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fuel injection (was:Doug, anyone, 400hp engines please...RAT

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brian-1927(at)lloyd.com
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Posted: Thu Mar 20, 2008 10:20 am Post subject: fuel injection (was:Doug, anyone, 400hp engines please...RAT

On Mar 20, 2008, at 5:51 AM, cjpilot710(at)aol.com wrote:

 	  Quote: 	
		   As I understand "fuel injection" the fuel is injected directly into  

 the cylinder.  Here I understand the advantage, and particularly  

 with the 'gami' (sp?) injectors that horizontal opposed engines use.


	


One does not inject gasoline directly into the cylinder. As KP said,  

that would produce detonation. One either uses multipoint fuel  

injection, where there is an injector for each cylinder, usually  

injecting the fuel right at the intake valve; or single-point fuel  

injection, where the fuel is injected well upstream and the engine  

depends on something else to ensure proper mixing of fuel and air.



(BTW, the GAMIjectors just vary the fuel flow from cylinder to  

cylinder in order to match the airflow differences between cylinders  

and ensure a uniform mixture in all the cylinders.)



 	  Quote: 	
		   Where does the extra power come from?  Super charger changes in  

 shape or rotation speed, gapless rings, domed pistons, and valve  

 design, I can understand but I just don't see the advantage here.


	


Fuel injection does not produce more horsepower per se. But fuel  

injection does give greater control over mixture thus making it  

possible to ensure that all cylinders receive the same mixture and  

therefore that all cylinders are producing their maximum power.

 	  Quote: 	
		   Of the different "changes" which one delivers the most horsepower?   

 The domed piston over the supercharger or just gapless rings?


	


It depends.



 	  Quote: 	
		  

 No one has ever explained to me why injecting fuel into the intake  

 pipe produces more power and is any different than injecting it at  

 the carb and my limited cranial volume can't seem to comprehend it.


	


The advantage of fuel injection over carburation comes from two things:



1. more uniform mixture distribution to all cylinders;



2. fewer restrictions in the induction system which leads to a higher  

MAP at high flow rates (full throttle).



This is one of those topics where the forest gets lost in the trees.  

We get so worked up about the details we forget the big picture. More  

*power* comes from burning more fuel. Period, end-of-report. If you  

can figure out how to get the engine to burn more fuel you will get  

more power. The only caveat to that is you must maintain the proper  

mass ratio of fuel to oxygen. You want just enough oxygen to ensure  

that all the molecules of fuel get burned.



Most people think that carburetors somehow "mix" the fuel and air.  

They do not. They roughly measure the flow of air based on a pressure  

drop in a venturi and then spray some amount of fuel into the air  

stream. Since liquid fuel and air have different densities, they do  

not necessarily go to the same places within the induction system with  

the same facility. This gives rise to mixture differences between the  

cylinders.



As an example, imagine a bunch of skinny, fit guys and a bunch of fat  

guys running a race in a twisting course. They are not going to arrive  

at the same place at the same time. This is what happens with the and  

(skinny guys) and the fuel (fat guys) in your induction system.



By injecting just the right amount of fuel at the inlet of each  

cylinder we avoid this problem as we force the fat guys to arrive at  

the same time as the skinny guys. The mixture in the cylinder is even.



But there is another way. That way is to run the skinny guys and fat  

guys through a blender right at the start of the race. That is what we  

do with our radial engines with superchargers. The blower serves to  

mix the fuel and air uniformly as well as help turn the fuel from  

liquid to vapor. (Liquid fuel will not burn. It must be turned to  

vapor first.) This ensures that the fuel:air mix that reaches the  

cylinders is a uniform mixture. This eliminates the need for  

individual injection of fuel at each cylinder.



So multipoint fuel injection is just a way to deal with the differing  

density of liquid fuel and air. If you can mix the proper ratio of  

fuel and air and then ensure that the liquid fuel vaporizes, you don't  

need the advantages of multipoint fuel injection. Single-point fuel  

injection upstream of the supercharger is just fine in that case.



Oh, and a pressure carburetor is nothing more than a single-point fuel  

injection system. It measures mass airflow and then admits the proper  

amount of liquid fuel. If one cares to look, one will find that there  

is almost no difference between a Bendix (Lycoming) or Airflow  

Performance fuel servo and a pressure carburetor. The only major  

difference is that the pressure carb has it output in a spray bar  

immediately following the servo (downstream of the throttle valve)  

while the other system send the fuel to a flow divider and then on  

down to an injector at each cylinder.



Now as for the other stuff, here is the skinny.

