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Turbocharged gasoline direct injection (TGDI) engines often have a flat torque curve with the maximum torque covering a wide range of engine speeds. Increasing the high-speed-end torque for a TGDI engine provides better acceleration performance to the vehicle powered by the engine. However, it also requires more fuel deliveries and thus longer injection durations at high engine speeds, for which the multiple fuel injections per cycle may not be possible. In this study, results are reported of an experimental investigation of impact of fuel injection on dilution of the crankcase oil for a highly-boosted TGDI engine. It was found in the tests that the high-speed-end torque for the TGDI engine had a significant influence on fuel dilution: longer injection durations resulted in impingement of large liquid fuel drops on the piston top, leading to a considerable level of fuel dilution.

The temperature distribution is studied theoretically in a battery module stacked with 12 high-power Li-ion pouch cells. The module is cooled indirectly with ambient air through aluminum heat-sink plates or cooling plates sandwiched between each pair of cells in the module. Each of the cooling plates has an extended cooling fin exposed in the cooling air channel. The cell temperatures can be controlled by changing the air temperature and/or the heat transfer coefficient on the cooling fin surfaces by regulating the air flow rate. It is found that due to the high thermal conductivity and thermal diffusivity of the cooling plates, heat transfer of the cooling plate governs the cell temperature distribution by spreading the cell heat over the entire cell surface. Influence of thermal from the cooling fins is also simulated.

Ideal operation temperatures for Li-ion batteries fall in a narrow range from 20°C to 40°C. If the cell operation temperatures are too high, active materials in the cells may become thermally unstable. If the temperatures are too low, the resistance to lithium-ion transport in the cells may become very high, limiting the electrochemical reactions. Good battery thermal management is crucial to both the battery performance and life. Characteristics of various battery thermal management systems are reviewed. Analyses show that the advantages of direct and indirect air cooling systems are their simplicity and capability of cooling the cells in a battery pack at ambient temperatures up to 40°C. However, the disadvantages are their poor control of the cell-to-cell differential temperatures in the pack and their capability to dissipate high cell generations.

The performance and life of Li-ion battery packs for electric vehicle (EV), hybrid electrical vehicle (HEV), and plug-in hybrid electrical vehicle (PHEV) applications are influenced significantly by battery operation temperatures. Thermal management of a battery pack is one of the main factors to be considered in the pack design, especially for those with indirect air or indirect liquid cooling since the cooling medium is not in contact with the battery cells. In this paper, thermal behavior of Li-ion pouch cells in a battery system for PHEV applications is studied. The battery system is cooled indirectly with liquid through aluminum cooling fins in contact with each cell and a liquid cooled cold plate for each module in the battery pack. The aluminum cooling fins function as a thermal bridge between the cells and the cold plate. Cell temperature distributions are simulated using a finite element analysis approach under cell utilizations corresponding to PHEV applications.

Thermal behavior of a lithium-ion (Li-ion) battery module for hybrid electrical vehicle (HEV) applications is analyzed in this study. The module is stacked with 12 high-power pouch Li-ion battery cells. The cells are cooled indirectly with air through aluminum fins sandwiched between each two cells in the module, and each of the cooling fins has an extended cooling surface exposed in the cooling air flow channel. The cell temperatures are analyzed using a quasi-dimensional model under both the transient module load in a user-defined cycle for the battery system utilizations and an equivalent continuous load in the cycle. The cell thermal behavior is evaluated with the volume averaged cell temperature and the cell heat transfer is characterized with resistances for all thermal links in the heat transfer path from the cell to the cooling air. Simulations results are compared with measurements. Good agreement is observed between the simulated and measured cell temperatures.

In this paper, a thermodynamic model is discussed for a single cylinder diesel engine with its intake and exhaust systems simulating a turbo-charged V8 diesel engine. Following criteria are used in determination of the gas exchange systems of the single cylinder engine (SCE): 1) the level of pressure fluctuations in the intake and exhaust systems should be within the lower and upper bounds of those simulated by the thermodynamic model for the V8 engine and patterns of the pressure waves should be similar; 2) the intake and exhaust flows should be reasonably close to those of the V8 engine; 3) the cylinder pressures during the combustion and gas exchange should be reasonably close to those of the V8 engine under the same conditions for the valve timing, fuel injection, rate of heat release and in-cylinder heat transfer. The thermodynamic model for the SCE is developed using the 1D engine thermodynamic simulation tool AVL BOOST.

