since you seem to have a good background on the subject, can you tell me this..
I know with a diesel the fuel is sprayed in under a lot more pressure I would imagine, and effectively ignited at the same time. this is why turbo diesel under a lot of boost doesn't pre detonate like gas does. essentially your adjusting the timing with the injection. since there is no fuel in the cylinder under compression until you place it there.

with DI can't they control detonation in a similar way? seems like the DI engines run higher compression. I was under the impression that you might be able to run more timing since the fuel is only in the cyl when its basically ready to be lit off.
or are they spraying the fuel into the cyl a lot sooner than the ignition point?

I think the DI motor with gas doesn't run nearly as high of fuel pressure as a diesel and I assume it takes longer to get the fuel in.

is there any logic to my theory?

 

Yeah that's pretty much it. The difference is the lack of the spark plug. Fuel injection in a DI engine must precede the ignition by a small amount of time, whereas a diesel ignites upon injection after vaporization. Since the gas engine has to inject the fuel into a high pressure environment and must remain until the spark plug fires, potential for preignition is still there.

A good thing to keep in mind is the difference between preignition and detonation. Detonation happens after the spark plug fires....preignition happens before the plug fires.

 

so is detonation only controlled with ignition? I know under boost your more at risk, they usually run colder plugs for that reason. and I would imagine no matter how late in the compression stroke you inject the fuel it still takes too much time to make it run in a similar fashion to a diesel?

I mean diesel burns while the fuel is injected, why cant a DI gas engine do the same thing? because of the burn speed of gas? or that its not possible to get the burn to start off with the spark at just the right time?

either way, ive always been impressed with the fact that ANY engine with direct injection can get all the fuel needed into the cyl in such a small fraction of a second at 6k+ rpm through such a small orifice and with such a degree of accuracy. mind blowing really when you think about it..

 

Well not exactly...detonation can come before the spark plug fires if preignition is soon enough. Detonation is when the stuff in the combustion chamber basically explodes and doesnt burn smoothly (deflagration). This usually happens when the mixture is ignited while the piston is still on the up stroke. Preignition may just mean the mixture starts to burn 1 degree sooner than expected. The hottest part of the chamber is usually the spark plug anyway, so it starts there like normal.

You can have preignition without detonation, but can never have detonation without preignition.


On the subject of compression ratio and DI....diesel fuel injection rail pressure is usually 15,000-20,000 psi while gas is usually 1500-2000psi. The difference has a lot of reasons; the volume of fuel required for diesels are higher (depending on how much coal you are rolling), the time required for diesel combustion is longer (hence the low rev range), the fuel itself is thicker (doesnt atomize as easily), and the compression ratio in addition to the boost diesels run makes it a much higher pressure environment to spray in to.

If you research diesel pre-injection where they pulse the fuel into the cylinder before the primary injection event, you can actually get real detonation in a diesel. Obviously this isnt the standard deal, but it is possible.

A big reason why DI gas engines have higher compression ratios is because they can get away with it....what I mean is if you can control precisely when the fuel is in there you can minimize the time it is allowed to hang out and could possibly auto-ignite. More compression means more heat, and heat is what ignites the fuel. So why dont gas DI engines have compression ratios similar to diesels (that are in the 18-20 SCR range)? Because the parts would have to be bigger to handle the stress (which adds weight), but more importantly the fuel would not be able to tolerate the heat in the chamber without preignition.

A big deal with DI engines is made about chamber swirl with piston design. The fuel air mixture needs to be as evenly distributed as possibly to limit excess lean or rich spots in the volume.

As far as volume of fuel injected of a comparable DI gas engine and a diesel of the same size, diesels usually run much leaner than stoich because of the wider flammability limits of diesel. So at normal cruise diesels use much less fuel than gasoline and are more thermodynamically efficient because of their higher compression ratio, which is how an 8,000lb diesel truck can get 20+ mpg all day long. When diesels run close to stoich they smoke like freight trains because the fuel takes much longer to burn.


Does that clear anything up or just make more questions?

 

thanks! this is awesome! so what AF ratio does a diesel usually run on the highway? and what is stoich for diesel?
if I understand you correctly, the reason it runs leaner unler light loads is because I would guess that since your spraying it in while it burns, there has to be a minimum amount of fuel needed to make combustion, once you get that in a cylinder whos temperature is hot enough to ignite (instantly) unless you need to make more power, your going to stop spraying which makes it lean.

where as a gas engine needs to light off a something near stoich because it's ignition point happens at a preset event, the spark.

if I am right, then why not make DI gas engines what have a spark plug that emits a constant spark?

 

Diesel stoich is actually a little bit richer (By mass) than gasoline at 14.5:1, however cruise AFR varies considerably depending on load and engine, but 25-50:1 is typical. Diesels have what is called a diffusion, or non-premixed flame. At the injector exit, there is 100% fuel. In the chamber far away there is 100% air. Somewhere in between you have an area called the combustion zone where the fuel ignites. This flame front just kind of sits there (although not really since turbulence and swirl in the chamber moves it all around) kind of like a candle. A gas engine is a premixed flame device; the air and fuel are evenly mixed, then it is ignited. Gas engines have to obey a rather narrow flammability margin ignite, whereas a diesel will burn any amount of fuel you throw in it. Lean limit would be defined as the minimum to keep the engine running and rich limit would be when you hydrolock the engine. Gas engines are far pickier.

Also diesel does not ignite instantly. It actually takes considerable time (in combustion world) to ignite, hence why you cant rev diesels that high; there is no time for it to burn! Diesels may or may not be injecting fuel while fuel is burning. I kind of doubt it personally, but the math may prove otherwise when you compare the start of the ignition event to the flow rate of the injectors and the amount of fuel required to the ignition delay of diesel.

Because gas engines are pre-mixed devices, even DI gas engines, they need an ignition source. If you tried to operate a gas engine like a diesel it wouldnt work since the fuel would not ignite without detonating.

A constant spark would be a waste of energy since the engine only uses 25% of its time actually making power (and needs a spark to ignite the engine). Once the mixture has started burning it is self sustaining until it is out of fuel or air.

 

Suppose I should have clarified... What most people use detonation as conversationally is intentionally igniting the mixture too soon. If preignition is leading to detonation all before the plug fires....you've done something horribly wrong You can have detonation without preignition, though. I'd wager its the most common source of knocking. Just add way too much timing and the spark will ignite it soon enough to detonate long before anything gets hot enough to cause preignition

 

Yea thats true....we should just be honest and clarify they are really separate mechanisms. Detonation is the result of the mixture burn rate going to a positive feedback cycle. For instance, preignition too early on the compression stroke. Mixture ignites > piston moves up, compressing the mixture > pressure and temperature increases >> combustion rate increases > pressure and temperature and compression increase combustion rate even faster > BAM (detonation).

Preiginition is just the mixture igniting before you want it to.

 

Well lets hope my tune is somewhat decent and no preignition or detonation occurring here.

I think i finally figured out these damn tq tables finally, so wont have to max those out and bastardize the tune anymore than necesary.

 

Id have to say this is incorrect, detonation occurs mostly AFTER the spark plug fires (post ignition) which is exactly why engines knock when you add a lot of timing, this isn't detonation caused by pre-ignition.