NO_x emissions from vehicles have been a substantial cause for concern due to their impact on urban air quality. In particular, despite reducing levels of permitted emissions legislatively, such reductions have not been observed in the real world. In this work, NO_x emissions from three vehicles—a Euro 5 car, a Euro V hybrid bus, and a Euro VI—bus have been measured in real driving conditions (and in the case of the buses—in full passenger service). A recently developed high spatio-temporal resolution technique combining very fast (10 ms) NO_x measurement with differential GPS accurate to 1 cm allows these emissions to be resolved to a distance of less than 10 cm (worst-case—dependent on vehicle speed). The results show that acceleration events for the vehicles play a significant part in their total NO_x emissions. In addition, standard events such as a speed bump and a bus stop are analysed. The temperature of any aftertreatment (catalytic converter) to reduce NO_x emissions is also observed to be of substantial significance. At idle, the passenger car was observed to near-double its NO_x emissions when the air conditioning was switched on. Finally, the real driving conditions are compared to the legislative compliance cycles for the certification of the buses, and those results used to further understand the observed NO_x emissions.

Adopting zero-carbon fuels, like hydrogen, can significantly reduce environmental harm and pave the way for a decarbonised trajectory with zero carbon emissions. The hydrogen internal combustion engine (ICE) technology has demonstrated its reliability and capacity to seamlessly integrate into the current ICE platform, originally designed for diesel and gasoline operation. The direct utilisation of pure hydrogen eradicates steady-state carbon dioxide and hydrocarbon emissions. It is important to highlight that efforts to comprehend and comprehensively tackle NO_x emissions are underway. A comprehensive study was carried out to assess the NO_x emissions for a hydrogen ICE with different injection modes from gasoline. The study involved varying the relative air-to-fuel ratio (AFR) from stoichiometric to the lean-burn limit in a boosted spark ignition (SI) engine fuelled with gasoline or hydrogen. A fast NO_x emissions analyser was employed to measure the instantaneous NO and NO₂ emissions in the engine exhaust. The study provides a detailed analysis of NO_x emissions, including steady-state averaged emissions, average crank angle domain NO_x distribution and emissions, in-cylinder pressure analysis, as well as time and cycle analyses of NO_x emissions’ temporal and cyclic variations. The primary discovery was that NO_x emissions are almost zero between lambda 2.75 and 3.7, and hydrogen produces 13.8% less NO_x emissions than gasoline at stoichiometric operation. Finally, the full NO_x time analysis revealed that the consistency of NO_x emissions is higher with hydrogen than with gasoline by using a novel approach by identifying the coefficient of variation of the NO_x emission of each cycle.

Engine-out NOx emissions from diesel engines continue to be a major topic of research interest. While substantial understanding has been obtained of engine-out (i.e., before any aftertreatment) NOx formation and reduction techniques, not least exhaust gas recirculation (EGR) which is now well established and fitted to production vehicles, much less data are available on cycle resolved NOx emissions. In this work, crank-angle resolved NO and NOx measurements have been taken from a high-speed light duty diesel engine at test conditions both with and without EGR. These have been combined with 1D data of exhaust flow, and this used to form a mass average of NO and NOx emissions per cycle. These results have been compared with combustion data and other emissions. The results show that there is a very strong correlation (R2 > 0.95) between the NOx emitted per cycle and the peak cylinder pressure of that cycle. In addition, the crank-angle resolved NO and NOx measurements also reveal that there is a difference in NO : NO2 ratio (where NO2 is assumed to be the difference between NO and NOx) during the exhaust period, with proportionally more NO2 being emitted during the blowdown period compared to the rest of the exhaust stroke.

Fast-response nitrogen oxide (NO _x) and carbon dioxide (CO₂) analysers have been used to study the effects of exhaust gas recirculation (EGR) delay within a modern diesel passenger car. These analysers have response times of the order of a few milliseconds, allowing the CO₂ analyser to be used to study the effects of the EGR transit delay during engine transients and the effect of sudden EGR valve actuation during a drive cycle. The effect of the EGR system delays on the NO _x emissions was measured with the fast-response NO _x analyser. The results indicate that the EGR delay contributes to the sharp short-duration spikes of NO _x. Tests have also been carried out to identify the transient cylinder-to-cylinder EGR distribution by comparing the CO₂ concentrations in the inlet ports, again using the fast-response CO₂ analyser. The results of these tests show a variation between the CO₂ concentrations in the intake ports, suggesting a poor EGR distribution under certain conditions. Measurements have been recorded of the EGR in the intake port runner of just one of the engine’s cylinders and have been compared with the exhaust port NO _x emissions. This provided some data on a cycle-by-cycle basis including the number of engine cycles required for the burned gas to complete the EGR circuit.

Existing ultra-fast response engine exhaust emissions analyzers have been adapted for on-board vehicle use combined with GPS data. We present, for the first time, how high spatio-temporal resolution data products allow transient features associated with internal combustion engines to be examined in detail during on-road driving. Such data are both useful to examine the circumstances leading to high emissions, and reveals the accurate position of urban air quality hot spots as deposited by the candidate vehicle, useful for source attribution and dispersion modelling. The fast response time of the analyzers, which results in 100 Hz data, makes accurate time-alignment with the vehicle's engine control unit (ECU) signals possible. This enables correlation with transient air fuel ratio, engine speed, load, and other engine parameters, which helps to explain the causes of the emissions spikes that portable emissions measurement systems (PEMS) and conventional slow response analyzers would miss or smooth out due to mixing within their sampling systems. The data presented is from NO and NO_x analyzers, but other fast analyzers (e.g. total hydrocarbons (THC), CO and CO₂) can be used similarly. The high levels of NO_x pollution associated with accelerating on entry ramps to motorways, driving over speed bumps, accelerating away from traffic lights, are explored in detail. The time-aligned ultra-fast analyzers offer unique insight allowing more accurate quantification and better interpretation of engine and driver activity and the associated emissions impact on local air quality.

Some of the most challenging aspects of preparing vehicles to comply with the new RDE (Real Driving Emissions) legislation are the calibration of engines to minimize emissions during numerous short lived transients. Fast response emissions analyzers are necessary to resolve rapid transient emissions and have been deployed simultaneously in the engine-out and tailpipe exhaust gas stream of several vehicles. The methodology presented and capability of the instrumentation enables fine resolution of emissions events leading to the opportunity for fine tuning of controller performance. Data has also been logged from the vehicles' ECUs and combined with accurate GPS data. The results show that certain road lay-outs (e.g., congested junctions and speed bumps) can provoke significant tailpipe NOx emissions; often emitting very high levels near schools where traffic calming measures have been adopted for road safety. The lean excursions and insufficient rich purge during such transients are generally shown to be the cause of such emitting events.