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MyCar.net |
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Series Hybrids In a series hybrid, the engine is used only to drive a generator, while the wheels are powered exclusively by an electric motor. A battery (or flywheel or ultracapacitor) is used to store energy, obtaining some energy input from regenerative braking, and most of the input from the engine/generator. The motor can be powered either directly by the engine/generator, by the battery, or by both simultaneously (at high-power demand). Strategy considerations about when to use the battery or the motor/generator lead to decisions about the relative power output of each unit and the energy storage capacity of the battery. The popular vision of a series hybrid has a small engine operating at constant output, providing the average power needed over the driving cycle, with a battery, flywheel, or ultracapacitor providing additional power when needed, such as for acceleration or hill-climbing. When the vehicle’s power needs are below the engine output, the excess energy goes to recharge the storage device. A careful examination of the vehicle’s energy requirements and the characteristics of the available power sources is necessary to show whether the popular vision will work in practice. First, examining an engine’s power characteristics does make it clear that the engine should be used to provide the total energy for driving, while the battery or other storage device should be sized to provide peak power. Although an ICE does have high specific power (power output per kilogram of engine weight) under normal operation, keeping the engine at its peak efficiency point sharply limits specific power. That is, a typical engine operating at its best efficiency point produces only about 40 percent of its peak output. Such an engine, combined with a generator, radiator, and other engine components, would weigh 7.5 to 8.5 kg/kW and have specific power about 117 to 130 W/kg.55 In contrast, advanced lead acid batteries of the semi-bipolar or bipolar type provide specific power of over 300 W/kg for a 30-second rating, while ultracapacitors and flywheels can provide 2,000 W/kg or more. That is, the storage devices can have higher specific power than the engine itself. Second, the storage mechanisms are limited in the amount of power they can provide, which has important implications for engine sizing and operations. The battery, for example, is capable of providing peak power in short bursts only, because of heat removal requirements. Ultracapacitors are limited by their low specific energy; they would have to be very large to provide high power for a long period. Consequently, while the storage devices can be used to satisfy high-power requirements that last a short period, the engine itself must be sized large enough to take care of any high-power requirements that may be of long duration. Consistent with the analysis for Electric Vehicles OTA has imposed the requirement that the vehicle be capable of sustaining a long climb of a 6 percent grade at 60 mph. Sizing the hybrid’s engine in this manner--to provide enough power to climb a long hill--implies that the engine, when operating at its most efficient speed, is providing a higher average power output than needed for most driving. This means that much of the time the engine is operating, it will be charging the battery or other storage device. When the storage device becomes fully charged, the engine must be turned off and the vehicle operated in the following manner: As long as power demands are moderate, the vehicle operates as an Electric Vehicle, until the storage is drawn down far enough to allow the engine to be turned on again. Depending on the energy storage capacity of the buffer, then, the engine might be turned off and on several times (for low-energy storage, such as with an ultracapacitor) or possibly just once during an average drive (with battery storage). The engine must be turned on well before the buffer is drained of its energy, however, because the buffer must still be available to provide a power boost, if needed. . During the period when the engine is turned off, it will have to be restarted, if there is a demand for power that exceeds the capacity of the buffer. In a hilly area, the engine may need to be restarted often. This operating mode is far more complex than implied by most discussions of series hybrids, which often give the impression that the engine runs at one speed during the entire trip, with the buffer providing occasional bursts of power on demand. Moreover, the need to turn the engine on and off may have important implications for pollution control. The imposition of a 6 percent grade-climbing ability at 60 mph, when coupled with the requirement that the engine run at constant output, has a startling impact on engine size and vehicle design. This grade-climbing capability requires about 30 kw/ton of vehicle and payload weight. Because attaining a desirable O to 60 mph acceleration time of about 12 seconds requires about 50 kW/ton of vehicle and payload (for a vehicle with an electric drivetrain), the batteries (or other storage devices) must supply (50-30) kW/ton for peak accelerations. Given these specifications, a mid-size Taurus hybrid would have the following characteristics: - Vehicle curb weight: 1843 kg - Engine output (nominal): 61.3 kw - Battery peak output: 40.9 kw - Battery weight: 136.2 kg - Battery type: semi-bipolar lead acid, 300 W/kg. The engine must be a 3.3L four-valve engine rated at 155 kw at its normal peak. The amazing result is that the engine must actually be substantially more powerful than that of the current Taurus. The reason, of course, is that the engine of the current Taurus already operates near the maximum efficiency point at a 6 percent grade climb at 60 