Andrew M. Williams

Turbo-discharging turbocharged internal combustion engines

Turbo-discharging is a novel approach that can better utilize the energy recoverable by a turbine (or series of turbines) mounted in the exhaust flow of internal combustion engines. The recovery of blowdown pulse energy in isolation of displacement pulse energy allows the discharging (depressurization) of the exhaust system to reduce engine pumping work and improve engine fuel economy. This is a novel approach to air system optimization that has previously been studied for naturally aspirated engines. However, to be successful, turbo-discharging should be applicable to turbocharged engines, as downsizing is a promising direction for future powertrain systems. This study uses one-dimensional gas dynamics modelling to explore the effect of turbo-discharging on a turbocharged gasoline engine, particularly focusing on the interaction with the turbocharging system. The results show that the peak engine torque is increased at low to mid speeds with high speed torque slightly reduced due to restrictions in engine breathing with low lift exhaust valves. The engine peak torque as a function of speed with a larger turbocharger and turbo-discharging was comparable to that of the smaller turbocharger without turbo-discharging. Fuel economy improvements were evident over most part-load regions of the engine map, with peak values varying from 2 to 7% depending on the baseline engine air system strategy. Hot trapped residual mass was consistently reduced across a large fraction of the engine map, with the exception of high power conditions, where the valve pressure drop effect dominated. This is expected to enable spark advance and further fuel economy benefit. The results from this study are promising and show that the use of some of the available exhaust gas energy for turbo-discharging in preference to turbocharging can have a positive effect on both part-load and full-load engine performance. There remains significant potential for further optimization with application of variable valve actuation and turbocharger control systems (for example, variable geometry turbines).

Correcting mass measurement of diesel particulate filters at non-ambient temperatures

Abstract Diesel particulate filters (DPFs) are becoming a widespread method for reducing the particulate matter (PM) emissions from both on-highway and off-highway automotive diesel engines. Mass measurements of DPFs are commonly used to determine rapidly both the amount of PM trapped by the filter and the amount regenerated (removed) by regeneration systems. To avoid issues with adsorption of atmospheric water the filters are often weighed at elevated temperatures. It is shown in this work that at elevated temperatures the filters weigh less than at lower temperatures as a direct result of the buoyant hot air within the filter substrate. This study shows that consideration of the buoyancy forces allows for correction of the mass measurement for the errors relating to the non-ambient temperature of the filter, allowing mass measurements at elevated temperatures while avoiding adsorption of atmospheric water on to the filter substrate and, therefore, improving the accuracy of mass-measurement-based studies of filtration and regeneration performance of DPFs. It is demonstrated that a filter with approximately 85 per cent overall porosity weighed at 150°C in ambient temperatures will have an error of about 0.3g/l (typically about 10 per cent of the trapped PM mass) in the mass measurement when not correcting for the temperature. By way of an example, this is shown to have potentially an important effect on the calculated trapped PM.

Turbo-Discharging: Predicted Improvements in Engine Fuel Economy and Performance

The importance of new technologies to improve the performance and fuel economy of internal combustion engines is now widely recognized and is essential to achieve CO2 emissions targets and energy security. Increased hybridisation, combustion improvements, friction reduction and ancillary developments are all playing an important part in achieving these goals. Turbocharging technology is established in the diesel engine field and will become more prominent as gasoline engine downsizing is more widely introduced to achieve significant fuel economy improvements. The work presented here introduces, for the first time, a new technology that applies conventional turbomachinery hardware to depressurize the exhaust system of almost any internal combustion engine by novel routing of the exhaust gases. The exhaust stroke of the piston is exposed to this low pressure leading to reduced or even reversed pumping losses, offering >5% increased engine torque and up to 5% reduced fuel consumption. This method has the distinct advantage of providing performance and fuel economy improvements without significant changes to the structure of the engine, the combustion system or lubrication system. The Turbo-Discharging concept is introduced and analyzed. A combination of filling/emptying models and 1-D gas dynamic simulations were used to quantify the energy flows and identify optimum valve timings and turbomachine characteristics. 1-D gas dynamic simulation was then used to predict primary fuel economy benefits from Turbo-Discharging. Secondary benefits, such as extended knock limits are then discussed.

