Really? I had the impression that the characteristic chittering
sound was due to the fact that the fuel/air mixture in a diesel
explodes spontaneously, while in a gasoline engine it burns rather
than explodes (however quickly) with a defined flame front.
"That woman speaks eight languages, and can't say 'no'
in any of them." -- Dorothy Parker
A diesel is a compression engine - that means its fuel is ignited by the
heat generated by the air that's compressed inside its cylinders. A
diesel's compression ratio is about 20:1 or 21:1 vs 8:1 to 11:1 for gas
engines. Gas engines need a spark to ignite their fuel and the higher
compression gas engines require premium fuel that ignites at higher
temperatures to avoid pre-ignition or "knocking."
A diesel ignition occurs at the instant that the fuel is injected into
the cylinder. '70s and '80s M-B diesels use a prechamber to control
their ignition and so the knocking. New diesels have electronic
controlled injection that allows direct injection into the cylinder for
greater power with less noise than the old engines that we drive.
Diesels don't have the quick revving of gas engines because the diesel's
moving parts pistons, connecting rods and crankshaft are heavier than
those used in gas engines due to the diesel's high compression.
Finally, diesels' reason for being is economy and durability, not high
revving performance - but they're getting there!
Current diesel cars on the world market have 16,5:1
(like Mercedes C 30 CDI or some Audis) to 23:1, newer
direct injection diesels typically 17:1 or 18:1, the
trend going to lower ratios.
Current gas engined cars range from 7,5:1 (VW Kombi
in Brazil) to 12,6:1 (Chevrolet Celta in Brazil),
the trend going to higher ratios with the newer
direct-injection gas engines.
Okay, this is where you folks lose me. Why should fuel igniting
in the preignition chamber sound any different from fuel that's
igniting in a traditional combustion zone? Either way the fuel
ignites instantaneously upon entering the cylinder, after all,
"Killing a wounded man while he retreated from battle is not an
action that most servicemen would brag about. But then again,
No, the prechamber's function is to reduce the "bang" of the igniting
fuel. It does that by containing the explosion to the prechamber and
then restricting the expansion of the burning gases to four or five
"burn holes" that direct the burning gases into the cylinder and against
When I see these relatively small burn holes it's amazing (to me) that
these engines have any power! But they do; it works.
Light duty engines started to move away from this system in the
Eighties and all road going diesel engines are now direct injection
with the combustion chamber built into the piston crown and fuel
directly injected into this [mainly] torroidal shaped indent.
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Undoubtedly it comes from the explosions in the cylinders. Air is
compressed to a high pressure and moderately high temperature in the
cylinder and the fuel is injected just when the piston starts to move
down. The fuel ingites with a small explosion and drives the piston down
and the crankshaft around. A four cylinder engine will have four
explosions per rotation of the crankshaft - that is connected to the
gearbox and moves the car forward.
The pressures in the cylinder are much higher than the pressures in a
gasoline cylinder, so the sound is louder. The speed of rotation of
diesels is usually quite a bit lower.
Diesel engines usually have a small number of cylinders and, rotating at
fairly low speeds, the sound goes chug chug chug...... If there were a
large number of cylinders and/or high speed the chugs would blend together.
One opinion and I hope it helps.
Sorry, but a four cylinder diesel engine only has an average of two
explosions per crank revolution unless it is a two cycle diesel like the
Detroit type which has to employ a blower in order to operate. Otherwise the
MB diesel is a four-stroke type, just like the typical gas engine. Many
differences but the easy ones are the compression ratio (diesel much higher)
and the method of inducing fuel into the combustion chamber. Incidentally,
the injection process actually begins before the engine reaches TDC, just as
the spark in a gas engine combustion chamber takes place before TCD. Both
have to get things going early since the whole process is taking place at a
pretty rapid rate.
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