combustion chamber design
2.26 CONSTANT-CHOKE CARBURETTORS
Figure 2.233 An air-bleed correction
2.26.5 Idle systems
A decreased airflow into the engine results in a reduction in air velocity at the venturi, and consequently the pressure at this point rises nearer to atmospheric. If the carburettor is properly corrected this will not alter the mixture strength, but it will result in a progressive coarsening of the spray owing to the reduced velocity of the airflow. Also, as the airflow is reduced, the velocity will eventually become insufficient to maintain the fuel droplets in suspension, owing both to their increased size and the low air velocity in the manifold. There is, therefore, a speed below which the engine will not run, and this depends primarily upon the size of the choke tube and, to a lesser extent, the diameter and length of the inlet
manifold. The conditions necessary for good low-speed operation are quite unsuitable for developing reasonably high power at high speeds, and there is obviously a limit to the extent to which power at high speed can be sacrificed to obtain good low-speed operation.
The extreme condition of low air velocity through the choke tube occurs when the engine is running but not driving the car. It then has merely to develop sufficient power to overcome its own internal friction.
This is the case whenever the vehicle is stationary, but may be required to move at any moment, as, for instance, when halted in traffic or at traffic lights. For economy and comfort, the engine should run slowly and quietly, but respond instantly to the opening of the throttle when the time comes for it to drive the vehicle.
This condition of engine operation is known as idling or slow-running. The airflow through the choke tube is then not only too slow to atomize the fuel, but there is insufficient depression in the choke tube to draw fuel from the jets.
To pass the amount of mixture needed to keep the engine running at idling speed, the throttle is barely open, and the air velocity is greatest where it passes through the very small gap around the edge of the throttle butterfly. A vacuum gauge will show that the depression in the manifold is at its highest, about 380–450 mm (15–18 inches of mercury) below atmospheric pressure.
An arrangement similar to that shown in Figure 2.234 is used to provide a suitable mixture for slow-running. A passage connects the float chamber, via the main jet, to an outlet positioned in the region of the throttle valve. Fuel flow is regulated by a slow-running jet or idling jet, and to ensure that fuel does not flow continually the top of the passage is taken above the fuel level in the float chamber. Siphoning is prevented by having an air bleed at the top of the inverted U of the passage and an adjusting screw adjacent to the throttle enables the volume of emulsified fuel entering the engine to be regulated to suit the condition of the engine. The speed at which the engine runs is determined primarily by the extent of the throttle opening; normally this is set by a throttle stop adjusting screw on the throttle linkage.
When the engine is idling a strong depression exists on the engine side of the throttle, causing fuel to be drawn from the idling system. This fuel mixes with the air spilling past the edge of the throttle butterfly.
2.26.6 Mixture strength for idle speeds
At idle speed, only a very small quantity of fuel is required for the engine to run. Under these conditions the cylinder contains a comparatively large quantity of exhaust gas. The exhaust gas dilutes the new charge, so in order to provide an ignitable mixture, a charge that is
slightly richer than normal is provided during slow running. The enrichment of the mixture is strictly limited because any unburned, or partially burnt fuel causes the exhaust emissions that pollute the atmosphere. Many engines are fitted with carburettors which will not idle at a speed less than about 900 revolutions per minute.
2.26.7 Acceleration from idle
Opening the throttle from the idling position causes more air to pass the throttle. This lowers the depression (raises the pressure) and reduces the fuel discharge from the slow-running outlet. At the same time, the airflow through the choke tube causes the main jet system to come gradually into operation. The changeover does not always occur smoothly, with the result that any deficiency in the fuel supply is accompanied by a sudden drop in engine power.
To avoid this ‘transfer flat spot’ the arrangement shown in Figure 2.235 is used. At a throttle position where the flat-spot would normally occur, one or more holes are drilled adjacent to the edge of the throttle valve to provide additional fuel discharge outlets. Fuel Figure 2.234 A slow-running (idle) system
Figure 2.235 Two-hole idling system
discharge from the upper hole is controlled by an interchangeable jet called a progression jet.
