Accurate measurement and modeling of mean exhaust gas temperature (EGT) in internal combustion engines (ICEs) is paramount to improve its emission and efficiency while maintaining component durability. Sheathed thermocouples provide a robust and cost-effective EGT measurement to the detriment of accuracy due to the junction's heat balance. Literature indicates that exposing and reducing the thermocouple junction diameter improves the mean measured EGT accuracy. However, such designs can affect its interaction with the pipe wall, the conventional heat sink for sheathed thermocouples. Furthermore, characteristics of the pulsating exhaust gas flow govern junction heat convection and heat conduction linked to the exposed wire length-to-diameter (l/d) ratio. Therefore, the complex interaction between thermocouple design attributes and the pulsating exhaust flow requires isolating the effects to derive the measurement accuracy benefits from exposed junction thermocouples. This study utilizes Type-K thermocouples in sheathed (6 mm) and multiwire exposed junction (51-254μm) constructions downstream of a single-pipe exhaust of a heavy-duty diesel engine. Isolated engine speed and load sweeps provided distinct pulsating exhaust flow conditions measured using a Pitot tube for mass flux and the exposed junction heating rate to indicate the true EGT. The study highlights fundamental differences in the junction-to-pipe wall thermal interaction between sheathed and exposed thermocouples, motivating the need for distinct heat sinks and error correction before comparing measurements. Moreover, the nature of the pulsating exhaust flow conditionally enhances or undermines gains in mean EGT accuracy.

The exhaust of internal combustion engines (ICEs) is characterized by rapid large amplitude exhaust gas temperature (EGT) pulsations that demand high-bandwidth measurements for accurate instantaneous and mean EGTs. While measurement technique challenges constrain on-engine EGT pulse measurements, reduced-order system simulations numerically estimate the EGT pulse and its mean to overcome the measurement limitation. Notwithstanding high-bandwidth pressure measurements, model calibration and validation for the EGT are confined to mean indications using sheathed thermal sensors like thermocouples and resistance thermometers. These EGT measurements are susceptible to errors caused by heat transfer, flow unsteadiness, and the thermal inertia of the sensor. Exposed thin-wire thermocouples provide an intermediate solution to the robustness-to-response tradeoff of thermal sensors. While the thermocouples' thermal inertia significantly affects the measured EGT pulse, the signal derivative (un-scaled dynamic error) provides greater insight by indicating the EGT waveform. This study utilizes a 50.8 μm Type-K thermocouple to contrast the exhaust pressure and EGT pulses through the measured signal and its derivative. Experiments in a single-pipe exhaust of a heavy-duty diesel engine with isolated engine speed and load sweeps present significant differences between the pressure and indicative EGT waveforms. It also highlights a rapid pre-pulse fluctuation unique to the EGT pulse waveform caused by exhaust gas-dynamics and impacted by heat transfer. The study motivates the need for increased bandwidth EGT measurements to improve model validation of EGT pulse estimates while showcasing the utility of thin-wire thermocouples.

Characterizing pulsating flow in high-temperature, high-pressure engine exhaust gas is crucial for the development and optimization of exhaust energy recovery systems. However, the experimental investigation of engine exhaust pulses is challenging due to the difficulties in conducting crank angle-resolved measurements under these unsteady flow conditions. This study contributes to characterizing mass flow pulses from an isolated cylinder exhaust of a heavy-duty diesel engine using a single-pipe measurement system, developed for pulsating flow measurement. A Pitot tube-based approach is adopted to measure exhaust mass flow pulsations, complemented by fast temperature measurements obtained using customized unsheathed thin-wire thermocouples. The on-engine experiment is performed by isolating the in-cylinder trapped mass and the valve opening speed to produce different exhaust pulse waveforms. The adopted approach’s sensitivity in resolving instantaneous mass flows is evaluated analytically and experimentally, considering attenuated temperature measurement effects. Based on exhaust flow measurements, mass flow pulses are analyzed with regard to blow-down and scavenge phases. Under the load sweep, the main waveform change occurs during the blow-down phase, with pulse magnitude increasing with the load. In contrast, as the engine speeds up with a comparable trapped mass, the exhaust mass distribution in the blow-down phase decreases from 75.5% at 700 rpm to 41.9% at 1900 rpm. Additionally, it is observed that cycle-to-cycle variations in mass flow pulses align with combustion stability during the blow-down phase and are predominantly influenced by gas-exchange processes during the scavenge phase.

