Investigation of late-cycle soot oxidation using laser extinction and in-cylinder gas sampling at varying inlet oxygen concentrations in diesel engines

[EN] This study focuses on the relative importance of O-2 and OH as oxidizers of soot during the late cycle in diesel engines, where the soot oxidation is characterized in an optically accessible engine using laser extinction measurements. These are combined with in-cylinder gas sampling data from a single cylinder engine fitted with a fast gas-sampling valve. Both measurements confirm that the in-cylinder soot oxidation slows down when the inlet concentration of O-2 is reduced. A 38% decrease in intake O-2 concentration reduces the soot oxidation rate by 83%, a non-linearity suggesting that O-2 in itself is not the main soot oxidizing species. Chemical kinetics simulations of OH concentrations in the oxidation zone and estimates of the OH-soot oxidation rates point towards OH being the dominant oxidizer.The authors gratefully acknowledge the Swedish Energy Agency, the Competence Center for Combustion Processes KCFP (Project number 22485-3), and the competence center METALUND funded by FORTE for financially supporting this research. The authors acknowledge Volvo AB for providing the gas-sampling valve and personally Jan Eismark (Volvo AB) and Mats Bengtsson (Lund University) for their technical support.Gallo, Y.; Malmborg, VB.; Simonsson, J.; Svensson, E.; Shen, M.; Bengtsson, P.; Pagels, J.... (2017). Investigation of late-cycle soot oxidation using laser extinction and in-cylinder gas sampling at varying inlet oxygen concentrations in diesel engines. Fuel. 193:308-314. https://doi.org/10.1016/j.fuel.2016.12.013S30831419

Review and Benchmarking of Alternative Fuels in Conventional and Advanced Engine Concepts with Emphasis on Efficiency, CO2, and Regulated Emissions

Alternative fuels have been proposed as a means for future energy-secure and environmentally sustainable transportation. This review and benchmarking show that several of the alternative fuels (e.g. methanol, ethanol, higher alcohols, RME, HVO, DME, and biogas/CNG) work well with several different engine concepts such as conventional SI, DICI, and dual fuel, and with the emerging concepts HCCI, RCCI, and PPC. Energy consumption is in most cases similar to that of diesel or gasoline, with the exception of methanol and ethanol that use less energy, especially in SI engines. Tailpipe emissions of CO2 with respect to engine work output (tank-to-output shaft) can be reduced by more than 15% compared to a highly efficient gasoline SI engine, and are the lowest with CNG / lean-burn SI and with alcohols in several engine concepts. Alternative fuels are considered safe and in most cases are associated with reduced risk with respect to cancer and other health and environmental issues. Apart from differences in handling depending on whether the fuel is gaseous or liquid, engine-out emissions of soot, NOx, HC, and CO vary between the fuels, although the levels typically are lower than for gasoline or diesel. The comparably small differences during engine operation indicate that production and distribution will be more important when it comes to the environmental performance and operating costs of the different alternative fuels. RME and ethanol are already established and work well in engines. So do biogas/CNG and RME. Diesel and gasoline already co-exist, and so there is good reason to use several alternative fuels in parallel. For example, increased amounts of RME in diesel and ethanol + methanol in gasoline (compatible with E85 vehicles) are relevant steps forward that essentially rely on current engine technology. New combustion engine concepts can be co-developed with new fuels and will lead to further reductions in energy consumption. Increased hybridization and integration with the electricity grid will provide better energy utilization as well as potential for further reductions in fuel consumption from new engine operation strategies. This enables realistic opportunities for sustainable alternative fuel production as well as energy-secure and environmentally sustainable transportation

Investigation of the Effect of Glow Plugs on Low Load Gasoline PPC

Low temperature combustion (LTC), is a promising alternative for combustion engines, because it combines the positive aspects of both CI and SI engines, high efficiency and low emissions. Another positive aspect of LTC is that it can operate with gasoline of different octane ratings. Still, higher octane gasolines prove to be difficult to operate at low load conditions leading to high combustion instability (COV) that leads also to high emissions. This drawback can be reduced by increasing the intake air temperature or increasing compression ratio, but it is not a viable strategy in conventional applications. For a diesel engine running under LTC conditions, a possibility is to use the existing hardware, glow plugs in this case, to increase the in-cylinder temperature at low loads and facilitate an improved combustion event. Here, an experimental investigation is performed, to investigate how glow plug operation can affect the combustion stability of an engine at steady state operation at low load, with different intake temperatures, as well the effect of them at higher loads. Results show that glow plugs are effective at reducing the required inlet air temperature for keeping stable combustion, with minimal effect on efficiency. The load limit that glow plugs are useful is around 7 bar IMEPg, or about 30 % of the maximum load. After that, the intake conditions can sustain combustion without the use of glow plugs