In this study, thermal runaway of a 3-cell Li-ion battery module is analyzed using a 3D finite-element-analysis (FEA) method. The module is stacked with three 70Ah lithium-nickel-manganese-cobalt (NMC) pouch cells and indirectly cooled with a liquid-cooled cold plate. Thermal runaway of the module is assumed to be triggered by the instantaneous increase of the middle cell temperature due to an abusive condition. The self-heating rate for the runaway cell is modeled on the basis of Accelerating Rate Calorimetry (ARC) test data. Thermal runaway of the battery module is simulated with and without cooling from the cold plate; with the latter representing a failed cooling system. Simulation results reveal that a minimum of 165°C for the middle cell is needed to trigger thermal runaway of the 3-cell module for cases with and without cold plate cooling.

A new fuel injection strategy is proposed for DME engines. Under this strategy, a pre-injection up to 40% demand is conducted after intake valves closing. Due to high volatility of DME, a lean homogeneous mixture can be formed during the compression stroke. Near TDC, a pilot injection is conducted. Combined fuel mass for the pre-injection and pilot injection is under the lean combustion limit of DME. Thus, the mixture is enriched and combustion can take place only in the neighborhood of sprays of the pilot injection. The main injection is conducted after TDC. Because only about half of the demand needs to be injected and DME evaporates almost immediately, combustion duration for the main injection plus the unburnt fuel in the cylinder should not be long because a large portion of the fuel has been premixed with air. With a high EGR rate and proper timing for the main injection, low temperature combustion could be realized.

The fuel saving benefit is analyzed for a class-8 truck diesel engine equipped with a WHR system, which recovers the waste heat from the EGR. With this EGR-WHR system, the composite fuel savings over the ESC 13-mode test is up to 5%. The fuel economy benefit can be further improved if the charge air cooling is also integrated in the Rankine cycle loop. The influence of working fluid properties on the WHR efficiency is studied by operating the Rankine cycle with two different working fluids, R245fa and ethanol. The two working fluids are compared in the temperature-entropy and enthalpy-entropy diagrams for both subcritical and supercritical cycles. For R245fa, the subcritical cycle shows advantages over the supercritical cycle. For ethanol, the supercritical cycle has better performance than the subcritical cycle. The comparison indicates that ethanol can be an alternative for R245fa.

On the basis of the molecular thermodynamics for fluids, the thermodynamic properties of DME are developed for pressure p ≤ 500 bar and temperature T ≤ 200 °C, which covers pressures and temperatures that a DME fuel system for the CI-engine application would experience. The properties cover subcooled, two-phase, and superheated/supercritical regions, including p-v-T properties, enthalpy, entropy, latent heat, heat capacity, speed of sound in vapor, liquid and two-phase mixtures, bulk modulus, and surface tension. A volume-cubic equation of state for DME also is developed, which allows calculating the DME density at any given pressure and temperature analytically. All the properties are given in equations as well as in charts. For convenience in two-phase-flow applications, e.g., design of the fuel tank and cavitation analysis, the saturated properties are also given in tables, listed in both pressure and temperature up to the critical point.

A novel fuel tank for storing liquid dimethyl ether (DME) has been developed. This fuel tank was made of cast aluminum with a water capacity of 40 liters. It contains two fluids: liquid DME and a vapor-liquid mixture of propane. A diaphragm separates the two fluids. The propane in the tank is a pressurizing fluid that pressurizes DME into a subcooled-liquid state; and, it also functions as a driving fluid that pumps the liquid DME from the tank to the injection pump using its vapor pressure. These features characterize the tank as a thermodynamic pump. Several hundred hours of tank tests at various temperatures have been conducted. Results of tank filling-discharge cycles simulating those in vehicle applications demonstrated that the concept of the two-fluid thermodynamic pump works and that the tank design is successful.

This paper analyzed chemical and thermophysical properties of dimethyl ether (DME) as an alternative fuel for compression-ignition engines. On the basis of the chemical structure of DME and the molecular thermodynamics of fluids, equations have been developed for most of the DME thermophysical properties that would influence the fuel-system performance. These equations are easy to use and accurate in the pressure and temperature ranges for CI engine applications. The paper also pointed out that the DME spray in the engine cylinder would differ significantly from that of diesel fuel due to the thermodynamic characteristics of DME. The DME spray pattern will affect the mixing and combustion processes in the engine cylinder, which, in turn, will influence emissions from combustion.

Compression ignition delay of DME is studied theoretically. Physical phenomena that would influence the ignition delay, characteristics of the DME spray and evaporation of DME droplets in the spray, are analyzed. It is found that the short ignition delay of DME revealed in engine tests is due largely to the short physical delay of DME: The evaporation rate of DME droplets is about twice that of diesel-fuel droplets at the same cylinder condition and, the stoichiometric mixture in a DME spray can be established immediately - in comparison, the stoichiometric mixture in a diesel-fuel spray cannot be established before temperatures of diesel-fuel droplets become higher than 225 °C. The high droplet evaporation rate of DME is also responsible for the irregular boundary and tip of the DME spray as observed by many investigators. On the basis of experimental data reported in the literature, cetane number of DME is estimated to be 68.

