Characterization of the Combustion of Light Alcohols in CI Engines : Performance, Combustion Characteristics and Emissions

Alternative fuels for combustion engines are becoming increasingly popular as society is pushing to phase out fossil energy to reduce CO2 emissions. The compression ignition (CI) engine has a high overall efficiency which makes it a valuable option for the transport fleet, despite the well known NOx and soot pollutants it emits. These two pollutants are emitted due to a combination of high local combustion temperature and low level of premixing prior to the combustion of diesel fuel. Based on previous work, it is well known that high research octane number (RON) fuels, such as gasoline, can be used in an CI engine to increase the premixing thus reducing the engine-out soot emissions, and to a certain extent, also NOx. Apart from reducing the regulated emissions, the automotive industry is also focusing on developing CI engines that run at a higher efficiency and emit less CO2, which can be achieved by using biomass based fuel, either neat or in blends. Methanol and ethanol are two good examples of such fuels. The idea of using light alcohols to run a CI engine did not arise recently; in Sweden, ethanol has been used in this engine type to run city buses since the mid 1980's. However, it is worth mentioning that the research of their use in CI engines has not been extensive. This work aims to investigate the performance, combustion characteristics and emissions of CI engines running on light alcohols, either neat or in blends with diesel, to study the advantages and drawbacks. The purpose is to better understand how the potential of these fuels can be further exploited while simultaneously finding ways to minimize the drawbacks of their use. The light alcohols, and in particular methanol, have a high heat of vaporization in combination with a low heating value. This contributes to a cooler combustion which also causes an extensive enleanment of the charge. The cooler combustion increases the efficiency by reducing the heat losses. The excessive enleanment, on the other hand, increases the total hydrocarbon (THC) and CO emissions. Moreover, the combustion instability increases. The findings of this work suggests that it is possible to counter these drawbacks, by increasing the intake temperature, Tin. This could be achieved by using a turbocharger without extensive intercooling. The higher Tin reduces the premixing period and improves the stability, resulting in increased oxidation of THC and CO. The drawback of this strategy is, however, an increased formation of NOx. For similar intake conditions, methanol combustion resulted in a 50 % reduction of NOx in comparison to iso-octane due to the its charge cooling effect. A double injection strategy can be used to reduce the required Tin, however, this will come at a cost of lower thermal efficiency due to the longer combustion duration. Another viable option to reduce the required Tin is by using a high compression ratio, rc. The resulting increase in NOx can be countered with EGR. However, if rc is too high, operating flexibility is reduced due to restrictions in structural integrity; for example, high lambda alongside high EGR rates will be limited to lower loads. The light alcohols do not produce black carbon soot when combusted, thus significantly lower particulate matter (PM) emissions, which makes them a good alternative to the heavier diesel fuels. On the other hand, the particle number (PN) emission is generally higher than that of conventional gasoline or diesel. It is worth noting that the emitted PN only consist of particles with a diameter 30 nm as measured with a fast particle analyzer. Furthermore, an observation of the emitted PM under a transmission electron microscope, using energy dispersive X-ray, strongly suggested that the origin of the PM was the lubrication oil rather than the combustion products of the light alcohols. The light alcohols have shown some noteworthy results in terms of efficiency and emissions. In this work, a gross indicated efficiency of approx. 53 % was achieved by using a high rc=27 piston and 50 % EGR at 6 bar IMEPg

Measurement of gasoline exhaust particulate matter emissions with a wide-range EGR in a heavy-duty diesel engine