A Study on the Effect of Elevated Coolant Temperatures on HD Engines

In recent years, stricter regulations on emissions and higher demands for more fuel efficient vehicles have led to a greater focus on increasing the efficiency of the internal combustion engine. Nowadays, there is increasing interest in the recovery of waste heat from different engine sources such as the coolant and exhaust gases using, for example, a Rankine cycle. In diesel engines 15% to 30% of the energy from the fuel can be lost to the coolant and hence, does not contribute to producing work on the piston. This paper looks at reducing the heat losses to the coolant by increasing coolant temperatures within a single cylinder Scania D13 engine and studying the effects of this on the energy balance within the engine as well as the combustion characteristics. To do this, a GT Power model was first validated against experimental data from the engine. Using a Water-PEG mixture as coolant, the coolant temperature was then varied from 60°C to 200°C for both the liner and the cylinderhead. This sweep was done for multiple combinations of engine loads and speeds as well as for different air-fuel ratios. It was found that at the higher air-fuel ratios, an increase in coolant temperature led to an increase in indicated efficiency as well as an increase in exhaust gas temperature and enthalpy. However at lower air-fuel ratios there is a decrease in indicated efficiency with higher coolant temperatures. It was also seen that ignition delay at higher temperatures was shorter with the combustion duration being longer. The change in combustion phasing was found to be dependent on engine load. While the higher coolant temperature simplifies heat recovery from the coolant itself, the consequently higher exhaust gas temperatures observed means that the heat losses are moved more towards the exhaust where energy recovery is easier

Influence of Injection Strategies on Engine Efficiency for a Methanol PPC Engine

Partially premixed combustion (PPC) is one of several advanced combustion concepts for the conventional diesel engine. PPC uses a separation between end of fuel injection and start of combustion, also called ignition dwell, to increase the mixing of fuel and oxidizer. This has been shown to be beneficial for simultaneously reducing harmful emissions and fuel consumption. The ignition dwell can be increased by means of exhaust gas recirculation or lower intake temperature. However, the most effective means is to use a fuel with high research octane number (RON). Methanol has a RON of 109 and a recent study found that methanol can be used effectively in PPC mode, with multiple injections, to yield high brake efficiency. However, the early start of injection (SOI) timings in this study were noted as a potential issue due to increased combustion sensitivity. Therefore, the present study attempts to quantify the changes in engine performance for different injection strategies. Simulations were performed on a heavy-duty multi-cylinder compression ignition engine fueled with methanol. Two operating conditions with different engine load were chosen from the European stationary cycle. Three different injection strategies were applied: 1) SOI > -160°ca aTDC 2) SOI > -40°ca aTDC 3) SOI > -25°ca aTDC. The engine settings were selected to maximize the brake efficiency for each case and the sensitivity of combustion to inlet conditions was analyzed. For the high load operating point, the brake efficiency was 2.2 %pt. higher for case 1 compared to case 3, while this difference was only 0.5 %pt. for the low load operating point. However, the combustion phasing for case 1 and 2 at the high load point proved to be very sensitive to inlet temperature, inlet pressure and oxygen concentration

Low Load Ignitability of Methanol in a Heavy-Duty Compression Ignition Engine

An increasing need to lower greenhouse gas emissions, and so move away from fossil fuels like diesel and gasoline, has greatly increased the interest for methanol. Methanol can be produced from renewable sources and eliminate soot emissions from combustion engines [1]. Since compression ignition (CI) engines are used for the majority of commercial applications, research is intensifying into the use of methanol, as a replacement for diesel fuel, in CI engines. This includes work on dual-fuel set-ups, different fuel blends with methanol, ignition enhancers mixed with methanol, and partially premixed combustion (PPC) strategies with methanol. However, methanol is difficult to ignite, using compression alone, at low load conditions. The problem comes from methanol's high octane number, low lower heating value and high heat of vaporization, which add up to a lot of heat being needed from the start to combust methanol [2]. This paper investigates methanol combustion at low load and compares it to diesel fuel, using a more classical diesel combustion strategy of diffusion combustion. This paper also investigates how a high compression ratio could aid the low load combustion of methanol. To get the methanol burning, with similar stability as diesel fuel, intake heating was used together with a pilot injection, of about a third of the main injection quantity. The resulting efficiencies were similar between diesel fuel and methanol, and for the emission measurements NOx was much lower for methanol than for diesel fuel. Increasing the compression ratio resulted in stable combustion without the need for intake heating and a pilot injection, at even lower loads. It also yielded higher efficiency without having a major effect on the emissions

Combined Low and High Pressure EGR for Higher Brake Efficiency with Partially Premixed Combustion

