1) Where does Diesel 'rattle' come from

2) Why do they only rev to approx 4750rpm ?

Cheers

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: 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.

I think there are 2 primary reasons why diesels don't rev:

1. The diesel is injected almost at TDC, and there is therefore a very limited window in which to injet the fuel and get it to burn before the piston goes too far past TDC and you lose the power stroke. As the engine speeds up, this window gets shorter, but the rate of injection remains the same, so there comes a point where you simply can't inject the fuel fast enough - unless you use larger injectors, but this causes problems with slow running.

2. The diesel burns in liquid phase as soon as it is injected, so it does not absorb a load of energy from the pistons during the compression phase like petrol does - run a diesel too fast and the pistons tend to melt

OK???

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: 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.

OK???

:orb_clap: nice one !!!

I also understood that diesels do not rev high because of the masses involved i.e. everything (mostly) has to be so much stronger (and thus heavier), thus they are governed down to save shaking themselves to bits or blowing up.

One little note to add to Chris's post.

Back in the old days, say 20 years ago, the diesel fuel was not injected into the top of the cylinder, but into a "swirl chamber" and hence out into the cylinder. On a modern design (VAG 1.9 and 2.0 in Fabia and Octavia for example), the fuel is injected directly into the cylinders, which allows the engine to rev a bit higher, reduces low rev diesel clatter and increases thermal efficiency to the point where diesel engines are actually approaching the maximum theoretical efficiency of the engine.

One little note to add to Chris's post.

Back in the old days, say 20 years ago, the diesel fuel was not injected into the top of the cylinder, but into a "swirl chamber" and hence out into the cylinder. On a modern design (VAG 1.9 and 2.0 in Fabia and Octavia for example), the fuel is injected directly into the cylinders, which allows the engine to rev a bit higher, reduces low rev diesel clatter and increases thermal efficiency to the point where diesel engines are actually approaching the maximum theoretical efficiency of the engine.

Good point Ken. Thanks

Direct injection makes the engine noisier.

The reason they are limited to 4-5k is that there is no point going past there as everything drops off. They could probably rev much higher than that if designed to do so, but again you would still change before there.