Hi all:

I just have a couple of newbie questions that I hope you guys don't mind me posting.. I've lurked and read a lot on the forums but I think there are just certain basic things that isn't just clicking:

1. With things like intake forced induction, I understand that the concept is to get as much oxygen into the chamber as possible for ignition. So with this in mind, it sounds like the oxygen is the "limiting reagent" and there is excess gasoline in the combustion reaction.

So one of two things can happen here, more oxygen gets mixed in and we get more of an explosion, which in result should turn into more power (hp).

OR the gasoline gets dialed back making it a leaner mixture, and there is just the same amount power produced, but since the reaction is more efficient, we SHOULD be saving gas because it was dialed back.

Is my theory correct here but the numbers or just insignificant or is there something something about cars and combustion that I'm completely forgetting about in the equation?

I ask all this because many of the Forced Inductions threads I see are trying to accomplish 2 things:

1. Fool the car to get the highest concentration of gasoline in the chamber.
2. Find some way to get as much air/oxygen into the chamber.

Also, I guess the other thing I was thinking about also, is, is there an "optimal" mixture of gasoline and air/oxygen that should be achieved? I would automatically assume that the most efficient mixture would be on the line curve where all the gasoline is consumed in the combustion and there would only be excess air/oxygen. (Of course I realize that this involves also the amount of actual gasoline that has been injected into a chamber.)

And I apologize in advance if I might have opened up any big cans of worms here with my really dumb down way of thinking of what goes on in the engine.

And thank you also to everyone that will take the time to clarify some of these things for me. I really do appreciate it.

 

I'm far from an expert, but that optimal mix is called http://en.wikipedia.org/wiki/Stoichiometric.

Too much air or fuel(or too little) will result in reduced efficiency.

Forced induction allows more air into the combustion chamber. To compensate the ECU meters more fuel via electronic fuel injection. It knows how much air by means of a Mass Air Flow sensor (MAF) and Manifold Air Pressure sensor (MAP) and auto adjusts for accuracy with the O2 sensor in the exhaust.

There should never be excess gasoline if the ECU and sensors are working correctly.

By increasing air and fuel at the correct ratio more power is achieved.

 

Cars have MAF and MAP sensors so they can dial up or down the quantity of gas.

Gasoline has a combustion ratio of about 14.7. This is where ALL fuel and air is burned. Optimal will vary a few points above or below that for any given car. Some folks go as high as 16 and as low as 9.

Cars are designed from the factory to run with specific air-to-fuel ratios.

When you force more air in, the car usually attempts to put more gas to match.

Though you can get some more power by running a car leaner than the factory ratio (modifying MAP/MAF sensor signal to trick the car into thinking there's less air than there really is), there's a limit. Too lean and you can damage the engine.

On the flip side, some engines can also produce more power by running richer instead.

Of course, some engines NEED to run a little rich, but not to produce power. Instead the excess fuel is used to prevent the car from damage due to excess heat... as the fuel is used for a "cooling effect" (evaporation process). This typically happens in a non-factory forced induction setup because factory road cars are designed to minimize pollution while maintaining longevity.

There's plenty more to this... but as a newbie myself, I can tell you this much.

 

Steve244, thanks for the clarification. I really appreciate it. So in troubleshooting, we can make the assumption that all the gasoline is consumed in the combustion reaction.

Now conceptually, if we were going for power (hp) gains, we would want to trick the ECU to inject more fuel into the chamber. Now how is this normally regulated by the car? How does the car know how much fuel to use?

I would imagine that in a fuel efficient car like our Fits, the ECU would just want to inject as little as possible to be fuel efficient, and just make sure that there isn't too much or too little air in the chamber? Because I can imagine scenarios where we can put as much air/oxygen as possible but if there isn't gasoline for it to react with, then there isn't a point in doing so...

 

Thanks for jumping in, Goobers.. So with what you said... What's the dependent and independent variable? Is the ECU/MAP/MAF reacting to the amount of air coming in?? Or is the ECU controlling the amount of air/oxygen also that comes into the engine somehow?

As for seeing the mixtures themselves, I could pretty much see that adding fuel into the system will almost always increase power, although obviously there will be a diminishing return.

As for cooling the engine with the gasoline, when the fuel is injected into the chamber, is it in gaseous form or liquid form? I would imagine that in that hot environment and with gasoline's low boiling point, that by the time it got close to that area, it would already be a gas, and really not be able to cool the engine as you state... (Not doubting you at all, just my rationale on things.)

