Fuel Injection Systems - difference in performance?
Discussion
Evening, all. 
I've just started learning about how engines work, and I've just been taught how two-stroke engines work, and despite their inefficiency and general weakness, I find them very interesting. But I've gone back to four-stroke, and I'm now reading about fuel injection systems. I know the difference between what a carburetor, direct fuel injection system and port fuel injection system does and how it works, but what I want to know now is how they affect performance.
I'm pretty sure that nearly all high performance cars use injection systems now, in fact maybe all cars do. For all I know carburetors are things of the past. My biggest question, however, is this: what is the difference in performance between direct fuel injection and port fuel injection, if any? I don't see that there can be much difference, as the distance between the two entry points is so little, but I'm throwing it open to the pool of knowledge that is this excellent community.
Thanks in advance.
Lucien
Edited to add: I've never visited this forum before. It's quite nice in here, isn't it?
Edited for a second time to correct friendly smiley!
I've just started learning about how engines work, and I've just been taught how two-stroke engines work, and despite their inefficiency and general weakness, I find them very interesting. But I've gone back to four-stroke, and I'm now reading about fuel injection systems. I know the difference between what a carburetor, direct fuel injection system and port fuel injection system does and how it works, but what I want to know now is how they affect performance.
I'm pretty sure that nearly all high performance cars use injection systems now, in fact maybe all cars do. For all I know carburetors are things of the past. My biggest question, however, is this: what is the difference in performance between direct fuel injection and port fuel injection, if any? I don't see that there can be much difference, as the distance between the two entry points is so little, but I'm throwing it open to the pool of knowledge that is this excellent community.
Thanks in advance.
Lucien
Edited to add: I've never visited this forum before. It's quite nice in here, isn't it?
Edited for a second time to correct friendly smiley!
Edited by lightningghost on Monday 14th August 23:54
Edited by lightningghost on Tuesday 15th August 00:23
Hi...we're a friendly bunch in here, unless you try to sell us snake oil then we take the piss a bit.
The position of the injector debate is not as simple as it looks. The earlier you put the fuel in in the intake the better the cooling effect if has on the air, however if you introduce it to early at low engine speeds, ie low air velocities, the fuel droplets will fall out of the air flow, with a detrimental effect on performance. Some systems have injectors close to the valve for low speeds which then swap over to the distant injectors at high speed.
Engines are a comprimise, what will you settle at?
Regards
Iain
The position of the injector debate is not as simple as it looks. The earlier you put the fuel in in the intake the better the cooling effect if has on the air, however if you introduce it to early at low engine speeds, ie low air velocities, the fuel droplets will fall out of the air flow, with a detrimental effect on performance. Some systems have injectors close to the valve for low speeds which then swap over to the distant injectors at high speed.
Engines are a comprimise, what will you settle at?
Regards
Iain
There are other fueling variations such as sequential vs bank fired and also single vs multipoint.
It is often said that sequential is only of benefit to economy & emissions but not performance, from my armchair I find that hard to believe. I can see that it would require more development time.
Single point injection was a transition stage from single carburetor. The manufacturers couldn't get a carb to achieve the emissions restrictions imposed so the quickest fix was to slap a single point injector system where the carb used to be.
There were also multi carburetor fueling systems like the Lambo Countach, six twin choke Webbers just imagine trying to balance that lot!
A comparison between carbs and efi could describe the metering needle in an SU carb as a mechanical fuel map.
It is often said that sequential is only of benefit to economy & emissions but not performance, from my armchair I find that hard to believe. I can see that it would require more development time.
Single point injection was a transition stage from single carburetor. The manufacturers couldn't get a carb to achieve the emissions restrictions imposed so the quickest fix was to slap a single point injector system where the carb used to be.
There were also multi carburetor fueling systems like the Lambo Countach, six twin choke Webbers just imagine trying to balance that lot!
A comparison between carbs and efi could describe the metering needle in an SU carb as a mechanical fuel map.
leorest said:
Single point injection was a transition stage from single carburetor.
The term "Direct injection" that the OP used would normally be used in the context of diesels which inject the fuel directly into the main combustion chamber. It doesn't apply to spark ignition engines, which always inject the fuel into the intake manifold.
