Please explain this difference between diesel and gasoline engines

[CalG]

CalG Gasoline engines don't need spark ignition to burn,

Then why do gas engines have spark plugs???????????

willy

Gasoline engines will Knock , I.E. auto ignition due to Hot spots or excessive compression.
Some folks call it dieseling. As I wrote, the engines will not last long under these conditions.

The spark plug is a great way to get the ignition off to a good start.

 

[number two]

Speaking of Diesel engines:

Did you know “Diesel” should always be spelled with a capital “D”?
The inventor of the Diesel engine was Rudolph Diesel.

Also, did you know Rudolph Diesel, who was almost as significant as Elon Musk back in the early 1900’s disappeared mysteriously from a passenger ship and was never found? His Diesel engine technology breakthrough was taking the world by storm. Rumor has it he was sharing his diesel technology with the British Empire to install in Royal navy submarines.

Many think he was murdered as he was costing billionaire moguls who produced coal fired ship engines their livelihoods as the diesel engine could produce much more power and for much less money. Or perhaps Der Kommisar from Germany wanted him gone for sharing diesel technology to Germany’s chief naval rival, the British.
Back then, it took many men in dangerous conditions to keep large coal fired ship engines running. Those men on the ships, sometimes as many as 200, had to housed, fed and paid to shovel coal and keep the engines running. Diesel engines made more power and took a smaller crew to keep running.

Diesel was taking over and still to this day 100 years later is still the go-to power source for most heavy tasks.

Rudolph Diesel is a very under rated inventor who changed the world forever with his Diesel engine technology and it’s still going strong today.

Another Glenn Beck listener!
I'm sure it helps with all that seat time.
Good Luck!

 

[MHarryE]

Gasoline air fuel mixture compressed and ignited heating the air which then pushes the piston. Diesel compresses air only, then fuel is injected over time giving a continuous push on the piston instead of a sharp push falling off. The diesel will pull better because of the constant push.

Examples illustrating the cycles. Gasoline is giving the piston a quick kick at the top, diesel is like standing on the piston. Simplified.

 

[MTGreen]

Diesel engines bring in a fixed volume of air per stroke, compress it anywhere from 14:1 to as high as 25:1 and then a very precise amount of fuel is injected under very high pressure to atomize it on the piston to combust. So, “the air fuel mixture is not all over the place”.

Diesel engines do bring in a fixed volume of air every stroke. It is only the fuel volume that varies. So the air/fuel mixture is all over the place. Because the fuel volume changes while the air volume does not. You post makes no sense.
Eric

My understanding from my fluid dynamics course (a long time ago) was that despite the piston displacing a certain mathematical volume each stroke, the air charge density (which is what really counts from a AFR perspective) is going to vary considerably dependent on temperature/outside air pressure, as well as turbulence/shear effects in the intake tract. The piston is not at full downstroke long enough for pressure to equalize with outside air pressure, so the charge density is also going to be variable by RPM. Intuitively, it would seem that the higher the RPM the less time to equalize and thus the lower the charge density prior to the start of the compression stroke.

In mechanical diesel injection, more throttle means more fuel per stroke is injected which results in higher RPM. So assuming the charge density does indeed decrease with RPM, this would indeed lead to even greater variability of air-fuel ratios. In a common rail ECU controlled solenoid injector system, injector on-time still controls RPM so AFR would still have substantial variation.

 

[rScotty]

I know someone here can help me with this question. Maybe the question should be spark ignition versus compression ignition, but I will just use gasoline spark ignition vs diesel compression ignition. Anyway, gasoline engines are very sensitive to air/fuel ratios. For example, gasoline engines when run too lean can run way too hot and this can result in pre-ignition, burnt valves, and melted pistons. I have personal experience with a melted piston crown, darn it. So the air/fuel mixture is closely controlled. Diesel engines don't control the air at all, the engines are always sucking in as much air as possible. This means that the air/fuel mixture is all over the place. So why is it that a compression ignition diesel engine can tolerate the very lean condition at low throttle settings? And, from what I have read here, diesel engines run hotter at higher throttle settings and running them at too low of a throttle setting, such as excessive idling, can cause problems such as "wet stacking". The reason being that the engine is running too cool and all the fuel is not burning completely. It's just idle curiosity on my part and I figure a good answer is likely to be found here.
Thanks,
Eric

Hello Eric, that's a good question you asked. Did you ever get an answer that you feel comfortable with?
If so...what?
rScotty

 

[etpm]

Hello Eric, that's a good question you asked. Did you ever get an answer that you feel comfortable with?
If so...what?
rScotty

I have not yet. Many answers did not address the question. Others tried to justify incorrect statements or had faulty logic. Like applying conditions to a diesel engine but ignoring the fact that a gasoline engine would have the same conditions. I guess I'm gonna need to really learn how the two different types of engines work and then post my findings. It looks like the library is in my future. Or I could get Chat GPT to write me some BS that I could then post here. Strike that. I'm heading to the library.
Eric

 

[Grumpycat]

I believe Direct Injected Gas motor introduce the gas before the compression is near completion. But I've been wrong before and will be again

Beside tech moves to fast for me, these days.

