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Scarce resources of fossil fuels and increasingly stringent exhaust emission legislation push towards a stronger focus to alternative fuels. Natural gas is considered a promising solution for small engines and passenger cars due to its high availability and low carbon dioxide emissions. Furthermore, natural gas indicates great potential of increased engine efficiency at lean-burn operation. However, the ignition of these lean air/fuel mixtures leads to new challenges, which can be met by fuel scavenged prechambers. At the Institute of Internal Combustion Engines of the Technische Universitaet Muenchen an air cooled natural gas engine with a single cylinder displacement volume of 0.5 L is equipped with a methane scavenged prechamber for investigations of the combustion process under real engine conditions. The main combustion chamber is supplied with a lean premixed air/fuel mixture.

Beside the main trend technologies such as downsizing, down speeding, external exhaust gas recirculation, and turbocharging in combination with Miller cycles, the optimization of the mechanical efficiency of gasoline engines is an important task in meeting future CO2 emission targets. Friction in the piston assembly is responsible for up to 45% of the total mechanical loss in a gasoline engine. Therefore, optimizing piston assembly friction is a valuable approach in improving the total efficiency of an internal combustion engine. The form honing process enables new specific shapes of the cylinder liner surface. These shapes, such as a conus or bottle neck, help enlarge the operating clearance between the piston assembly and the cylinder liner, which is one of the main factors influencing piston assembly friction.

This SAE Technical Paper gives a summary of the essential findings in the development and operation of a test engine dedicated to the measuring of the friction between the piston group and the liner. Firstly the fundamental demands on the high-precision and close to real engine operation friction measuring are laid out. Subsequently the basic engine, the measuring system based on the floating liner method including a gas balance device, as well as the implemented measuring technique are specified. Major influencing variables on the friction of the piston assembly and its interference variables are also summarized. Extensive information about the systematic and strategies for the test engine's operation startup are given in acknowledgement of influencing and interference variables. This strategy reduces the developmental and startup process of an engine dedicated to the measuring of piston group friction.

Presented within the framework of this SAE Technical Paper are the highly accurate results of friction experiments, performed upon a floating-liner, single-cylinder test engine with a capacity 0.5 liters and crank angle resolution during motored and fired operation. This allows for the measurement of mixed friction zones at the dead centers. These mixed friction zones can result in friction losses and lead to wear in the components in-volved. The strength of the friction forces in any given mixed friction zone is largely dependent on the operating point. This is why the influence of each of the most important operating parameters - speed (rpm), load, oil and coolant temperature - is individually analysed, before the interactions, which are depicted in the resultant engine map, are discussed.

Natural gas and especially biogas combustion can be seen as one of the key technologies towards climate-neutral energy supply. With its extensive availability, biogas is amongst the most important renewable energy sources in the present energy mix. Today, the use of gaseous fuels is widely established, for example in cogeneration units for combined heat and power generation. In contrast to conventional spark plug ignition, the combustion can also be initialized by a pilot injection. In order to further increase engine efficiency, this article describes the process for a targeted optimization of the pilot fuel injection. One of the crucial points for a more efficient dual fuel combustion process, is to optimize the amount of pilot injection in order to increase overall engine efficiency, and therefore decrease fuel consumption. In this connection, the injection system plays a key role.

The use of modern simulation tools in the engine product development process is explained using the layout of a piston and its piston ring-pack of AUDI V6 SI-engine as example. Based on the requirements for piston rings in a SI-engine the possible trade-offs are explained. A base layout for a ring-pack for the specific engine is presented. Further the validity of the simulation model is rated as the simulation output is compared to actual dynamometer measurements of the blow-by map of the engine. Additionally a test setup is presented, which measures piston ring movement and the pressures between the rings and in the ring grooves. Also these measurement results are compared to the simulation. Using DOE (design of experiments) on the base layout potentials for optimization are shown and applied. To identify the positive effects in the engine pistons with piston rings are fabricated in accordance with the DOE recommendations.

This SAE Technical Paper contains detailed data which are relevant for the calculation of the friction forces of the piston assembly in internal combustion engines. Useful ways of employing calculations besides measurements are exactly classified for the optimization of the piston assembly system in order to reduce friction losses. In the first step the theoretical basics for the calculation of the tribological system are introduced. Referring to the theory, the paper goes into detail about the basic set-up and the modeling degree of the calculation program. Furthermore, measured and calculated curves of friction forces are compared for different operating points. In addition, analysis of the crank-angle resolved friction force are presented with varying engine speeds, oil temperatures and loads and a detailed interpretation of the results is given.

