Page 391 - Understanding Automotive Electronics

2735 | CH 11 Page 378 Tuesday, March 10, 1998 1:30 PM

11 FUTURE AUTOMOTIVE ELECTRONIC SYSTEMS

Other keys to optimal emission control include precise mixture
distribution and ignition control. Mixture distribution is controlled by
regulating (under electronic control) the fuel injection timing. Electronic
controls have proved to be the only cost-effective means of mixture distribution
and ignition timing control.
Should the highly sophisticated fuel control system for the 2-stroke/cycle
engine meet all emissions and performance goals, the engine should have a
great future in passenger car applications.

Alternative-Fuel Engines
In order to reduce dependence on imported oil, there has been a
considerable development effort devoted to finding alternatives to gasoline
fuels. The two leading candidates are natural gas and alcohol fuels (methanol
and ethanol). The latter fuel is made from grain and is a renewable fuel source.
Brazil has operated large fleets of ethanol-fueled cars for years, though at great
government subsidy. For the foreseeable future, ethanol does not appear to be a
practical alternative to replace gasoline in the United States (although it is often
mixed with gasoline, forming so-called gasohol).
Methanol is an alcohol fuel that is potentially available at relatively low
cost and can replace gasoline as a motor fuel. It can be manufactured from coal,
which is in abundant supply in the United States. It is routinely used as a fuel
for racing cars because it has a relatively high octane rating.
Unfortunately, methanol has a number of disadvantages for passenger use.
For example, it has roughly half the energy of an equivalent amount of gasoline.
Thus, for a given range, the fuel tank must be roughly double the size of a fuel
tank for gasoline. In addition, methanol-fueled cars do not start readily in cold
weather, which is an obvious disadvantage in the northern United States.
Moreover, methanol is toxic and highly corrosive. The entire fuel system must
be made from stainless steel in order to cope with the severe corrosion.
Yet another disadvantage of methanol is the lack of a distribution system
(including gas stations equipped to sell this fuel). This latter problem can be
potentially overcome with the advent of engines that can operate on either
gasoline, methanol, or a mixture of the two. Vehicles that are so equipped are
called flexible fuel vehicles (FFV).
Electronic fuel controls are vital to the success of any FFV. Fundamental
to any FFV is a sensor that can measure the alcohol content of the fuel. There
are two types of practical alcohol sensors available at the present time. One of
these measures alcohol content optically by sensing the index of refraction for
the mixture, which is a unique function of the alcohol content. The other
senses alcohol with a capacitive sensor. This latter sensor is based on the
differences in dielectric constant (see Chapter 5) of gasoline and methanol.
Once the alcohol content of fuel has been measured, the electronic fuel
control system can properly regulate fuel delivery to stoichiometric mixture

378 UNDERSTANDING AUTOMOTIVE ELECTRONICS