Limits of actual cars
M.Cricchio, F.D'Aniello
(*) - G.Rizzo (**)
(*)
Istituto Alfano I - (**) DIMEC, Università di Salerno
Summary
The study of suitable alternatives to conventional cars is not a recent
task. Several solutions have been analyzed through the years, without
achieving significant applicative results. Until now, internal combustion
engines have been preferred with respect to the possible alternatives,
mainly due to the their higher reliability and mature technological stage.
Nevertheless, the issues associated with fossil fuel depletion, together
with the combustion-related environmental problems, increased the research
efforts towards studying and developing innovative propulsion systems.
Historical aspects
Atmospheric Engine
Car was invented more than two centuries ago and many researchers tried
to reinvent it since then. In 1909, an inventor stated: “...Automobile
technology has achieved its final technological stage…”. The automobile
history is strictly connected to that of the internal combustion engine,
whose ancestor was the Leonardo’s “gunpowder engine”.
| |
| Leonardo's Engine, precursor
of the modern internal combustion engines |
Following the first “Atmospheric Engine” prototype, several other propulsion
systems were invented. In the 19th century, they were already at a satisfactory
development stage and their use widespread, thus significantly contributing
to the Industrial revolution. Nevertheless, they were still too large,
heavy and low-efficient.
Four-stroke Engine
The real innovation into design engine comes from Beau de Rochas, who
suggested the four-stroke engine, and Otto, who carried out his project
in 1876 reducing the overall bulk, improving the efficiency and power.
Since the first decade of the century, Internal combustion engines are
most commonly used for mobile propulsion systems, overcoming competition.
In fact the first speed record was set in 1902 by a steam automobile (external
combustion engine).
| |
|
| The first Four-stroke
Engine, Nikolaus August Otto (1876) |
Serpollet's
Steam Engine, which has set the world record of speed at 121 Km/h
(Nizza, 1902) |
In summary, the main advantages of the conventional internal combustion
engines are:
- high power to weight ratio and efficiency;
- high energy density of the fossil fuels, resulting in a large cruising
range of the car;
- low cost of fuel (before
energy crisis);
- a large and diffuse fuel distribution network.
| |
| Ford T Model , first vehicle
producted with an assemby line process (1914) |
Engine Development
Even if the basic function of every engine is essentially the same, during
the last thirty years, the prevention of pollution of the environment
and rapid changes in the micro-electronic technology, have enabled to
develop systems minimizing pollution, fuel consumptions, and improving
safety and comfort.
In the spark ignition engines, the carburetors were gradually substituted
by electronic control injection systems because they were not able to
guarantee the limits on pollutant emissions. For compression ignition
engines also, the old injection systems are substituted by Common Rail
ones. With this improvement, the engine is defined by some famous authors
like “…a computer with a mechanical actuator...”.
| |
 |
|
Since 1970, the limits of the exhaust
emissions have been reduced by two order of magnitude |
Constructive outline
of a carburetor (Click to Enlarge) |
Alternatives
The need of a continuous reduction of CO2 emissions has grown the research
activity on alternative powertrain system. Depending to the different
energy source utilized, the powertrain systems can be divided into thermal,
electrical
and hybrid ones. The
analysis on the last two categories will be presented separately. Thus,
taking into account the former, some of the alternatives were examined:
- Brayton engine (gas turbine): favourable in terms of weight-power
ratio (usually, it is used for aeronautical purposes), lower efficiency
than the traditional engines ( particularly in variable load conditions);
it is characterised by high cost material.
- Rankine engine (steam turbine): utilized for fixed applications, such
as thermal-electric and cogeneration systems; the high weigh, low adaptability
of the engine to operating conditions.
- Stirling engine: characterised by many advantages, due to using the
thermodynamic regenerative cycle with high efficiency; Philips Research
Labs, a leader in Stirling engine development, studied to make this
engine a competitor of conventional internal combustion engines.
- Wankel engine: a rotary internal combustion engine, which has lower
size and vibratioms than a traditional internal combustion engine, but
which exhibits higher consumption and emissions.
| |
|
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| Rotary Wankel Engine |
"Turboflite"
Chrysler Modelr, equipped by a gas turbine(1961) |
Prototype of Ford
Torino, equipped by a Stirling engine |
Limits of the thermal engines
What are the major limitations of actual cars, with regard to environmental
aspects? They are related to the way to convert the energy required to
car motion. The required energy is determined by car weight and speed,
and may be limited by reducing friction in transmission system, tyres
resistance and using aerodynamic car bodies.
Thermodynamic Limits
A thermal engine (in most cases, a reciprocating internal combustion
engine) converts chemical energy of the fuel into thermal energy (by a
combustion process) and into mechanical energy and in other kinds of energy
(the sum of energy in output equals the input energy, as stated by the
First
Law of Thermodynamics). The efficiency represents the ratio
between what you spend and what you get while transferring energy.
What happens to the chemical energy not converted into mechanical energy?
- About 25-30% is dissipated by the cooling system, designed to let
the engine work in a given range of temperature.
- The biggest quantity of energy, from 30 to 40%, is dissipated with
the exausted gases.
- The engine itself while working is very warm and wastes its heat in
a process called "radiating" that represents about 2-8% of the chemical
energy.
- Another fraction (about 10 - 15%) is represented by mechanical energy
lost into the engine due to friction (and in turn converted into heat),
and by the energy given to the auxiliaries.
The energy fractions absorbed by different mechanisms are largely indicative,
depending on the kind of engine (Spark Ignited, Diesel), its power and,
for a given engine, on operating conditions (torque, rpm, temperature).
In order to improve the engine efficiency, inner attrite have to be reduced
as well as the pumping loss, and the thermodynamic cycle has to be optimized.
But, even with an ideal engine, the conversion ratio from thermal to mechanical
energy (the efficiency) is less than the unit, and in real cases not greater
than 0.40, due to the limitations stated by the Second
Law of Thermodynamics. A significant amount of energy is therefore
lost and wasted to the environment, at low temperatures.
Combustion Process
The thermal energy in input is obtained by the combustion
of fossil fuels, a mixture of hydrocarbons,
containing carbon and hydrogen. This process produces carbon dioxide (CO2),
that is responsible of the Greenhouse
Effect and Global Warming. Moreover, the combustion process
also produces some pollutant
emissions: unburned hydrocarbons, nitrogen oxides, carbon monoxide
(in spark ignition engines) and particulate (in Diesel engines). Their
effect is particularly dangerous in urban areas. These emissions are regulated
in many countries by severe laws and reduced by complex on-board emission
control systems.
Moreover, the combustion process also produces some pollutant emissions:
unburned hydrocarbons, nitrogen oxides, carbon monoxide (in spark ignition
engines) and particulate (in Diesel engines). Their effect is particularly
dangerous in urban areas. These emissions are regulated in many countries
by severe laws and reduced by complex on-board emission control systems.
Link
http://library.thinkquest.org/C006011/english/sites/index.php3?v=2
http://www.aardvark.co.nz/pjet/chrysler.shtml
http://www.stanleysteamers.com/serpollet.htm
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