Search Results

EGR coolers are effective to reduce NOx emissions from diesel engines due to lower intake charge temperature. EGR cooler fouling reduces heat transfer capacity of the cooler significantly and increases pressure drop across the cooler. Engine coolant provided at 40–90 C is used to cool EGR coolers. The presence of a cold surface in the cooler causes particulate soot deposition and hydrocarbon condensation. The experimental data also indicates that the fouling is mainly caused by soot and hydrocarbons. In this study, a 1-D model is extended to simulate particulate soot and hydrocarbon deposition on a concentric tube EGR cooler with a constant wall temperature. The soot deposition caused by thermophoresis phenomena is taken into account the model. Condensation of a wide range of hydrocarbon molecules are also modeled but the results show condensation of only heavy molecules at coolant temperature.

The operating range in HCCI mode is limited by the excessive pressure rise rate and therefore high combustion induced noise. The HCCI range can be extended with turbocharging which enables increased dilution of the charge and thus a reduction of combustion noise. When the engine is turbocharged the intake charge will have a high temperature at increased boost pressure and can then be regulated in a cooling circuit. Limitations and benefits are examed at 2250 rpm and 400 kPa indicated mean effective pressure. It is shown that combustion stability, combustion noise and engine efficiency have to be balanced since they have optimums at different intake temperatures and combustion timings. The span for combustion timings with high combustion stability is narrower at some intake temperatures and the usage of external EGR can improve the combustion stability. It is found that the standard deviation of combustion timing is a useful tool for evaluating cycle to cycle variations.

The soot distribution as function of ambient O₂ mole fraction in a heavy-duty diesel engine was investigated at low load (6 bar IMEP) with laser-induced incandescence (LII) and natural luminosity. A Multi-YAG laser system was utilized to create time-resolved LII using 8 laser pulses with a spacing of one CAD with detection on an 8-chip framing camera. It is well known that the engine-out smoke level increases with decreasing oxygen fraction up to a certain level where it starts to decrease again. For the studied case the peak occurred at an O₂ fraction of 11.4%. When the oxygen fraction was decreased successively from 21% to 9%, the initial soot formation moved downstream in the jet. At the lower oxygen fractions, below 12%, no soot was formed until after the wall interaction. At oxygen fractions below 11% the first evidence of soot is in the recirculation zone between two adjacent jets.

Long chain alcohols possess major advantages over the currently used ethanol as bio-components for gasoline, including higher energy content, better engine compatibility, and less water solubility. The rapid developments in biofuel technology have made it possible to produce C 4 -C 5 alcohols cost effectively. These higher alcohols could significantly expand the biofuel content and potentially substitute ethanol in future gasoline mixtures. This study characterizes some fundamental properties of a C 5 alcohol, isopentanol, as a fuel for HCCI engines. Wide ranges of engine speed, intake temperature, intake pressure, and equivalence ratio are investigated. Results are presented in comparison with gasoline or ethanol data previously reported. For a given combustion phasing, isopentanol requires lower intake temperatures than gasoline or ethanol at all tested speeds, indicating a higher HCCI reactivity.

High-load HCCI operation is typically limited by rapid pressure-rise rates (PRR) and engine knock caused by an overly rapid heat-release rate (HRR). Exhaust gas recirculation (EGR) is commonly used in HCCI engines, and it is often stated in the literature that charge dilution with EGR (or high levels of retained residuals) is beneficial for reducing the PRR to allow higher loads without knock. However, EGR/retained-residuals affect other operating parameters such as combustion phasing, which can in turn influence the PRR independently from any effect of the EGR gases themselves. Because of the multiple effects of EGR, its direct benefit for reducing the PRR is not well understood. In this work, the effects of EGR on the PRR were isolated by controlling the combustion phasing independently from the EGR addition by adjusting the intake temperature. The experiments were conducted using gasoline as the fuel at a 1200 rpm operating condition.