A large number of measurement techniques have been developed or adapted from other fields to measure various parameters of engine particulates. With the strict limits given by regulations on pollutant emissions, many advanced combustion strategies have been developed towards cleaner combustion. Exhaust gas recirculation (EGR) is widely applied to suppress nitrogen oxide (NOx) and reduce soot emissions. On the other hand, gasoline starts to be utilized in compression ignition engines due to great potential in soot reduction and high engine efficiency. New engine trends raise the need for good sensitivity and suitable accuracy of the PM measurement techniques to detect particulates with smaller size and low particulate mass emissions. In this work, we present a comparison between different measurement techniques for particulate matter (PM) emissions in a compression ignition engine running on gasoline fuel. A wide-range of EGR was used with lambda varied from 3 down to 1. The compared equipment includes AVL smoke meter, AVL Micro Soot Sensor, Pegasor and Cambustion Differential Mobility Spectrometer (DMS). The goal of this paper is to compare the recorded values and show the sensitivity of the instruments to soot properties altering, in both lean and stoichiometric combustion situations

Heat Loss Analysis for Various Piston Geometries in a Heavy-Duty Methanol PPC Engine

Partially premixed combustion (PPC) in internal combustion engine as a low temperature combustion strategy has shown great potential to achieve high thermodynamic efficiency. Methanol due to its unique properties is considered as a preferable PPC engine fuel. The injection timing to achieve methanol PPC conditions should be set very close to TDC, allowing to utilize spray-bowl interaction to further improve combustion process in terms of emissions and heat losses. In this study CFD simulations are performed to investigate spray-bowl interaction for a number of different piston designs and its impact on the heat transfer and the overall piston performance. The validation case is based on a single cylinder heavy-duty Scania D13 engine with a compression ratio 15. The operation point is set to low load 5.42 IMEPg bar with SOI -3 aTDC. After satisfactory agreement with experiments in terms of combustion phasing, in-cylinder pressure and heat release rate, the effect of piston bowl geometry is investigated by performing several CFD simulations with modified piston bowl geometry while keeping the compression ratio, CA50 and injection conditions the same as the baseline case. The influence of the wall temperature gradient, the near wall effective conductivity and the piston bowl area on the heat transfer is studied. It was observed that the flow structures that re-direct the hot vapor away from the in-cylinder walls will reduce the wall area that actively transfer the heat. The final piston performance comparison showed that piston bowl designs with a reduced area to volume ratio does not guarantee lower heat loss. Therefore, the mixing process as the result of the spray-bowl interaction and the resulting fuel distribution are considered as the main mechanisms to minimize the total heat losses

Experimental investigation of methanol compression ignition in a high compression ratio HD engine using a Box-Behnken design

Methanol is an alternative fuel offering a lower well-to-wheel CO2 emission as well as a higher efficiency, given that the fuel is derived from biomass. In addition to reduced CO2, methanol does not emit soot particles when combusted which is a great advantage when attempting to reduce NOX levels due to the effectively non-existing NOX-soot trade-off. The engine setup used was a Scania D13 engine modified to run on one cylinder, utilizing a high compression piston with a rc of 27:1. This study analyzes the effects of four control parameters on gross indicated efficiency and the indicated specific emissions; CO, THC and NOX. The control parameters chosen in this work was common rail pressure (PRAIL), EGR, λ and CA50, running at 6 bar IMEPG and 1200 rpm. The effects of the control parameters on performance and emissions was analyzed using a surface response method of the Box-Behnken type. Predictive mathematical models were obtained from regression analysis performed on the responses from the experiments. The highest gross indicated efficiency achieved was ∼53%, when a high level of EGR was applied together with the combustion phasing set to its low level at CA50 = 6 CAD ATDC. The control parameters influencing the CO emissions are λ and the interaction between PRAIL and λ, while THC is only controlled by PRAIL and EGR. NOX emissions was, as expected, influenced mainly by EGR and λ, although PRAIL and CA50 also had minor effects. The effect of increased PRAIL, increased THC emissions which in its turn reduced the gross indicated efficiency. Throughout the experiment, THC concentration never decreased below ∼150 ppm due to utilization of high rc in combination with the volatility of methanol. It was also concluded that a rc = 27 is rather high if operation flexibility is required, especially at the higher load range

