The work described in this thesis aims to conduct a systematic study of the two stage turbocharging system to improve the Diesel engine transient performance as well as NOX and CO2 emissions with a focus on the improved turbocharger matching and the control of the charging system, through the use of high fidelity engine models backed by experimental results. To perform the analytical study, commercial 1D simulation software has been used in the process of system characterisation and control strategy design. To validate the analytical results, a two stage turbocharging system was installed on a production diesel engine and tested on a transient engine test bench. The test results were then used to further calibrate the 1D engine/turbocharger model. Several other technologies were also investigated in simulation to explore their potential to further improve the system. Unlike most studies in the literature, this project focused on the system benefit of the engine and turbochargers, instead of conducting optimisation solely at the component level. The engine global parameters, such as the engine fuel consumption, emission levels and the transient response were the main parameters to be considered and were also best suited to the strengths of the 1D simulation method. The interactive use of both the analytical and experimental methods was also a strong point of this study. A novel control strategy for the system was proposed and demonstrated in the simulation. Experiments confirmed the validity of this control strategy and provided data for further model calibration. The comparison of the test results of the baseline engine to those obtained with the two stage turbocharged engine system verified the benefits of the novel turbocharging arrangement and control scheme. Transient response (T1090) was improved, with a 50% faster torque rise at 1000 rpm; the fuel consumption over the NEDC was 4% lower and NOx emissions over the NEDC were 28% lower. In the meantime, the study also revealed shortcomings of the system, such as the lack of EGR control at low speed, low load condition and a mid-speed fuel consumption deterioration of 13% on average at 3000 rpm due to excessive back pressure. With a novel 1D model corroborated using test results, exploratory simulation was done to rectify the aforementioned shortcomings and to further improve the system. Simulation results showed that by implementing VGT and ball bearing technology in the high pressure stage of the two stage system, the EGR controllability at low speed was regained and the excessive back pressure at high speed was improved. Consequently, the fuel consumption was only increased by 1.3% compared to the baseline NEDC operation and the transient response was on par with the original two stage system, with only 0.05s slower in torque rise at 1000 rpm, and still 48% faster than the baseline VGT system. Furthermore, the NOx emission can be expected to be greatly improved in the upcoming more intensive drive cycles compared to the NEDC cycle, with simulation showing NEDC NOX emissions dropped by 1%, comparing to a substantial reduction of 11% in WLTC.

Internal combustion engines developments are driven by emissions reduction and energetic efficiency increase. To reach the next standards, downsized/downspeeded engines are required to reduce fuel consumption and CO2 emissions. These techniques place an important demand on the charging system and force the introduction of multistage boosting architectures. With many possible arrangements and large number of parameter to optimize, these architectures present higher complexity than current systems. The objective of this thesis has thus been to investigate the potential of two-stage boosting architectures to establish, for the particular case of passenger car downsized/downspeeded Diesel engines, the most efficient solutions for achieving the forthcoming CO2 emissions targets. To respond to this objective, an exhaustive literature review of all existing solutions has first been performed to determinate the most promising two-stage boosting architectures. Then, a new matching methodology has been defined to optimize the architectures with, on the one hand the development of a new turbine characteristic maps representation allowing straight forward matching calculations and, on the other hand, the development of a complete 0D engine model able to predict, within a reduced computational time, the behavior of any boosting architecture in both steady state and transient operating conditions. Finally, a large parametric study has been carried out to analyze and compare the different architectures on the same base engines, to characterize the impacts of thermo-mechanical limits and turbocharger size on engine performance, and to quantify for different engine development options their potential improvements in term of fuel consumption, maximum power and fun to drive. As main contributions, the thesis provides new modeling tools for efficient matching calculations and synthesizes the main trends in advanced boosting systems to guide future passenger car Diesel engine develop / Varnier ., ON. (2012). Trends and Limits of Two-Stage Boosting Systems for Automotive Diesel Engines [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/16880 / Palancia

I jämförelse med sina föregångare, har moderna lastbilsmotorer genomgått en betydandeutveckling och har utvecklats till effektiva kraftmaskiner med låga utsläpp genom införandet avavancerade avgasbehandlingssystem. Trots att de framsteg som gjorts under utvecklingen av lastbilsmotorer har varit betydande, så framhäver de framtida förväntningarna vad gällerprestanda, bränsleförbrukning och emissioner behovet av snabba samt storskaliga förbättringar av dessa parametrar för att förbränningsmotorn ska fortsätta att vara konkurrenskraftig och hållbar. Utmaningen i att uppfylla dessa till synes enkla krav är den invecklade, ogynnsammabalansgång som måste göras mellan parametrarna. Förbränningsmotorns kärna är förbränningsprocessen, som i sin tur är kopplad till motorns luftbehandlings- och bränsleregleringssystem. I denna studie undersöks Millercykeln som en potentiell lösning till att nå de motstridiga kraven för framtida lastbilsmotorer, framförallt med fokus på potentialen att förbättra prestandan samtidigt som NOx-emissionerna hålls på konstantnivå. Traditionellt har utvärderingen av Millercykeln utförts på encylindriga forskningsmotorer, vilket också har utgjort utgångspunkten i denna studie. Även om studier på flercylindriga simuleringsmodeller och forskningsmotorer har gjorts med konstanta inställningar för Millercykeln, så utförs de