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tThis may not be quite true when the additive is methanol because of the formaldehyde in the exhaust. With ethanol, the exhaust instead contains some acetaldehyde, a relatively innocuous substance.

Currently, flex-fuel cars are popular in Brazil where both pure ethanol (E-100) and gasoline are usually available at any refueling station. Flex-fuel engines will automatically adjust themselves to the fuel being used.

3.7 Hybrid Engines for Automobiles

Automobile emission standards are established individually by each state, but the leader is the California Air Resources Board (CARB), which has proposed the most stringent emission specifications in the country. These include a requirement that, by a given date, 2% of the vehicles sold in California be zero emission vehicles (ZEVs). This requirement was later postponed. Automobile manufacturers have spent considerable effort in the exegesis of the expression ZEV.

Clearly, a purely electric vehicle (EV) emits no noxious gases. However, it consumes electricity generated in part by fossil fuels, producing pollutants. An EV pollutes, albeit very little compared with a conventional internal combustion vehicle (ICV). Some argue that if an automobile equipped with an internal combustion engine emits the equivalent amount of pollutants (or less) as the total emission from an EV, then such an ICV should also be considered a "zero emission" car. The great popularity of the Toyota Prius, attests to the general interest in this type of vehicle.

A hybrid vehicle is an electric car equipped with an additional fuel-driven power source. Hybrids lead to a substantial lowering of emission for several reasons:

1. Whereas a normal automotive engine has to operate over a wide range of speeds, from idling to full acceleration, the engine of a hybrid is optimized for operation at, when possible, constant speed and can be fine-tuned for maximum efficiency and minimum pollution.

2. There is no waste during the frequent idling periods in normal city driving—the gas engine, instead of idling, is actually turned off.

3. Regenerative braking that returns power to the battery during deceleration can be implemented in a relatively simple manner.

There are two categories of hybrid vehicles: series and parallel. In series hybrids, the power applied to the wheels comes entirely from the electric motor(s). The fuel-driven component simply recharges the battery.

In parallel hybrids, wheel power is derived from both electric and IC motors. Clutches are used to couple these different power plants to the wheels according to the requirements of the moment.

Series hybrids are relatively simple but require large electric motors capable of delivering full acceleration power. In addition, they must have auxiliary systems to maintain battery charge. Thus series hybrids have large drive motors, a charging engine, and a generator. The sum of the powers of these three components substantially exceeds the power necessary to drive the vehicle. This can be expensive.

In parallel hybrids the electric motors can be much smaller, and the additional surge power comes from the IC power plant. However, the extra power plant in a hybrid does not have to be a heat engine. Fuel cells may prove ideal for such an application.

Many consider the hybrid car as a intermediary solution in the transition from the current IC engine to the future purely electric car whose advent still depends on battery improvements, among other things. It is easy to accept that the next step for the automotive industry will be the plug-in hybrid, a car that has batteries large enough for much of the daily city driving but can rely on its internal combustion engine for longer trips. For purely urban driving, no fuel is ever needed because the batteries are fully charged overnight using electricity supplied by the local utility.

3.8 The Stirling Engine

Had the early automobile developers opted for a Stirling engine rather than an Otto, present-day vehicles would be more efficient and less polluting. A quirk of history tipped the scales away from the Stirling. Nevertheless, in at least one application, there will be a revival of this old technology. As we are going to see in Chapter 5, space missions to the outer planets use as a source of power the Radioisotope Thermal Generator (RTG) because the feebleness of the sunlight in those faraway regions makes photovoltaics unpractical. RTGs use thermoelectric generators and have effective efficiencies of some 10%. By replacing thermocouples by free-piston Stirling engines, NASA plans to raise the efficiency to 30% and thus reduce both the mass and the very high cost of the plutonium heat source. Instead of the 10kg of plutonium (millions of dollars), the radioactive fuel will be reduced to only 2 kg. Free-piston Stirling engines have longevity required by these missions. NASA expects to use them probably around 2012.

Stirlings have the following advantages:

1. They are more efficient than Otto and Diesel engines.

2. They can operate with a wide variety of fuels.

3. Being an external combustion engine, they tend to generate fewer pollutants. They still produce large amounts of carbon dioxide, but, owing to their greater efficiency, they produce less than current automotive engines of equivalent power. They can operate well with fuels having a low carbon-to-hydrogen ratio, thus producing more energy per unit amount of carbon emitted.

4. They are low-noise devices because no explosions are involved.

3.8 The Stirling Engine 117 Table 3.7 Several Stirling Engine Configurations

{Alpha (two cylinders, two pistons)

Beta (one cylinder with piston and displacer) „ ,

Gamma (one cylinder with piston, another with displacer)

Free-piston _ Ringbom

In addition to its application to engines, the Stirling cycle can be adapted for refrigeration without needing Chlorinated Fluorocarbons (CFCs).