Sensitivity Analysis of Partially Premixed Combustion (PPC) for Control Purposes

Partially Premixed Combustion (PPC) is a promising advanced combustion mode for future engines. In order to investigate the sensitivity of PPC to exhaust gas recirculation (EGR) rate, intake gas temperature, intake gas pressure, and injection timing, these parameters were swept individually at three different loads in a single cylinder diesel engine with gasoline-like fuel.A factor of sensitivity was defined to indicate the combustion's controllability and sensitivity to inlet gas parameters and injection timings. Through analysis of experimental results, a control window of inlet gas parameters and injection timings is obtained at different loads in PPC mode from 5 bar to 10 bar IMEPg load at 1200 rpm.To further study the PPC controllability with injection timing, main injection timing was adjusted to sustain steady combustion phasing subject to perturbation of inlet gas state. Experimental results show that the main injection timing can resist the interference of intake parameters and maintain constant combustion phasing. Injection timing control is a promising approach to maintain high engine efficiency and low emission levels during transient operation

Quantification and Analysis of the Charge Cooling Effect of Methanol in a Compression Ignition Engine Utilizing PPC Strategy

The charge cooling effect of methanol was studied and compared to that of iso-octane. The reduction in compression work due to fuel evaporation and the gain in expansion work were evaluated by the means of in-cylinder pressure measurements in a HD CI engine. A single injection strategy was utilized to obtain a longer premixing period to adequately capture the cooling effect. The effect was clear for both tested fuels, however, methanol generally caused the pressure to reduce more than iso-octane near TDC. It was found that the contribution of reduced compression work to the increased net indicated efficiency is negligible. Regarding the expansion work, a slower combustion with higher pressure was obtained for methanol in comparison to that of iso-octane due to the cooling effect of fuel evaporation. As a result from this, a lower heat transfer loss was obtained for methanol, in addition to the significantly lower NOx emissions

Effect of Start of Injection on the Combustion Characteristics in a Heavy-Duty DICI Engine Running on Methanol

Methanol as an alternative fuel in internal combustion engines has an advantage in decreasing emissions of greenhouse gases and soot. Hence, developing of a high performance internal combustion engine operating with methanol has attracted the attention in industry and academic research community. This paper presents a numerical study of methanol combustion at different start-of-injection (SOI) in a direct injection compression ignition (DICI) engine supported by experimental studies. The aim is to investigate the combustion behavior of methanol with single and double injection at close to top-dead-center (TDC) conditions. The experimental engine is a modified version of a heavy duty D13 Scania engine. URANS simulations are performed for various injection timings with delayed SOI towards TDC, aiming at analyzing the characteristics of partially premixed combustion (PPC). The simulations are based on a relatively detailed chemical kinetic mechanism and a well-stirred reactor (WSR) approach, accelerated using a so-called chemistry coordinate mapping (CCM). The injection of the fuel is treated with Lagrangian Particle Tracking (LPT) method. A baseline case with SOI of -20 after TDC (ATDC) was studied experimentally; this case was chosen to validate the model and a good agreement between the experiments and the simulation is found after adjustment of the initial pressure and temperature condition. In all injection conditions the combustion phasing is kept the same, i.e. with the 50-percentage heat release at the same crank angle (CA50) by adjusting the intake temperature. It is shown that as SOI is delayed the combustion characteristics changes significantly leading to a high maximal pressure-rise-rate (MPRR). The SOIs between -20 and -7 ATDC results in a combustion process governed by auto-ignition with propagating ignition fronts. The MPRR increases with SOI due to the rapid heat release caused by ignition at lean but increasingly richer conditions towards stoichiometry. The diffusion controlled, diesel like combustion (CDC), starts to occur around SOI -3 ATDC. The first portion of injected fuel ignites with a delay at leaner conditions, and then forms a diffusion flame. The amount of fuel consumed in the ignition process is larger than the amount of fuel consumed in the diffusion flame. Thus, contribution to the total heat release from the ignition process is larger and more rapid from that when using diesel or gasoline in the same CDC injection. Such behavior is attributed to a longer ignition delay time, large latent heat value and higher stoichiometric mixture fraction for methanol than hydrocarbon fossil fuels. It is concluded that a single main injection strategy of methanol may not be preferable due to the high MPRR thus other injection strategies, e.g., multiple injections should be used