When the engine is idling a high depression on the engine side of the choke causes petrol to discharge from the lower drilling. During this phase the progression hole will be above the throttle, so air will enter this hole, pass through the progression jet and bleed into the slow-running mixture to assist in the emulsifying process. As the throttle is opened, the rush of air past the edge of the throttle will create a depression in this region and will cause fuel to discharge from the progression outlet.
2.26.8 Idle speed adjustment
To meet emission regulations, final adjustment of the slow-running mode requires a tachometer to set the engine speed accurately and an exhaust gas analyser to measure the quantity of carbon monoxide and hydrocarbons present in the exhaust gas.
The slow-running adjustment varies with different engines and carburettors, but the main points are as follows:
1 Check that the ignition system is serviceable and that the ignition timing is correct.
2 Warm up the engine to its normal operating temperature.
3 With the engine running, adjust the throttle stop screw to obtain the recommended idle speed.
4 Adjust the volume control screw until the highest and smoothest speed is found.
5 Set the exhaust emission level to the manufacturers specification.
6 Reset the throttle stop screw to make the engine slow-run at the recommended speed.
2.26.9 Emission control
To meet emission regulations, the slow-running system of a carburettor is designed to take into account the following:
● Tamper-proof adjusting screws
Screws provided for mixture adjustment are arranged so that non-qualified personnel are deterred from altering the mixture. The adjusting screw is normally hidden in a recessed hole, which is sealed by a metal or plastic plug.
● Carbon monoxide (CO), hydrocarbon (HC) content in the exhaust gas
An exhaust gas emission test, taken after the idle speed has been set, should show that the CO and the HC content is below the value specified by the manufacturer.
● Running on
When the ignition is switched off the driver expects the engine to stop immediately. Sometimes an engine
continues to run very erratically for a period of time or until it is stalled by the driver. During this running-on period combustion is initiated by a hot-spot within the combustion chamber, such as a valve or sparking plug electrode. The extraordinary high temperature of a component is often produced when the engine is operated on the weak mixture required to meet the emission regulations.
Various arrangements are used to overcome this problem. The method shown in Figure 2.236 is an electric solenoid which cuts off the slow-running mixture when the ignition is switched off. The system is often referred to as an run-on valve’ or ‘anti-dieseling valve’.
Figure 2.236 Anti-run-on valve
2.26.10 Economizers and power systems
Under ideal conditions a carburettor that supplied a chemically correct mixture (approximately 15:1) would cause the engine to produce maximum power and economy at all speeds and loads. In practice the physical problems associated with correctly vaporized fuel distribution to each cylinder makes it impossible to achieve this ideal. Even if it could be achieved, the difficulty of bringing each particle of fuel into intimate contact with the correct amount of oxygen needed to burn it completely to carbon dioxide (CO2) and water (H2O) makes the ideal unachievable.
Varying the mixture strength supplied to an engine that is operating under load at a fixed speed causes the engine performance to change as shown in Figure 2.237. The graph plots the ‘fuel used per unit of power’
against the ‘torque output’.
When the air/fuel mixture is weak the fuel economy is very poor and the torque output is low. As the mixture is enriched both the economy and torque improve, but after the lowest point of the curve (maximum economy) has been reached, further enrichment produces a slight increase in torque at the expense of economy. Continuing to enrich the mixture past the maximum torque point produces a drop in torque and a considerable rise in fuel consumption.
Three points on this graph should be noted:
1 The chemically correct (CC) mixture strength gives neither maximum economy nor maximum power.
2 Maximum power (P) requires a mixture that is slightly rich.
3 Maximum economy (E) requires a mixture that is slightly weak.
Whereas a carburettor without an economy system has small reserves of fuel on the outlet side of the main jet to allow for sudden increases in speed or load, an economy-type carburettor generally needs an
‘acceleration pump’. The pump delivers fuel into the mixing chamber when the throttle is plunged open.