In this paper, dependence of liquid-DME viscosity on temperature and pressure was studied theoretically. It was found that in the saturated-liquid state, the DME viscosity is 0.37 cSt at - 40 ° C and it drops to 0.17 cSt when temperature increases to 80 ° C. In the subcooled-liquid state, viscosity varies linearly with pressure at a given temperature; at 20 ° C, viscosity of the subcooled liquid is 0.23 cSt at 5.3 bar and it increases to 0.33 cSt at 500 bar. The predicted liquid-DME viscosity and its pressure dependence agree with those obtained by measurement. Lubricity of liquid DME also was studied. Polar-headed, long-chain alcohols and fatty acids with chain length of C15 ∼ C22 were found to be candidates of lubricity additives for DME. Castor oil (chemically, it is basically a C18 fatty acid) was found to be a good additive for improving the DME lubricity.

Waste heat recovery (WHR) has been recognized as a promising technology to achieve the fuel economy and green house gas reduction goals for future heavy-duty (HD) truck diesel engines. A Rankine cycle system with ethanol as the working fluid was developed at AVL Powertrain Engineering, Inc. to investigate the fuel economy benefit from recovering waste heat from a 10.8L HD truck diesel engine. Thermodynamic analysis on this WHR system demonstrated that 5% fuel saving could be achievable. The fuel economy benefit can be further improved by optimizing the design of the WHR system components and through better utilization of the available engine waste heat. Although the WHR system was designed for a stand-alone system for the laboratory testing, all the heat exchangers were sized such that their heat transfer areas are equivalent to compact heat exchangers suitable for installation on a HD truck diesel engine.

Good understanding of fuel sprays in the engine cylinder is crucial to optimizing the operation of direct injection gasoline engines. In this paper, a detailed analysis is conducted on direct gasoline injection sprays from a multi-hole injector. Penetrations and angles of the sprays are characterized with a homogeneous model for the fuel spray. The drop size distributions in the sprays are analyzed using an empirical distribution model. Predicted spray penetrations, spray angles, and drop size distributions under three different injection pressures are compared with the measurements for injection pressures = 40, 100 and 150 bar and good agreements are observed. Transient evaporation rates are also studied for fuel drops in an environment simulating the cylinder condition during the intake stroke of a direct injection gasoline engine.

Emissions from CI engines fueled with dimethyl ether (DME) were discussed in this paper. Thanks to its high content of fuel oxygen, DME combustion is virtually soot free. This characteristic of DME combustion indicates that the particulate filter will not be needed in the aftertreatment system for engines fueled with DME. NOx emissions from a CI engine fueled with DME can meet the US 2007 regulation with a high EGR rate. Because 49% more fuel mass must be delivered in each DME injection than the corresponding diesel-fuel injection, and the DME injection pressure is lower than 500 bar under the current fuel-system technology, the DME injection duration is generally longer than that of diesel-fuel injection. This is unfavorable to further NOx reduction. A multiple-injection strategy with timing for the primary injection determined by the cylinder temperature was proposed.

This paper discusses the performance of the radial plunger pump used in the contemporary diesel common-rail fuel systems for rail-pressure supply. On the ground of the pump mechanism, the transient flow, drive torque, and efficiency of the pump are analyzed for various operation conditions. The analysis shows that the number of plungers and utilization of the pump capacity govern fluctuations in the pump discharge. The pump flow can be characterized by a discharge function which applies to both full- and part-capacity pump flows. At the full pump capacity, the discharge fluctuation is determined solely by the number of plungers: a pump with an odd number of plungers has more ripples and lower amplitudes in its discharge than a pump with an even number of plungers does. A pump operates at a part capacity has more fluctuations in the discharge than when at the full capacity.

A comparative study of characteristics of diesel fuel and dimethyl ether sprays was conducted on the basis of momentum conservation. The analysis reveals that the DME spray in the diesel combustion system may not develop as well as that of diesel fuel at high engine loads and speeds due primarily to the following reasons. (1) Because 42% more fuel volume must be injected into the engine to reach the diesel-fuel equivalent and because the DME injection pressure is lower than that of diesel fuel, longer injection duration for DME is needed even if with the enlarged orifice diameters.

A variable-displacement, 275-bar dimethyl-ether pump for a common-rail injection system has been developed successfully. The pump is an inlet-throttled, wobble-plate-actuated, multi-plunger system. Results of the pump tests/simulations show that the pump can deliver fuel according to the engine requirement at different speeds due to its variable-displacement feature, which is obtained by controlling the discharge phase angle via the two-phase filling characteristic of the pump. Although the pump is designed for dimethyl ether, its concept is general and thus may be applied to the common-rail systems for other fuels.