The potential of boosted HCCI for achieving high loads has been investigated for intake pressures (Piⁿ) from 100 kPa (naturally aspirated) to 325 kPa absolute. Experiments were conducted in a single-cylinder HCCI research engine (0.98 liters) equipped with a compression-ratio 14 piston at 1200 rpm. The intake charge was fully premixed well upstream of the intake, and the fuel was a research-grade (R+M)/2 = 87-octane gasoline with a composition typical of commercial gasolines. Beginning with Piⁿ = 100 kPa, the intake pressure was systematically increased in steps of 20 - 40 kPa, and for each Piⁿ, the fueling was incrementally increased up to the knock/stability limit, beyond which slight changes in combustion conditions can lead to strong knocking or misfire. A combination of reduced intake temperature and cooled EGR was used to compensate for the pressure-induced enhancement of autoignition and to provide sufficient combustion-phasing retard to control knock.

Using alternative fuels like Natural Gas (NG) has shown good potentials on heavy duty engines. Heavy duty NG engines can be operated either lean or stoichiometric diluted with EGR. Extending Dilution limit has been identified as a beneficial strategy for increasing efficiency and decreasing emissions. However dilution limit is limited in these types of engines because of the lower burnings rate of NG. One way to extend the dilution limit of a NG engine is to run the engine on Hythane (natural gas + some percentage hydrogen). Previously effects of Hythane with 10% hydrogen by volume in a stoichiometric heavy duty NG engine were studied and no significant changes in terms of efficiency and emissions were observed. This paper presents results from measurements made on a heavy duty 6-cylinder NG engine. The engine is operated with NG and Hythane with 25% hydrogen by volume and the effects of these fuels on the engine performance are studied.

This article presents an investigation of the sources combustion-generated noise and its measurement in HCCI engines. Two cylinder-pressure derived parameters, the Combustion Noise Level (CNL) and the Ringing Intensity (RI), that are commonly used to establish limits of acceptable operation are compared along with spectral analyses of the pressure traces. This study focuses on explaining the differences between these two parameters and on investigating the sensitivity of the CNL to the ringing/knock phenomenon, to which the human ear is quite sensitive. Then, the effects of independently varying engine operating conditions such as fueling rate, boost pressure, and speed on both the CNL and RI are studied. Results show that the CNL is not significantly affected by the high-frequency components related to the ringing/knock phenomenon.

This study explores the use of two conventional ignition improvers, 2-ethylhexyl nitrate (EHN) and di-tert-butyl peroxide (DTBP), to enhance the autoignition of the regular gasoline in an homogeneous charge compression ignition (HCCI) engine at naturally aspirated and moderately boosted conditions (up to 180 kPa absolute) with a constant engine speed of 1200 rpm. The results showed that both EHN and DTBP are very effective for reducing the intake temperature (Tin) required for autoignition and for enhancing stability to allow a higher charge-mass fuel/air equivalence ratio (ϕm). On the other hand, the addition of these additives can also make the gasoline too reactive at some conditions, so significant exhaust gas recirculation (EGR) is required at these conditions to maintain the desired combustion phasing. Thus, there is a trade-off between improving stability and reducing the oxygen available for combustion when using ignition improvers to extend the high-load limit.

A detailed understanding of the various factors affecting the trends in gross-indicated thermal efficiency with changes in key operating parameters has been carried out, applied to a one-liter displacement single-cylinder boosted Low-Temperature Gasoline Combustion (LTGC) engine. This work systematically investigates how the supplied fuel energy splits into the following four energy pathways: gross-indicated thermal efficiency, combustion inefficiency, heat transfer and exhaust losses, and how this split changes with operating conditions. Additional analysis is performed to determine the influence of variations in the ratio of specific heat capacities (γ) and the effective expansion ratio, related to the combustion-phasing retard (CA50), on the energy split. Heat transfer and exhaust losses are computed using multiple standard cycle analysis techniques. The various methods are evaluated in order to validate the trends.