The development of today's powertrains focuses on the reduction of CO₂ emissions. Therefore several new technologies for internal combustion engines have been established. A further tendency is the successive electrification of powertrains in hybrid vehicles. However, these trends lead to increasing system costs which are a very important aspect at the market segment of compact cars. At the Institute of Internal Combustion Engines of the Technical University of Munich a drivetrain concept for urban and commuter traffic is under development. It is based on a lean-burn air-cooled two-cylinder natural gas engine which is combined with a hydraulic hybrid system. The paper contains detailed information about the engine as well as the hybrid vehicle powertrain in parallel structure. Particular characteristics and innovations of the hydraulic hybrid system compared to systems known so far are shown.

A new constant volume combustion chamber (CVCC) apparatus is presented that calculates the cetane number (CN) of fuels from their ignition delay by means of a primary reference fuel calibration. It offers the benefits of low fuel consumption, suitability for non-lubricating substances, accurate and fast measurements and a calibration by primary reference fuels (PRF). The injection system is derived from a modern common-rail passenger car engine. The apparatus is capable of fuel injection pressures up to 1200 bar and requires only 40 ml of the test fuel. The constant volume combustion chamber can be heated up to 1000 K and pressurized up to 50 bar. Sample selection is fully automated for independent operation and low levels of operator involvement. Capillary tubes employed in the sampling system can be heated to allow the measurement of highly viscous fuels.

In the near future, pollutant and GHG emission regulations in the transport sector will become increasingly stringent. For this reason, there are many studies in the field of internal combustion research that investigate alternative fuels, one example being oxygenated fuels. Additionally, the design of engine components needs to be optimized to improve the thresholds of clean combustion and thus reduce particulates. Simulations based on PRiME 3D® for dynamic behaviors inside the piston ring group provide a guideline for experimental investigation. Gas flows into the combustion chamber are controlled by adjusting the piston ring design. A direct comparison of regular and synthetic fuels enables to separate the emissions caused by oil and fuel. This study employed a mixture of dimethyl carbonate (DMC) and methyl formate (MeFo).

Polyoxymethylene dimethyl ethers (OME) are promising alternative diesel fuels with a biogenic or electricity-based production, which offer carbon neutral mobility with internal combustion engines. Among other e-fuels, they stand out because of soot-free combustion, which resolves the trade-off between nitrogen oxide (NOx) and soot emissions. Additionally, long-chain OME have a high ignitability, indicated by a cetane number (CN) greater than 70. This opens up degrees of freedom in the injection strategy and enables simplifications compared to the operation with fossil diesel. This study investigates the hydraulic behavior of two solenoid injectors with different injector geometry for heavy-duty applications on an Injection Rate Analyzer (IRA) in diesel and OME operation. For OME, both injectors show longer injection delays in all injection pressure ranges investigated, increasing with rail pressure.

Oxygenated synthetic fuels such as oxymethylene ether (OME) are a promising approach to reduce the emissions of diesel engines and to improve sustainability of mobility. The soot-free combustion of OME allows an optimization of the combustion process to minimize remaining pollutants. Considering the injection system, one strategy is to decrease the rail pressure, which has a positive impact on the reduction of nitrogen oxides without increasing the particle formation. Furthermore, due to the reduced lower heating value of OME compared to diesel fuel, an adaptation of the injector nozzle is recommended. This work describes a method for analyzing the injection process for OME, using the Mie scattering effect in an optically accessible heavy-duty diesel engine. The design of the 1.75 l single cylinder engine allows operation up to 300 bar peak cylinder pressure, providing optical access through the piston bowl and through a second window lateral below the cylinder head.

In this study, a fully optically accessible single-cylinder research engine is the basis for the visualization and generation of extensive knowledge about the in-cylinder processes of mixture formation, ignition and combustion of oxygenated synthetic fuels. Previous measurements in an all-metal engine showed promising results by using a mixture of dimethyl carbonate and methyl formate as a fuel substitute in a DISI-engine. Lower THC and NOx emissions were observed along with a low PN-value, implying low-soot combustion. The flame luminosity transmitted via an optical piston was split in the optical path to simultaneously record the natural flame luminosity with an RGB high-speed camera. The second channel consisted of OH*-chemiluminescence recording, isolated by a bandpass filter via an intensified monochrome high-speed camera.

Polyoxymethylene dimethyl ethers (OME) are synthetic fuels, which offer the property of sustainability because the reactants of production base on hydrogen and carbon dioxide on the one hand, and the air pollution control in consequence of a soot-free combustion in a diesel engine on the other hand. High exhaust gas recirculation (EGR) rates are a promising measure for nitrogen oxide (NOx) reduction without increasing particle emissions because of the resolved soot-NOx trade-off. However, EGR rates towards stoichiometric combustion in OME operation reveals other trade-offs such as methane and formaldehyde emissions. To avoid these, a lean mixture with a combination of EGR and exhaust after-treatment with selective catalytic reduction (SCR) is useful. The limitation of urea dosing due to the light-off temperature of SCR systems requires heating measures.

