Vehicle Pollutant Emissions | Encyclopedia MDPI

Vehicle Pollutant Emissions: History

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The combustion of common petroleum fuels like petrol and diesel in IC engines releases the following major species: nitrogen, water, CO₂, O₂, NO_x, CO, unburned hydrocarbons (VOCs), and PM in the exhaust. Secondary species like SO₂, N₂O, aldehydes, and ammonia can also be produced. CO₂ is a GHG and is thus harmful to the global environment but in the amounts leaving automotive exhaust, it is not harmful to human health directly. Water and nitrogen are benign species. The remaining species (CO, NO_x, VOCs, PM, and SO₂) are pollutants and have harmful health implications. In addition to these exhaust emissions, non-exhaust emissions (as PM) are also produced by vehicles, most notably from brake, tyre, and road wear, and re-suspension of previously deposited roadside dust.

urban air quality
air pollution
transport emissions

1. Introduction

The global transport system accounts for around 16% of the world’s greenhouse gas (GHG) emissions, and around half of these originate from road passenger vehicles, which are at present, mostly powered by internal combustion (IC) engines running on fossil fuels [1]. Moreover, harmful pollutants emitted by vehicles are significant contributors to local air pollution. Globally, around 27% of urban air pollution is attributed to road transport [2]. Pakistan, the world’s fifth most populous country with a population of over 240 million [3], contributes minimally to the global warming problem with its humble sub-0.5% share of the total GHG emissions [4]. Around 23% of Pakistan’s GHG emissions originate from road transport [5]. The transport emissions are also associated with dangerously high levels of air pollution present in Pakistan’s urban centres [6,7,8]. The country’s air quality is currently ranked as the worst (180 out of 180) in the world [9]. Air pollution has been estimated to reduce the average life expectancy in Pakistan by nearly four years [10]. However, significant uncertainty exists about the extent of the road transport sector’s contribution to air pollution in Pakistan’s urban centres. Some source apportionment/sectoral inventory studies have estimated the share of transport to be below 5% [5], while others have reported values above 70% [6,11]. This large variation is believed to result from: (i) the scarcity of high-fidelity, primary pollutant emissions data, which necessitates indirect estimation approaches, and (ii) differences in interpretation and reporting of the contributions of various pollutant species.

2. Vehicle Pollutant Emissions

2.1. Pollutant Production in IC Engines

Oxides of nitrogen (NO_x) are formed by the combination of nitrogen and oxygen from the air at the high temperatures present during combustion. Engine designs and combustion regimes that lower combustion temperatures reduce NO_x emissions.

CO and VOCs are products of incomplete combustion, and can be reduced by improving the mixing of fuel and air to promote complete combustion. VOCs also arise from misfiring combustion, wall wetting, crevice release, and direct evaporation from fuel tanks, especially in warm climates. Unburned hydrocarbon emissions from compressed natural gas (CNG) vehicles also include methane, which is a very potent GHG and contributes to ground-level O₃ formation.

Particulate emissions result from the combustion of locally concentrated (rich), especially liquid, hydrocarbon fuel parcels. Most PM from the exhaust is composed of very fine particles (transient nuclei) but they grow rapidly via VOC (most notably polycyclic hydrocarbons, PAHs) condensation to form larger (accumulation range) particles [16]. Previous studies have found that PAHs in combustion engines are formed due to incomplete combustion during thermal synthesis [44]. PAHs are therefore considered molecular markers for combustion sources in source apportionment studies [15]. PM emissions are higher for engines with poor air–fuel mixing.

SO₂ emissions result from the oxidation of sulphur in the fuel or lube oil. Sulphur is a naturally occurring component of crude oil but is removed during fuel refinement to comply with regulatory limits (Table 1). Thus, SO₂ emissions are controlled not via combustion and emissions control technologies (as is the case for NO_x, PM, CO, and VOC emissions) but by removing the feedstock needed for its generation. SO₂ emissions are negligible in markets with strong fuel regulations. In the UK, for example, atmospheric SO₂ levels dropped by 95% from 1970 to 2014 because of the removal of sulphur from petroleum fuels [1].

Table 1. Sulphur level limits in European fuel regulations [45].

Regulation	Application Date (in Europe)	Sulphur Level Limit (ppm)
Euro 2	1996	500 (diesel)
Euro 3	2000	350 (diesel), 150 (petrol)
Euro 4	2005	50 (diesel, petrol)
Euro 5	2008	10 (diesel, petrol)
Euro 6	2013	10 (diesel, petrol)

2.2. Determinants of IC Engine Emissions

Some important engine design and operating parameters that affect pollutant production are listed below [1,46].

