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Flexible-fuel vehicles now being developed by the automotive industry b-12 5 -G; ' require smart sensors that detect alcohol concentrations ___c

in the fuel supply and alert the engine to make the adjustments necessary for eftjcient combustion. 2 6 CJ 8 7 0 . ~ ~

13ensors in Flexible-Fuel Vehicles Bill Siuru

oday's automotive engineers can T choose from a variety of alternative fuels: methanol (methyl alcohol), ethanol (ethyl alcohol), compressed natural gas, propane, and hydrogen. The consensus is that methanol is the most likely near- term candidate for widespread use as an alternative to both unleaded gasoline and diesel fuel. Natural gas and propane will probably be limited to fleet opera- tions and niche markets. Hydrogen, although an attractive option, will most likely be put on hold until the twenty- first century. Ethanol, already a near- term alternative fuel, must compete with more conventional uses for corn, sugar cane, and the other raw miterials from which it can be made.

Methanol is being touted as a way to reduce America's deDendence on in-

gasoline. M85 allows ignition down to -20"F, and provides a couple of safety advantages as well. Pure methanol burns with an invisible flame, increasing the danger of an undetected fire resulting from an accident or a fuel spill; the addi- tion of gasoline produces a combustion with a visible flame. Also, the gasoline smell of M85 discourages people from drinking the substance and encourages them to wash up after handling it.

The infrastructure needed to store, transport, and distribute methanol or M85, however, will probably not be in place for many years. In the interim, automakers are developing flexible-fuel vehicles, or FFVs, with engines that will operate on M85, ethanol, gasoline, or any combination thereof. The driver

need not specify a choice, and a single fuel tank would accommodate whatever is selected. FFVs could be run on M85 for cleaner urban operation, and on gasoline for long-distance travel and eas- iei'cold weather starting.

One essential component of an FFV system is the sensor that detects alcohol concentration levels in the fuel tank. This information is supplied to the en- gine electronic unit, which automati- cally adjusts the fuel flow rate, exhaust gas recirculation, and spark timing to achieve the best results with the particu- lar fuel in use. These adjustments cor- rect for the different air/fuel ratios required by various fuels. For example, the stoichiometric aidfuel ratio for gaso- line is 14.7:l; for methanol it is -6.5:l.

ported oil. I t is also a cleaner burning fuel with fewer smog-producing hydro- carbons in the exhaust by-products. On the negative side, methanol can produce potentially carcinogenic aldehydes. Most internal combustion engines need only minor modifications to run on mcthanol; it is more corrosive than gasoline, how- ever, so fuel system components must be corrosion resistant. h4ethanol is uscd in racc cars for the 5-7% power advantage over gaqoline, but on a per-gallon basis it provides roughly 30?40 lecq energy/gal. This means, of course, either largcr fuel tanks or shorter distances on a tdn!,.

Starting up a methanol-burning engine at sub-Lero temperaturcs i q very di f f i -

cult, if not impossible. \lost manufac- turers dTC therefore dcsigning vehicles to operate on a fucl called h185, d mixture of 85% methanol and 1 5 % unledded

SENSORS September 1992 47

I ' CONDUIT TO INNER E L E C T R O D E 2 . OUTER CYLINDER YLINDER FORMS ONE ELECTRO HER ELECTRODE

FUEL IN- < > -+FUEL OUT + Figure 1. The capacitance-type dielectric sensor is placed in the fuel line. (Courtesy of Siemans AG.)

Oxygen sensors are used for final trim- ming on gasoline engines to achieve the best aidfuel ratios and could, in princi- ple, determine fuel composition as well. For accurate readings, however, the 0, sensor must reach normal operating temperatures. Most automakers there- fore use both 0, detectors and alcohol sensors in their FFV designs.

ALCOHOL MEASUREMENT

An alcohol concentration sensor could theoretically be used to distinguish gaso- line from alcohol-based fuels by measur- ing any of a number of properties, including differences in conductivity,

dielectric constant, or sonic velocity. There are also measurable differences in optical properties, e.g., indexes of refrac- tion and absorption in the visual, microwave, or IR wavelengths. The high-volume nature of auto production makes some of these techniques too expensive or too complicated to imple- ment. Other methods are overly sensi- tive to fuel impurities, fuel system com- ponent aging, and even fuel product variations among suppliers.

