At first glance, a diesel engine looks like a heavy-duty gasoline engine, minus spark
plugs and ignition wiring (Fig. 2-1). Some manufacturers build compression ignition
(CI) and spark ignition (SI) versions of the same engine. Caterpillar G3500 and G3600
SI natural-gas fueled engines are built on diesel frames and use the same blocks,
crankshafts, heads, liners, and connecting rods.
But there are important differences between CI and SI engines that cut deeper
than the mode of igniting the fuel.
Compression ratio
When air is compressed, collisions between molecules produce heat that ignites
the diesel fuel. The compression ratio (c/r) is the measure of how much the air is
compressed (Fig. 2-2).
Compression ratio = swept volume + clearance volume \ swept volume
Swept volume = the volume of the cylinder traversed by the piston in its
travel from top dead center (tdc) to bottom dead center (bdc)
Clearance volume = combustion chamber volume
Figure 2-3 graphs the relationship between c/r’s and thermal efficiency, which
reaffirms what every mechanic knows: high c/r’s are a precondition for power and
fuel economy.
At the very minimum, a diesel engine needs a c/r of about 16:1 for cold starting.
Friction, which increases more rapidly than the power liberated by increases in compression,
sets the upper limit at about 24:1. Other inhibiting factors are the energy
required for cranking and the stresses produced by high power outputs. Diesels with
c/r’s of 16 or 17:1 sometimes benefit from a point or two of higher compression.
Starting becomes easier and less exhaust smoke is produced. An example is the
The Yanmar 1GM10, shown with a marine transmission, provides auxiliary power for small sailboats. The 19.4 CID unit develops
9 hp and forms the basic module for two- and three-cylinder versions.
Compression ratio is a simple concept, but one that mathematics and pictures express better than words
Caterpillar 3208 that has a tendency to smoke and “wet stack,” that is, to saturate its
exhaust system with unburned fuel. These problems can be alleviated with longer
connecting rods that raise the compression ratio from 16.5:1 to 18.2:1.
It should be noted that a compressor, in the form of a turbocharger or supercharger,
raises the effective c/r. Consequently, these engines have c/r’s of 16 or 17:1,
which are just adequate for starting. Once the engine is running, the compressor provides
additional compression.
Gasoline engines have lower c/r’s—half or less—than CI engines. This is
because the fuel detonates when exposed to the heat and pressure associated with
higher c/r’s. Detonation is a kind of maverick combustion that occurs after normal
ignition. The unburned fraction of the charge spontaneously explodes. This sudden
rise in pressure can be heard as a rattle or, depending upon the natural frequency
of the connecting rods, as a series of distinct pings. Uncontrolled detonation destroys
crankshaft bearings and melts piston crowns.
plugs and ignition wiring (Fig. 2-1). Some manufacturers build compression ignition
(CI) and spark ignition (SI) versions of the same engine. Caterpillar G3500 and G3600
SI natural-gas fueled engines are built on diesel frames and use the same blocks,
crankshafts, heads, liners, and connecting rods.
But there are important differences between CI and SI engines that cut deeper
than the mode of igniting the fuel.
Compression ratio
When air is compressed, collisions between molecules produce heat that ignites
the diesel fuel. The compression ratio (c/r) is the measure of how much the air is
compressed (Fig. 2-2).
Compression ratio = swept volume + clearance volume \ swept volume
Swept volume = the volume of the cylinder traversed by the piston in its
travel from top dead center (tdc) to bottom dead center (bdc)
Clearance volume = combustion chamber volume
Figure 2-3 graphs the relationship between c/r’s and thermal efficiency, which
reaffirms what every mechanic knows: high c/r’s are a precondition for power and
fuel economy.
At the very minimum, a diesel engine needs a c/r of about 16:1 for cold starting.
Friction, which increases more rapidly than the power liberated by increases in compression,
sets the upper limit at about 24:1. Other inhibiting factors are the energy
required for cranking and the stresses produced by high power outputs. Diesels with
c/r’s of 16 or 17:1 sometimes benefit from a point or two of higher compression.
Starting becomes easier and less exhaust smoke is produced. An example is the
9 hp and forms the basic module for two- and three-cylinder versions.
Caterpillar 3208 that has a tendency to smoke and “wet stack,” that is, to saturate its
exhaust system with unburned fuel. These problems can be alleviated with longer
connecting rods that raise the compression ratio from 16.5:1 to 18.2:1.
It should be noted that a compressor, in the form of a turbocharger or supercharger,
raises the effective c/r. Consequently, these engines have c/r’s of 16 or 17:1,
which are just adequate for starting. Once the engine is running, the compressor provides
additional compression.
Gasoline engines have lower c/r’s—half or less—than CI engines. This is
because the fuel detonates when exposed to the heat and pressure associated with
higher c/r’s. Detonation is a kind of maverick combustion that occurs after normal
ignition. The unburned fraction of the charge spontaneously explodes. This sudden
rise in pressure can be heard as a rattle or, depending upon the natural frequency
of the connecting rods, as a series of distinct pings. Uncontrolled detonation destroys
crankshaft bearings and melts piston crowns.
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