A finite-volume based two-dimensional wall-flow diesel particulate filter regeneration model

Abstract Many existing diesel particulate filter (DPF) models do not sufficiently describe the actual physiochemical processes that occur during the regeneration process. This is due to the various assumptions made in the models. To overcome this shortcoming, a detailed two-dimensional DPF regeneration model with a multistep chemical reaction scheme is presented. The model solves the variable density, multicomponent conservation equations by the pressure implicit with splitting of operators (PISO) scheme for inlet and outlet channels as well as the porous soot layer and filter wall. It includes a non-thermal equilibrium (NTE) model for the energy equation for porous media. In addition, for the first time, experiments on the DPF were conducted to determine the interstitial heat transfer coefficient inside the DPF porous wall. The results compare well with an in-house one-dimensional model and subsequently this was used in the new two-dimensional model. By using this detailed two-dimensional model, some interesting observations of the DPF regeneration process were revealed. These included flow reversals and asymmetry in the filter channels.

On the measurement and modelling of high pressure flows in poppet valves under steady-state and transient conditions

Flow coefficients of intake valves and port combinations were determined experimentally for a compressed nitrogen engine under steady-state and dynamic flow conditions for inlet pressures up to 3.2 MPa. Variable valve timing was combined with an indexed parked piston cylinder unit for testing valve flows at different cylinder volumes whilst maintaining realistic in-cylinder transient pressure profiles by simply using a fixed area outlet orifice. A one-dimensional modelling approach describing three-dimensional valve flow characteristics has been developed by the use of variable flow coefficients that take into account the propagation of flow jets and their boundaries as a function of downstream/upstream pressure ratios. The results obtained for the dynamic flow cases were compared with steadystate results for the cylinder to inlet port pressure ratios ranges from 0.18 to 0.83. The deviation of flow coefficients for both cases is discussed using pulsatile flow theory. The key findings include: 1. For a given valve lift, the steady-state flow coefficients fall by up to 21 percent with increasing cylinder/manifold pressure ratios within the measured range given above; 2. Transient flow coefficients deviated from those measured for the steady-state flow as the valve lift increases beyond a critical value of approximately 0.5 mm. The deviation can be due to the insufficient time of the development of steady state boundary layers, which can be quantified by the instantaneous Womersley number defined by using the transient hydraulic diameter. We show that it is possible to predict deviations of the transient valve flow from the steady-state measurements alone.

Modelling gas flow pressure gradients in gelcast ceramic foam diesel particulate filters

Abstract New mathematical models are proposed that predict fluid flow pressure gradients in gelcast ceramic foam diesel exhaust particulate filters by considering the foam structure conceptually as serially connected orifices. The resulting multiple orifice mathematical (MOM) model is based on the sum of a viscous term derived from an extended Ergun model and the kinetic energy loss derived from the Bernoulli and conservation of mass equations. The MOM model was calibrated using experimental data obtained from measuring the air flowrate and pressure drop across a physical large-scale three-dimensional model of a cellular foam structure produced using rapid manufacturing techniques. The calibrated model was then validated using fluid flow data obtained from gelcast ceramic foam filters of various cell sizes and was found to require no empirical recalibration for each gelcast ceramic foam sample. The MOM model for clean filters was extended to predict pressure gradients of filters loaded with particulate matter (PM). The prediction of pressure gradients through gelcast ceramic filters using the MOM model for clean and PM-loaded cases was shown to be in reasonable agreement with experimental data. The models were finally applied to design a filter for a turbocharged, charge-cooled, 2.0l, four-stroke, common rail, direct injection passenger car diesel engine.

