Why do diesel engines rattle?

Question

Must be the mechanical parts inside that make the noise! whats going on there?

Answer 1

It is essentially pinging, or pre-ignition. On a gasoline engine, if there is a hot spot in the combustion chamber anywhere, it will ignite the fuel/air mixture before the spark is fired to do this, hence the "pre" in pre-ignition. This pre-ignited flame will begin to travel across the top of the piston. Meanwhile, the spark plug will fire when it is supposed to, thereby igniting a second flame, which begins to travel across the piston. This all happens VERY fast, and VERY violently! When these two flame paths (semi-controlled explosions) collide, the resulting concussion is heard as a ping, or "spark knock." What does this have to do with a diesel engine? Well, a diesel engine has no spark plug to ingite the fuel/air mixture. Instead, it has a very high mechanical compression ratio (how much the fuel/air mixture is squeezed before it ignites,) which purposely creates hot spots within the combustion chamber. Often, there are even spots, such as bumps or ridges, engineereed and built into the top or edge of the piston, which are designed to further retain heat, which increase the pistons ability to ignite the fuel/air mixture. It is this specifically engineered hot spot that ignites the fuel/air mixture in a diesel engine. The compression ratio is so high (2-1/2 to3 times a gasoline engine,) that the combustion explosion is so violent, that spark knock (in essence, if not exactly in correct engineering terms since there is no spark plug) happens all the time...it is engineered to do this. As technology progresses, some of the sound has been engineered out of some diesel engines, as many people actually prefer something quieter (imagine that?) You will hear about glow plugs in a diesel engine, but they are not spark plugs at all. They are like the element on your stove. They heat up a rod that gets red hot, and ingites the fuel/air mixture when a diesel engine is first fired up. Why? Simply because a cold diesel engine has not run at all, and therefore has not had time to build up any hot spots to do the igniting needed. By the way, the hot spots build up in a matter of a few seconds in an operating engine, rather than the minutes one might imagine. Also, some diesel engines don't use glow plugs, but instead pre-heat the fuel to a very high temperature, which makes it much easier to ignite with high mechanial compression ratios, even without it being run for a few seconds to warm it up. I hope this helps.

Note on earlier comments: A diesel engine is NOT a two-stroke (which uses no valves, and each individual piston fires on every revolution of the engine.) It is a four-stroke engine, which uses valves, and only fires on the compression stroke of the piston, or every other revolution of the engine. It still has a non-firing exhaust stroke, which clears the combustion by-products from the engine. Nor does it have loose tolerances. With such high mechanical compression ratios, and corrresponding cylinder pressures, the tolerances are actually probably closer than most gasoline engines. Also, diesel fuel is no longer lesser refined. With the new ultra-low sulfer diesel mandated in 2007, diesel fuel will be even more highly refined, and vastly more clean burning as a result.

Answer 2

Diesel Engine Rattle

Answer 3

You have some interesting answers here. No, diesels are NOT two-strokes, in talking car terms, ( model aircraft use compression/ ignition, 'diesel' engines but they run on ether). Commer used to make a two-stroke diesel many years ago, with a double acting engine, (two pistons in one cylinder, horizontally opposed). Everyone appears to have missed the reason a diesel 'rattles' compared to a petrol engine and that is because there is a very high pressure, fast moving, reciprocating pump, called, believe it or not, a fuel pump, forcing the fuel into the engine, and it is that which is making the 'rattling' sound.It has nothing to do with pre-ignition, which would indicate an incorrectly tuned engine, compressed air, as all engines compress air, or being built loose, (in fact the opposite because of the higher compression ratios).

Answer 4

Diesel engines are known as compression ignition engines.They have a very high compression ratio and no spark plugs.To ignite the fuel/air mixture,the air in the cylinder is heated as it is compressed and at the end of the compression stroke,the fuel is injected and ignition takes place.The reason for the difference in sound is simply because the explosion(ignition) is more violent in diesel engines due to the higher compression pressures in the cylinder.

Answer 5

Its more like a spark knock like when you put low grade fuel in a car that needed high tess. Diesel engines don't have spark plugs they have glow plugs that stay at a temp that will light the fuel when it is sprayed into the cylinders. If you notice the newer more efficient diesels don't rattle near as much.

Answer 6

wow scary responses, a diesel can be 2 or 4 stroke,it's noisy because the air only is compressed to a very high temperature , then the fuel is injected and it all goes off at once giving the rattling noise

Answer 7

not all do, I have two and they are both quieter than a petrol engine.

One trick that you might like to try is to take a piece of wood, suggest about 3 feet long and one inch round. put one end on the engine block, the other pressit against the flap of your ear. You can then hear the vibrations in different parts of an engine, you can hear the pistons moving, timing chains and bearings

Answer 8

the way it was explained to me is that the engine is put to gether looser than a gas engine because it gets oiled both by the oil in the engine and the fuel this engine is only turning hundreds not thousands of rpm so it do'nt build up the heat that agas engine makes

Answer 9

A Diesel is a two stroke engine, it has loose tolerances, and very High compression, much higher than a gasoline powered motor, so high that it has no spark plugs, the high compression from the piston, ignites the fuel.

Answer 10

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