Internal combustion engines are still widely used for propulsion in modern vehicles. Upcoming emission legislation imposes stricter limits on exhaust emissions. One method to achieve emission compliance is by using a three-way catalyst (TWC), which offers excellent emission reduction if the mixture is stoichiometric. However, stoichiometric mixtures in spark-ignited engines have the drawback of increased knock propensity. Knock can be mitigated by using water injection, which serves as both a diluent and utilizes latent heat to reduce the temperature, thereby reducing the reactivity of the unburned mixture. Methanol as a fuel has received more attention thanks to its high research octane number (RON) and its potential to contribute to decarbonization when produced as e- or bio-methanol. In the current study, Direct Water Injection (DWI) was evaluated on a Heavy-Duty (HD) single-cylinder research engine fueled by methanol. This work aims to fill a research gap on methanol-fueled engines with water injection. A direct injection system of water was chosen as it offers the freedom to inject during the closed cycle. Furthermore, a chemical kinetic study on the oxidation of stoichiometric methanol–water mixtures was conducted based on findings in the literature suggesting that, under certain conditions, water mixed with alcohol (in this case, ethanol) can reduce the ignition delay. The experimental results demonstrate that DWI effectively suppresses knock and reduced Nitrogen Oxides (NOx), albeit with deteriorated combustion efficiency. The chemical kinetic study suggested that at lower to intermediate temperatures, water acts as an efficient third-body collider, which lowers the ignition delay. However, this effect is not significant for the typical timescales encountered in HD engines.

Methanol as a fuel is gaining popularity due to its favorable properties and potential for sustainable production as bio- or electro-methanol. By operating according to the Spark-Ignited (SI) principle with a Three-Way Catalyst (TWC), low emissions can be achieved. The main phenomena limiting the efficiency of the SI engine when operating with stoichiometric mixtures are knock and, occasionally, pre-ignition. One method to suppress both knock and pre-ignition is water injection. This study explores the possibility of suppressing knock in-cycle using direct water injection for cycles with an elevated risk of knocking. The prediction was based on the observation that, at knock-limited operation, only cycles with the most advanced combustion phasing knock. Furthermore, at knock-limited loads, combustion predominantly consisted of a single combustion mode: deflagration. The results demonstrated partial knock suppression and allowed for a combustion phasing advancement of 1.5°at loads of 10 and 15 bar gross indicated mean effective pressure. The earliest practical point during the combustion cycle to confidently determine if knock will occur was when about 10%–20% of the fuel had been consumed. However, theoretically, in a best-case scenario, this could be as early as when 5% of the fuel was consumed. An experiment simulating pre-ignition also demonstrated the ability to detect such cycles and partially suppress the ensuing knock. A major limitation of the method is that the window between detecting a cycle with a high likelihood of knock and knock onset was less than 7°at 1000 rpm.

Reaching the particle emissions regulatory limits for the combustion engine is a challenge for developers.Particle filters have been the standard solution to reduce particle emissions, but filters arelimited in storage capacity and need to be regenerated, a process emitting more carbon dioxide(CO2) as more fuel is consumed to regenerate the filter. In previous research, it was found that theengine can emit large spikes in particle numbers (PNs) under stationary operating conditions. Thesespikes were several orders of magnitude higher than for the base particle emissions level and occurredseemingly at random. The source of the spikes was believed to be the cylinder-piston-ring systemand as 50–99% of the particles stemmed from these spikes, the influence on the particle emissionsmade it an interesting investigation to find the root cause of it. The experiments were performedfor different piston ring loads, locked ring positions, and different oil compositions. The resultsindicate a possibility to control the PN emissions through the experiment alterations, with lockedpiston rings having the greatest influence at a higher load. There was no clear relation between ringrotation and flutter with the spikes observed. The locked piston ring configurations did indicate thering gap not to be the main contributor to the spiking as fully aligned gaps did not result in thehighest levels of particle emissions. Variations to the oil composition indicate that a high-volatilityoil will emit higher levels of small, sub-10 nm particles compared to the standard baseline oil. Ahigh-viscosity oil instead lowers the particle emissions, possibly due to the higher inner friction athigh temperatures reducing the oil ingress into the combustion chamber. The link between the PNspiking phenomenon and the oil pathway past the piston ring was not established through theexperiments reported in this publication.

A viable option to reduce global warming related to internal combustion engines is to use renewable fuels, for example methanol. However, the risk of knocking combustion limits the achievable efficiency of SI engines. Hence, most high load operation is run at sub-optimal conditions to suppress knock. Normally the fuel is a limiting factor, however when running on high octane fuels such as methanol, other factors also become important. For example, oil droplets entering the combustion chamber have the possibility to locally impact both temperature and chemical composition. This may create spots with reduced octane number, hence making the engine more prone to knock. Previous research has confirmed a connection between oil droplets in the combustion chamber and knock. Furthermore, previous research has confirmed a connection between oil droplets in the combustion chamber and exhaust particle emissions. However, the co-variation between oil originating particle emissions and knock has not been investigated. The current study examines the connection between knock and particle number in the exhaust, when running on fuel with low soot production. A single cylinder spark ignited heavy-duty engine was used. It was equipped with port fuel injection and fueled with methanol, which produces very little soot at lambda 1. Consequently, the measured exhaust particle numbers were assumed to origin essentially from engine oil. Three grades of oil, in combination with three piston ring configurations, were used to vary the amount of oil entering the combustion chamber. Results from knock limited operation at both medium and high engine load showed that an increased number of particles in the exhaust was associated with an increased likelihood of knock. The authors find the hypothesis that an increase in particle number correlates with an increase in auto-ignition tendency to be confirmed.

