Philip Keller

Influence of Pre Turbo Catalyst Design on Diesel Engine Performance, Emissions and Fuel Economy

This paper gives a thorough review of the HC/CO emissions challenge and discusses the effects of different diesel oxidation catalyst designs in a pre turbine and post turbine position on steady state and transient turbo charger performance as well as on HC and CO tailpipe emissions, fuel economy and performance of modern Diesel engines. Results from engine dynamometer testing are presented. Both classical diffusive and advanced premixed Diesel combustion modes are investigated to understand the various effects of possible future engine calibration strategies. INTRODUCTION / LITERATURE REVIEW Utilizing modern, premixed Diesel combustion concepts to achieve low NOx and soot while maintaining excellent fuel economy, leads to increased engine out HC/CO emissions as well as lower exhaust temperatures. One option to improve the performance of the Diesel oxidation catalyst system is to position considerable catalyst volume upstream of the turbo charger turbine. This allows taking advantage of faster catalyst light-off after a cold start and of increased average catalyst bed temperature during steady state and transient engine operation. However, apart from packaging constraints, the installation of catalytic converters upstream of the turbine is challenging for engine performance due to thermal inertia before the turbine and other effects on the turbo charger. To explain the need for pre turbine catalysts, various future challenges will be reviewed in detail the following paragraphs. THE EMISSIONS CHALLENGE Over the last years the focus of the Diesel engine development community has clearly been to reduce NOx and Particulate matter emissions into the environment. The discussion, whether to reduce NOx through engine internal measures, for example using high rates of cooled EGR or variable valve timing or through NOx exhaust after treatment (such as NOx-Absorber or SCR systems), is undecided. Up to EURO4 the available DOC (“Diesel Oxidation Catalyst”) technology in conjunction with significant improvements in engine technology, especially advances in common rail fuel injection technology [9], was able to control tailpipe HC and CO emissions to meet the regulations. “Critical” vehicles, such as highly powered sedans with low average engine loads in the test cycle were equipped with heavily Pt-loaded multiple-catalyst systems to minimize end of useful life HC/CO emissions increases due to catalyst aging and poisoning. For EURO5, new and more stable catalyst coatings [11, 12] are being introduced allowing lowering the (precious metal) costs of the DOC. Coated wall flow Diesel Particulate Filters with good HC/CO conversion capability have been introduced in almost all vehicle categories. This either allows reducing catalyst displacements and loadings further or to eliminate any additional DOC’s required. Up to EURO5, Diesel engines have been calibrated to run mostly in classical “diffusive” Diesel combustion mode. HC and CO emissions are traditionally low to moderate, because only little unburned Diesel fuel is dispersed in the piston bowl and therefore is not combusted. Most fuel is oxidized in or near the flame front at extremely high local temperatures. Ignition delays are typically very short, misfiring, which leads to high HC emissions, only occurs at very high exhaust gas recirculation rates and is usually due to EGR control imperfections or a very late center of combustion. In some limited operating areas of current EURO4 and EURO5 applications, extreme EGR rates in combination with high EGR cooling are calibrated to purposely lead to long ignition delays and therefore to partial premixing before combustion, which drives up HC and CO emissions. Furthermore, to achieve NOx targets, compression ratios are being reduced to as low as 16:1, which also increases the HC engine out emissions [9], especially during cold start at low ambient temperatures. For EURO6 / Tier 2 Bin 5 engine calibrations, the areas of the engine operating map where premixed combustion modes (=“PCCI” or “LTC”) are used to further lower engine out NOx emissions, will increase greatly. Future combustion concepts, shown for example in [7,6] will utilize partially to fully premixed combustion modes in a wide area of the engine map, while relying on classical Diesel combustion in the high load / speed areas. These combustion concepts rely heavily on advanced fuel injection and EGR/boosting systems as described in [1]. Engine out HC and CO emissions at loads < 2 BMEP (which typically correspond to cruising at speeds < 50 kph) are up to 5 times higher than for typical EURO4 calibrations as shown in FIG.1. FIG.1: Ricardo’s Tier 2 Bin 5 Emissions concept [7] Going one step further to fully premixed combustion (“HCCI”) in a wide engine operating area could mean increasing the equivalence ratio to near the stoichiometric value. FIG.2, taken from [4], shows that at equivalence ratios near one, considerable fuel energy content in form of HC & CO is not combusted but leaves the engine through the exhaust. This energy could not be recovered with current catalyst / turbo charger layouts. DOC’s downstream of the turbo charger oxidize these emissions – any “valuable” exhaust enthalpy increase is lost. At stoichiometric conditions however the lack of oxygen in the exhaust system would prevent any oxidation of HC/CO. Some form of secondary air introduction into the exhaust system before catalyst could be required. FIG.2: HC/CO energy and Energy flux in the “PCCI” Diesel engine Engine out temperatures with premixed combustion are expected to be similar or even slightly lower than with classical “low NOx” Diesel combustion. The reason for this is the faster heat release rate, maintained around TDC for good fuel economy, with highly premixed combustion compared to diffusive combustion, using high EGR rates, with longer burn duration combined with later injection timing. Also, advanced high power EGR coolers reduce the intake manifold temperature, which also manifests in lower engine out exhaust temperature. Tailpipe exhaust mass flow will decrease due to higher exhaust gas recirculation rates [1, 13]. Depending on the catalyst position and the type of EGR system used (high pressure or low pressure EGR) the DOC could see significantly lower enthalpy flow. This will result in a longer catalyst bed warm up time and consequentially in higher test cycle HC/CO emissions. FIG.3 shows that depending on the vehicle class (sedan, Minivan etc), the transmission (manual, step automatic, dual clutch type etc.) and the drive train (2WD or 4WD), the exhaust gas temperatures in the ECE part of the NEDC are below or near the light off temperature (50% conversion efficiency) of most modern catalyst formulations (aged). FIG. 3: Exhaust gas temperatures in the ECE part of the NEDC for various passenger vehicle categories EMISSIONS LEGISLATION The development of test cycle emissions in Europe in the last years was very focused on lowering NOx and PM HC and CO numbers were left “virtually untouched”. While the absolute HC limits have not significantly changed, the factor HC/NOx has therefore been steadily increasing for the European Emission legislation. This fact has encouraged engine calibrations with higher HC/CO engine out emissions. But the fact that HC + NOx are limited as well as NOx alone, generally limits the tolerable increase in HC when lowering NOx tailpipe emissions. In general, HC & CO do not present a significant problem for most EURO5 applications. But EURO6 will require a much closer look on HC & CO engine out emissions. US Tier 2 legislation, currently very focused on NOx and Particulate emissions, has rather loose limits on CO, which in general can be easily met by modern Diesel engines. To comply with Tier 2 Bin 2 regulations however, HC tailpipe emissions must be drastically reduced. This will present a great challenge to any catalytic system. The Japanese emissions legislation introduced in 2005 also imposes strict limits on NMHC, while CO limits are similar to the EURO5 regulation. FIG.4 and FIG.5 show the progression of HC/CO and NOx test cycle limits in Europe and the US over the next years. It can generally be said that reducing HC/CO tailpipe emissions will become more important in the future. European Union: NEDC cycle 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.5

