Car Parts and Accessories

Car Filtering Dust from the Air

All modern cars are fitted with an air cleaner on the carburettor intake and most manufacturers have opted for the dry element type rather than the oil bath unit. Its most important function is to prevent dust from getting into the carburettor and engine cylinders.

An average car engine uses about 3-6 m³ of air a minute, and a filter is essential to prevent particles of dust or dirt blocking air jets and causing wear to pistons and cylinder bores.

Filters offer a resistance to air flow and will affect carburettor performance as they become blocked with dirt. They should be replaced or cleaned at regular intervals—say every 20 000 km. Very dusty conditions may necessitate attention at more frequent intervals.

The air cleaner also acts as a silencer. The filter itself muffles the hissing noise from the carburettor air intake; and the casing and air-intake tube are designed to damp out the resonance of pressure fluctuations in the inlet manifold.

Most engines are fitted with a closed crankcase breathing system, mandatory in countries with vehicle pollution laws, which prevents the engine crankcase fumes reaching the atmosphere.

Many air cleaners have summer and winter settings. In winter, the air-cleaner entry tube is swung round to pick up warm air from the exhaust manifold. This helps cold starting and prevents carburettor icing; but, since the air density is reduced, there is a slight loss of power.

Most systems have air cleaners incorporating temperature-control devices, to ensure that the engine receives a supply of air at a constant temperature.

The cleaners in common use today have a paper element, which is discarded and replaced when no longer serviceable— usually when it becomes partially blocked.

The housings for the elements are often made of plastic, which is corrosion-free and less resonant than pressed-steel units.

The problem of Auto Smog

Public concern over air pollution by motor vehicles arose mainly from the problem in the Los Angeles area. Unlike the smogs which used to exist in London, the Los Angeles experience showed that a smog may consist of photochemical action as well as particulate emissions in the form of dirt, sulphur and soot. In a geographical basin, the combination of unburned hydrocarbons (HC), oxides of nitrogen (NOx) and sunlight will create smog. This occurs when the basin-like region retains air without a prevailing wind to blow the pollutants away.

Similar conditions exist to a certain extent in Cape Town, which suffers from a smog problem. Overseas, initial steps that were taken to reduce the output of carbon monoxide (CO), which is a colourless, odourless and highly toxic gas, were followed by measures to reduce the motor vehicle-generated levels of unburned HC and NOx from the atmosphere. Gradual reductions in permissible levels of pollutants in the exhaust gas have been accompanied by stricter penalties for running a vehicle which fails to meet various tests, or tampering with emission control equipment.

The last in the line of pollution control requirements was designed to reduce the levels of lead in the exhaust gas. Lead is used to allow petrol to burn without detonating and passes out of the exhaust system. It is now considered a health hazard in many countries.

Cleaning up the Car Exhaust

The initial methods used by manufacturers to reduce pollution levels were to incorporate restrictive devices. These generally reduced engine performance and increased fuel consumption. This was followed by improved combustion chamber design with the addition of ‘after-engine‘ techniques such as catalytic converters fitted to exhaust systems. Cars running on unleaded petrol are likely to use a combination: control of CO and NOx by greater control over combustion; control over HC levels by correct combustion and closed engine crankcase and fuel tanks; reduced lead by a change in fuel specifications and the incorporation of a gas catalyser in the exhaust system.

The diagram below shows some of the methods used. Newer engines, using a computer to monitor and control the combustion process to very fine limits, use fewer `add-on’ components. The positive crankcase ventilation valve (PCV), the charcoal canister and the canister control valve reduce hydrocarbon emissions. The throttle positioner, and the air injection system reduce unburned HC and CO while the spark control system and the exhaust gas recirculation valve (EGR) reduce NOx and unburned HC emissions.

The automatic hot air intake valve, choke breaker, auxiliary acceleration pump and the choke all aid drivability when the engine is cold. The fast idle cam improves drivability when it is cold.

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