What are the factors that influenced emission?

The emissions depend on a wide range of factors. The most important of these are:

Fuel used. Emissions depend on the fuel used to power the vehicle. For example, a car powered by petrol will emit more carbon monoxide (CO) and VOCs and be less fuel efficient than a similar one powered by diesel. However, the diesel car will emit more NOx and particulate matter. Carbon dioxide emissions depend on fuel consumption and the carbon content of the fuel.

Maintenance: Poorly maintained vehicles consume more fuel and emit higher levels of CO and VOCs than regularly serviced ones.

Driver behavior: Emissions are not constant, but vary depending on how the vehicle is being driven. NOx emissions increase when the engine is under load such as during rapid acceleration and when travelling at high speeds, while CO and VOC emissions will increase when it is necessary to run rich, for example, when the engine is cold, and during accelerations. Thus, in general, emissions will be lowest when a car is driven at a steady speed. In the stop-start driving conditions that characterise congested urban areas, emissions will be higher than at the same average speed but under free flow conditions.

Cold starts. Most car journeys are very short and are in urban areas, emissions from cars are particularly high when first driven from a cold start.

Control Measures

Control of tailpipe emissions

Optimizing air-fuel ratio: In a typical automobile engine with no air pollution controls, a mixture of fuel and air is fed into a cylinder by the carburettor and is compressed and burned by a spark from the spark plug. The explosive energy of the burning mixture moves the pistons. The piston’s motion is transmitted to the crankshaft that drives the car.

One kilogram of petrol can burn completely when mixed with about 15 kg of air. For maximum power, the proportion of air to fuel (A/F) must be less.

The A/F ratio is fairly easy to regulate. It has a direct effect on emission of CO, HC and NOx. A/F ratio of 14.6 is the stoichiometric mixture for complete combustion. At low ratios, both CO and HC emissions increase. At very lean mixtures (high A/F ratio), the NOx emission begins to decrease. Thus one of the approaches taken to control emission is to set the carburettor at a very lean setting.

Catalytic converter: The greatest step in controlling emissions from road vehicles was the introduction of closed loop (or controlled) three-way catalysts for petrol vehicles. These remove 80-90% of the emissions of CO, VOCs and NOx. The reactions taking place on the catalyst are shown below:

Automotive catalysts are typically made of platinum and rhodium. For efficient removal of all three pollutants, the A/F ratio needs to be close to the stoichiometric ratio (i.e. 14.7). Cars fitted with these catalysts require an oxygen sensor to monitor the exhaust gas composition and electronically controlled fuel management system to control the A/F ratio.

This technology cannot be used in the oxygen-rich exhaust of a lean burn petrol or diesel engine. For these engines, CO and HC emissions can be reduced using a simple oxidation catalyst.

Control of evaporative emission

The HC-containing vapours from the carburettor and the fuel tank pass through an activated carbon bed that removes the HC before those vapours are vented to the air. When the engine is running at other than idle or very low speeds, air is sucked through the bed in the reverse direction, removing the HC from the activated carbon preparing it for its next service. This regenerated air is returned to the air entry of the engine, where the HC it contains is burned. Suitable valves maintain the flow in the proper direction. The activated carbon bed has a low-enough flow resistance that the pressure in the carburettor float chamber is close is close enough to atmospheric pressure for the proper operation of the carburettor.