Date: Tue, 26 May 1998 10:43:33 -0700
Reply-To: Björn <bratjen@DIRECT.CA>
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: Björn <bratjen@DIRECT.CA>
Subject: Diesel and Gasoline facts (long)
Content-Type: text/plain; charset="iso-8859-1"
The recent flurry of "I like my gas engine" versus "I like my Diesel" should
be enhanced with some facts or maybe cease to take up space.
Here are some of the facts for everyone:
Combustion engines convert energy at about 25% to 35% efficiency. This means
that most of the energy is dissipated through the mechanics of the engine
and heat. The difference between gasoline and Diesel engines is the
combustion process.
Gasoline engines:
The compressed and heated (through the thermodynamics of compression) fuel
and air mixture is ignited with a spark which creates a sudden combustion.
If you present the combustion in a graph the increase in force is quite
rapid, more pronounced than in a Diesel. This is not a completely even wave.
The explosion travels through the combustion chamber and "echoes" of the
walls. In this rather chaotic split second you can have different flame
fronts racing towards each other and create additional smaller bursts of
energy which can lead to a counterproductive knocking in the engine. A
higher compression ratio can increase the energy output yet increases the
danger of knocking. Therefore inhibitors are added to the engine which
results in the octane level rating of fuels.
If the fuel/air mixture is lean (less fuel) the burning process tends to be
hotter and therefore can lead to local overheating, i.e. burning pistons or
valves. If the mixture is too rich you end up with an increase in carbon
deposits. In Gasoline engines there are always some carbon deposits which
oil companies try to reduce by adding their secret formula detergents to the
fuel. Remember whatever is in there is transformed in the combustion process
and ends up in the air we breathe. Therefore efforts have been made to
optimize the fuel consumption through electronic ignition systems and by
adding catalytic converters which act almost like an afterburner creating an
ion exchange which reduces some of the components (up to about 80%) we
believe to be harmful.
To look at a fuel injected combustion process (takes a while to load):
http://www.itm.rwth-aachen.de/staff/antoni/gti.schlieren.html
Diesel engines:
The air is compressed about 2.5 times more than gasoline engines and
therefore becomes much hotter. When the fuel is injected the heated gases
ignite and create a combustion process that is somewhat less pronounced than
the gasoline engines. The comparative graph would show a lesser slope. For
this process the fuel needs to be less refined and actually a number of
different fuels can be used in this process. The problem area with Diesel
engines is getting an even mixture of fuel and air (which can lead to excess
carbon particle output). This is usually done in a chamber by creating a
turbulence in the cylinder head. TDI (Direct Injection) engines use
electronic controls and create a much finer mist using shaped indentations
in the piston.
Additive are sometimes being used to lubricate the injection pump. Again
whatever they are, their components end up in our atmosphere.
Because the combustion process is "softer" a Diesel can stand much more
pressure and therefore compression is often enhanced by a turbocharger which
uses the kinetic energy of the exhaust gases to blow more air into the
combustion chamber. This creates more heat and to compensate and make the
engine even more efficient the intake air can be cooled (intercooler) to
create higher density of the gases. The emissions of a Diesel are compared
with a gasoline engine lower in harmful gases but richer in carbon
particles. Catalytic converters can also be added to Diesel engines but they
are only about 40% effective.
To see a TDI combustion process in action (takes a while to load):
http://www.itm.rwth-aachen.de/staff/antoni/tdi.schatten.html
Comparison:
Gasoline engines have a harder combustion process and are harder on the
components involved (compare the impact of a metal and a rubber hammer).
They create different frequencies (partially also due to their lower
pressure) and create less vibrations. Therefore gasoline engines appear to
be quieter and smoother. The other side of this equation is that Diesel
engines will outlast gasoline engines.
Their torque curves are usually lower and less even than Diesel engines,
which means again that gas engines run best at higher rpm and their power
drops of more quickly once you get to lower rpm. Because of the high and
more even torque generated by a Diesel you can get away with less total
power. Diesel engines are using smaller amounts and less refined fuel which
means that overall less energy has to be used to propel a vehicle.
The environmental discussion favours the Diesel mainly because of less fuel
consumption and less overall emissions. However, efficiency can have unknown
side effects. Little is known about the impact of the ions released in a
catalytic converter. The new TDI engines are again more efficient but they
create carbon particles smaller than our lungs can filter. We don't know
what effects this will have.
The only sensible environmental alternative is the use of different fuels
(or different energy conversions). Diesel engines are more versatile here
because they are more fuel tolerant and can even use vegetable oils whereas
the gasoline engine would at least need whisky.
Hope this adds some fuel for thought (not necessarily for written words) for
everyone.
Björn Ratjen
1987 Syncro Est Alia (self camperized)
1995 Passat TD
(1982 Vanagon Diesel)
1979 VW Bus (self- camperized)
