Diesel Engine Sound

I thought we covered that in a previous thread.

air compression

Tony Blair asked:

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.

Geoff

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.

Juergen

The majority of that knocking sound you hear is because of the fuel igniting in the preignition chamber... the small chamber above the combustion zone of piston.

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, right?

Geoff

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 the piston.

When I see these relatively small burn holes it's amazing (to me) that these engines have any power! But they do; it works.

igniting

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.

Huw

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.

RF

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.