Loss Analysis of a HD-PPC Engine with Two-Stage Turbocharging Operating in the European Stationary Cycle

Partially Premixed Combustion (PPC) has demonstrated substantially higher efficiency compared to conventional diesel combustion (CDC) and gasoline engines (SI). By combining experiments and modeling the presented work investigates the underlying reasons for the improved efficiency, and quantifies the loss terms. The results indicate that it is possible to operate a HD-PPC engine with a production two-stage boost system over the European Stationary Cycle while likely meeting Euro VI and US10 emissions with a peak brake efficiency above 48%. A majority of the ESC can be operated with brake efficiency above 44%. The loss analysis reveals that low in-cylinder heat transfer losses are the most important reason for the high efficiencies of PPC. In-cylinder heat losses are basically halved in PPC compared to CDC, as a consequence of substantially reduced combustion temperature gradients, especially close to the combustion chamber walls. Pumping losses are on the other hand three times higher than for CDC due to the increased mass flow rate over the valves from the charge dilution and the high amounts of EGR. Friction losses remain uncertain with respect to the direct injection of gasoline instead of diesel, but have been estimated to be slightly higher than for CDC in this work. A sensitivity analysis demonstrates that further reductions of in-cylinder heat transfer losses are possibly the most beneficial for further increases in brake efficiency. Further improvements can also be reached by reducing exhaust port and manifold heat transfer losses and optimized gas exchange and boosting systems. A PPC engine with 57% gross indicated efficiency is likely to reach more than 50% brake efficiency.