GreenV8S said:
leorest said:
Single point injection was a transition stage from single carburetor.
The term "Direct injection" that the OP used would normally be used in the context of diesels which inject the fuel directly into the main combustion chamber. It doesn't apply to spark ignition engines, which always inject the fuel into the intake manifold.leorest said:
GreenV8S said:
leorest said:
Single point injection was a transition stage from single carburetor.
The term "Direct injection" that the OP used would normally be used in the context of diesels which inject the fuel directly into the main combustion chamber. It doesn't apply to spark ignition engines, which always inject the fuel into the intake manifold.Yes, I suspect that the OP did really mean single point injection, and I agree with your description of that. Just wanted to point out that DI strictly means something else. As you were, move along please, nothing to see here ...
lightningghost said:
...In a GTi, is the injection into the air going in, or into the cylinder itself?...
With reference to GreenV8S, chassis 33 and virgil's comments the injection is into the air going in, not into the cylinder. lightningghost said:
...I'm guessing that it's just like a carburetor, but the fuel is injected into the air as opposed to be sucked out...
Nicely put and that's my take on it too! Of coarse the "elastic trickery"method allows closed loop control etc..lightningghost said:
Cheers guys.
In a GTi, is the injection into the air going in, or into the cylinder itself? I'm guessing that it's just like a carburetor, but the fuel is injected into the air as opposed to be sucked out...
In a GTi, is the injection into the air going in, or into the cylinder itself? I'm guessing that it's just like a carburetor, but the fuel is injected into the air as opposed to be sucked out...
Yes and no:
The single point fuel injection found on small engine cheaper cars where the injector is at the combined end of the intake manifold (like a single carb setup would be) is just like a carb...one squirt and the travelling gas pulls the fuel into the revelent cylindar.
The multipoint fuel injection - found on most decent engines and most if not all performance engines is a but more selective in that there is one (or more) injector per inlet runner - ie te tube that goes to the intake port, so only fuels that cylindar.
There are then two types of this set up.
Batch fired - cheaper aftermarket systems use this and squirt fuel from all the injectors at once then wait for the moving air to pull it into the cylindar. This works well as a general compromise.
Sequential fired - more tunable and each inhector is timed to squirt fuel either into an open valve, or just before it opens. This obviously takes a lot more setting up, better hardware and software but ultimately must make the best power and lowest emissions.
The other good thing about EFI is that you generally have mapped ignition too making the time of the spark more acurate rather than relying on the distributor mechanics.
Hope this helps a little.
Virgil
You of course all forget the mechanical fuel injection designed by Porsche in the late 60's.
Fitted to race and road cars, it produced monumental horsepower, directly injecting fuel in to the combustion chamber, the injectors supplied by a belt driven pump run off the Camshaft, and running seperate butterfly stacks for the air intakes.
An incredibly efficient system, absolutely designed around producing maximum power.
Fitted to race and road cars, it produced monumental horsepower, directly injecting fuel in to the combustion chamber, the injectors supplied by a belt driven pump run off the Camshaft, and running seperate butterfly stacks for the air intakes.
An incredibly efficient system, absolutely designed around producing maximum power.
Batch fired - cheaper aftermarket systems use this and squirt fuel from all the injectors at once then wait for the moving air to pull it into the cylindar. This works well as a general compromise.
Sequential fired - more tunable and each inhector is timed to squirt fuel either into an open valve, or just before it opens. This obviously takes a lot more setting up, better hardware and software but ultimately must make the best power and lowest emissions.
Don't be under the misapprehension that I was until recently in as much as I thought the same about Sequential injection. It only works in this way up to a point when the injectors are open for more time than they are shut and the injection system then works as a batch fire. The main purpose of sequential is to improve emissions and economy at low speeds, not to enhance performance.
Sequential fired - more tunable and each inhector is timed to squirt fuel either into an open valve, or just before it opens. This obviously takes a lot more setting up, better hardware and software but ultimately must make the best power and lowest emissions.
Don't be under the misapprehension that I was until recently in as much as I thought the same about Sequential injection. It only works in this way up to a point when the injectors are open for more time than they are shut and the injection system then works as a batch fire. The main purpose of sequential is to improve emissions and economy at low speeds, not to enhance performance.
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