Page 100 - Essentials of physical chemistry

62 Essentials of Physical Chemistry

FIGURE 4.2 A modern ‘‘fire piston’’ based on an ancient device used by natives of the South Pacific to start
fires. It is said that Rudolf Diesel was inspired to invent the diesel engine after learning of this device. (The
picture is from Wilderness Solutions at http:==www.wildersol.com=. With permission of Mr. Jeff Wagner,
proprietor of that company.) This picture shows the small hollow in the tip of the piston shaft where tinder is
inserted prior to one quick thrust of the plunger into the wooden cylinder. This model shows the original wound
string design but other models use a modern rubber O-ring. Mr. Wagner estimates temperatures of about 8008F
are achieved in this wooden device.

device using rapid compression of air to ignite tinder shown as a modern version in Figure 4.2. We
calculate a typical temperature for such a compression later.
In Figure 4.3 [5] we can see some details of a diesel engine illustrating the heavy design and the
lack of a spark plug. It is common for diesel engines to use compression ratio of 22:1 or more and
some of the technical design is in the inverted hemispherical combustion chamber shape in the top
of the piston. The compression ratio (CR) is the total volume swept out by the movement of the
piston from the lowest position to the top position plus the volume of the small combustion volume
(in the top of the piston here) divided by the volume of the combustion chamber,

V swept þ V comb
:
CR ¼
V comb
Some of the early diesel engines destroyed themselves due to the high compression stresses, which
led to stronger, heavier designs. Although there is no electrical spark apparatus it should be pointed
out that the small rotating bump labeled ‘‘camshaft’’ is the effective brain of the device in that it
regulates the timing of the opening and closing of the spring-loaded valves (there are separate lobes
for intake and exhaust valves). In a modern engine, there would also be some mechanical
synchronization of the fuel spray some time after the closing of the intake valve. It is likely both
valves would be closed when the piston is traveling up in the cylinder to compress the air rapidly
and then the fuel will be sprayed into the combustion chamber when the air is at the highest
temperature. In modern internal combustion engines, including diesel engines, much of the high
technology would be optimized to tight tolerances in the shape of the camshaft lobe (bump). The
opening of the exhaust valve by the camshaft also has to be timed to allow exit of exhaust gases after
completion of the power stroke of the explosive expansion but then close about the time the intake
valve opens to admit a new charge of air. Since the engine may typically be operating at 3000 rpm or
faster up to 5000 rpm, considerable engineering research has gone into machining=grinding the
shape of the camshaft ‘‘bump’’ to optimize the timing of valve operation in the four-cycle sequence.
The ideal fuel for a diesel engine is cetane (n-hexadecane, C 16 H 34 ) with a linear chain structure,
which is essentially double the molecular weight of octane (C 8 H 18 ) used in gasoline. Other related
fuels can be burned in diesel engines but there is an interesting contrast with gasoline engines.
Gasoline engines (with spark plugs) need a highly branched octane (2,2,4-trimethyl pentane,