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Whole space combustion for diesel light duty vehicles

Risultati finali

To successfully undertake this project it was necessary to develop a new diesel injection system, which is able to provide a homogeneous diesel and air fuel mixture, using the widest space of the combustion chamber and avoiding cylinder wall wetting by the fuel. This injection system is based on an appropriate injection control strategy based on IFP "pre-existing know-how" including multiple injection events per stroke combined with an appropriate "space" spray pattern also based on IFP "Pre-existing know-how". This injection system is an essential result to achieve the main result of the project of new combustion process. This injection system should be further developed and manufactured by CRT.
Investigations made during Space Light project have shown that for PCCI combustion a fuel, in order to suit the needs of homogeneous combustion should meet the following characteristics: - Fast vaporisation. - Self-ignition features. - Low aromatic content. - Ability to be injected at high pressures. This will ensure a good homogeneity of mixture and an almost simultaneous burn begin in the whole burning chamber. Such a fuel must be newly developed that is why a closed co-operation between automotive and fuel industry is required. During SPACE LIGHT the first steps were already made in that direction. The results obtained during this project have shown that there is potential for the development of HIMCCI even with actual fuels, that is however in a limited engine range. By using new developed fuels this range can be increased and eventually extended to the whole engine range when using also appropriate injection strategies and fully variable valve timing. In the coming years, attention should be given to transient engine conditions and also in car tests, with a suitable fuel. The availability of a fully variable timing system suitable for in-car test will ease the development of such burning processes. Costs for this system should be in an acceptable range and they hang up on the whole system complexity. When such a system combined with an HIMCCI burning process will be cheaper to implement as the traditional after treatment system.
To develop new diesel engine combustion processes, such as HCCI, it is necessary to have a technological solution that allows significant flexibility in the control of the engine operating conditions. Importantly, for this purpose it is essential to have flexibility in the control of the intake and exhaust valve events. Of the project partners, LOTUS has from its "Pre-existing Know-how" an innovative electro-hydraulic variable valve actuation technology (VVA) that can be adapted, at least as a research and development tool, to be used in a diesel HCCI combustion engine. It has been combined with IFP's "Pre-existing Know-how" and allowed to earn "Knowledge" during the project on the valve control strategies suitable for HCCI.
This result consists in the application of the VVA technology (currently never applied to a diesel engine) in combination with the innovative diesel engine combustion process HIMCCI (which can provide a significant NOx engine-out emission reduction, as shown during this project, compared to state of the art diesel engines). With this implementation the HC and CO emission (which are one of the drawbacks of HIMCCI combustion process) can be significantly reduced. It is also possible to increase the exhaust gas temperature, which will help the catalytic after-treatment process. However, unburned HC and CO emissions are still remaining at a high level, and a particulate trap is in any case necessary to reach very low particulate emission values. Also the noise generated by the combustion has to be put under control during the further developments.
Premixed Charge Compression Ignition (PCCI) combustion was realised in a single-cylinder research engine with a port-fuel injection. Heptane was used as the test fuel as it has a similar cetane number (60) to diesel fuel and low boiling temperature. In addition, heptane can be easily injected via a standard port-fuel injector and Heptane as the surrogate fuel. Comprehensive results were obtained using n-heptane and an engine speed of 1500rpm. The effect of compression ratios and intake charge temperature on engines performance and emission was studied in detail. In general, the PCCI combustion of heptane is limited by its low IMEP output at low load (leaner mixture) and by the knocking combustion at high load (richer mixture) for the same amount of EGR. As EGR increases, both the low and high-load limits are reduced in terms of air/fuel ratios and over a certain percentage misfire occurs. When one increases the compression ratio from 12:1 to 18:1, the operational range of PCCI combustion increases and the misfire limit is extended to higher EGR rates. However, the maximum IMEP value that can be obtained with HCCI combustion is reduced at higher compression ratio due to the onset of knocking combustion at a leaner air/fuel ratio. As the compression ratio increases, the combustion takes place earlier and combustion duration becomes shorter - thus the HC and CO emissions are reduced slightly. Operational range of PCCI combustion is increased at higher intake charge temperature. The increased charge temperature also leads to earlier beginning of combustion. However, the temperature effect on combustion duration is small. Similar to the compression ratio effect, the HC and NOx emissions are lower at increased charge temperature.
Several turbocharger-engine configurations were investigated through numerical simulation. While in HCCI mode - mainly at medium load -, the turbo matching is quite tricky since high EGR rates and charge cooling are needed in the cylinder. This could lead to reduced mass flow rates and higher compression ratios through the turbo compared to a normal Diesel operation. The turbo working conditions approach the surge line and actions have to be taken to avoid stalling. A computer model was thus developed in order to predict the turbocharger working conditions and the overall engine performances while in HCCI mode. In parallel, the innovative injection system developed by IFP was fully characterised in an experimental test rig with multiple optical access windows. Thermodynamic conditions similar to those existing in the cylinder during advanced fuel injection were generated in a vessel and the spray development was recorded and analysed trough laser imaging. Correlations among fuel pressure, injection pulse duration, air density, air temperature and spray penetration were generated mainly for those situations that are poorly covered in the literature (i.e. small fuel injected quantities and high air temperatures).
This result consists of the development of an innovative diesel engine combustion process based on IFP "Pre-existing know-how" and where the internal combustion process takes place in the whole space of the combustion chamber. This can be achieved thanks to an appropriately prepared highly diluted Homogeneous Charge Compression Ignition (HCCI). With these characteristics, the HCCI combustion process can provide very low level of particulates combined with near zero level of NOx emissions while maintaining the high efficiency of the best state of the art diesel engines. This result is achieved on different single cylinders steady state operation, at the end of the project. The transient behaviour and the HCCI limited zone have been pointed out as further work necessity, during the very last phases of the project. When fully operational, the HCCI combustion process could be a solution to provide a bright future to the diesel engine as a low CO2 power-train with near zero pollutant emissions.

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