4101 Petrol Engine Emissions

Petrol is a hydrocarbon fuel that is composed of approximately 85% carbon and 15% hydrogen, by mass. In order for the petrol to burn efficiently and release the energy that drives the engine, it must be supplied with the chemically correct amount of oxygen. The oxygen that is used for combustion comes from atmospheric air and this contains approximately 23% oxygen and 77% nitrogen, by mass. The chemical equations that govern the combustion of fuel areas follows.

For correct (stoichiometric) combustion this can be interpreted as: 1 kg of carbon requires 2.67 kg of oxygen and produces 3.67 kg of carbon dioxide.

For hydrogen 2H2 + O2 = 2H2O

Fig. 4.22 A typical wiring diagram

In this case stoichiometric combustion is achieved when 1 kg of hydrogen is supplied with 8 kg of oxygen to produce 9 kg of H2O (steam).

In this simplest case, there are two main products of combustion that appear in the exhaust gas, CO2 and steam. The steam is eventually converted back to water in the atmosphere and it does not figure in the analysis of exhaust gas. This leaves the CO2, but this is not the complete picture because the air contains nitrogen and this also appears in the exhaust gas. This leaves us with two main constituents of exhaust gas, carbon dioxide and nitrogen (CO2 and N2).

Arising from the composition of the fuel and the atmospheric air that provides the oxygen, correct combustion requires approximately 14.7 kg of air for each 1 kg of fuel. The air to fuel ratio is calculated from the mass of air divided by the mass of fuel. When this ratio is 14.7:1, it is given the value of lambda = 1. When the mixture is rich, i.e., extra fuel, lambda is less than 1 and when the mixture is weak, i.e., less fuel, lambda is greater than 1. The value of lambda is much quoted in literature that deals with catalytic converters and engine management.

During its operation, a petrol engine meets a range of different conditions and this causes the air-fuel ratio to vary either side of 14.7:1 (lambda less than 1 to lambda greater than 1). These variations from the chemically correct air-fuel ratio result in other gases such as carbon monoxide (CO) and unburnt hydrocarbons (HC) appearing in the exhaust gas. Thus, under operating conditions, the exhaust gas will contain small amounts of CO and HC in addition to the CO2 and N2.

Unfortunately, under high temperature conditions that occur in the engine cylinder, nitrogen reacts with oxygen to form oxides of nitrogen (NOx) and this also appears in the exhaust gas, mainly under high engine load. NOx is reduced by a three-way catalyst and other measures, such as exhaust gas recirculation and possibly secondary air injection into the exhaust system.

In the UK, a vehicle is tested annually for exhaust emissions and enforcement authority officials can stop a vehicle at any time to carry out spot checks. An exhaust gas analyser is used for these tests and most garage equipment manufacturers make, or market this type of equipment. They are called four-gas anlysers because they are able to detect quantities of CO2, HC, CO and HC that are in a sample of exhaust gas. In most cases, the exhaust gas analyser will also give a value of the air-fuel ratio, or lambda, and this can be a very useful aid when attempting to trace emissions-related faults, such as defective exhaust gas oxygen sensor signals, catalyst problems, blocked air filter, and faulty fuel injectors.

For Department of Transport tests, the test stations use manufacturers' figures to confirm compliance with the standards and these vary slightly. In this case exact data is a prerequisite for testing. However, if the system is out of limits at idle speed, most systems include a means of adjusting the mixture to bring the CO% reading back inside the limits. Figure 4.23 gives and indication of the type of adjustment that is provided on an early model Rover car.

In addition to the diagnostic work that can be performed with the aid of the exhaust gas analyser, the self-diagnostic capability of the ECM also provides

Fig. 4.23 The idle air adjustment to correct CO%

valuable information through the fault codes as it constantly monitors the performance of the engine management system.

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