Autor: Andreas Wegmann

Abstract The reduction of carbon dioxide emissions is discussed by politicians as well as in general public today. The efforts that are taken are not limited to Germany or Europe, but take place in most parts of the world. In terms of vehicle propulsion combustion engines contribute a noteworthy part of the worldwide CO2-emission. The present thesis was intended to investigate the CO2-reduction potentials of gaseous fuels used in a combined diesel-gas-combustion on the basis of state-ofthe- art Diesel engines and their control systems. As gaseous fuels both compressed natural gas (CNG) as well as liquefied petroleum gas (LPG) were used. Due to their lower carbon ratio, they provide a potential of reducing CO2 by 11% (LPG) and 24% (CNG) if combusted with an equal effective efficiency compared to Diesel engines. Former investigations on the sector of the diesel-gas-combustion were made on the basis of diesel engines that are almost not state-of-the-art today. These engines were equipped with mechanical systems for dosing diesel and gas and were not or only low-turbocharged. These investigations showed the feasibility of this type of combined combustion, but also identified a strongly increasing emission of hydrocarbons and a decrease of the engine´s efficiency. Investigations for improving these effects were implemented by varying the diesel injection timing as well as throttling the air mass flow to lower the air fuel ratio. Today’s state-of-the-art diesel engines are equipped with electronically controlled diesel injection systems that allow multiple injections per stroke and operate with high injection pressures. Furthermore cooled exhaust gas recirculation systems (EGR) as well as flaps for influencing the air inlet swirl can be used to affect the combustion. Additionally, electronic control devices allow realizing a cylinder selective and sequentially operating gas dosing. Therefore, the present investigation is focusing in its experimental part on the reduction of CO2- and HC-emissions by using all available state-of-the-art air, diesel and gas control devices. In its theoretical part, a modified method for analyzing the diesel-gas-combustion was developed. This calculation method shows a good reproducibility and comparability among the three research engines that were used. The engines are all six cylinder diesel engines. Two of them belong to the heavy duty sector; one is a passenger car engine. In a first step the aim was to check whether the correlations that are known from literature can be transferred to state-of-the-art engines. The results for the engine M1 (electronically controlled inline injection pump) and M2 (CommonRail Injection system) showed an enormous increase of hydrocarbon (factor 29 for engine M1 and 25 for M2) and carbon monoxide (factor 13 for M1 and 2.5 for M2) emission while increasing the energetic gas ratio up to 70%. II Abstract At the same time particle emissions could be reduced by 80% in maximum and the carbon dioxide emissions, depending on the operating point, by 24%. The increase of HC and CO can be explained by lean air fuel ratios in the air-gasphases that lead on the one hand to an expiration of the flame (so called flame quenching) and on the other hand to unburned air-gas-areas at the outer zone of the combustion chamber near to the wall (wall quenching). Simultaneously the combustion center was delayed by several degCS, what is, besides the increase of unburned fuel, the second reason for increasing specific fuel consumption by maximum 4%. In total, these results correspond to those that are described in literature, so it can be noted that these main relations are valid for state-of-the-art diesel engines too. While increasing the gas rate the particle concentration drops down. One new result of this work is represented by the result that the particle spectrum always stays within the spectrum of reference diesel combustion, so that a shifting of the particle spectrum to smaller particles that might easier enter the lungs can be excluded. At the same time on engine M2 cylinder pressure gradients could be significantly reduced compared to a single diesel operation. Although special acoustic measurements were not carried out, the noise reduction could be clearly observed. With respect to the aspired emission certification of the engines M1 and M3, some student assistant investigations were conducted that specially focused on the improvement of the diesel-gas-combustion regarding an optimization of the diesel main injection timing, whereas the focus of this thesis was put on the interaction of diesel-gas-combustion with modern engine control systems. The engine´s load showed a big influence on fuel consumption and emissions. HC and CO emissions decrease significantly with higher loads. The fuel consumption in combined diesel-gas-combustion at very low loads of for example 2bar is slightly higher than in diesel operation, but converges for increasing loads. At 10bar the fuel consumption of the DG-combustion is even 1% lower than in diesel mode. Concerning the CO2-emissions the diesel-gas-combustion enables a reduction of maximum 16% at medium and high loads compared to diesel combustion only. Further investigations concerning a variable air management via throttle as well as a turbocharger with variable turbine geometry show that there is a potential to reduce hydrocarbon emission by a factor 2.5 and carbon monoxide by a factor 2. The main effect is that lower air fuel ratios lead to a postponement of the combustion from the first combustion section (phase of pre-mixed fuels) to the second one (phase of diesel metering) due to an increased ignition delay time. For the first time the present work reveals the interactional effects of a multiple diesel injection and diesel gas combustion. The quantity of an early pilot injection (PiI2) and a pilot injection close to TDC (PiI1) shows a typical NOx-particle-trade-off at medium Abstract III gas parts and part load. An important new finding is that an early timing of both pilot injections leads to a reduction of nitrogen oxides and particles simultaneously. At the same time the pilot injection that is close to TDC (PiI1) which leads to the ignition of the air-gas-mixture can be used to uncouple the beginning of the gas combustion from the diesel main injection. Detailed investigations concerning exhaust gas recirculation and inlet swirl show that their effects still remain independent from each other also for combined diesel-gascombustion. EGR can be distinctively used to decrease nitrogen oxide emissions by maximum 60% at part load while particle emission increases following a typical NOxparticle- trade-off. HC and CO emissions increase by maximum 20 respectively 30% whereas the fuel consumption (-4%) and the CO2-emission could be reduced (-7%). The increase of efficiency results from a shorte-ning of the ignition delay time and a de-throttling due to EGR. The inlet swirl at part load shows an optimum regarding all emission for medium flap positions. An increase of inlet turbulences reduces especially hydrocarbon and carbon monoxide emissions so that it can be used to control their behavior in a mixed diesel-gas-application. Furthermore the inlet swirl system can be applied to move on the typical NOx-particle-trade-off on a lower absolute level than without. As a further control system a cylinder selective and sequentially operating gas dosing unit was investigated. Used for combined diesel-gas-combustion, a reduction of hydrocarbons by 35% in comparison to a centrally dosing gas system can be realized, whereas all other limited emissions were kept neutral or also reduced. All the results from the different subsystems (air and EGR management, inlet swirl system and cylinder selective and sequentially operating gas dosing) built the basis for in total three emission certifications on the engines M1 and M3. Whereas the diesel-LPG application for M3 without any additional oxidation catalyst only allowed low maximum gas parts up to 34%, the diesel-CNG certification for M1 with an additional methane selective oxidation catalyst leads to medium energetic gas ratios of 56% and a large average CO2-reduction by 13%. The present thesis may also be the basis for further research in the field of combined diesel-gas-combustion. Visual analysis of the combustion as well as mechanical optimization of the combustion chamber and the camshaft timing will reveal more potential to reduce CO2-emissions. Also the potential of a direct gas injection and the combination of all presented technologies will be worth to investigate. At the same time the question comes up whether the results can be also transferred to other engine applications. For example engines in block heat power plants might be diesel-gas-engines that can manage changing fuel availability by a variable energetic fuel mix of gas and a liquid fuel.