Ethanol, as the most produced renewable biofuel, is considered a promising low-carbon alternative to petroleum-based fuels in the transport sector due to its high energy density and auto-ignition resistance. The lean-burn combustion in spark-ignition (SI) engines has the potential to further improve thermal efficiency in regard to knock mitigation and the reduction of combustion temperature. However, the characteristics of lean-burn combustion in an ethanol-fueled engine in relation to the combustion losses and the gas-exchange process remain unclear, especially for high-load operation. This study contributes with a deeper understanding of the high-load performance of an ethanol-fueled heavy-duty SI engine using lean-burn combustion. Based on the experimental results from a single-cylinder engine test, a 6-cylinder engine model is built by integrating a validated predictive combustion model to characterize the lean-burn combustion process. The engine’s thermal efficiency and combustion phasing are evaluated for knock limited operation and then compared to the theoretical optimum which is regardless of knock. The energy and exergy balances are applied to evaluate the effect of dilution with excess air ratios up to 1.8. Losses through heat transfer, exhaust flow, and incomplete combustion are quantified. In addition, entropy generated through combustion is discussed to identify the relationship between exergy destruction and different operating conditions. In the context of lean-burn combustion, the thermal efficiency at high-load operation incrementally increases from 40.4% at stoichiometric condition to 47.3% at an excess air ratio of 1.8. At the same time, the exergy destruction through combustion increases by 3.3 percentage points across the selected dilution range. Furthermore, the challenging requirements to realize lean-burn combustion with lower exhaust gas temperatures and higher intake boost pressures is assessed through an exergy analysis of the turbocharging system.

Accurate exhaust gas temperature (EGT) measurements are vital in the design and developmentprocess of internal combustion engines (ICEs). The unsteady ICE exhaust flow and thermal inertia of commonly used sheathed thermocouples and resistance thermometers require high bandwidth EGT pulse measurements for accurate cycle-resolved and mean EGTs. The EGT pulse measurement challenge is typically addressed using exposed thin-wire resistance thermometers or thermocouples.The sensor robustness to response tradeoff limits ICE tests to short durations over a few exhaust conditions. Larger diameter multiwire thermocouples using response compensation potentially overcomes the tradeoff. However, the literature commonly adopts weaker slack wire designs despiteindications of coated weld taut wires being robust. This study experimentally evaluates the thin-wirethermocouple design placed in the exhaust of a heavy-duty diesel engine over wide-ranging exhaust conditions for improving both sensor robustness and accuracy of the measured EGT. The assessed design parameters included the wire diameter (51 μm to 254 μm), the exposed wire length, and thewires placed slack or taut with coated weld faces. All taut wires with ceramic-coated weld faces endured over 3 h of engine operation, while similar diameter slack wires (51 μm and 76 μm) were sensitive to the exhaust condition and exposed wire length. Reducing the wire diameter from 76 μmto 51 μm significantly impacted response improvements as evidenced at certain test conditions bya peak-peak EGT increase of 92 °C, a mean EGT drop of 26 °C, and a doubling of the sensitivity ofmean EGT cycle-to-cycle variations to ±12 °C. Increasing the exposed wire length showed less significant response improvements. The Type-K thin-wire thermocouples showed negligible drift, thereby indicating the possibility of using smaller and longer wires built taut with coated weld facesfor improved accuracy of EGT measurements in ICEs.

Stoichiometric operation of a Port Fueled Injection (PFI) Spark-Ignited (SI) engine with a three-way catalytic converter offers excellent CO2 reduction when run on renewable fuel. The main drawbacks with stoichiometric operation are the increased knock propensity, high exhaust temperature and reduced efficiency. Knock is typically mitigated with a reactive knock controller, with retarded ignition timing whenever knock is detected and the timing then slowly advanced until knock is detected again. This will cause some cycles to operate with non-ideal ignition timing. The current work evaluates the possibility to predict knock using the measured and modelled temperatures at Inlet Valve Closing (IVC) and Top Dead Center (TDC). Feedback effects are studied beyond steady state operation by using induced ignition timing disturbances. The approach is based on a deterministic controller where the timing is advanced beyond steady state knock limited operation or vastly retarded to produce warmer residuals in the following cycle. The results indicate that for the current engine there is no feedback effect. Chemical kinetics explains the lack of feedback due to lack of reactivity at TDC conditions. The chemical kinetic study in conjunction with the established auto ignition models described by Livengood-Wu reveals that the charge mixture entered a region of reactivity around the 50% burned point. It was also found that knocking and non-knocking cycles can have overlapping thermodynamic trajectories but for knocking cycles there is less dispersion. The study uses a solver which corrects the IVC temperature to minimize the error between observed knock onset and the point where the Livengood-Wu expression reaches unity for a knocking cycle. The corrections were found to have a correlation to uncaptured evaporation effects. Combined experimental and modelling results were in line with previous findings, namely that cycle-to-cycle combustion variations are plausibly explained by early flame propagation.