inte i samband med undersökningar av encylindriga motorer. Dessutom så möts inte kraven från insugssystemet på samma sätt mellan de olika motorkonfigurationerna. Denna studie undersöker och jämför potentialen för ökad prestanda med Miller-cykeln mellan encylindrig och flercylindrig motorkonfiguration för en lastbilsmotor med ett tvåstegs turboladdningssystem, som representerar ett realistiskt insugssystem som möjliggör implementeringen av Millercykeln. För att undersöka motorprestationen så används i denna studie den kommersiella mjukvaran GT-Power. Ytterligare resultat från studien innefattar kvantifiering av prestandakraven för ett högeffektivt tvåstegs turboladdningssystem och dess inverkan på temperaturen i inloppet till avgasbehandlings-systemet. En kvalitativ förståelse av betydelsen av interaktionen mellan cylindrar och effekten på cylinder-cylinder variationer med Millercykel utfördes också i simuleringar med flercylindrig motorkonfiguration. Studien utvärderade Millertiming inom ett intervall på -90 till +90 graders vev vinkel från utgångsvinkeln för stängning av insugsventilen. Utvärderingen utfördes vid systemjämvikt vid en fullastpunkt (1000RPM), där basfallet för både encylindrig och flercylindrig motor för utvärdering av Millercykeln var det välkända fallet med konstant specifik NOx. Ett ytterligare fall framhäver NOx-reduktionspotentialen med Miller vid konstant EGR-flöde på en encylindrig konfiguration. Fallen med ökad prestation realiserades genom att öka lufttillförseln, bränslemängden och det geometriska kompressionsförhållandet. Maximal prestandaökning observerades i fallet med ökad bränslemängd, och endast i detta fall utvärderades även konfigurationen med fler cylindrar för jämförelse av prestationsförbättringen med en encylindrig motsvarighet med Millertiming. Den flercylindriga motorn innefattade EGR som en lågtryckskrets, och medan detta antagande förenklade i avseende på modellering och kontroll, så var det till fördel för konfigurationen meden flercylindrig motor (jämfört med encylindrig) på grund av reducerade pumpförluster. Som påföljd gjordes en jämförande undersökning med encylinder-modellen med motsvarande mottryck för flercylinder-modellen inställt som gränsvärde. Resultaten visar att encylindermodellen representerar medelvärdet för cylindrarna i flercylinder-motorn när lämpligagränsvillkor tillämpas som kontrollparametrar. Studien ger en grund för jämförelse av Millertiming på encylindrig samt flercylindriga konfigurationer, samtidigt som kraven på insugssystemet fastställs och utgör en utgångspunkt föratt utvärdera Millercykeln och bestämma insugssystemets krav för hela motorns arbetsområde. / Modern heavy-duty engines have undergone considerable development over their predecessors and have evolved into efficient performance machines with a reducing emission footprint through the incorporation of advanced aftertreatment systems. Although, the progress achieved in heavy-duty engine development has been significant, the future expectation from heavy-duty engines in terms of performance, fuel consumption and emissions stresses the need for rapid large-scale improvements of these metrics to keep the combustion engine competitive and sustainable. The challenges in resolving these apparently straightforward demands are the intricate unfavourable trade-off that exists among the target metrics. The core of the combustion engine lies in the combustion process which is inherently linked to the air handling and fuel regulating systems of the engine. This study explores adopting the Miller cycle as a potential solution to the conflicting demands placed on future heavy-duty engines with an emphasis on the performance enhancement potential while keeping the specific NOX emission consistent. Traditionally, evaluation of the Miller cycle is performed on single-cylinder research engines and formed the starting point in this study. While studies on full-engine simulation models and test engines with fixed Miller timing have been evaluated, they appear to be performed in isolation of the favoured single-cylinder approach. Additionally, the charging system requirements are not consistently addressed between the two approaches. This study investigates and contrasts the performance enhancement potential of the Miller cycle on single-cylinder and serial enginemodels of a heavy-duty engine along with a two-stage turbocharging system to represent a realistic charging system that enables implementation of Miller timing. The commercial engine performance prediction tool GT-Power was used in this study. Additional outcomes of the study included quantifying the performance demands of a high efficiency two-stage turbocharging system and its impact on the inlet temperature of the exhaust aftertreatment system. A qualitative understanding of the significance of cylinder interaction effects on cylinder-cylinder variations with Miller timing was also performed on the serial engine cases. The study evaluated Miller timing within a range of -90 to +90 CAD from the baseline intake valve close angle. The evaluation was performed at steady-state operation of the engine at one full load point (1000RPM) wherein both the single-cylinder and serial engine Miller evaluation included a base case which characterises the Miller effect for constant specific NOX. An additional case highlights the NOX reduction potential with Miller for a constant EGR rate on the single-cylinder configuration. The performance enhancement cases were realised by increasingthe air mass, fuel mass and the geometric compression ratio. Maximum performance increase was observed in the increased fuel mass case and only this case was evaluated on the serial engine for contrasting single-cylinder and serial engine performance enhancement with Miller timing. The serial engine incorporated EGR as a low-pressure circuit and while this simplified modelling and controller considerations, it led to biasing of results in favour of the serial engine configuration (over the single-cylinder) due to reduced pumping loss. A subsequent comparison case was evaluated on the single-cylinder model with backpressure settings from the serial engine model. The results show that the single-cylinder model is representative of the cylinder averaged responses of the serial engine when appropriate boundary conditions are imposed as controller targets. The study provides a basis for contrasting Miller timing on single-cylinder and serial configurations while determining the charging system requirements and presents a starting point to evaluate Miller timing and determine air system demands over the entire engine operating range.