Supercharging and Turbocharging

Supercharging and Turbocharging

Engine performance has been increased, without an increase in weight, by the use of supercharging and turbocharging. Other emission-control devices that

Turbocharger

correct the ignition timing and fuel delivery are covered in the appropriate learning programmes. These devices improve the performance of those systems, as well as reduce harmful exhaust emissions.

 State FOUR benefits of electronic control.

 Explain why pollution should be controlled.

 Look back over the previous section and write out a list of the key bullet points here:

AIR-SUPPLY SYSTEM AND INTAKE AIR TEMPERATURE CONTROL

Introduction The air-supply system has to provide clean air in sufficient quantity to the engine. Also, it must supply equal quantities of air to each cylinder. This will assist fuel vaporization and an even mixture distribution. Creating a swirl in the airflow as it enters the cylinders is also desirable. A system of warm air for cold starts, followed by temperature- controlled air for normal running, is essential. Finally, the system must silence the airflow and provide a flame trap in the event of fire in the inlet manifold.

System Components The air-supply systems for most vehicles are similar. They consist of an air-intake duct, an

Throttle body Airflow meter

air-temperature control mechanism, an air-cleaner housing and filter, an inlet

manifold and inlet ports. A position for Ducting an exhaust-gas recirculation system may

Air filter

also be included. For multi-point, or port, fuel-injection engines, the system will also include a throttle-body housing and an airflow meter.

Clean Air Clean air is required in the engine to prevent particles of dust and grit from damaging, or blocking, engine and fuel-supply components. Air Air filter is filtered through an element in the air cleaner. Most air-cleaner elements are made from micro- porous paper, which allows a good flow of air but traps airborne dust. Other elements have included oiled wire gauze and foam rubber. The air-cleaner housing and the filter elements are cleaned, or replaced, at scheduled service intervals.

Paper Elements Paper elements are folded to provide a large surface area and long service life. The element can be wrapped to form a circular

Circular-type

element if required. The outside edges are sealed air filter with an integral, or separate, rubber sealing ring.

Air-Cleaner Housings Air-cleaner housings have internal ducting to distribute the air over the full surface of the filter. The airflow in some filter

Filter in its

housings is made to swirl so that airborne dirt is housing thrown out and falls into a dust trap in the base of

the filter. The airflow into flat filters is from the underside so that dirt falls out from below, rather than into the top of, the filter.

Inlet Manifolds The inlet manifolds on modern engines are usually of the same

length and diameter to enable all

Modern inlet Old inlet manifold manifold

cylinders to be supplied with the same volume and airflow characteristics. Early engines, with manifolds using pipes of differing lengths, often produced slightly different combustion patterns in each of the cylinders.

Throttle Plate ฀฀ At the entrance to the inlet manifold is the throttle plate controlling the flow and quantity of air entering the engine. Diesel engines do not use a throttle plate unless a vacuum is required for the control, or operation, of other systems.

Intake-Air Heating For Cold Engines Mixture composition occurs in the inlet manifolds of carburettor and monopoint fuel-injection engines. Plastic inlet

These manifolds are heated to aid the atomization manifold and distribution of the fuel in the air charge. This

is particularly important when the engine and the air supply are cold. Inlet manifolds were made from aluminium, which readily conducts heat and warms evenly and quickly. However, thermoplastic is now being used more often.

Manifold Heating On older types of engine with the inlet manifold positioned over the exhaust manifold, an exchange of heat was provided by

Water-heated

connecting the two manifolds together. This design inlet manifold is not suitable for cross-flow and "V" engines. One

method of inlet-manifold heating on these engines uses the engine-cooling system. Water passages in the manifold are connected to the water jacket so that coolant flows as soon as the engine is started.

Electrical Heating Another method, which does not use the cooling system, has an electric heater element under the centre of the manifold operating Electric

when the engine is started from cold. A manifold

heater

temperature-sensing switch in the engine coolant cuts off the electrical supply when the engine temperature rises.

Heated-Air Supply On some engines, the incoming air supply is heated. Two designs have been used for this. One heats the air below the carburettor, Heated air

and the other before it enters the air cleaner. The improves

vaporization

older fuel-evaporative system, used on some American-vehicle engine designs, had an electric heater element below the carburettor to heat the air flowing into the manifold. The heater element was supplied with an electric current through a relay and controlled by an engine temperature- sensing switch.

Air-Temperature Control Heating the air entering the inlet duct assists in atomization and fuel distribution in the air charge. To warm the air, it is Pick-up for

passed over the exhaust manifold before being hot air on an

exhaust

drawn into the air duct. This is only necessary

manifold

when the air is cold. When the engine temperature increases, the air density, and therefore mass, would be reduced if heating of the air were continued. At an engine temperature of about 50ºC, the full air supply is drawn from a cold position in the engine compartment, or from the front of the vehicle. Between a cold engine and 50ºC, progressive mixing occurs.

Flap Control ฀฀ The ducting of warm, or cool, air is controlled by a flap in the air-cleaner intake. This provides either a normal airflow, or one from over the exhaust manifold. Two designs of thermostatically-controlled air-cleaner operation are used. One type uses a vacuum motor and bimetallic vacuum valve and the other uses a wax-pellet actuator.

Vacuum System ฀฀ The layout of the vacuum system is shown here. The bimetallic valve responds to the temperature of the incoming air stream and opens or closes the vacuum supply from the inlet manifold to the vacuum motor. The motor reacts to the vacuum supply to move the flap and mix warm and cool air.

Wax-Pellet Actuator The wax-pellet actuator is set in the warm-air-supply duct and, when cold, holds the flap across the cool-air duct. As the wax Wax pellet

pellet is heated up by the warmed air, it expands warm-air

control

and forces out the insert pin, or piston. The pin is connected to a lever which pulls the flap open to allow in a cool airflow. The lever and flap are held by a calibrated return spring and actuated by the force of the expanding wax pellet.