Air-to-fuel ratio: If more air is supplied than is needed to completely oxidise the available fuel (the stoichiometric amount), the mixture is considered fuel-lean and leads to more complete and cooler combustion. This reduces emissions of VOCs, CO, and NO_x. Burning stoichiometric or rich is desirable from a drivability perspective, even though doing so can impose an efficiency (fuel consumption) penalty.

Engine combustion mode: The two most common engine combustion systems are compression ignition and spark ignition systems. Engines employing these are colloquially referred to as diesel and petrol engines, respectively, because of the predominant fuels traditionally used in them even though spark ignition engines are commonly used to burn other fuels (e.g. CNG, LPG, ethanol) as well. Compression ignition engines operate fuel-lean but are direct fuel-injected, whereby locally rich fuel–air parcels combust in a globally lean combustion process that is limited by the mixing of the injected fuel with air. This leads to the formation of soot and high PM emissions, but low CO and VOC emissions. Petrol engines mostly operate on stoichiometric mixtures and have relatively high VOC and notable CO emissions.

Fuel metering system: Fuel can be admitted into an engine via (direct or indirect) injection, or carburetion. Injection-based metering provides precise control over mixture air–fuel ratios. Direct (in-cylinder) injection can form spatially stratified mixtures and improve engine efficiency and stability, which reduces CO and VOC emissions. This can, however, lead to higher PM emissions. Petrol engines have in the past been carburetted or indirectly injected, but modern engines increasingly have direct injection systems. Resultantly, PM emissions, which have traditionally been very low for petrol engines, have become notable. Modern emissions regulations like Euro 6 thus have strict PM (4.5 mg/km) and particle number (6 × 10¹¹/km) limits [43].

Engine operating cycle: IC engines usually operate on a two-stroke (2SC) or a four-stroke cycle (4SC). Although 2SC engines have the advantage of having higher specific power (better drivability) and simpler design (higher reliability, lower cost), they have traditionally suffered from high exhaust pollutant emissions, especially the small engines used in 2/3 wheelers. Such engines are crank-case scavenged, whereby they cannot have a lube oil sump and thus require oil to be mixed with fresh air (at a ratio of around 1:40) [47]. The combustion of this lube oil leads to high VOC, PM [15], and SO₂ [48] emissions. Moreover, traditional 2SC designs suffer from low levels of mixing of fuel with air, which leads to increased CO, PM, and VOC emissions.

Fuel properties: The chemical and physical properties of fuels also affect the production of emissions, e.g. fuels with higher sulphur and aromatic (benzene derivatives) compounds produce more PM emissions, fuels with low carbon to hydrogen ratio (e.g. natural gas) produce low CO emissions, gaseous fuels produce lower PM emissions, and oxygenated fuels (e.g. ethanol-blended petrol) tend to have higher aldehyde emissions. Requirements about fuel composition are defined by fuel quality standards (Table 1). A fuel’s propensity to generate PM emissions can be indexed by so-called PM Indices [49].

After-treatment systems: If pollutant concentrations cannot be reduced via combustion management, they should be lowered in the vehicle exhaust through “after-treatment” systems. Some common after-treatment devices used in countries with strict emissions regulations are:

Three-way catalysts simultaneously reduce exhaust concentrations of CO, VOCs, and NO_x in spark ignition engines. They require stoichiometric engine operation.
Selective catalytic reduction catalysts are used in lean-burning engines, mostly diesel engines, to reduce NO_x emissions. An on-board reservoir of an ammonia-releasing compound like urea is required for their operation.
Oxidation catalysts oxidise unburned fuel and products of incomplete combustion (CO and VOCs) to CO₂. They are also known as two-way catalysts or catalytic converters [1].
Particulate filters trap PM emissions and regenerate periodically via rich burning events. Originally made for diesel engines (as diesel particulate filters, DPFs) such filters are now increasingly being used for direct injected petrol engines (as gasoline particulate filters, GPFs).

Many of these after-treatment systems use precious metals as catalysts that can be “poisoned” if the fuel is not of sufficiently high quality, e.g. diesel particulate filters and selective catalytic converters require sub-50 ppm levels of sulphur to operate effectively as “soot-free” vehicles, and three-way catalysts can be poisoned by high sulphur content fuel [45]. The lack of modern standard fuels thus not only causes high emissions production (e.g. of SO₂, PM) directly but also indirectly by hindering the adoption of vehicles with advanced after-treatment systems. A related example is from 2017 when Honda stopped the production of its Civic sedan in Pakistan purportedly because of high manganese levels in the fuel [50].

This entry is adapted from the peer-reviewed paper 10.3390/air1040018

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