Diesel engines, at least in cars, sound so much different than Otto-cycle engines. Why?
Diesel engines run at a much higher compression and generally idle at lower RPMs. A diesel engine does use the Otto cycle. The main difference is that the fuel/air mix is ignited without a spark plug in a diesel engine. The compression of the gases alone cause the fuel to ignite.
I’m wrong. Diesel engines do not use the Otto cycle. It is very likely that I have other misconceptions about diesel engines. Hopefully someone will be along shortly to clear this up.
Why would that result in the clatter?
I am pretty sure cylinder pressures in deisels get higher than in gasoline engines, and that the pistons are made taller and often with more rings than those in gasoline engines. My deisel tractor makes a deep mechanical clattering or knocking sound that I think sounds like heavy metal parts hitting. I imagine the pistons shifting back and forth in the cylinders as the crank passes top dead center and the piston rod changes which side of the piston it’s loading.
As usual, Wikipedia has the answer: “As the piston approaches top dead center (TDC), fuel oil is injected into the cylinder at high pressure, causing the fuel charge to be atomized. Owing to the high air temperature in the cylinder, ignition instantly occurs, causing a rapid and considerable increase in cylinder temperature and pressure (which is where the characteristic Diesel “clatter” comes from). The piston is driven downward with great force, pushing on the connecting rod and turning the crankshaft.”
Nothing mechanical about it.
Newer Diesels have a “graduated” fuel injection, or pre-combustion chamber, which makes them much quieter.
Actually, diesel engines do use the Otto cycle. The difference between diesel engines and gasoline engines is the means of ignition. Diesel engines are compression ignition and gasoline engines are spark ignition. A big reason why diesels operate at higher pressure is that they need that higher pressure to ignite the fuel. The higher pressure also makes them more fuel efficient.
That will teach me to trust Wikipedia. The wiki article seems to make a distinction between the Diesel cycle and the Otto cycle even though they are the same except for the way the fuel ignites.
There is a difference to be heard between low and high compression gasoline engines. Sports cars sound louder because in many cases because their engines are running at higher compression. More compression = more bang. Even very high compression gas engines don’t run as high as average diesel engines.
Running lower RPM’s but at higher torque also means that you have fewer cycles per second which should account the lower pitch of the engine. I don’t think it’s vastly different from the difference between a snare and bass drum.
I read in a car magazine, many years ago, that the combustion of the diesel engine in partly what you hear, but a lot of it is the cylinder walls vibrating and “ringing” from the combustion pulse.
(my bolding)
Wikipedia is not quite right here. If ignition instantly occured, diesels would be much quieter. The fuel burn rate would be limited by the injection rate, and no detonation would be possible.
The ignition does NOT occurr instantly though. There is a delay, and a significant amount of fuel is injected during this delay. When ignition finally occurs, all the fuel injected during the ignition delay detonates. THAT is the source of the characteristic diesel “rattle-rattle” noise.
One way to reduce the noise is “split shot” injection. A small amount of fuel is injected, then there is a delay, then the rest of the fuel charge is injected. The delay allows time for the initial shot to ignight without adding more fuel to detonate. Once the initial shot has heated and raised the pressure in the combustion chamber, ignition of the follow-on shot occurs with minimal delay.
Ford began using the above technique on thier PowerStroke ™ diesel trucks starting with 1997 CA models. All 1999-on models employ split-shot injection. Owing to a design flaw in the low pressure fuel system, it often does not work properly on one particular cylinder resulting in a characteristic noise known as “fuel cackle” The bandaid for this problem was a specially ported injector for that cylinder.
Otto cycle vs. Diesel Cycle: In thermodynamics, there is a particular cycle known as a Diesel cycle. To obtain this, fuel would need to be injected over the entire power stroke. This leads to poor effiency. Because the fuel in real diesel engines is all injected at the beginning of the power stroke, an Otto cycle is obtained. Modern Diesel engines don’t really operate as thier namesake inventor intended.
The main difference between the Otto cycle and the Diesel cycle is that, in the ideal case, the former uses fixed-volume combustion while the latter uses fixed-pressure combustion. That is, in a normal gasoline engine the combustion happens very quickly (instantaneously in the ideal theoretical case) near top dead center, while with a Diesel engine the combustion happens as the fuel is injected into the cylinder over time as the piston descends.
I suspect I know why diesels tend to clatter at idle, but I can’t prove it. If the injectors squirt fuel into the cylinders too quickly it can build up and then detonate (i.e. all burn at once in an explosion). This is most likely at idle since at this speed the pistons spend more time near top dead center (where the pressure and temperature due to compression is highest). Older engines with mechanical fuel injectors are more prone to this than those with modern electronic injectors because their control systems are cruder.
And diesels have a lower revolutions limit. Why is that?
This is not entirely correct. The cycles are superficially similar, but the ideal Otto cycle is thermodynamically somewhat different to the ideal diesel cycle. Your NASA link only shows the Otto cycle.
I refer you to a previous thread for a lot more detail:
So do gasoline engines. The rotating mass, including the flywheel, has to have enough kinetic energy to push the next piston up to TDC.
I own a Honda Civic that burns compressed natural gas (and only CNG). Because that fuel has an octane rating of 130, Honda designed the engine with a 16:1 compression ratio, approaching diesel range. At idle it sound, ever so faintly, like a diesel. It has injectors (though rather different than liquid fuel injectors, I’d bet) and sparkplugs like any EFI gasoline engine.