Reflexiones sobre la experiencia con la literacidad en quechua y castellano

To understand how a hybrid may save energy, it is necessary first to examine how conventional vehicles use energy:

In order to maintain movement, vehicles must produce power at the wheels to overcome aerodynamic drag (air friction on the body surfaces of the vehicle, coupled with pressure forces caused by the air flow), rolling resistance (the resistive forces between tires and the road

surface), and any resistive gravity forces associated with climbing a grade. Further, to accelerate, the vehicle must overcome the natural resistance of its mass to acceleration, called inertia – most of the energy expended in acceleration is then lost as heat in the brakes when the vehicle is brought to a stop.16_{And in addition, the vehicle must provide power for accessories such as} heating fan, lights, power steering, and air conditioning.

Finally, a vehicle will need to be capable of delivering power for acceleration with very little delay when the driver depresses the accelerator, which may necessitate keeping the power source in a standby (energy-using) mode.

A conventional engine-driven vehicle uses its engine to translate fuel energy into shaft power, directing most of this power through the drivetrain to turn the wheels. Substantial amounts of energy are lost along the way. Within the engine, for example, moving parts – especially pistons, crankshaft, and valves – create friction; there are a number of aerodynamic and fluid drag losses (“pumping losses”) because air must be pumped through air cleaner, intake manifold, valves, and exhaust system, and, most importantly, because spark-ignition engines reduce their power output by throttling the air flow which causes additional aerodynamic losses that are very high even at light loads. Much of the heat generated by combustion cannot be used for work and is wasted, both because heat engines have theoretical efficiency limits, and because attaining even these limits is impossible because some heat is lost through cylinder walls before it can do work, and some fuel is burned at less than the highest possible pressure (OTA 1995).

Fuel is also burned while the engine is experiencing negative load (during braking) or when the vehicle is coasting or at a stop, with the engine at idle.

Although part of engine losses would occur under any circumstances, part occur because in conventional drivetrains, engines are sized to provide very high levels of peak power for the acceleration capability expected by consumers17_{– perhaps 10 times the power required to cruise}

16_{Some of this energy is also lost as aerodynamic drag and rolling resistance losses.}

17_{For a mass market family car in today’s market, the ability to accelerate from 0 to 60 mph in 12 seconds}

at 60 mph – but are operated at most times at a small fraction of peak power where they are quite inefficient. Having such a large engine also increases the amount of fuel needed to keep the engine operating when the vehicle is stopped or during braking or coasting, and increases losses due to the added weight of the engine, which increases rolling resistance and inertial losses. Even gradeability requirements (example: 55 mph up a 6.5% grade) require only about 60 or 70% of the power needed to accelerate from 0 to 60 mph in under 12 seconds.18_Multispeed

transmissions allow the engine to operate within a fairly narrow speed regime across the range of vehicle speeds, allowing the engine to stay in the most efficient parts of the engine map more of the time than would be the case with fewer gears – but at the cost of losses in the transmission itself.

Figure 2-1 shows how fuel energy is translated into work at the wheels for a typical midsize vehicle in urban and highway driving as represented by U.S. Environmental Protection Agency (EPA) driving cycles.19_{The part of the figure at the right represents the tractive losses incurred} by the vehicle as a fraction of the total fuel energy. Some highlights of the figure are:

ƒ At best, only one-fifth of the fuel energy reaches the wheels and is available to overcome the tractive forces, and this is on the highway cycle when idling losses are at a minimum, braking loss is infrequent, and shifting is far less frequent.

ƒ Braking and idling losses are extremely high in (EPA cycle) urban driving and even higher in more congested driving, e.g., within urban cores during rush hour. Braking loss, that is, the shedding of the kinetic energy of motion through heat generated by the brakes, represents 46% of all tractive losses in urban driving. Idling losses represent about one sixth of the fuel energy on this cycle.

ƒ Losses to aerodynamic drag, a fifth or less of tractive losses in urban driving, are more than half of the tractive losses during highway driving.

18_{Depending on relative inertia and other losses and on the vehicle load specified for both acceleration and}

gradeability requirements.

19_{Note that the box labeled “kinetic” represents the potential energy built up in accelerating the vehicle.}

This energy is lost as heat in braking and in the rolling and aerodynamic losses that occur as the vehicle decelerates. In other words, regenerative braking’s “target” for recapture is not the full potential energy of the vehicle, because rolling resistance and aerodynamic drag will always be responsible for part of the braking forces when the vehicle is slowed. Another factor that can limit the amount of kinetic energy recoverable is the number of axles driven. For safety, both axles must brake, so only four-wheel drive cars can target the full braking energy. Front-wheel drive cars fare a bit better than rear-wheel drive cars because, in most cars, the front brakes do the larger share of the braking.

Figure 2-1 Energy Flow (from fuel energy to power at the wheels) for a Midsize Automobile