High compression:



Increasing the compression ratio improves efficiency up to a point.  

The problem is how to prevent detonation in engines using gasoline as  

a fuel. We accomplish that by using fuels that have a greater  

detonation margin (measured in octane). In any gasoline-powered engine  

you want the compression to be as high as possible without detonation.  

Less than that and you are losing efficiency.



(It might be interesting to note that diesel engines use really high  

compressions as they are intended to operate in this detonation  

region. This is one of the reasons that they are so much more  

efficient than gasoline engines.)

Rings:



We want to seal the cylinder to keep our compressed burning mixture in  

the cylinder. The closer we can get the rings to the top of the  

piston, the less volume in the combustion chamber and the higher we  

can maintain the compression ratio. We also keep the hot burning gases  

above the piston where their pressure can produce power. Rings will a  

smaller area will have less friction with the cylinder wall. Rings  

will a smaller gap will leak less gas.

Supercharging:



Supercharging, either through the use of a compressor stage  

mechanically powered by the engine, or by a compressor stage powered  

by an exhaust-gas turbine (turbo-charging), greatly improves  

efficiency and power.

First, the supercharger crams more fuel/air mix into the cylinder.  

More fuel/air means more power. Period. Second, it is easier to get a  

gas to flow in a pipe by blowing on one end than by sucking on the  

other. The supercharger does a much better job of getting fuel/air  

mixture into the cylinder than the piston does during its intake  

cycle. In addition to aiding the fuel/air mixture get to the cylinder,  

the mechanics of doing the compression aid in converting the fuel from  

liquid to vapor and also aid in mixing the resulting fuel vapor with  

the air.



Supercharging covers a multitude of ills.

Valve size, valve timing, and head design:



Our fuel/air mix has mass. That means it requires force to start it  

moving and force to stop it again. (Good ol' Newton's second law.)  

Since we want lots of fuel and air in our cylinder we want to get the  

combustion mixture moving as fast as possible and then have a lot of  

it moving. We want the valve to be big enough to not restrict the  

flow, hence we like bigger valves where possible. In addition, we want  

no restriction to the flow so we want the pathway to have as little  

aerodynamic drag as possible. This is what "flowing the heads" does --  

reduces aerodynamic drag so the fuel/air mix can move as quickly as  

possible into the cylinder.



There is an extra benefit from getting the flow as fast as possible;  

the momentum of the fuel/air mix helps cram more fuel/air into the  

cylinder. It takes more force to stop the flow of something that is  

moving faster. (If you don't believe me, run out in front of your car  

and try to stop it when it is going 2mph and then again when it is  

going 20mph.)



Remember, it takes a fixed amount of time to fill up the volume of the  

cylinder with fuel/air mix. Also remember that, as RPM increases,  

there is less time to fill up the cylinder. This is why engines tend  

to lose efficiency at higher RPMs. OTOH, the faster you turn the  

engine, the more total fuel/air mix you can burn in the engine. (More  

bangs per second.) The trick here is to somehow keep the flow of fuel/ 

air up and keep the cylinder filled. To help with this we mess with  

valve timing. This is where you start to talk about high-lift and long- 

duration camshafts. The high-lift cam causes the valve to open wider  

and provide less resistance to gas flow. Long-duration allows the  

intake valve to stay open longer to allow the mass of the gas flow  

(fuel/air mixture) to cram more gas into the cylinder even though the  

compression stroke has begun.



So we can go on and on here but it all boils down to that one thing I  

mentioned at the beginning: cram more fuel/air mix into the combustion  

chamber where it can burn and produce power.



And no, I am not going to get into ignition timing. I'll save that for  

later.

 	  Quote: 	
		  

 Jim "Pappy" Goolsby



 Create a Home Theater Like the Pros. Watch the video on AOL Home.


	


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brian HYPHEN 1927 AT lloyd DOT com    Cameron Park, CA 95682

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warner.wesley(at)gmail.co
Guest

Posted: Thu Mar 20, 2008 10:45 am Post subject: fuel injection (was:Doug, anyone, 400hp engines please...RAT

 	  Quote: 	
		    One does not inject gasoline directly into the cylinder. As KP said,

  that would produce detonation. One either uses multipoint fuel

  injection, where there is an injector for each cylinder, usually

  injecting the fuel right at the intake valve; or single-point fuel

  injection, where the fuel is injected well upstream and the engine

  depends on something else to ensure proper mixing of fuel and air.


	


Not trying to nitpick but if I'm not mistaken, they do inject directly

into the cylinders on some 2-strokes.  I believe Mercury and Aprilia

both have systems for this.



Wes

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