mph. Hence, if the engine of the hybrid electric vehicle (HEV) is sized in the same proportion, it must be larger to provide the increased power to overcome the weight associated with the motor, battery, electrical system, and generator, which adds 800 lbs to the weight--and the larger engine also adds to the vehicle’s weight. The result is that the Taurus hybrid weighs over 900 pounds more than the current Taurus. This is only one of the unattractive aspects of limiting engine operation to only one output level. Another problem is that on the FTP city cycle, the engine operates for a very brief duration. The 23-minute cycle requires about 2.3 kWh of energy at the motor to cover the cycle, which means that the engine needs to run about 1.1 minutes,57 and be shut off the rest of the time. Hence, cold-start fuel consumption will add a significant penalty to total fuel consumption. Interestingly, because the battery is capable of storing 5.7 kwh, the vehicle could be run as an Electric Vehicle over the entire FTP cycle, if it started with the battery fully charged--though its performance would be quite limited. The above analysis clearly indicates that restricting the engine in a series hybrid to operating only at its most efficient point is not a practical strategy; the theoretical advantage in efficiency is overwhelmed by both the requirement for a very large engine and he energy and emissions penalties from turning the engine on and off during operation. A more practical alternative is to use a smaller engine running at its most efficient point most of the time, with short-term high-power needs met by the battery (or other storage device) and longerterm power needs, such as hill climbing, met by allowing the engine to increase its output. In other words, if high-peak loads persist for over 20 or 30 seconds, the control logic can allow the engine to provide more power rapidly (albeit with lower efficiency) so that the batteries are not taxed too heavily. To avoid too large an efficiency loss, the engine can be constrained to stay within 10 percent of the maximum efficiency--a constraint that still allows a substantial increase in available power. The only disadvantage of this strategy is that the battery must be somewhat bigger, to provide maximum peak short-term power with the engine operating at lower power than the previous, larger engine. Electric Vehicleen this has some benefits, however, because the larger storage capacity of the battery reduces the need to turn the engine on and off, thus reducing the adverse emission consequences. For the same Taurus example, we have the following Hybrid Electric Vehicle specification: - Vehicle curb weight : 1385 kg - Engine peak output : 44.7 kW - Continuous output : 19.0 kW - Engine plus generator weight : 167 kg - Battery : peak output : 59.1 kW - Motor output : 79.3 kW In other words, the hybrid reasonable. Its engine is now litres, and total vehicle weight with a relaxed engine-operating quite small, with a 44.7 kW peak very similar to the current Taurus. strategy appears much more rating and displacement of 1.0 On the urban cycle, the engine would be on 28 percent of the time, and shut off during the rest of the cycle. On the highway cycle, the engine is on for 62 percent of the time, and the engine would be operating continuously at 70 mph cruise on level ground. This is favorable for fuel efficiency because the engine would be operating at its near optimal point, and energy can flow directly from generator to motor without going through the battery. The effects on fuel consumption can be estimated with reasonable accuracy using the methodology presented in appendix A. The major assumption here is that the engine can be operated at close to optimal efficiency, or else be turned off. The computation, described in box 4-2 and table 4-11, shows that urban fuel economy for the Hybrid Electric Vehicle “Taurus” is 32.7 mpg, highway fuel economy is 41.2 mpg, and composite fuel economy is 36.1 mpg, which is about 30 percent better than the current Taurus. Most of the improvement is in the urban cycle, with only a small (8.4 percent) percentage improvement on the highway cycle--not a surprising result because engine efficiency is quite high at highway speeds. The 30 percent improvement is an optimistic value for current technology, since the efficiencies of Electric Vehicleery one of the components have been selected to be at 2005 expected values, which are higher than the actual observed range for 1995. It also assumes the availability of a semi-bipolar battery that can produce high-peak power for acceleration. In the absence of such high-peak power capability, fuel economy drops precipitously. If a normal lead acid battery with a peakpower capability of 125 W/kg is used, composite fuel economy is only 24.5 mpg, which is almost 12 percent lower than the conventional Taurus. These findings are in good agreement with the observed fuel efficiency of some Hybrid Electric Vehicles with conventional lead-acid batteries. As noted, both Nissan and BMW reported lower fuel economy for their series hybrid vehicles, which used nickel-cadmium batteries with specific peak power of 125 to 150 W/kg.59 Table 4-12 presents detailed assumptions and results for analyses of several series hybrid vehicles that might be ready for introduction by the years 2005 and 2015. For these vehicles, ICES were combined with bipolar lead acid batteries, ultracapacitors, or flywheels using the same flexible operating regime Electric Vehiclealuated above. The main focus of the results should be on the last five rows in the table, which lists urban, highway, and composite fuel economy, range as a pure Electric Vehicle with the engine off, and the amount of time the storage mechanism can put out maximum power if it begins with a full charge. In 2005, improvements to engine peak