Low Power Autoselective Regeneration of Monolithic Wall Flow Diesel Particulate Filters

This paper presents research into a novel autoselective electric discharge method for regenerating monolithic wall flow diesel particulate filters using low power over the entire range of temperatures and oxygen concentrations experienced within the exhaust systems of modern diesel engines. The ability to regenerate the filter independently of exhaust gas temperature and composition significantly reduces system complexity compared to other systems. In addition, the system does not require catalyst loading and uses only massproduced electronic and electrical components, thus reducing the cost of the after-treatment package. Purpose built exhaust gas simulation test rigs were used to evaluate, develop and optimise the autoselective regeneration system. On-engine testing demonstrated the performance of the autoselective regeneration process under real engine conditions. Typical regeneration performance is presented and discussed with the aid of visual observations, particulate mass measurements, back pressure measurements and energy consumption. The research demonstrates the potential of the novel autoselective method for diesel particulate filter regeneration. The autoselective process does not require an exhaust by-pass and enables the system to be low power, catalyst-free and exhaust temperature independent.

SI engine combustion wall thermal management potential without the presence of control limitation

Tight future CO2 emission targets have encouraged extensive research in options for improving internal combustion engine efficiency. Amongst those, engine thermal management is a promising area to improve fuel economy, engine power and even reliability. Earlier studies have shown that engine thermal management was not just protecting engine from overheating but it also can improve engine performance, fuel consumption and even emissions. However, the effects and limits of thermal management are highly complex, and a better understanding is required to reach the full potential. The aim of this paper is to demonstrate the potential of manipulating combustion wall temperature for improving engine efficiency. A 1D numerical model of a 2.2L natural aspirated engine was developed using GT-Suite software for this purpose. The spark timing and fuelling in the engine model was also recalibrated to explore the indirect influence of thermal management influence on engine efficiency. The model assumes that the optimal temperature can be achieved at all times, ignoring some of the control implementation issues for now. The results show that optimized combustion wall temperature produces significant fuel consumption improvements at low to medium engine speed at both low and high load. The comparison with conventional temperature control was made using 7 legislated and academic test cycles. The highest fuel economy improvement of about 4% was recorded in urban test cycles. A smaller improvement of more than 2% was found for motorway driving. The results are due to improved combustion and lubrication only, not including reduced hydraulic losses.

IC Engine Air System Uni-, Bi-, and Tri-Directional Energy Flow Optimisation: Turbocharging, Turbocompounding and Turbodischarging

Air systems are becoming increasingly complex and important for achieving IC engine performance and emission targets. Turbocharging is becoming increasingly prevalent enabling high power density engines, improved pumping work and improved fuel economy. Turbo-compounding allows turbine energy to contribute directly to crankshaft work with the aim of improving fuel economy. Turbodischarging allows turbine energy to be used to extract exhaust gases from the engine reducing pumping work and residual gas fraction while simultaneously increasing the amount of energy that can be recovered by the turbine(s). The optimum energy flow split between turbocharging, turbodischarging and turbocompounding has not previously been explored. This paper presents results of a study investigating the potential of tri-directional energy flow optimisation in comparison to uni-directional optimisation and bi-directional optimisation (i.e. using all three approaches, any two approaches or turbocharging alone).Thermodynamic analysis demonstrates the potential of bi-directional optimisation to achieve realistically 4% fuel consumption benefit for both turbocharging and discharging, and turbocharging and compounding on gasoline engines from pumping work alone. The peak benefit of the former occurs at a slightly lower engine torque than the latter as the energy cost of a unit fuel consumption benefit with turbodischarging increases with increasing levels of exhaust depressurisation. The Tri-directional optimisation shows a complex optimum position utilising all three systems and achieving a realistic peak benefit of 4.4% fuel consumption improvement. Optimisation on diesel engine architectures suggests significantly lower potential in the order of 1% benefit while lean burn gas engines showed up to 2.6% benefit. Sensitivity to compression and expansion efficiencies, exhaust manifold volume and system temperatures are presented.The future hybridisation of IC engine air systems may enable energy storage. This paper offers fundamental insight into the marginal fuel cost of capturing energy from the three systems and the marginal fuel value of using stored energy in the air system.Copyright

Schlieren visualization of spark generated shockwaves in narrow channels

Summary form only given. Recently there has been an increased interest in shock wave behavior and shock-wall interaction in microscale channels and tubes. Micro- and millimeter sized shock waves are of significance in a variety of modern applications, such as miniature combustion, jets and shock tubes, explosives, laser shock processing and medical devices. Due to the small scale of the confinement heat transfer to the walls and friction play an important role for shock behavior.