Effect of inlet vanes on centrifugal compressor acoustics and performance

The effect of inlet guide vanes (IGVs) on the acoustic and performance characteristics of an automotive centrifugal compressor is studied on a steady-flow turbocharger experimental facility. Broadband noise accompanying flow separation occurs as the flow rate of the compressor is reduced and the incidence angle of the flow relative to the leading edge of the inducer blades increases. The addition of IGVs upstream of the inducer imparts a tangential (swirl) velocity component in the same direction of impeller rotation, which improves the incidence angle particularly at low to mid-flow rates. In the present study, experimental data is compared for three compressor inlet geometries, including a no-swirl baseline along with two different IGV configurations. IGVs were shown to slightly improve the surge line at the highest rotational speed considered in this study, while compromising the maximum flow rate at all rotational speeds. In the high-flow range, IGVs are observed to increase compressor inlet noise levels over a wide frequency range. At the more common low to mid-flow range, however, broadband whoosh noise is reduced with the addition of IGVs in the 5-12 kHz band.

Hybrid coolant pump with electric and mechanical drive

The new hybrid coolant pump from BorgWarner combines the advantages of electrically and mechanically driven pumps in a single system. In electric mode, the pump can be flexibly controlled, while in mechanical mode it operates with a high efficiency. Extensive simulation by BorgWarner has demonstrated the potential of the hybrid pump for achieving further improvements in fuel economy.

Hybrid-Kühlmittelpumpe mit Elektrischem und Mechanischem Antrieb

Die neue Hybrid-Kuhlmittelpumpe von BorgWarner vereint die Vorteile elektrisch und mechanisch angetriebener Pumpen in einem System: Im elektrischen Modus ist sie flexibel regelbar, im mechanischen Modus arbeitet sie mit hohem Wirkungsgrad. Umfangreiche Simulationen von BorgWarner zeigen das Potenzial der Hybridpumpe zur weiteren Verbrauchseinsparung auf.

Development of a 48V P0 demonstration vehicle with eBooster® air charging

A demonstration vehicle is presented where improvements to the electrical and air induction systems are made which provide increased performance with improved fuel economy. This is made possible by a 48 V architecture which enables the deployment of new components, specifically a belted motor generator unit (MGU) and electricallydriven compressor (eBooster®). The synergy between these components enables energy efficient means to collect regenerated energy and provide added torque, faster engine response, and extended engine off operation among a list of added features. Control features and strategy are highlighted along with simulation and vehicle test data. Resultant performance and fuel economy benefits are reviewed which support the contention of 48 V being a cost effective architecture to enable CO2 reduction relative to a high voltage hybrid.

Simulation of surge in the air induction system of turbocharged internal combustion engines

A computational approach has been developed to accurately predict compression system surge instabilities within the induction system of turbocharged internal combustion engines by employing one-dimensional, nonlinear gas dynamics. This capability was first developed for a compression system installed on a turbocharger gas stand, in order to isolate the surge physics from the airborne pulsations of engine and simplify the ducting geometry. Findings fromthe turbocharger stand study were then utilized to create a new model of a twin, parallel turbocharged engine. Extensive development was carried out to accurately characterize the wave dynamics within key induction system components in terms of transmission loss and flow losses for the individual compressor inlet and outlet ducts. The engine was instrumented to obtain time-resolved measurements for model validation during surge instabilities, and simulation results agree well with the experimental data, in terms of both the amplitude and frequency. The present quasi-one-dimensional approach relaxes many of the assumptions inherent to earlier lumped parameter surge models; therefore, it provides the flexibility to model advanced boosting systems with multiple turbochargers and complex ducting geometry.