Low-temperature gasoline combustion (LTGC), based on the compression ignition of a premixed or partially premixed dilute charge, can provide thermal efficiencies (TE) and maximum loads comparable to those of turbo-charged diesel engines, and ultra-low NOx and particulate emissions. Intake boosting is key to achieving high loads with dilute combustion, and it also enhances the fuel's autoignition reactivity, reducing the required intake heating or hot residuals. These effects have the advantages of increasing TE and charge density, allowing greater timing retard with good stability, and making the fuel ϕ- sensitive so that partial fuel stratification (PFS) can be applied for higher loads and further TE improvements. However, at high boost the autoignition reactivity enhancement can become excessive, and substantial amounts of EGR are required to prevent overly advanced combustion.

The improvement of the indicated thermal efficiency of an argon power cycle (replacing nitrogen with argon in the combustion reaction) is investigated in a CFR engine at high compression ratios in homogeneous charge compression ignition (HCCI) mode. The study combines the two effects that can increase the thermodynamic efficiency as predicted by the ideal Otto cycle: high specific heat ratio (provided by argon), and high compression ratios. However, since argon has relatively low heat capacity (at constant volume), it results in high in-cylinder temperatures, which in turn, leads to the occurrence of knock. Knock limits the feasible range of compression ratios and further increasing the compression ratio can cause serious damage to the engine due to the high pressure rise rate caused by advancing the combustion phasing.

Engine knock is an abnormal phenomenon, which places barriers for modern Spark-Ignition (SI) engines to achieve higher thermal efficiency and better performance. In order to trigger more controllable knock events for study while keeping the knock intensity at restricted range, various spark strategies (e.g. spark timing, spark number, spark location) are applied to investigate on their influences on knock combustion characteristics and pressure oscillations. The experiment is implemented on a modified single cylinder Compression-Ignition (CI) engine operated at SI mode with port fuel injection (PFI). A specialized liner with 4 side spark plugs and 4 pressure sensors is used to generate various flame propagation processes, which leads to different auto-ignition onsets and knock development. Based on multiple channels of pressure signals, a band-pass filter is applied to obtain the pressure oscillations with respect to different spark strategies.

This work explores how the high-load limits of HCCI are affected by fuel autoignition reactivity, EGR quality/composition, and EGR unmixedness for naturally aspirated conditions. This is done for PRF80 and PRF60. The experiments were conducted in a single-cylinder HCCI research engine (0.98 liters) with a CR = 14 piston installed. By operating at successively higher engine loads, five load-limiting factors were identified for these fuels: 1) Residual-NOx-induced run-away advancement of the combustion phasing, 2) EGR-NOx-induced run-away, 3) EGR-NOx/wall-heating induced run-away 4) EGR-induced oxygen deprivation, and 5) excessive partial-burn occurrence due to EGR unmixedness. The actual load-limiting factor is dependent on the autoignition reactivity of the fuel, the EGR quality level (where high quality refers to the absence of trace species like NO, HC and CO, i.e. simulated EGR), the level of EGR unmixedness, and the selected pressure-rise rate (PRR).

Partially Premixed Combustion (PPC) has shown its potential by combining high combustion controllability with emission characteristics that are close to those of an HCCI engine. In order to get PPC the ignition delay needs to be long enough for the fuel and air to mix prior to combustion. This can be achieved by injecting the fuel sufficiently early while running with high EGR. In order to find out where and how PPC occurs a map that shows the changes in combustion characteristics with injection timing and EGR was created. The combustion characteristics were studied in a six cylinder heavy duty engine where the Start of Injection (SOI) was swept from early to late injection over a wide range of EGR levels. The emissions were monitored during the sweeps and in the most promising regions, with low emissions and high efficiency, additional changes in injection pressure and engine speed were applied to get a more versatile picture of the combustion.