Two types of alcohol sensor are cur- rently being tried out in FFVs: capaci- tance-type detectors that measure changes in the dielectric constant of the fuel, and optical sensors that measure

changes in the index of refraction of the fuel blend.

DIELECTRIC CONSTANT SENSORS

The relative dielectric constants of pure gasoline and pure methanol are an order of magnitude apart (see Table 1). Gasoline's low dielectric constant as compared to that of methanol allows variations due to different brands or types to be ignored. Indeed, there is only about a 2% variation in dielectric con- stant over the range of current commer- cial gasolines including those that con- tain oxygenates like MTBE and ethanol. While the dielectric constant is affected by both fuel temperature and conductiv- ity changes caused by impurities, these effects can be measured and compen- sated for.

Several types of sensors can be used to measure differences in dielectric con- stant. Siemens AG has developed a sim- ple device that creates a capacitor out of the fuel as it flows between two elec- trodes; the capacitance depends on the fuel's dielectric constant and thus its alcohol concentration. Figure 1 shows a

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18)

0 2 4 6 8 10 12 14 16 18 20 FREOUENCY IN MHz

Figure 2. In a single-capacitor circuit (A), the ratio of input to output voltage is a function o f frequency for different alcohol concentrations (B). (Courtesy of Ford Motor Co.)

cylinder capacitor designed to be housed in the fuel line. One electrode is formed by the central element and the other one by the surrounding cylinder. The fuel mixture, i t . , the dielectric medium, fills the area between the center and the outer electrode.

There are also various ways to convert capacitance measurements related to the dielectric constant to an output signal that can be supplied to the fuel injection system. Figure 2 shows a simple, single- capacitor circuit with a high-frequency AC voltage and a resistive load. The ratio of input to output voltage is used to dis- cern the composition of the fuel; in the example in Figure 2, the device operates

Photo 1. Chrysler’s smart sensor determines metha- nol concentration by using a capacitor to measure the dielectric constant of the fuel. The sensor also relays the information to the engine computer. (Courtesy of Chrysler Motors.)

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I

I 1 Figure 3. A more advanced capacitance-type device uses three capacitors in a K filter network. (Courtesy of Ford Motor Co.)

at a fixed frequency of 10 MHz. A ther- mocouple or thermistor could be added to the circuit to measure fluid tempera- ture, and one of several methods could be used to measure conductivity. A microprocessor uses the three measure- ments, Vout, conductivity, and tempera- ture, to produce an output signal cor- rected for temperature and fuel conduc- tivity. The smart sensor in Photo 1 uses a capacitor to measure dielectric constant, and instantaneously relays to the engine the exact fuel concentration detected.

Investigations have been conducted on other, more complex capacitance-type sensors that could provide greater accu- racy. Ford engineers have developed a couple of these. One (see Figure 3) uses three capacitors and two inductors con- nected in an electrical K filter network.

I

This sensor circuity results in better res- olution, i.e., it produces a larger change in output signal as the fuel changes from gasoline to methanol.

Another detector, based on a resonant- cavity design, consists of two cylindrical conductors forming a coaxial RF trans- mission line. Although resonant-cavity sensors usually operate at several hun- dred MHz, the high relative dielectric constant of methanol allows this device to be used in the 20-100 MHz frequen- cy range, allowing it to be short enough for practical fuel systems. Among the advantages the design offers are that both conductors can be at D C ground potential, and that they exhibit relatively low sensitivity to conductivity variations due to impurities in the fuel.

OPTICAL SENSORS Optical sensors designed for FFVs

measure differences in the index of re- fraction. The sensor shown in Figure 4 discerns methanol concentration by measuring the amount of reflected light reaching a receiver. The light path, and thus the amount of light, depend on the index of refraction of the fuel blend.

OPTICAL SENSOR SCHEMATIC

LIGHT FUEL OUT RECEIVER

LIGHTPATHJ L WITH GASOLINE [

I LIGHT PATH k WITH METHANOL

FUEL IN I I LIGHT EMITTER

Figure 4. The received light increases with increased methanol in this optical fuel composition sensor. (Courtesy o f Ford Motor Co.)

