Question:How come a diesel engine doesn't have spark
plugs?
Answer:Because diesel engines use the heat of compression
to ignite fuel instead of a spark. Diesel engines have very high
compression ratios, typically 16 to 1 and higher. When the piston
comes up on the compression stroke, therefore, it creates very
high temperatures in the combustion chamber. A diesel engine
has no carburetor or throttle like a gasoline engine, so only
air is drawn into the cylinders. The fuel is then injected directly
into the combustion chamber through a mechanical fuel injector.
When it hits the hot air, it ignites and burns just like gasoline
ignited by a spark. It's a relatively simple design that delivers
high fuel efficiency (typically 30 to 40% higher than a gasoline
engine of equal displacement) and low carbon monoxide emissions
(but a lot of soot).
GLOW PLUGS
Most passenger car and light truck diesel engines have something
called "glow plugs" to aid cold starting. Because the
diesel engine relies solely on heat to ignite the fuel, generating
enough heat while cranking a cold engine can be a problem. So
each cylinder has a small electric heating element (the glow plug)
screwed into an area of the combustion chamber called the "precombustion
chamber" to help get the engine started.
A glow plug resembles a miniature spark plug except that it
has no electrodes. On the end that screws into the head is a
metal bulb with a resistor inside that glows red hot when voltage
is passed through it. The glow plugs are wired to a relay and
a timer and are used only when the engine is first started.
The plugs go out once the engine starts.
On older diesels, a "WAIT" light comes on when the
ignition is first turned on to allow time for the glow plugs to
heat up. When the WAIT light goes out (which may take from 5
to 20 seconds depending on the application), the engine can be
cranked and started.
Glow plugs can eventually burn out and may have to be replaced.
A symptom of bad glow plugs would be hard cold starting problems.
