3D CFD Analysis of an Abnormally Rapid Combustion Phenomenon in Downsized Gasoline Engines

Soaring oil prices combined with increased environmental awareness maintain the drive for fuel-efficient and CO2-friendly powertrains. Even if hybrids seem to be the media friendly solution, there is still much work to be done on combustion systems. For gasoline engines aggressive downsizing seems promising and IFP proposes an innovative approach. However, new difficulties arise as specific power outputs are increased. Various combustion phenomena are encountered from simple knock to rumble and must be understood and mastered in order to fully optimize the combustion system. Abnormally rapid yet non destructive and seemingly stable combustion is one of these new phenomena. 3D CFD (Computational Fluid Dynamics) simulation tools were used in order to investigate and under- stand what lies behind this particular combustion

Combustion Approach for Downsizing: the IFP Concept

Individual transport is facing more and more constraints. In the past decade regulations and car manufacturers have focused above all on decreasing pollutant emissions. Even though it has always been a major part of research, fuel economy has now become the number one priority because of the CO2 greenhouse effect. In the long term, hybridisation presents the best potential but in the short term, downsizing will allow for a substantial reduction in gasoline engine fuel consumption. To obtain the full benefits from downsizing, IFP has developed a combustion concept based on high knock resistance and high low-end torque potential due to scavenging. This approach has initially been developed by combining gasoline direct injection (GDI) variable valve timing and twin-scroll technologies (for 4-cylinder engines). This technology is used to develop lambda one partly stratified combustion due to split injection. These stratified mixtures improve engine knock resistance but with conventional injection systems they induce severe drawbacks on pollutant emissions. IFP's answer to this problem has shown very high potential according to the 1.89 MPa at 1500 rpm reached by a 2 litre engine with compression ratio of 11.2. In the car industry, cost is one of the most restrictive criteria. IFP has taken this into account when proposing 2 concrete solutions to reduce the cost of turbocharged gasoline engines. The first step was to replace twin-scroll turbine housings by mono-scroll turbines. Then the challenge was to maintain scavenging on 4-cylinder engines. Results have shown that this approach seems to be a very good trade-off between cost and performance. The 1.8 litre engine performs either 2.4 MPa BMEP at 2000 rpm and 90 kW/l or 2.4 MPa BMEP at 1400 rpm and 83 kW/l. In the latter case, the engine has also been evaluated in full lambda one operation with maximum upstream turbine temperature of 1050°C. Even if limited by lambda one combustion, maximum power density reaches 73 kW/l with a very attractive 260 g/kWh specific fuel consumption. As the cost of small gasoline engines is critical, IFP has started to develop the scavenging process in the case of port fuel injection. Current results show that it is possible to avoid fuel by-pass with a specifically designed engine that points the way to very interesting prospects

Analysis of the part load helical vortex rope of a Francis turbine using on-board sensors

The cavitation helical vortex rope arising at Francis turbine partial load induces pressure fluctuations at the rope precession frequency, which can be decomposed into two components; the convective one and the synchronous one. The latter acts as an excitation source for the hydraulic system and is amplified in case of resonance. The present paper aims to highlight the impact of both components on the mechanical behaviour of the runner by performing on-board measurements. It is shown that the use of on-board pressure sensors enables to naturally separate the components of the pressure fluctuations. In addition, the synchronous component has little impact on the mechanical behaviour of the runner, even for the case in which hydro-acoustic resonance occurs

Mechanical impact of dynamic phenomena in Francis turbines at off design conditions

At partial load and overload conditions, Francis turbines are subjected to hydraulic instabilities that can potentially result in high dynamic solicitations of the turbine components and significantly reduce their lifetime. This study presents both experimental data and numerical simulations that were used as complementary approaches to study these dynamic solicitations. Measurements performed on a reduced scale physical model, including a special runner instrumented with on-board strain gauges and pressure sensors, were used to investigate the dynamic phenomena experienced by the runner. They were also taken as reference to validate the numerical simulation results. After validation, advantage was taken from the numerical simulations to highlight the mechanical response of the structure to the unsteady hydraulic phenomena, as well as their impact on the fatigue damage of the runner

Unsteady hydraulic simulation of the cavitating part load vortex rope in Francis turbines

For Francis turbines at part load operation a helical vortex rope is formed due to the swirling nature of the flow exiting the runner. This vortex creates pressure fluctuations which can lead to power swings, and the unsteady loading can lead to fatigue damage of the runner. In the case that the vortex rope cavitates there is the additional risk that hydro-acoustic resonance can occur. It is therefore important to be able to accurately simulate this phenomenon to address these issues. In this paper an unsteady, multi-phase CFD model was used to simulate two part-load operating points, for two different cavitation conditions. The simulation results were validated with test-rig data, and showed very good agreement. These results also served as an input for FEA calculations and fatigue analysis, which are presented in a separate study