Investigation of effects of intake temperature on low load limitations of methanol partially premixed combustion

Methanol has unique properties as a fuel, and partially premixed combustion has promising results with high engine efficiency and low emissions. Low load studies with methanol partially premixed combustion are scarce, and the effect of intake temperature on low load methanol partially premixed combustion still remains an intriguing question. This study aims to investigate the effect of intake temperature on low load limitations of methanol partially premixed combustion by an experimental study. The engine was operated at 800 rpm under two different loads. The low load condition was performed at 3 bar Indicated mean effective pressure (IMEP), and the idle condition was commenced at 1 bar IMEP. From the results, it was seen that the intake temperature affected engine stability, engine performance, and engine emissions. The combustion stability decreased with the decrease of intake temperature. The ignition delay became longer and the peak cylinder pressure became smaller with lower intake temperature. The combustion efficiency reduced with the decrease of intake temperature from 0.99 to 0.96 at 3 bar IMEP, whereas it decreased from 0.99 to 0.98 at 1 bar IMEP for the single injection case and the split injection case. The thermodynamic efficiency remained constant at 0.43 at 3 bar IMEP, decreased from 0.30 to 0.28 at 1 bar IMEP for the single injection case, and reduced from 0.26 to 0.24 at 1 bar IMEP for the split injection case. The gross indicated efficiency increased from 0.41 to 0.42 at 3 bar IMEP, whereas it reduced from 0.29 to 0.28 and 0.26-0.24 at 1 bar IMEP at single injection and split injection, respectively. Total hydrocarbon emission increased, NOX emission decreased or remained constant, and CO emission remained constant with the decrease in intake temperature. Finally, the combustion phasing study was performed at 1 bar IMEP at constant intake temperature to determine the effect of the start of injection timing on the engine's performance and the emissions under the idle condition

Performance and emissions of diesel-gasoline-ethanol blends in a light duty compression ignition engine

An approach to reduce CO2 emissions while simultaneously keeping the soot emissions down from compression ignition (CI) engines is to blend in short chained oxygenates into the fuel. In this work, two oxygenated fuel blends consisting of diesel, gasoline and ethanol (EtOH) in the ratio of 68:17:15 and 58:14:30 have been utilized and studied in a single cylinder light duty (LD) CI engine in terms of efficiency and emissions. The reasons of utilizing gasoline in the fuel blend is due to the emulsifying properties it has while increasing the total octane rating of the fuel to be able to run the engine with a higher fraction of premixed flame. When performing the experiments, the control parameters were set as close as possible to the original equipment manufacturer (OEM) EU5 calibration of the multi-cylinder engine to study the possibility of using such blends in close to stock LD CI engines. With the oxygenates, in particular the fuel with the higher concentration of EtOH achieved an indicated net efficiency of ∼51% inf comparison to ∼47% for diesel at 8 bar BMEP. The NOX emissions increased slightly for the double injection strategy at 13 bar BMEP from ∼13.5 g/kW h to ∼14.5 g/kW h when going from diesel fuel to the higher ethanol blend. However utilizing single injection strategy at lower loads reduces the NOX. Highest soot mass measured for diesel was ∼0.46 g/kW h in contrast to ∼0.1 g/kW h for the oxygenates. Also, soot production when running the engine on the ethanol containing fuels was not significantly affected by EGR utilization as in the case of diesel. Considering particle size distribution, the particles are reduced both in terms of mean diameter and quantity. At 1500 rpm and 2 bar BMEP an increase of over ∼300% in THC and CO was measured, however, increasing the speed and load to above 2000 rpm and 8 bar BMEP respectively, made the difference negligible due to high in-cylinder temperatures contributing to better fuel oxidation. Despite having lower cetane numbers, higher combustion stability was observed for the oxygenates fuels