efficiency, higher battery peak-power, and body-weight reductions are expected to provide significant improvements to the fuel efficiency of an Hybrid Electric Vehicle with battery storage (using a bipolar lead acid battery); fuel economy increases to 48.5 mpg. This however, is only a 25 percent improvement in fuel economy over the 2005(m) scenario vehicle using the same body, aerodynamic, and rolling resistance improvements. The reduction in fuel economy benefit relative to the advanced conventional car--the benefit in 1995 was 30 percent -- occurs primarily because engine technologies such as variable valve timing (VVT) and lean-bum help part-load fuel efficiency more than peak efficiency. Hence, a crucial advantage of the series hybrid--maintaining engine efficiency close to the highest point--is steadily eroded as part-load efficiencies of the IC engine are improved in the future. Several of the Hybrid Electric Vehicles Electric Vehiclealuated in table 4-12 can, if necessary, operate for a while as an Electric Vehicle, though with reduced performance and limited range. With a bipolar lead acid battery, for example, the 2005 series hybrid has a range of about 28 miles maximum, or 22 miles realistically. The use of an ultracapacitor, if it is sized only to provide peak power requirements for acceleration, reduces the range to less than one mile, owing to the ultracapacitor’s high power-to-energy ratio. In fact, if sized this way, the ultracapacitor stores only 0.1 kWh, so that it can deliver the required peak acceleration power of 40 kw for only eight seconds, which clearly is impractical. In OTA’s analysis, the ultracapacitor size is tripled from the size needed for power. The result--peak acceleration capability of 24 seconds and Electric Vehicle range of 2.4 miles--still seems inadequate, however, because the ultracapacitor will not be able to support long, repeated accelerations, which maybe necessary on the highway, and on most trips the engine would have to be shut down and restarted several times, which may adversely affect emissions. If flywheel storage becomes commercially practical by 2005, the composite fuel economy of an ICE/flywheel hybrid will be similar to that of the ultracapacitor-based hybrid--about 60 mpg. With the flywheel sized to provide the necessary 40 kW of peak power, it can provide this power level for about 54 seconds or allow travel in an Electric Vehicle mode for about five miles. The peaking capability may be on the margin of acceptability, though it is doubtful whether there will be enough power for rapidly repeated accelerations. In OTA’s analysis, the flywheel size is doubled from the size required just to meet peak power requirements. By 2015, the use of a lightweight aluminum body with low drag and low rolling resistance tires, and the use of a high-efficiency engine permits the Hybrid Electric Vehicle with a bipolar battery to be 280 lbs lighter than the advanced conventional vehicle, although the engine must be a 0.7 litre, twocylinder engine with the attendant noise and vibration problems of such engines. The advanced bipolar lead acid battery, rated at 500 W/kg of specific power, weighs only 82 kg. Electric Vehicleen so, the fuel efficiency of the vehicle at 65.3 mpg is less than 23 percent better than the equivalent 2015 advanced vehicle with a conventional drivetrain. The ultracapacitor and flywheelequipped vehicles are estimated to be Electric Vehicleen lighter and more fuel efficient at 71 to 73 mpg, but the problems of energy storage still persist. Assuming that the ultracapacitor meets the DOE long-term goal of a specific energy storage capacity of 15 Wh/kg, it can still provide peak power for only about 25 seconds starting from a fully charged condition, if sized for peak power. Similarly, a flywheel sized for peak power can provide this peak power for only 65 seconds. Such low values makes it impossible for a vehicle to have repeatable acceleration characteristics, if they are subjected to two or three hard accelerations in the duration of a few minutes. As done in OTA’s analysis for 2005, the flywheel capacity is doubled and the ultracapacitor size is tripled to provide sufficient energy storage, with resulting cost and weight penalties. At their expected levels of energy storage, ultracapacitor’s would have to be substantially oversized (with respect to their power capability) to be used with an Hybrid Electric Vehicle, as Electric Vehicleen a tripling of ultracapacitor size provides peak power for only about one minute from a fully charged state. At this time, a high peak-power lead-acid battery appears to be a better storage technology for a series Hybrid Electric Vehicle than an ultracapacitor or flywheel, although the battery will be less efficient If developers can substantially increase the specific energy storage capability of ultracapacitors and flywheels, however, they will become far more practical as hybrid vehicle energy storage devices. The estimated fuel economies attained by the hybrids are sensitive to the assumptions about the efficiency of the electric drivetrain components. Although the component efficiencies assumed in the above analysis are superior to the best current values, the PNGV is aiming at still higher efficiencies. A sensitivity analysis of the results displayed in table 4-12 indicates that improving motor/generator efficiencies by increments of 2 percent will boost fuel economy by a similar percentage. For example, for the 2015 lead acid hybrid, a 2 percent boost in engine efficiency raises vehicle fuel economy from 65.3 to 66.9 mpg; an additional 2 percent boost raises it to 68.5 mpg. Similarly, a 2 percent engine efficiency boost for the ultracapacitor hybrid raises fuel economy from 71.2 mpg to 73.1 mpg, with an additional 2 percent boost yielding 74.9 mpg.
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