Mitsubishi Electric's optical fuel-com- position sensor uses parallel light beams emitted by an IR LED (see Figure 5). The collimated light passes through an

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optical-glass rod prism and is refracted at an angle proportional to the fuel’s index of refraction. The light is reflected by a mirror and refracted again as it passes through the same boundary surface be-

a different angle from the first passage. A condensing lens converges the beam

onto a position-sensitive detector. The light beam’s position on the detector is a function of the refractive index and, therefore, of the alcohol concentration. A high-response thermistor is used for

l I l

fore traveling back through the prism at

I

temperature compensation. One of the advantages of this device is its resistance to contamination in the optical system.

Another optical sensor design devel- oped by NGK Spark Plug places the bot- tom of a transparent column into con- tact with the fuel flow. An LED light beam strikes the interface between the bottom of the column and the liquid. Light impinging on the interface at less than the critical angle is refracted, while light impinging at greater than the criti- cal angle is totally reflected and escapes

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from the other side of the column. A photodiode on the other side of the col- umn generates an output that is a func- tion of the amount of light received. The amount of light received depends on the critical angle, which in turn de- pends on the index of refraction of the fluid; the fuel alcohol concentration in the fuel can be thus determined.

Optical techniques present certain challenges to sensor designers. The opti- cal surfaces are susceptible to film de- posits. Errors can be introduced when the fuel undergoes a phase separation, e.g., at lower temperatures, and when small amounts of water in the fuel cause it to cloud, making detection difficult. In addition, the index of refraction of com- mercial gasolines can vary by 25 %.

FLEXIBLE FUEL DEMONSTRATORS

The Big Three US. automakers are ac- tively pursuing FFV technology. Within the next year or so, Chrysler plans to build as many as 2000 Dodge Spirit and Plymouth Acclaim FFVs, to be supplied to federal government fleets in Califor- nia, Washington, DC, and Chicago, in addition to fleet and retail customers in California. Chevrolet’s plans include up to 4000 Lumina sedans that can run on M85, and Ford will manufacture 2500 Taurus-based FFVs. (A sidenote of inter- est is that after several years of testing optical sensors in a fleet of 200 FFVs, Ford intends to switch to a capacitance sensor for reasons of increased accu-

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LIGHT PATH FOR LARGE i i i F i i i i i i W E iNOEX R E F R K i t v E lNutX

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a i2.41-/ v)

z 1.75

0 - I

1 a 1 1 - I

z

v) 0

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1.33 1.375 1.42 REFRACTIVE INDEX OF FUEL

Figure 5. This optical fuel comparison sensor detects changes in the index of refraction. (Courtesy of Mitsubishi Electric.)

Saab Does it Differently Saab does not use a methanol/gasoline

concentration sensor in its environmen- tally friendly Saab 9000 prototype FFV; instead, the standard 0, sensor located in the exhaust system identifies the fuel blend being burned. Information from the 0, sensor is supplied to the fuel injection system to supply the optimum air/fuel ratio for each blend.

Two requirements must be satisfied for this arrangement to work: the fuel must blend very slowly during operation, and provisions must be made for cold-start conditions while the 0, sensor warms up to operating temperatures.

Saab uses an additional 6-liter "blending tank" in conjunction with the main fuel tank; the blender ensures the slow, care- fully controlled mixing of gasoline, methanol, or ethanol that allows the 0, sensor to measure exhaust products from an essentially constant a i r h e 1 mixture.

While the 0, sensor is warming up, the fuel blend is taken from the "slave tank," containing a known mixture detected dur- ing the engine's previous run and stored in computer memory. Once the 0, sensor reaches the proper operating temperature, new fuel is pumped from the main tank.

racy.) As might be expected, most of the Chrysler and Ford products are destined for California,

Finally, European manufacturers in- cluding Mercedes-Benz, Saab, Volvo, and Volkswagen, and Japanese automak- ers such as Nissan, Toyota, Mazda, and Mitsubishi, have all demonstrated FFVs that are aimed at the U.S. market.

William D. Siuru, Jr., Ph.D., PE, is an automo- tive and aviation journalist. He can be reached at 6341 Galway Dr., Colorado Springs, C O 80918; 719-528-1980.

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