Aging effects such as coking or cavitation in the nozzle of common rail (CR) diesel injectors deteriorate combustion performance. This is of particular relevance when it comes to complying with emission legislation and demonstrates the need for detecting and compensating aging effects during operation. The first objective of this paper is to analyze the influence of worn nozzles on the injection rate. Therefore, measurements of commercial solenoid common rail diesel injectors with different nozzles are carried out using an injection rate analyzer of the Bosch type. Furthermore, a fault model for typical aging effects in the nozzle of the injector is presented together with two methods to detect and identify these effects. Both methods are based on a multi-domain simulation model of the injector. The needle lift, the control piston lift and the pressure in the lower feed line are used for the fault diagnosis.

Measurement techniques for piston ring rotation, engine oil emission and blow by have been implemented on a single-cylinder petrol engine. A novel method of analysis allows continuous and fast real-time identification of the piston ring rotation of the two compression rings, while the mass-spectrometric analysis of the exhaust gas delivers the cylinder oil emission instantly and with a high temporal resolution. Only minor modifications to the piston rings were made for the insertion of the γ-emitters, the rings rotate freely around the circumference of the piston. The idea of this setup is that through online observation at the test bench, instant feedback of the measured variables is available, making it possible to purposefully select and compare measurement points. The high time resolution of the measurement methods enables the analysis of dynamic effects. In this article, the measurement setup and evaluation method is described.

Measuring and simulating the contact between piston pin and connecting rod (conrod) is very complex. The pin can rotate freely in the conrod as well as in the piston. Further, there is no defined oil supply with a constant pressure as it is for example in main bearings. A tribometer test bench was adapted to measure friction between pin and conrod. The system is loaded with a constant force and oil supply is realized as defined deficient lubrication. During one part of the schedule, the rotational speed is defined as ramp to measure friction coefficient over speed, in another part the speed was pivoted from positive to negative speed within less than 500 milliseconds. With this measurement method, the different friction coefficients between radial slider and pivot bearings could be quantified. The measurements were conducted for four different pin-coatings.

At the Institute of Internal Combustion Engines of the Technische Universitaet Muenchen a drivetrain for urban and commuter traffic is under development. The concept is based on a lean-burn air-cooled two-cylinder natural gas engine which is combined with a hydraulic hybrid system. The engine is initially mechanically charged which results in an engine speed dependent torque. Turbocharging the natural gas fuelled engine derives increased engine torque especially at low engine speeds and exploits the potential of better knock resistance of natural gas compared to gasoline fuel. The paper presents a turbocharging concept for the two-cylinder engine at first. The firing order of 180/540°CA due to the crank shaft design and the lean-burn combustion are challenging restrictions to cope with. The consequences of the uneven firing order are investigated using 1D-simulation and the matching of the exhaust gas turbocharger is shown.

The reduction of both harmful emissions (CO, HC, NOx, etc.) and gases responsible for greenhouse effects (especially CO2) are mandatory aspects to be considered in the development process of any kind of propulsion concept. Focusing on ICEs, the main development topics are today not only the reduction of harmful emissions, increase of thermodynamic efficiency, etc. but also the decarbonization of fuels which offers the highest potential for the reduction of CO2 emissions. Accordingly, the development of future ICEs will be closely linked to the development of CO2 neutral fuels (e.g. biofuels and e-fuels) as they will be part of a common development process. This implies an increase in development complexity, which needs the support of engine simulations. In this work, the virtual modeling of real fuel behavior is addressed to improve current simulation capabilities in studying how a specific composition can affect the engine performance.

The synthetic fuels dimethyl ether (DME) and polyoxymethylene dimethylether (POMDME or OME) are promising oxygenated fuels to meet the rising challenges of air pollution control, CO2-neutrality, and sustainability. The sootless combustion and high ignitability of DME and OME represent ideal properties for an application in diesel engines. However, recent investigations of oxygenates reported an increase of nanoparticles, which are known to have fatal effects on human’s health. Besides nanoparticles, ongoing discussions about future emission legislation focus on a drastic reduction of NOx. For this reason, the present work investigates different measures to reduce NOx emissions using DME/OME and a paraffinic diesel fuel (PDF) as reference. Different rail pressures, exhaust gas recirculation (EGR) rates, and injection timings are evaluated, considering the effectivity on NOx reduction and the impact on other emissions, especially on nanoparticles.