The Stirling cycle consists of an isothermal compression, an isometric heat addition, an isothermal expansion, and an isometric heat rejection (cf. Table 3.4). Its great efficiency results from the possibility of heat regeneration described in more detail later in this chapter. A number of Stirling engine configurations have been tried. See Table 3.7.

All configurations employ two pistons, in some cases a power piston and a displacer. The distinction will become clear when we examine examples of the engine.^ Kinematic engines use pistons driven by the crankshaft, in general through connecting rods. In the free-piston configuration, the pistons are not mechanically connected to any part of the engine. The Ringbom uses one kinematic and one free piston.

3.8.1 The Kinematic Stirling Engine The Alpha Stirling Engine

Since the alpha configuration is the easiest to understand, we will examine it in more detail.

Consider two cylinders interconnected by a pipe (Figure 3.11 and 3.12). One cylinder (labeled "Hot") is continuously heated by an external source— flame, radioisotopes, concentrated solar energy, and so on. The temperature of the gas in this cylinder is TH. The other cylinder (labeled Cold) is continuously cooled by circulating water or blowing cool air or, perhaps simply by convection. The temperature of the gas in this cylinder is TC. There is, as in any heat engine, a source and a sink of heat.

The space above the pistons is filled with a working gas (in practical engines, this may be hydrogen or helium). In order to follow the cycle, we will use a specific example. A gas with a 7 = 1.40 is used. The volume of each cylinder can, by moving the piston, be changed from 10~3 m3 to 0 m3— that is, from 1 liter to 0 liters.

t Power pistons compress or do work on expansion. Displacers do no work. All they do is transfer fluid from one region of the machine to another.

Hot Cold

State 0

State 0

State 1

Figure 3.11 The first two states of an alpha Stirling cycle.

State 2

State 3

Figure 3.12 The final states of an alpha Stirling cycle.

Initially (State 0), the "cold" piston is all the way down. The volume in this cylinder is VCo = 10~3 m3, the temperature is TCo = Tc = 300 K, and the pressure (in both cylinders) is pCo = pHo = 105 Pa or 1 atmosphere.

From the perfect gas law, pV = ¡RT, we calculate the amount of gas in the "cold" cylinder as 40.1 x 10~6 kilomoles. The amount of gas in the connecting pipe and in the "hot" cylinder (VHo) is assumed to be negligible.

Phase 0—y 1 (Isothermal compression)

The "cold" piston is moved partially up so that the gas volume is now VCl = 10~4 m3 (a compression ratio, r = 10). Since the cylinder is in contact with the heat sink, the heat generated by the compression is removed and the temperature remains unchanged. In other words, the compression is isothermal. The energy required is

The temperature did not change while the pressure increased 10-fold. State 1 of the gas is

Phase 1^2 (Gas transfer, followed by isometric heat addition)

The "cold" piston goes all the way up, and the "hot" piston goes partially down so that VH2 = 10-4, VC2 = 0. The total volume of the gas does not change. Theoretically, there is no energy cost to this gas transfer, but the gas is now in contact with the hot source and will start heating up. Assume, arbitrarily, that the temperature rises to 652 K. For this to happen, the heat source must deliver to the gas an amount of heat,

A gas whose 7 = 1.4 has a cv of 20.8kJ K-1 kmole-1. Hence, the heat necessary to raise the temperature from 300 to 652 K, while keeping the volume unchanged (isometric heat addition), is

= 40 x 10-6 x 20.8 x 103(652 - 300) = 293 J. (3.21)

Since the gas temperature went up without a change in volume, the pressure must have increased. State 2 of the gas is

652 6 6

1 2 300

Phase 2^3 (Isothermal expansion)

The high pressure pushes the "hot" piston down until the volume in the cylinder reaches 10-3 m3. The corresponding 10:1 expansion would cool the gas down, but heat from the external source keeps the temperature constant—we have an isothermal expansion that delivers 500 J to the crankshaft:

We have arbitrarily chosen a TH of 652 K in the preceding phase, so that the energy delivered to the crankshaft comes out a round number. The heat input required is, of course, Q2^3 = Qexpan = 500 J.

State 3 of the gas is

Phase 3^0 (Isometric heat rejection)

Finally, the pistons return to their initial position. The gas volume does not change but, owing to its transfer to the "cold" cylinder, it cools isometrically to 300 K and thus returns to State 0. This completes the cycle. The heat removed during this phase is

Qa^o = lcvAT = 40 x 10~6 x 20.8 x 103(652 - 300) = 293 J. (3.23)

exactly the same as Qi^2.

In one cycle, the crankshaft receives 500 J from the "hot" piston (W2^3) and returns 230 J used in the compression phase (W0^i). A net mechanical energy of 500 - 230 = 270 J constitutes the output of the machine. This happens at a cost of two heat inputs, Qi^2 and Q2^a, amounting to 793 J.

The efficiency of the device is

The efficiency of a Carnot cycle working between 652 and 300 K is

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Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained


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