Internal Combustion Engines

Transcription

1 Emissions & Air Pollution Lecture 1 1

2 Outline In this lecture we will introduce the three major regulated gaseous pollutants produced by I.C. engines: Carbon Monoxide (CO) Unburned Hyrocarbons (HC) Nitrogen Oxides (NO x ) We will discuss their properties, their health and environmental hazards and their causes. 2

3 Carbon Monoxide (CO) CO is the most dangerous pollutant to human health. It is a colourless, odourless and highly poisonous gas (silent killer). It has an affinity to haemoglobin in blood of around 300 times that of O 2 and thus it can dramatically reduce the oxygen carrying capacity of O 2 of the blood. Therefore, it was the first pollutant to be regulated by emission standards around the world. 3

4 Carbon Monoxide (CO) At concentrations of less than 100 ppm, its effects are barely noticeable. It causes headaches, fatigue and respiratory problems at concentrations around ppm. Concentrations around 4000 ppm are fatal in less than one hour. Typical gasoline engine-out concentrations of CO are around 1.5% (vol.) or ppm. Therefore, it is extremely dangerous to operate IC engines in confined spaces. 4

5 Carbon Monoxide (CO) CO is formed in IC engines when inadequate O 2 is present in the A/F mixture (rich mixture conditions). This problem becomes significant during cold-start and acceleration conditions. As a result, it would be expected that diesel engines would naturally produce less CO in comparison to gasoline engines since they always run on lean mixtures. 5

6 Carbon Monoxide (CO) 6

7 Carbon Monoxide (CO) Theoretically, CO production should be zero at stoichiometric or lean conditions. However, this is not the case due to the following reasons: Cylinder-cylinder maldistribution of the charge or local rich regions in the cylinder. Dissociation of CO 2 to CO and O 2 at high temperatures. CO can also be formed when there is insufficient time to complete the oxidation of CO to CO 2. It can also be produced as an intermediate product during the oxidation of hydrocarbons. 7

8 Unburned Hydrocarbons (HC) Unburned Hydrocarbons (HC) are derived from fuel which was not totally oxidised in the cylinder. HC is a general term used to describe virtually hundreds of hydrocarbon species found in the exhaust gas. The atmosphere naturally contains a wide range of hydrocarbons, with methane being the most prominent. Beside IC engines, HC are produced from natural sources (biogenic sources). However, road transportation accounts for around 35% of atmospheric HC. 8

9 Unburned Hydrocarbons (HC) In addition to that, certain hydrocarbons produced by IC engines, especially aromatics, are considered to be carcinogenic and pose extreme risk to health especially in dense urban areas. For example, benzene (C 6 H 6 ) is a class A carcinogen, which is particularly linked to leukaemia and is subjected to stringent handling precautions when used in a laboratory. Through partial oxidation, benzene can be produced in the exhaust gas even if it is not present in the fuel supplied to the engine. 9

10 Unburned Hydrocarbons (HC) 10

11 Unburned Hydrocarbons (HC) As shown in the previous figure, the A/F ratio has a similar effect on HC as it does on CO (levels of HC increase as the A/F ratio is reduced below stoichiometry). However, even with lean mixtures, the level of HC does not reach zero, this is due to a combination of reasons: Incomplete mixing of the air/fuel mixture. Flame quenching at the walls of the combustion chamber Crevice volumes in the cylinder such as: the spark plug threads, valve seats, cylinder head gaskets and the piston rings. Burning of lubrication oils. Deposits on the walls of the combustion chamber. Valve overlapping. 11

12 Unburned Hydrocarbons (HC) On the other hand, if the mixture is leaned excessively, the levels of HC increase because of incomplete combustion. This is either because the mixture is too lean to support the combustion, or because the temperature is too cool, or both. 12

13 Nitrogen Oxides (NO x ) The term NO x is generally used to describe two components which are often found together: NO and NO 2. There are other oxides of nitrogen such as N 2 O, but they are much less common in I.C. engines. The overwhelming majority of NO x produced by I.C. engines consists of NO. Road transportation contributes over 50% of atmospheric nitrogen oxides. 13

14 Nitrogen Oxides (NO x ) NO is colourless, odourless and relatively nontoxic. While NO 2 is reddish-brown, and if inhaled at high concentrations, it can cause acute inflammation of the respiratory tracts and can damage lung tissues. NO converts to NO 2 in the atmosphere. Problems caused by NO x are not limited to direct health effects. The most significant problem is an environmental one. 14

15 Nitrogen Oxides (NO x ) NO x reacts with HC in bright sunlight to form ozone which is a major atmospheric pollutant and results in the formation of smog in urban areas that suffer from natural lack of ventilation. Ground-level ozone can cause eye irritation chest problems and asthma. This problem has been the driving force behind the first emission legislations. NO x can also react in the atmosphere to form nitrates and nitric acid resulting eventually in acid rain. 15

16 Nitrogen Oxides (NO x ) NO, which forms almost 95% of NO x emissions, is produced by the reaction between atomic oxygen and nitrogen: O + N 2 NO + N followed by: N + O 2 NO + O The initiation of this reaction requires a large amount of energy and so the level of NO x is determined by the peak cylinder temperature (>2000 K) in addition to the amount of oxygen available. 16

17 Nitrogen Oxides (NO x ) 17

18 Nitrogen Oxides (NO x ) In general, maximum NO x levels occur at slightly lean A/F ratios. This poses a serious problem for gasoline engines which need to run at such ratios. However, NO x emissions can be reduced through decreasing the combustion temperature by retarding the ignition timing or by decreasing the compression ratio. Other means of reducing NO x emissions will later be introduced. 18