Originally Posted by smokeshow

With port injection, a good portion of the fuel doesn't even have any positive contribution to the actual combustion. When you richen it up to 11.5 AFR on gasoline, basically all of the fuel added between 12.5 and 11.5 is for knock prevention. During the compression stroke, that additional fuel is vaporizing, absorbing heat from the compressing air and metal surfaces in the cylinder. It also serves to slow the combustion down to make sure your peak cylinder pressure occurs at a certain crank angle to maximize torque on the power stroke.

When the fuel is direct injected, it doesn't need to be injected in surplus to absorb heat. All it needs to do is do its job in combustion and that's it. So a lot less fuel is needed. Like Area57 said, the optimal range to shoot for is high 12s AFR. 12.8, plus or minus 0.3.

Originally Posted by Bass Mechanic

Not surprised to hear this, keep in mind on a DI engine the fuel is sprayed into an already closed and in compression cylinder.
As a result your adding 11.5 parts of additional fuel to an already full charge of air. The net result is in a way, your supercharging with the pressurized fuel. Even though it's richer you have more volume stuffed into the cylinder.

Compare that to multi port. Let's say for easy math a cylinder is 100 cc's your going to draw in at best 100 cc's of air and fuel.
With DI you get 100 cc's of air and your adding additional fuel on top of that already full cylinder. The richer you make it to a point the more total volume you have to ignite.

This is the same reason flex fuel engines that use DI make more power on e85 because your stuffing yet 33% more fuel into the same cylinder. It's nearly the same as adding 33% more air and fuel into the same space as a multi port and a small supercharger

Not to mention the additional cooling effect of spraying the fuel directly into the cylinder.

Originally Posted by Atomic

Ok mister engineering expert, care to correct the rest of the world with your vast knowledge?

Originally Posted by smokeshow

Lmao. That dude is a joke.

Originally Posted by ayousef

Classis Ad hominem attack guys just classic, regardless of what you studied if you don't know what you're talking about someone with more knowledge on the subject will inevitably correct you.

Now intead of the name calling, why don't you let me and everyone else know why I am incorrect, show us what you studied engineering for.

For Atomic: You need to explain to everyone how you cannot have detonation without pre-ignition like you claimed in your previous post.

As far as Smokeshow is concerned I think I made enough of a fool of you on your facebook page, which is why you deleted me and added me 50 times in a row lmao, don't want you to get too butthurt on the forum as well.

Originally Posted by Atomic

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?

Originally Posted by smokeshow

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

Originally Posted by ayousef

Classis Ad hominem attack guys just classic, regardless of what you studied if you don't know what you're talking about someone with more knowledge on the subject will inevitably correct you.

Now intead of the name calling, why don't you let me and everyone else know why I am incorrect, show us what you studied engineering for.

For Atomic: You need to explain to everyone how you cannot have detonation without pre-ignition like you claimed in your previous post.

As far as Smokeshow is concerned I think I made enough of a fool of you on your facebook page, which is why you deleted me and added me 50 times in a row lmao, don't want you to get too butthurt on the forum as well.

Originally Posted by ayousef

Ops, I just realized that smokeshow took a shot at you even before I did, you didn't seem to care though.



So, tell me again how can you not have detonation without pre-ignition, im still waiting for your counter argument buddy. You now have two options:

1- admit you were wrong and apologize to everyone for providing them with misleading information as "facts"

2- backup your statement, given that you will have to turn against your bestie smokeshow in the process.