There are two main types of acceleration pump, mechanical and vacuum-operated.
Mechanical acceleration pumps
The mechanical pump shown in Figure 2.238 is controlled by a linkage connected to the throttle. This linkage acts on a diaphragm, which draws fuel via a one-way valve from the float chamber and pumps it past a delivery valve fitted in the outlet passage. A separate jet, or restriction, in the passageway regulates the flow of fuel delivered by the pump.
Figure 2.237 Effect of varying air/fuel ratio
To achieve both maximum power and greatest economy the mixture strength supplied by the carburettor must be varied to suit the operating conditions, i.e. it must provide a slightly rich mixture for power and slightly weak mixture for economy. Emission regulations normally prevent the use of mixtures richer than 15:1, because they do not fully burn during combustion, so the engine power output at 15:1 is the maximum and the operating mixture range is on the weak side of this value.
The devices used in a carburettor to vary the mixture strength to suit either the power or cruising conditions are called ‘economizer or power’ systems.
In all cases the weak cruising mixture is obtained by reducing the petrol flow through the main jet system.
This can be achieved either by using a smaller jet or by reducing the pressure difference across the jet.
Restoration of the air/fuel ratio to 15:1 for power is achieved either by providing an additional path from the float chamber to the choke, or by exposing the main jet to the normal pressure difference. This is achieved through the use of a mechanically operated or vacuum-operated ‘power valve’.
2.26.11 Acceleration pumps
Whenever the throttle is plunged open, there is a sudden weakening of the mixture. One of the reasons for this is the delay caused by the reluctance of the fuel to respond compared with the fast action of the air. If the carburettor is already delivering a slightly weakened mixture to provide maximum economy, the further weakening that occurs when the throttle is opened causes a delay before the engine responds (in some cases it will cause the engine to misfire) resulting in a flat spot.
Figure 2.238 A mechanical accelerator pump
When the fuel outlet pipe terminates in a region where the pressure is lower than atmospheric there is a risk that fuel will discharge continuously, so a device such as a weighted valve is used to prevent this flow.
Most pump designs use a spring in the pump operating linkage to increase the time that the fuel in the pump chamber is being discharged. This additional fuel also improves exhaust emissions and economy.
Vacuum acceleration pumps
The pump shown in Figure 2.239 operates using the sudden change in manifold pressure that occurs when the throttle is suddenly opened. The pumping action is produced by the diaphragm spring, which is compressed when a high depression acts in the vacuum chamber. A sudden collapse of this depression allows the spring to discharge fuel into the mixing chamber.
2.26.12 Cold starting
Provisions in the fuel system are necessary to start a cold engine, as the low cranking speed of the engine is insufficient to allow the venturi to create an adequate depression. In addition, the slow-moving air entering the engine causes a large amount of fuel to be deposited
on the walls of the inlet manifold. Since the manifold and cylinders are cold, very little vaporization of the fuel takes place, so ignition of the charge under these conditions is very difficult.
Note: Petrol consists of many different hydrocarbons (HC) and the fractions (types of HC) that make up the fuel have different boiling points in the range 85 to 220°C. A light-fraction fuel boils (vaporizes) at a low temperature whereas a heavy-fraction fuel does not boil until the temperature approaches 220°C.
The provision of extra fuel during cold starting ensures there is an adequate supply of light-fraction fuels that are able to vaporize in the cold engine. If the quantity of gas formed by this vaporization is sufficient, it is possible to ignite the gas and produce enough heat to drive the engine over, even though the cold oil is causing a large drag.
Cold-starting systems used with fixed choke carburettors are normally of the ‘strangler’ type.
A strangler-type cold-starting system
Figure 2.240 shows the principle of a strangler. It consists of a flap valve which is positioned at the air entry point of the carburettor. When the valve is closed,
the strangulation of the air supply intensifies the depression felt at the venturi. As a result, extra fuel is supplied to provide a very rich mixture (typically 8:1).
