburettor is also equipped with a by-pas which mixes air and fuel in a circuit parallel to the main venturi, the idling system.
Under acceleration there is extra power needed. To get the mixture into the optimum power range additional fuel is added by means of an acceleration pump.
And finally a Choke is incorporated into the carburettor body. The choke valve is being closed when the engine is cold. This causes a great pressure drop over the valve. As a result the pressure at the venturi is very low and the mixture will be very rich. This enables the engine to start when it is cold.
As you can see there are a lot of systems in a static carburettor. And the bigger carburettors with double venturis can be very very complex. However once set up properly they function very reliably because there are very few moving parts in the system. That also explains the name "static" carburettor...doesn't sound very wild and dynamic does it?...so that is why the British have invented the following genious device..The constant vacuum carburettor....Ehm, yes I am biased towards Albion products, so what?, sue me
The Constant Vacuum carburettorThe constant vacuum carburettor....it's a different approach to the same problem...how to mix air and fuel under different engine conditions.
The basic principle is quite straightforward as we can see in the pictures at the right. Over the years the principle basically remained the same as in the first model of 1904.... Never change a winning team.
At low engine speed (idle) The air inlet is almost closed by the piston. The piston was pressed down by gravity in old SU designs using heavy pistons but with the later, lighter, aluminium piston designs an additional spring was added (spring not shown in pictures for clarity).
When the throttle is is opened more air flows towards the engine because the intake becomes less restrictive. The air speed across the piston increases resulting in a larger pressure drop over the piston. The pressure above the piston decreases and the piston is being sucked upwards. The opening area increases and air speed and pressure drop across the piston decreases until there is a new balance between the underpressure above the piston and the weight of the piston and spring load. More flow means the piston moves higher into the carb. until the balance is restored. This is why the pressure across the piston (and the airspeed) is virtually constant...... and the carburettor is called a constant vacuum carburettor.
Perhaps the most beautiful set-up with SU carburettors can be found in the Series 1 Jaguar E-types. The 3.8 ltr engine was equipped with three HD 8 carburettors. Power output was rated by Jaguar at 265 bhp. However a figure of 210 bhp would be closer to the mark
The genius of the design is to locate a tapered needle at the underside of the piston which works in a jet filled with petrol. (The fuel level in the jet is determined by the float). Now the higher this needle (and the piston) goes the less it restricts the fuel flow because of its tapered design. Thus the more air enters the higher the piston moves upwards and more fuel is supplied. Simple and effective.
The shape and characteristics of the metering needle governs the fuel air mixture ration for all speed and load conditions. You can see the needle as a simple mechanical equivalent of the memory chip from a digital EFi system.
A thicker oil gives more damping and as a result a richer mixture under acceleration (But still the original mixture at constant throttle openings). Because viscosity of the oil changes a bit with ambient temperature the acceleration mixture is also somewhat dependend on ambient temperature. The use of synthetic oil in the dashpot can reduce this effect because the viscosity changes less with temperature.
Cold start enrichment
Some types use an electrically operated auxiliary enrichment carburettor. This is actually a small separate carburettor which by-passes the main carburettor and admits fuel directly to the manifold when the unit is activated by a switch on the dashboard.
The throttle valve and the cold start enriching mechanism are interconnected so when the cold start is pulled out, the engine idle speed is increased. The first few degrees of the cold start mechanism opens the throttle valve only. Further movement lowers the fuel jet and opens the throttle valve a further amount.
The SU evolution
The H (Horizontal) SU carburettor is one the most used carburettors on British cars in the early fifties. The picture with the SU in three positions is an H-type carburettor. Typical details of this model were:
The HS was introduced in the late fifties. It was produced together with the H and HD series. differences are:
After the H, the HD (Horizontal Diaphragm-jet) was introduced.
The HIF (Horizontal Integral Float chamber)
SU CARBURETTORS Pt. 2