They do funny things with DI, that is why they go to the effort to implement Direct Injection. They can inject multiple short bursts to control combustion speed for better efficiency and emissions.

Ford did same starting with the 6.4L Powerstroke diesel. Controlled the timing of the injection(s) up to 5 per cycle. Eliminated the traditional diesel rattle. But the really nasty thing they did was use DI during the exhaust cycle to dump raw fuel into the DPF for regeneration. Much of this fuel washed the cylinder walls and diluted the oil.

Fuel dilution of oil is a common malady of gasoline DI engines.

My F-150 has DI and traditional port injection. My Subaru has only DI

 

[ericm979]

I have not yet. Many answers did not address the question. Others tried to justify incorrect statements or had faulty logic. Like applying conditions to a diesel engine but ignoring the fact that a gasoline engine would have the same conditions. I guess I'm gonna need to really learn how the two different types of engines work and then post my findings. It looks like the library is in my future. Or I could get Chat GPT to write me some BS that I could then post here. Strike that. I'm heading to the library.
Eric

In a spark ignition engine the fuel is mixed with air before the compression stroke. Near the end of the compression stroke, the spark plug(s) fire and ignite the mixture. The engine designer's goal is to make the fuel/air ratio the same throughout the combustion chamber at the time of combustion. There are a lot of ways to help that including tumble or swirl intake ports which encourage the fuel/air to circulate in the cylinder as they're being compressed, and squish bands which help the fuel/air mix as TDC approaches. The more homogeneous the mix the better it burns but that also makes detonation possible.

In a diesel, the air is compressed and then the fuel is injected near the end of the compression stroke, using a high pressure injector which atomizes the fuel. This makes a cloud of fuel/air in the middle of the chamber. Due to the high compression the air is already so hot that the atomized fuel spontaneously ignites where it's the right (stiochiometric) ratio with air. (Compressing air heats it up as anyone who has touched the fins on their air compressor knows). But the mix across the entire chamber is not homogeneous. There's a cloud of fuel, which is surrounded by air.

Spark engine detonation happens when the engine gets too hot. Something inside the chamber can overheat and serve as an ignition source. But ignition can also happen spontaneously due to the heat of compression, if the air in the chamber started out too hot. This early ignition puts more heat into the combustion chamber cylinder and piston, which compounds the problem for the next cycle.

Diesels are resistant to detonation because the fuel cloud burns up as its being sprayed out, so there's not unburnt fuel/air in far corners of the chamber to be ignited by a hot chamber. Compared to a spark engine the pressure rise is slower, which reduces the likelyhood of detonation. They can get detonation if there's too much fuel injected or the timing is off but those usually only when those are well outside the normal operating parameters.

 

[pmsmechanic]

I'd like it if someone could answer the original question which in a nutshell is this: Why is a lean mixture catastrophic for a gas engine but not for a diesel? So far the majority of the discussion has not been about this.

I'm going to venture a guess that it has to do with the differences in burning characteristics between gas and diesel. But I have no idea if I'm right or not. It's just a hunch at this point.

 

[etpm]

Diesels are resistant to detonation because the fuel cloud burns up as its being sprayed out, so there's not unburnt fuel/air in far corners of the chamber to be ignited by a hot chamber. Compared to a spark engine the pressure rise is slower, which reduces the likelyhood of detonation. They can get detonation if there's too much fuel injected or the timing is off but those usually only when those are well outside the normal operating parameters.

Your statement above makes some sense. I'm gonna need to think about it and learn about how diesel burns compared to gasoline. I think the way the fuels burn is the reason, or main reason, for the differences in temperature. You also stated that when the diesel is injected it starts to burn when the right (stoichiometric) ratio is achieved. I think you are partly mistaken. I think the fuel starts to burn as soon as it can and doesn't care about the stoichiometric ratio. I still am trying to learn why the very lean burn condition of a diesel engine doesn't lead to the very hot conditions that very lean burning gasoline engines experience. And just to be clear, I am not talking about diesel engines with electronic controls, but instead diesel engines with mechanical injection. Like the engine in my 43 year old Yanmar. As near as I can tell even gasoline engines with fuel injection still burn too hot when running too lean.
Thanks,
Eric