Homogeneous Lean Combustion in Downsized Spark-Ignited Engines

Emissions of greenhouse-gasses and noxious compounds from internal combustion engines propelling personal transportation vehicles is an imminent issue in the society. Therefore, it is vital to find means of reducing these emissions to decrease the impacts of transportation. Despite the current rapid electrification of the light duty vehicle fleet, it is expected that there will still be a substantial share of vehicles, produced and sold, that are propelled either solely or partly by combustion engines in the next decades to come. An advantage of combustion engines is that they consume hydrocarbon fuels, which are energy dense and can be produced from renewable sources enabling elimination of net carbon emissions. These fuels can be distributed using the current infrastructure, allowing for a fast transition into a low-carbon transportation system. The sources of renewables are however limited, and production of renewable fuels requires energy, which is why the fuel efficiency of combustion engines is key.This thesis addresses the need for reduced emissions from personal transportation vehicles by investigating homogeneous lean combustion in downsized spark-ignited engines as a means of improving combustion engine fuel efficiency. Lean combustion offers substantial efficiency improvements to the current already well-developed combustion systems. However, historically, it has been proven difficult to achieve robust lean combustion that achieves both efficiency improvements and sufficiently low emissions of nitrogen oxides. In this thesis, the focus has been to investigate the potentially synergetic combination of high engine loads above 10 bar brake mean effective pressure, a common attribute of downsized engines, and lean combustion. The idea is that lean combustion reduces knocking combustion, a harmful event that limits engine efficiency due to cylinder pressure limitations. Simultaneously, it is hypothesized that higher engine loads will lead to faster and more stable combustion, allowing important reductions in nitrogen oxides.Using engine experiments and simulations, homogeneous lean combustion has been investigated. From the experiments it could be concluded that lean combustion can be sustained at high loads. One of the world’s first two-stage turbochargers designed solely for lean combustion was utilized for this purpose and found to be successful. However, it was discovered that lean combustion does not eliminate knocking combustion completelyKeywords: engine, efficiency, emissions, lean, combustion, nor did high load operation eliminate cyclic dispersion of combustion, which imposes limitations. Using improved in-cylinder charge motion and alternative fuels, these limitations can be mitigated, allowing for stable, efficient, low nitrogen oxide high load lean combustion

Cold-Start Modeling and On-Line Optimal Control of the Three-Way Catalyst

We present a three-way catalyst (TWC) cold-start model, calibrate the model based on experimental data from multiple operating points, and use the model to generate a Pareto-optimal cold-start controller suitable for implementation in standard engine control unit hardware. The TWC model is an extension of a previously presented physics-based model that predicts carbon monoxide, hydrocarbon, and nitrogen oxides tailpipe emissions. The model axially and radially resolves the temperatures in the monolith using very few state variables, thus allowing for use with control-policy based optimal control methods. In this paper, we extend the model to allow for variable axial discretization lengths, include the heat of reaction from hydrogen gas generated from the combustion engine, and reformulate the model parameters to be expressed in conventional units. We experimentally measured the temperature and emission evolution for cold-starts with ten different engine load points, which was subsequently used to tune the model parameters (e.g. chemical reaction rates, specific heats, and thermal resistances). The simulated cumulative tailpipe emission modeling error was found to be typically − 20% to + 80% of the measured emissions. We have constructed and simulated the performance of a Pareto-optimal controller using this model that balances fuel efficiency and the cumulative emissions of each individual species. A benchmark of the optimal controller with a conventional cold-start strategy shows the potential for reducing the cold-start emissions

A Control-Oriented Spatially Resolved Thermal Model of the Three-Way-Catalyst

The three-way-catalyst (TWC) is an essential part of the exhaust aftertreatment system in spark-ignited powertrains, converting nearly all toxic emissions to harmless gasses. The TWC’s conversion efficiency is significantly temperature-dependent, and cold-starts can be the dominating source of emissions for vehicles with frequent start/stops (e.g. hybrid vehicles). In this paper we develop a thermal TWC model and calibrate it with experimental data. Due to the few number of state variables the model is well suited for fast offline simulation as well as subsequent on-line control, for instance using non-linear state-feedback or explicit MPC. Using the model could allow an on-line controller to more optimally adjust the engine ignition timing, the power in an electric catalyst pre-heater, and/or the power split ratio in a hybrid vehicle when the catalyst is not completely hot. The model uses a physics-based approach and resolves both axial and radial temperature gradients, allowing for the thermal transients seen during heat-up to be represented far more accurately than conventional scalar (i.e. lumped-temperature) real-time models. Furthermore, we also use a physics-based chemical kinetics reaction model for computing the exothermic heat of reaction and emission conversion rate which is temperature and residence-time-dependent. We have performed an experimental campaign with a standard spark-ignited engine and a commercial TWC, where we measured steady-state operation and cold-start transient behavior. This experimental data allowed us to tune the model, where we found excellent matching between the measured and modeled tailpipe emissions. Modeling the radial temperature gradient improved the relative accuracy of the conversion efficiency by 15%, and simulations indicate the potential for an absolute improvement by 15 percentage points for some cases. Furthermore, the modeled TWC temperature evolution for a cold-start was typically within \ub110 \ub0 C of the measured temperature (with a maximal deviation of 20 \ub0C). The proposed model thus bridges a gap between heuristic models suited for on-line control and accurate models for slower off-line simulation