The concept of Partially Premixed Combustion (PPC) in internal combustion engines has shown to yield high gross indicated efficiencies, but at the expense of gas exchange efficiencies. Most of the experimental research on partially premixed combustion has been conducted on compression ignition engines designed to operate on diesel fuel and relatively high exhaust temperatures. The partially premixed combustion concept on the other hand relies on dilution with high exhaust gas recirculation (EGR) rates to slow down the combustion which results in low exhaust temperatures, but also high mass flows over cylinder, valves, ports and manifolds. A careful design of the gas exchange system, EGR arrangement and heat exchangers is therefore of utter importance. Experiments were performed on a heavy-duty, compression ignition engine using a fuel consisting of 80 volume % 95 RON service station gasoline and 20 volume % n-heptane. A wide range of engine speeds and loads were run using a low pressure EGR system. The experiments served as a validation basis for a one-dimensional simulation model. Using the model, a comparison between low pressure EGR, high pressure EGR and combined low and high pressure EGR was performed. The results showed that the combined low and high pressure EGR configuration could reach an average brake efficiency of 41.6 % while the low pressure EGR and high pressure EGR reached 39.5 % and 39.9 % respectively. The combined configuration reached a higher efficiency because it decreased the mass flow range in which the turbine and compressor needed to work which resulted in a higher overall turbocharger efficiency. The effect of varying the EGR and charge air cooler gas outlet temperatures was also studied. It was concluded that a higher cooler temperature decreased both the brake efficiency and the maximum achievable load due to a higher in-cylinder heat transfer loss

Investigating the potential of an integrated coolant waste heat recovery system in an HD engine using PPC operation

With the increasing focus on reducing emissions and making fuel efficient vehicles within the automotive industry over the past few years, new methods are constantly being investigated to improve the efficiency of the powertrain. One such method is recovering waste heat from the exhaust gases as well as the coolant using a thermodynamic cycle such as a Rankine cycle. However, most studies looking into low temperature or coolant heat recovery investigate the use of a separate secondary cycle for the recovery of waste heat itself. This has the disadvantage of having the working fluid at a lower temperature than the coolant which reduces the recovery efficiency. This paper investigates the potential of an integrated Rankine cycle waste heat recovery system where the coolant also acts as the refrigerant and is integrated with the exhaust gas recirculation waste heat recovery. The refrigerant/coolant used for this study is ethanol, while being used in two modes for low temperature/coolant recovery: using the engine as the preheater and using it as an evaporator. Using a combination of GT Power and Matlab, a Scania D13 engine was simulated in partially premixed combustion operation with a waste heat recovery system. For the engine load-speed range, the coolant flow rate, pressure ratio and superheat were swept for determining the optimal values for maximizing output power. It was seen that while using the engine both as a preheater and as an evaporator the recoverable power increased in comparison to using only the exhaust gas recirculation heat for recovery. When using the engine for preheating, the recoverable power increased marginally with an indicated efficiency gain of less than 0.5 percentage points whereas when using the engine for the evaporation of the coolant, the indicated efficiency showed gains of up to 1.7 percentage points in comparison to using EGR-only heat recovery with a total gain in indicated efficiency of up to 5.5 percentage points. This larger gain in recoverable power while using the engine as an evaporator in comparison to as a preheater is due to the location of the pinch point in analyzing the heat exchange process. The system behavior was also studied with regards to the pressure ratio, the mass flow rate of coolant and the superheat. It was generally observed that at higher loads and speeds these parameters increased as more waste heat was available for recovery for the system

The relevance of different fuel indices to describe autoignition behaviour of gasoline in light duty dici engine under ppc mode

Partially premixed combustion (PPC) with gasoline fuels is a new promising combustion concept for future internal combustion engines. However, many researchers have argued the capabilities of research octane number (RON) and Motor Octane Number (MON) to describe the autoignition behaviour of gasoline fuels in advanced combustion concepts like PPC. The objective of this study is to propose a new method, called PPC number, to characterize the auto ignition quality of gasoline fuels in a light-duty direct injected compression ignition engine under PPC conditions. The experimental investigations were performed on a 4-cylinder Volvo D4 2 litre engine. The ignition delay which was defined as the crank angle degrees between the start of injection (SOI) and start of combustion (SOC) was used to represent the auto ignition quality of a fuel. The ignition delays of primary reference fuels PRF (blends of n-heptane and iso-octane) were used to develop a reference curve where a PPC metric for gasoline could be based on. The PPC number of a specific gasoline is defined as the octane number of the PRF, which has the same ignition delay as gasoline under the same operating condition. Twelve different gasolines, having RON values between 55 and 95, were tested at two different operating conditions of 0% exhaust gas recirculation (EGR) and 40% EGR levels namely Case0 and Case40 respectively. The intake pressures of Case0 and Case40 were 1.5 bar and 1.8 bar respectively with a constant inlet temperature of 110 o C. The PPC numbers of all gasolines were measured and the relevancy of the other indices such as RON, MON, Octane Index and HCCI number were assessed. When the indices were compared, PPC number showed consistence and continues correlations with ignition delays in both conditions. Results also revealed that the spray target and piston geometry gave a big impact to the auto ignition quality of fuels