Improve hybrid bus fuel efficiency
High-power servo motor |
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This information will be use to create a simple fuel consumption model for the bus. The model will be use to answer some question about optimizing the servo motor for maximum fuel and dollar savings. The paper will introduce a few concepts before pulling the full bus model together. Conclusions will be drawn based on what the model tells us. Hybrid bus overview The momentum of a bus changes constantly as the bus travels on its route, picking up and discarding passengers. The momentum of the bus increases as the bus accelerates between a low speed and a high speed. The increase in momentum of the bus is a result of work done by the diesel engine. The diesel engine consumes fuel at some rate while it converts the fuel energy into mechanical energy. When the bus decelerates between a high speed and a low speed, the momentum change of the bus is converted to heat by application of the brake. Any normal speed corrections that result from turning a corner, starting and stopping, or simply adjusting to traffic flow speed, will result in a momentum change of the bus. Every momentum cycle from low speed to high speed and back results in fuel being burned to accelerate the bus, and then the bus momentum being converted to waste heat in the brake pads. In other words, every speed adjustment consumes fuel and therefore costs money. In a hybrid bus platform, a traction servo motor is used to accelerate the bus from battery power. In series hybrid, the servo motor undertakes the entire task of accelerating the bus. In a parallel hybrid the servo motor assists the diesel engine in accelerating the bus. The main difference between the traditional diesel-only platform and the hybrid platform is that the traction servo motor is also used to slow the bus momentum during changes in speed. The energy captured during deceleration is stored in a capacitor bank or a battery. During the next acceleration cycle, the energy, previously captured and stored in the battery, is converted back into momentum of the bus. Energy re-capture concept The energy re-capture process using the servo motor system appears simple and efficient. However, effective re-capture rate is lower than one might imagine. Consider the component average efficiencies provided in Table 2. Note that the average efficiency can vary considerably from the “peak” efficiency often provided from the component manufacturer. For example, a servo motor has a low efficiency if it is commanded to produce high torque at low speed, and an amplifier consumes watts of energy even if the commanded current to the motor is zero. Therefore, the motor efficiency in Table 2 is not the peak motor efficiency; it is the average efficiency of the motor over the range of operation that includes operating the motor at low speed high torque points.
Let’s assume that the bus slows from a high speed of Vhigh to a lower speed of Vlow due to a change of traffic flow. During the speed adjustment, the energy contained in the bus’ momentum changes. The change of kinetic energy can be calculated from the bus velocity and mass, as indicated by equation . However, if the bus ends up at a different elevation, one may also need to include the bus potential energy; however, we are not going to include potential energy in order to simplify concepts.  The traction servo motor system is going to attempt to recapture this change in energy as the bus slows. At some later time the bus will re-adjust it speed from the low velocity back to the high velocity. The traction system will reuse the captured energy to bring the bus back up to the original speed. Figure 1 contains a simple schematic of the system. Using a very simple model the energy recaptured, r E , is given in equation . If we consider the efficiencies in Table 2, the recaptured energy, r E , will calculate as indicated in equation.  Note that only 77.5% of the energy was recovered. If we then send the energy back to the bus momentum at the same efficiency level as we captured it, the amount of energy converted back into momentum is only 60% (60% = 0.775*0.775*100%). Since the efficiency during capturing and reuse of the energy is not 100%, some external power needs to be supplied during acceleration in order to retain the original speed. This is depicted by the power input in Figure 1. important. Having an easy method of measuring efficiency can provide some valuable insight. By knowing the mass of the bus, the velocity of the bus, and measuring the additional input power during acceleration, one can determine the efficiency of the entire system without ever knowing how much energy was recaptured and recycled. We don’t even need to known what the individual component efficiencies are. A derivation will follow. Measuring system efficiency  Also note that during the energy recapture cycle the power input from the outside is zero and that the energy recapture from the bus momentum to electrical energy occurs at an efficiency of e . This results in that equation can be rewritten as equation . The assumption that power in is zero during the recapture is actually not necessary; it was done to simplify the math; the result of equation is the same even it power is nonzero re-capture. The parameter v L is defined as the energy leverage. The energy leverage is a number greater than one for a system that is effectively recapturing and reusing energy. The energy leverage is the change in kinetic (and potential) energy of the bus divided by the net energy input over a compete energy cycle. An energy cycle is defined as a change in velocity from 1 V to 2 V and then back to 1 V . If one thinks about the structure of the equation, it appears that we have a system where we are getting more energy out.  In fact, we are, because we are recapturing and reusing a portion of the input energy. A pendulum is a good example: it has a very high energy leverage number, a small amount of input energy over one cycle and it used to maintain a much larger kinetic energy change over the same cycle. The process of measuring the system efficiency would be quite simple: Run the bus through several velocity cycles on a level surface while monitoring and recording the energy going into the servo motor drive from the external power source; this is equal to in P . (However, this would only be only feasible on a series hybrid because the engine is decoupled.) The energy leverage is calculated from the left side of equation . The right side of equation is solved for e , as show in equation . The efficiency is calculation from equation .  The efficiency of the servo system will be very important. It ultimately will be responsible for a significant proportion of the overall fuel savings. Equations and can also be used to determine the theoretical efficiency for the same system. Later in this paper, the average efficiency is determined for a servo motor; the energy leverage method is used to determine the theoretical efficiency.Next a simple model will be developed that will be used as a tool to determine the dollar value of an efficiency point. The values in table 1 are used as a starting point. The assumptions that were selected for our bus are presented in Table 2. Our bus will weigh 40,000 lbs (20 tons), will have a diesel engine with an average efficiency of 20%, and a fuel economy of 2.4 mpg highway (1.7 mpg city).
The bus model in this paper is assumed to be used primarily in the city to pick and drop off customers very frequently. Without having an actual move profile of a city bus over its 300,000-mile journey, some simplifying assumptions will be made. Detail: Coefficient of efficiency fuel in hybrid system Source: en.usp-ltd.com.ua References to similar articles:
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