 

More fuel without more air (oxygen) won't give more power. Just more unburned hydrocarbons being burned in the cat.

The only way to get more power is more fuel and more air.

It knows how much fuel to meter based on the MAF, MAP, and O2 sensors.

Aftermarket tunes may "trick" the ECU to make more power. They may alter timing and fuel mix to get some power at the expense of efficiency and increased pollutants, but gains are minor without forced induction. These are intended for forced induction applications where real gains may be enjoyed.

There are piggy-back devices sold to alter the signal from the MAF and or MAP sensors, but these are snake-oil. The O2 sensor will put it back in the correct range or it will throw a code and go into limp mode if it fails to correct.

Honda designed our little engines well. There really aren't any measurable gains to be had without going to forced induction.

Aftermarket CAIs may make more noise, but the car already has a factory CAI so the theory that colder denser (more oxygen) air is fed to the engine is already accounted for.

 

We cool by evaporating water. A jet of fuel vaporizing in the engine also has a cooling effect. Increased fuel to maximize cooling allows spark timing advances that might damage the engine otherwise (engine knock resulting from heat) but increase power. But there's not a lot to be had there...

 

When fuel is injected... it's a mist. As it vaporizes into an actual gas and mixes with the fuel, it cools down the mixture before it heats up again due to the engine or act of compression.

It's basically... cool air getting hot vs hot air getting hotter.

The ECU can both react and control the amount of air. In the intake path, the MAF sensor is "ahead" of the air (first thing you encounter if you were the air travelling in)... so that's the reaction. However, the next thing is the throttle body... which the ECU controls. So that can open up wider or close the passage way to control the amount of air coming in. And finally, there's a MAP sensor after the TB.

On the other end of the combustion process... there's the AFR sensor in the exhaust pipe. And sometimes, this also needs to be altered to achieve richer/leaner ratios.

After all that, the ECU decides on how much fuel to inject.

 

I'd say your foot alters the position of the throttle plate allowing more air to pass and the ECU reacts by adjusting the fuel injection based on the readings from the MAF and MAP sensors.

The ECU plays with the throttle plate at idle but that's about it.

Forced induction can be controlled, allowing higher boost on demand. But the ECU still monitors the amount of air using the MAF and MAP sensors to calculate fuel. More air being forced at higher pressure needs more fuel to maintain stoichiometry.

 

Due to its "drive by wire" nature, you could just as easily make the argument that the pedal tells the ECU roughly how much power you want from the car and it opens and closes the throttle accordingly.

The point still stands, regardless of WHY the ECU is opening or closing a specific amount, it's still controlling the throttle.

edit:
Just wanted to add... this holds true unless you are in OPEN loop (typically, WOT, or sometimes in deceleration in gear above supposed "DFCU" lower threshold).

Because, in open loop, the ECU ignores the O2 sensor and then relies only on the MAF/MAP.

 

Others have explained things better than I could. While the means of accomplishing things are much different, carbureted engines and carburetors are designed to try to do the same basic thing—maintain a more or less consistent and efficient mixture under whatever operating conditions the engine is under. Closed-loop computer control via EFI just does a much more reliably accurate job.

It is perhaps worth noting that diesel engines operate with completely different principles in this area. A diesel generally has no throttle as such; it is always getting a more or less fixed amount of air in the combustion chamber, and only varying the amount of fuel delivered. At idle, a diesel is running a very lean mixture indeed. (There is some control of the amount of air on modern diesel engines through diddling with the turbo controls and wastegates and such; but the general principle isn't vastly changed, so far as I know.)

 

The only thing the aftermarket filter units offers is the possibility of more AIR being introduced to combustion chambers due to less restriction to intaske air flow. In theory correct but in reality only at very high revs will it really matter. The computerized air flow measiurement adds more fuel but for the most part the dyno very rarely demonstrates more tha a hp or gain. Street driving rarely has any effext on power or mpg.
Forced intake induction, supercharging or turbocharging offers much greater intake of air to the combustion chamber and of course uses the computerized air intake flow to add more fuel. More power and less mpg is the result. Really good systems offered by manufacturers can do both more power, typically 10 to 24%, and when not abused offer good mpg on the street.
Other methods such as enriching the fuel air ratio with nitrous oxide will do likewise but not fot the street.
Incidently there is an iptimum air (o2 as part of air) to fuel ratio for maximum power so just adding ore fuel, aka by larger injectors for example, aren't much use use. Fiooling the computer rarely offers an advantage.
In the old days we could do a lot of things for a lot more power but today's federal limits on emissions makes engine changes for more power almost criminal at least.