Once the engine has started, the richness of the mixture must be reduced to a point where the engine runs smoothly. This reduces the risk of rapid cylinder wear caused by fuel washing the oil film from the top part of the cylinders. Excessive fuel also ‘fouls’ the sparking plugs and prevents them functioning in the normal manner.
For historical reasons the strangler is sometimes called the ‘choke’, which is the reason why this is occasionally marked on the driver’s control knob. The control cable acts on the strangler valve, the first movement of the ‘choke’ control opens the throttle a small amount to give a ‘fast idle’ to allow for the extra drag on the engine when it is cold.
The driver must not ‘over-choke’ the engine, because this action ‘floods’ the engine with fuel vapour and starves it of air. Combustion cannot occur under these conditions and this becomes apparent when a driver attempts to start a hot engine after it has not been used for a few minutes. During this time the fuel in the manifold vaporizes and drives out the air. So when the starter is operated, the engine will not fire until sufficient air has been induced. If the driver misreads the temperature condition and pulls out the ‘choke’, the result will be that the engine becomes flooded and most probably the plugs will be fouled. To clear the over-choked condition, the driver should push in the ‘choke’
and then keep the throttle fully depressed as the starter is operated. This action allows the engine to start after a few seconds unless the sparking plugs have been flooded with petrol.
To minimize the problems associated with over-choking, the strangler normally incorporates some arrangement to weaken the mixture as soon as the engine starts. Figure 2.241 shows an offset strangler valve that allows the moving air to act on the valve and partially open it after the engine has started.
Figure 2.239 A vacuum-operated accelerator pump
Figure 2.240 A simple strangler system Figure 2.241 Offset strangler
Automatic strangler
Incorrect use of the choke by the driver, especially the delay in returning it to the ‘off’ position during driving, causes emission problems as well as those drawbacks previously outlined. To overcome these problems, the automatic choke was introduced.
Figure 2.242 shows the principle of one system, which uses a bimetallic strip to sense the temperature of the coolant. When the engine temperature is less than about 15 °C the bimetal strip pulls the strangler to the closed position.
After the engine has started, the strangler is partly opened by the diaphragm in the vacuum chamber; this is activated by the high depression created in the inlet manifold.
As the engine warms up, the rise in coolant temperature causes the bimetallic strip gradually to reduce its pull on the strangler. Due to the rate at which the bimetallic strip heats up, the richness and the fast-idle action are slowly changed to suit the engine temperature.
2.26.13 Constructional features for later types of carburettor
The importance of emission control has meant that a carburettor must supply air and finely atomized fuel which is evenly distributed in the proportion needed to suit the engine operating conditions.
These requirements can be met fairly easily if the engine operates at one constant speed, but wide speed limits coupled with other variable factors (such as
engine load, engine temperature, changes in atmospheric temperature, pressure and humidity, etc.), make the carburettor’s task very difficult.
Single, fixed-choke carburettors
This type has been described in detail in the previous section. Alterations during the development of the carburettor include an ‘auxiliary venturi’. Placing a small venturi in a position such that its outlet is in the waist of the main venturi causes the fuel to mix with the air in two stages.
Petrol joining the air flowing through the auxiliary venturi gives a petrol-rich initial mix, but on meeting the main airflow at the auxiliary venturi outlet the fuel is distributed evenly throughout the air mass and a constant air/fuel ratio is obtained.
Twin-choke carburettor
Engines such as V6 and V8-types normally require two carburettors. The two carburettors are effectively joined together to share a common float chamber.
Normal practice is to arrange the manifold so that one venturi system supplies one set of cylinders and the other venturi the remaining cylinders. Each venturi requires a system for cold starting, slow-running, cruising and power.
The two throttle valves are interconnected and should be synchronized to give the same opening at all
The two throttle valves are interconnected and should be synchronized to give the same opening at all