The urban areas of Pakistan exhibit some of the world’s highest levels of air pollution, primarily due to sub-2.5 μm particulate emissions. This issue significantly impairs both the country’s economy and the quality of life of its residents. Road transport is a significant contributor to anthropogenic air pollution but there are discrepancies about the extent of its share. Source apportionment and sectoral inventory studies attribute anywhere between 5 and >80% of the total air pollution to vehicular sources. This uncertainty propagates into the transport policy interventions that are informed by such studies and can thus hinder the achievement of desired pollution mitigation targets. In an effort to reconcile such discrepancies and guide future studies and policy-making efforts, this paper critically reviews source apportionment studies conducted in the urban centres of Pakistan over the past two decades. The strengths and weaknesses of different approaches are compared, and results from the studies are discussed based on the emissions profile of Pakistan’s automotive fleet that emerges. Inconsistencies in the reporting of pollutant concentrations and interpreting their impacts without accounting for the relative disease burden of different pollutant species are found to be the major reasons for the large variations in the reported sectoral shares. At the end, a framework for regular air pollution monitoring and source tracking is proposed in which high-fidelity receptor-based studies inform lower-fidelity but economical sectoral inventory assessments.

It is vital to have accurate predictions of the gas exchange behavior of an engine in order to reliably study engine performance and emissions using engine simulation models. There are a multitude of factors, both upstream and downstream of the engine cylinder, which influence its gas exchange characteristics. Quite often these influences are interconnected in a non-linear manner that results in complicated feedback loops, which can introduce significant errors in the computed thermodynamic state of the post-breathing cylinder mixture. The effects of such bi-directional movement of pressure pulses are particularly pronounced in two-stroke engines. This study investigates the importance of exhaust system design on the scavenging characteristics of a piston-scavenged, cross-flow, two-stroke engine. A validated one-dimensional predictive model is used to study the effects of changing the exhaust port timing, exhaust system length, and exhaust port efficiency on the breathing performance of the engine, along with the consequent effects on the thermal efficiency and NO_x emissions. Exhaust pressure waves and mass flows across ports are used to understand and explain the observed changes. The results show that while making design changes, thermodynamic efficiency considerations can act as a barrier to improving the scavenging efficiency of the engine; in addition, a trade-off between the two has to be considered in the design process to meet engine performance targets. The effects of such a trade-off on the NO_x production are analyzed and two exhaust system modifications are discussed.

Gasoline engines employing the spatially distributed auto-ignition combustion mode, known as controlled auto-ignition (CAI), are a prospective technology for significantly improving engine efficiency and reducing emissions. This review paper provides an overview of developments in various gasoline CAI technologies and discusses their attendant strengths and weaknesses. Hybrid propulsion systems powered by high-efficiency gasoline CAI engines can provide a low-carbon pathway for mobility sector decarbonisation. Therefore, this paper focuses on the challenges and opportunities of CAI implementation, especially for electrified powertrains. Different control actuators that can extend the CAI operating range are discussed, and opportunities for synergistic operation between thermal and electric components of hybridised powertrains are identified. Such synergies can remove impediments in the way of CAI system adoption and can, thus, support CAI adoption and maximise efficiency gains from its implementation. The prospects of supporting CAI combustion for different powertrain electrification levels, hybrid architectures, engine size, and energy management systems are discussed. Load levelling offered by electrified powertrains through CAI-favouring energy management strategies has the potential to substantially relax the operating point requirements for CAI-based thermal propulsion units and to remove the need for expensive actuators. The highly flexible spark-assisted partially premixed compression ignition hybrid mode (SACI-PPCI) emerges as a promising CAI strategy for conventional powertrains, and the moderately flexible spark-assisted compression ignition (SACI) configuration can be a cost-effective thermal propulsion mode for electrified powertrains.

The chemical composition of the trapped fuel-air-residual gas mixture controls the nature of combustion in internal combustion engines and thus serves as a key determinant of the ensuing emissions and work production processes. A frequently used trapped composition metric for engine control is the trapped equivalence ratio. Unfortunately, in two-stroke engines, it is unfeasible to accurately determine this using traditional intake flow and exhaust emissions measurements because of the simultaneous occurrence of intake and exhaust processes, which causes: (1) exhaust emissions to be diluted by the slippage (short-circuiting) of fresh air through the exhaust ports, i.e., trapping inefficiencies, and (2) high residual combustion product retainment, i.e., scavenging inefficiencies. The current paper supplements scavenging efficiency data obtained in a previous study for a cross-scavenged, natural-gas, two-stroke engine with experimental trapping efficiency data from the same engine, and characterizes the overall gas-exchange (scavenging + trapping) behavior of the engine at two engine loads, three speeds, and three spark timings. The trapping efficiency experiments use natural gas as a tracer for fresh charge and the total gas-exchange data is used to compute the trapped equivalence ratio. The trapping performance of the engine – which improves with speed and load increase, and spark retardation – along with scavenging, volumetric, thermal, and combustion efficiency changes determine the trapped equivalence ratio of the engine. The relationship between trapped equivalence ratio and NO_x emissions is presented and the importance of accounting for scavenging and trapping inefficiencies in accurately determining equivalence ratio is discussed.