Investigation of Homogeneous Lean SI Combustion in High Load Operating Conditions

Homogeneous lean combustion (HLC) can be utilized to substantially improve spark ignited (SI) internal combustion engine efficiency. Higher efficiency is vital to enable clean, efficient and affordable propulsion for the next generation light duty vehicles. More research is needed to ensure robustness, fuel efficiency/NOx\ua0trade-off and utilization of HLC. Utilization can be improved by expanding the HLC operating window to higher engine torque domains which increases impact on real driving. The authors have earlier assessed boosted HLC operation in a downsized two-litre engine, but it was found that HLC operation could not be achieved above 15 bar NMEP due to instability and knocking combustion. The observation led to the conclusion that there exists a lean load limit. Therefore, further experiments have been conducted in a single cylinder research DISI engine to increase understanding of high load lean operation. HLC is known to suppress end-gas autoignition (knock) by decreasing reactivity and temperatures, but during the experiments knock was observed to be prominent and increasing in severity when engine load was increased despite operating ultra-lean close to lambda 2. Knock is normally mitigated by spark retardation which decreases peak cylinder pressure. However, to maintain stable combustion at lambda = 2 the combustion phasing had to be kept close to TC which resulted in high peak cylinder pressures. Therefore, the combustion event had to be balanced in a window where early combustion promoted knock and late resulted in instability and partial burns. A tumble flap was introduced to increase in-cylinder tumble which reduced knock and improved combustion stability. It could be observed that for most load-points assessed; increased tumble could suppress knock and increase the air-dilution limit which proved beneficial in decreasing the NOx\ua0emissions. The highest engine load that could be achieved with highly diluted combustion was 16 bar NMEP

Influence of Trapped Residual Gasses in Air-Diluted Spark Ignited Combustion

Homogeneous air-diluted spark ignition is one advanced combustion concept that is recognized for its potential of efficiency improvement which can be utilized in the next generation, affordable, light duty propulsion systems. However, cyclic dispersion and high load end-gas autoignition of diluted combustion remains a challenge, preventing necessary nitric oxide emission suppression which in turn obstructs market penetration. It is well known that trapped residual gasses in the cylinder influence the cyclic dispersion of combustion by contributing to the total amount of charge dilution which influence the total mean flame speed and thereby the combustion sensitivity to cyclic perturbations. However, the amount of trapped residual gasses in the cylinder is difficult to measure and therefore its influence is complicated to assess. In lean combustion research, the presence of residual gasses is often acknowledged, but few studies have investigated the influence of a combined dilution of residual gasses and air. This paper aims at assessing the influence of trapped residual gasses on lean combustion through combining engine experiments with 1D-computer simulations using a three-pressure analysis. A three-pressure analysis utilizes experimentally acquired data such as crank angle resolved port- and cylinder pressures to minimize the scope of discretization and predictability of the model, to improve accuracy. The experimental results were replicated in the simulation and quantities, such as residual gas fraction and total trapped in-cylinder mass, were estimated.From the performed engine experiments and corresponding simulations, it has been concluded that residual gasses have a substantial influence on combustion. The total dilution, the blend of residuals and air, is highly correlated to NOx emissions at all investigated operating conditions. Additionally, at low loads, the total dilution correlates with the dilution stability limit, rather than air-dilution solely. At high loads, residuals contribute little to the total dilution, but has been linked to increased propensity of knock

