A new test rig has been designed, built and commissioned, and is now jointly pursued to facilitate experimental investigations into advanced combustion processes (i.e., dual fuel, multi-mode) under turbulent conditions at high, engine-like temperature and pressure levels. Based on a standard diesel engine block, it offers much improved optical access to the in-cylinder processes due to its separated and rotated arrangement of the compression volume and combustion chamber, respectively. A fully variable pneumatic valve train and the appropriate preconditioning of the intake air allows it to represent a wide range of engine-like in-cylinder conditions regarding pressures, temperatures and turbulence levels. The modular design of the test rig facilitates easy optimizations of the combustion chamber/cylinder head design regarding different experimental requirements. The name of the new test rig, Flex-OeCoS, denotes its Flexibility regarding Optical engine Combustion diagnostics and/or the development of corresponding Sensing devices and applications. Measurements regarding in-cylinder gas pressures, temperatures and the flow field under typical operating conditions are presented to complete the description and assessment of the new test rig.

No abstract

Natural gas is a promising alternative fuel for internal combustion engines, it allows for a reduction of engine-out emissions without impairing high engine efficiencies. Although this approach is already utilized from small to large engine classes, it is almost exclusively based on the combustion of a premixed, homogeneous charge. For ignition, small engines use standard spark-plugs or pre-chambers, while large and lean-operated engines use pre-chambers and pilot injections. Direct high-pressure gas injection is a more recent, alternative way to operate gas engines which offers benefits compared to premixed operation such as high compression ratio, high combustion pressures, lean operation, quantity regulation, among others. However, in contrast to diesel direct injection, the compression temperatures are too low for the auto-ignition of the gas jets. Therefore, an additional ignition system is required, usually a pilot injection system is used. In this study, the usability and performance of three ignition strategies for direct injected high-pressure gas jets have been investigated in an optically accessible test-rig that is able to operate at engine-like conditions. The first type of ignition system is a pilot injection with a liquid fuel, the second is a glow-plug located near the main gas jet, and the third system is a pre-chamber with a nozzle hole aimed at the main gas jet. Results show that all three strategies are feasible options under the studied conditions. Ignition by a pilot fuel injection is a safe and reliable way to ignite the main fuel. The glow-plug is less reliable and leads to high cycle-to-cycle variation. The best option in the present study is the pre-chamber, it is very reliable, delivers the highest peak cylinder pressure and exhibits the lowest cyclic variability. The good performance is attributed to the intense mixing of the main gas jet with the hot jet exiting the pre-chamber.

Natural gas is a promising alternative fuel for internal combustion engines, it allows for a reduction of engine-out emissions without impairing high engine efficiencies. Although this approach is already utilized from small to large engine classes, it is almost exclusively based on the combustion of a premixed homogeneous charge. For ignition, small engines use standard spark plugs or pre-chambers, while large and lean-operated engines use pre-chambers and pilot injections. Direct high-pressure gas injection is a more recent, alternative way to operate gas engines which offers benefits compared to premixed operation such as high combustion pressures, leaner operation, easier quantity regulation, and higher compression ratios, among others. However, in contrast to diesel injection, the compression temperatures are too low for the auto-ignition of the gas jets. Therefore, an additional ignition system is required, usually a pilot injection system is used. In this study, the usability and performance of a scavenged pre-chamber used for the ignition a high-pressure gas jet has been investigated in an optically accessible test-rig that is able to operate at engine-like conditions. Results show that the turbulent hot jet generated by the pre-chamber was able to ignite the high-pressure gas jet under a wide range of operating conditions. Moreover, it also appears to be a promising ignition strategy for very early direct injection during the compression stroke as well as for port injection. The good performance is attributed to the intense mixing of the main gas jet with the hot jet exiting the pre-chamber.

Internal combustion engines will continue to play a role during a transitional phase, especially in heavy-duty or marine applications. In this context, lean-burn gas/dual-fuel combustion is an attractive concept to reduce CO2, combined with considerably lower particulate and NOX pollution, and with efficiencies comparable to diesel combustion. However, ignition processes still pose considerable challenges, with pre-ignition in particular being a major issue. The underlying mechanisms are probably based on self-ignition of lube oil in hot zones. In order to investigate fundamentals of such phenomena in optically accessible test rigs, a novel injector was specifically developed to induce pre-ignitions artificially. The so-called “PieZo-Droplet-Injector” (PZDI) enables dosing of minor amounts of lubricating oil or even the injection of single droplets with diameters in the range of 100–200 µm. The working principle relies on a needle actuated with a piezo stack, which pushes a certain amount of lube oil in a bore so that (even single) droplets can be ejected through an adjustable nozzle. To confirm the PZDI functionality and to investigate droplet characteristics based on adjustable operating parameters, tests were performed under ambient conditions as well as in a constant volume combustion chamber under reasonable pressure and temperature conditions. Overall, the PZDI showed an excellent behavior in terms of capabilities to inject small amounts or even single droplets of lube oil. At last, this specially developed injector allows selective lube oil addition in an optically accessible engine test facility for upcoming examination of pre-ignition phenomena under real operating conditions.