High EGR rates combined with turbocharging has been identified as a promising way to increase the maximum load and efficiency of heavy duty spark ignition engines. With stoichiometric conditions a three way catalyst can be used which means that regulated emissions can be kept at very low levels. Obtaining reliable spark ignition is difficult however with high pressure and dilution. There will be a limit to the amount of EGR that can be tolerated for each operating point. Open loop operation based on steady state maps is difficult since there is substantial dynamics both from the turbocharger and from the wall heat interaction. The proposed approach applies standard closed loop lambda control for controlling the overall air/fuel ratio for a heavy duty 6-cylinder port injected natural gas engine. A closed loop load control is also applied for keeping the load at a constant level when using EGR.

This work explores the high-load limits of HCCI for naturally aspirated operation. This is done for three fuels with various autoignition reactivity: iso-octane, PRF80, and PRF60. The experiments were conducted in a single-cylinder HCCI research engine (0.98 liter displacement), mostly with a CR = 14 piston installed, but with some tests at CR = 18. Five load-limiting factors were identified: 1) NOx-induced combustion-phasing run-away, 2) wall-heating-induced run-away, 3) EGR-induced oxygen deprivation, 4) wandering unsteady combustion, and 5) excessive exhaust NOx. These experiments at 1200 rpm show that the actual load-limiting factor is dependent on the autoignition reactivity of the fuel, the selected CA50, and in some cases, the tolerable level of NOx emissions. For iso-octane, which has the highest resistance to autoignition of the fuels tested, the NOx emissions become unacceptable at IMEPg = 473 kPa.

Low-sulfur “clean” diesel fuel has been mandated in the US and Europe. However, quality of diesel fuel, particularly the sulfur content, varies significantly in other parts of the world. Due to logistical issues in various theaters of operation, the Army is often forced to rely on local fuel supplies, which exposes vehicles to diesel fuel or jet fuel (JP-8) with elevated levels of sulfur. Modern engines typically use cooled Exhaust Gas Recirculation (EGR) to meet emissions regulations. Using high-sulfur fuels and cooled EGR elevates problems associated with cooler fouling and corrosion of engine components. Hence, an experimental study has been carried out in a heavy-duty diesel engine running on standard JP-8 fuel and fuel doped with 2870 ppm of sulfur. Gas was sampled from the EGR cooler and analyzed using a condensate collection device developed according to a modified ASTM 3226-73T standard. Engine-out emissions were analyzed in parallel.

Homogeneous charge compression ignition (HCCI) combustion with fully premixed charge is severely limited at high-load operation due to the rapid pressure-rise rates (PRR) which can lead to engine knock and potential engine damage. Recent studies have shown that two-stage ignition fuels possess a significant potential to reduce the combustion heat release rate, thus enabling higher load without knock. This study focuses on three factors, engine speed, intake temperature, and fuel composition, that can affect the pre-ignition processes of two-stage fuels and consequently affect their performance with partial fuel stratification. A model fuel consisting of 73 vol.% isooctane and 27 vol.% of n-heptane (PRF73), which was previously compared against neat isooctane to demonstrate the superior performance of two-stage fuels over single-stage fuels with partial fuel stratification, was first used to study the effects of engine speed and intake temperature.

Isopentanol is an advanced biofuel that can be produced by micro-organisms through genetically engineered metabolic pathways. Compared to the more frequently studied ethanol, isopentanol's molecular structure has a longer carbon chain and includes a methyl branch. Its volumetric energy density is over 30% higher than ethanol, and it is less hygroscopic. Some fundamental combustion properties of isopentanol in an HCCI engine have been characterized in a recent study by Yang and Dec (SAE 2010-01-2164). They found that for typical HCCI operating conditions, isopentanol lacks two-stage ignition properties, yet it has a higher HCCI reactivity than gasoline. The amount of intermediate temperature heat release (ITHR) is an important fuel property, and having sufficient ITHR is critical for HCCI operation without knock at high loads using intake-pressure boosting. Isopentanol shows considerable ITHR, and the amount of ITHR increases with boost, similar to gasoline.