Influence of Trapped Residual Gasses in Air-Diluted Spark Ignited Combustion

Homogeneous air-diluted spark ignition is one advanced combustion concept that is recognized for its potential of efficiency improvement which can be utilized in the next generation, affordable, light duty propulsion systems. However, cyclic dispersion and high load end-gas autoignition of diluted combustion remains a challenge, preventing necessary nitric oxide emission suppression which in turn obstructs market penetration. It is well known that trapped residual gasses in the cylinder influence the cyclic dispersion of combustion by contributing to the total amount of charge dilution which influence the total mean flame speed and thereby the combustion sensitivity to cyclic perturbations. However, the amount of trapped residual gasses in the cylinder is difficult to measure and therefore its influence is complicated to assess. In lean combustion research, the presence of residual gasses is often acknowledged, but few studies have investigated the influence of a combined dilution of residual gasses and air. This paper aims at assessing the influence of trapped residual gasses on lean combustion through combining engine experiments with 1D-computer simulations using a three-pressure analysis. A three-pressure analysis utilizes experimentally acquired data such as crank angle resolved port- and cylinder pressures to minimize the scope of discretization and predictability of the model, to improve accuracy. The experimental results were replicated in the simulation and quantities, such as residual gas fraction and total trapped in-cylinder mass, were estimated.From the performed engine experiments and corresponding simulations, it has been concluded that residual gasses have a substantial influence on combustion. The total dilution, the blend of residuals and air, is highly correlated to NOx emissions at all investigated operating conditions. Additionally, at low loads, the total dilution correlates with the dilution stability limit, rather than air-dilution solely. At high loads, residuals contribute little to the total dilution, but has been linked to increased propensity of knock

Homogeneous Lean Combustion in a 2lt Gasoline Direct Injected Engine with an Enhanced Turbo Charging System

In the quest for a highly efficient, low emission and affordable source of passenger car propulsion system, meeting future demands for sustainable mobility, the concept of homogeneous lean combustion (HLC) in a spark ignited (SI) multi-cylinder engine has been investigated. An attempt has been made to utilize the concept of HLC in a downsized multicylinder production engine producing up to 22 bar BMEP in load. The focus was to cover as much as possible of the real driving operational region, to improve fuel consumption and tailpipe emissions. A standard Volvo two litre four-cylinder gasoline direct injected engine operating on commercial 95 RON gasoline fuel was equipped with an advanced two stage turbo charger system, consisting of a variable nozzle turbine turbo high-pressure stage and a wastegate turbo low-pressure stage. The turbo system was specifically designed to meet the high demands on air mass flow when running lean on higher load and speeds. Also, a dual coil ignition system was used for enhanced ignition ability and a lean NOx emissions exhaust after-treatment system (EATS) dummy was fitted downstream the turbo to receive representative exhaust pressures and temperatures for further development purposes. The engine was mapped running lean in various load points in the operational area of interest. It was found that the engine could sustain a high degree of dilution in lower engine speeds and intermediate loads. Fuel consumption improvements of 12% were obtained running at 1500 rpm and 10 bar BMEP at lambda 1.8. At higher engine loads, above 10 bar BMEP, it was found that the combustion stability deteriorated. The ignition could not be optimized due to knocking combustion and at the same time, combustion duration, measured in crank angle degrees, increased with increasing en-leanment and engine speed, leading to late combustion phasing and large variation in cycle-to-cycle of NMEP. This is currently limiting the operational region of lean combustion of the engine used. The load limit in lean operation was investigated, assessing combustion variations and knock phenomena under different operating conditions

High Load Lean SI-Combustion Analysis of DI Methane and Gasoline Using Optical Diagnostics with Endoscope

Homogeneous lean spark-ignited combustion is known for its thermodynamic advantages over conventional stoichiometric combustion but remains a challenge due to combustion instability, engine knock and NOx emissions especially at higher engine loads above the naturally aspirated limit. Investigations have shown that lean combustion can partly suppress knock, which is why the concept may be particularly advantageous in high load, boosted operation in downsized engines with high compression ratios. However, the authors have previously shown that this is not true for all cases due to the appearance of a lean load limit, which is defined by the convergence of the knock limit and combustion stability limit. Therefore, further research has been conducted with the alternative and potentially renewable fuel methane which has higher resistance to autoignition compared to gasoline. Operation with a gaseous fuel on high load was achieved by high pressure direct injection and boosting in a single cylinder research engine. To analyse the combustion further, an endoscope allowing optical access to the combustion chamber was utilized to acquire combustion chamber flame images. High load lean operation with methane could confirm the hypothesis that without a knock limit, optimal ignition timing could be maintained resulting in high combustion stability, and the lean load limit mitigated. Instead, limitation was reached due to peak cylinder pressure. Direct injected methane resulted in overall higher combustion stability compared to gasoline. However, methane also provided an overall lower fuel conversion efficiency by 1-2 %-units compared to gasoline. Despite higher combustion stability using methane, the maximum air-dilution could only be marginally extended. Flame images using the endoscope revealed that the flame growth post ignition was prohibited, possibly due to flame quenching, at high turbulence conditions