How Temperature Compensation In Carburetor Occurs
Like most liquids, petrol flows more easily when hot. Therefore in hot weather or in slow-moving traffic, when under bonnet temperatures are high, fuel flow from the carburetor increases, producing a rich mixture.
Emission carburetors employ various adaptations to prevent this. On a vacuum SU HIF carburetor, the main jet is carried at the end of a bi-metallic strip.
The strip bends as the temperature rises and is so arranged that, as the temperature increases, the strip pushes the jet upwards, positioning it at a wider part of the metering needle and weakening the mixture.
Alternatively, the base of the main jet may incorporate a wax capsule that expands when heated with a similar result. Zenith CD and some fixed-choke carburetors use a bi-metallic blade attached to a conical valve.
As the blade heats up and bends, the valve is drawn outwards, opening a channel that allows extra air to enter the carburetor barrel. Slightly weakening the mixture.
All emission control devices are non-adjustable and operate automatically. Some recent carburetors make use of the fact that a turbulent flow of fuel is not affected by variations in temperature.
By adding a small quantity of air to the fuel before it leaves the jet, the flow becomes turbulent and needs no temperature compensation.
Hint: ‘as it gets hot, fuel flows more easily. To prevent over-richness, the SU HIF carburetor uses a bi-metallic spring in the fuel that bends upwards as the temperature rises, raising the jet to maintain the correct mixture.’
To help warm up the engine as soon as possible and reduce the need for a choke, many cars now have a dual air intake to the air cleaner, one taking cool air from the front of the engine bay and one taking hot air ducted from the exhaust area.
On starting from cold, a valve at the joint of the two intakes blanks off the cold air pipe and allows the carburetor to take only hot air from the exhaust duct.
The warm air helps vaporize the fuel and enables the choke to be dispensed more rapidly.
When the engine is warm-up, the valve which may be wax-operated or worked by a temperature-sensitive bi-metallic blade switches to the cold intake.
Cold air occupies less space than hot air, and the cold intake charge fills the cylinders more efficiently, enabling the engine to develop more power.
Engines without automatic air intake control usually have a ‘summer’ and ‘winter’ manual setting for the air cleaner to pick up pipe.
The winter setting, which picks up warm air from near the exhaust manifold, should be selected during the colder months.
In addition to Poppet’s ‘by-pass’ valves, pollution is also high when an engine is decelerating on a closed throttle. With the throttle shut, any fuel clinging to the manifold walls as a liquid will suddenly be sucked into the cylinders.
For a moment this makes the mixture far too rich, then when this fuel has been used, the mixture becomes too weak. Each extreme gives a sharp increase in the pollutants in the exhaust gas.
A simple cure for this is to place a spring-loaded poppet valve in the throttle plate.
Under all other conditions but deceleration, the poppet valve remains shut; during deceleration, the particular high vacuum in the inlet manifold opens the poppet valve, allowing a moderate amount of mixture to enter.
This avoids the sudden burst of liquid fuel being drawn into the cylinders and enables the engine to continue to fire cleanly.
A similar result is achieved by a ‘by-pass’ valve. A channel is built into the carburetor which runs from the atmosphere side of the throttle plate to the engine side.
Flow through the channel is obstructed by a diaphragm-operated valve connected to the inlet manifold.
At very high vacuum levels, the diaphragm unseats the valve, allowing a certain proportion of mixture to flow down the channel into the manifold. Once again, the rich ‘snap’ followed by excessive weakness is avoided.
