Fuel Injection or Caburettion
I have touched on this previously, but wish to dwell on it a little longer. On the Intake stroke of a 4-cycle petrol motor a negative pulse in the intake is caused by the Piston going down with the Intake valve being open. This Valve action is typically from 32 BTDC until 68 ATDC on a performance motor, which naturally only makes Power on top, while conservative cam times from 20 degrees before TDC until 50 Degrees after bottom dead center. The bigger the overlap is of the exhaust valve closing, while the Intake is opening the more of a performance (high RPM) cam you have got. Why am I nattering on about cams and their Timing? Well arguably the most important aspect is Exhaust Duration Followed by Intake duration and that has an effect on the rest of the Motor.
OK- when your Intake valve opens, your exhaust side is also open and if designed right, it will have created a negative wave (see it as a vacuum) in the cylinder to draw the charge in. The earlier it opens, the more chance you have of a positive pressure differential pushing the Exhaust Gasses out into the Intake tract, especially at low RPM. Thatís part of the reason why a "Hot" cam makes the Motor Rumpetyrump. Alright- the negative wave has started to suck the mixture in creating a Finite Amplitude wave in the Intake that gets the charge going into the Head. The piston going down also "sucks" in the mixture.
The length and the dia. of the intake runners Determine at what RPM range the Waves will be standing and in harmony, which creates the strongest pulse towards the head. Long thin dia. Runners = Torque at low RPM, while short fat runners, such as on Sidedrafts, Make more top end Horses. Ok that means the Piston can move past Bottom dead centre and come up into Compression Stroke until the Pressure in the sleeve is greater than that of the incoming charge. Since this point changes in the RPM band, letting the Exhaust valve close at a Particular point is a compromise. Mixture being forced back into the intake runner can go through the carb, while picking up even more fuel from the ventury. This is called Fuel standoff; Beetles were famous for this. At idle it can cause surging RPM and the Engine to die.
In a perfect system the Air flows through a carburetor and picks up Fuel in the ventury (a narrowing of the bore- increasing Air speed and mixability). Thatís why choosing a too big Carb is bad- because you can never get the fuel mixture right. Also that is why most carbs are progressive so the First Bore is small and made for good mixing at Low RPM while giving you Fuel Economy and the second Barrel is a big hole that pours in fuel for High RPM performance. Now- the Autolite 2 barrel we sell is made for Economy (each barrel feeds Ĺ of the V8), while the Holley 500 CFM 2 barrel that everybody seems to pick up cheap is made for Performance and therefore has a healthy thirst. Some Figures on the Autolite my customer gets 7 Km/L while on the Holley he got 4 Km/L on his 307 Chevy. While testing in the USA showed a 300 Hp Chevy to loose close to 40 Hp using the Autolite over the Holley. Thatís why you go "4 keel" and then you have the best of both worlds, as shown above.
Now for Fuel Injection- we get Single point, which is a glorified Carburetor. Multi point which means 1 or more Injectors per Cylinder and then the Ultimate; Sequential Multi point injection, meaning that each Injector squirts fuel at a certain Crank degree. Then we have mechanical aíla Hilborn or Electronic as per Motronic. Can you also see what I am getting at: Spark positioning- right on brother. OK a EFI system needs certain Inputs, such as RPM, Throttle Position (TPS), Manifold pressure (MAP), Crank Trigger, Air Flow Meter, Air and Water Temperature Sensor.
All these inputs are fed into a computer where they are stored in memory and Utilized by a Computer that then tells your Ignition when to fire and your Injectors when to open and for how long. EFI is normally split into Air Volume (Air flow meter) and Air pressure (MAP sensor) systems. Since Temperature, Volume and Pressure are rigidly related it does not matter what system you use. Lets take a look at the graph below, that could just as well work for injector Timing:
There are 3 types of Injection system:
Constant Flow where just the Volume gets changed (mostly mechanical)
Pulsed Injectors where All Injectors get pulsed at the same time
Sequential where every single Injector gets pulsed at a certain Crank/Camshaft degree.
I personally chose a Pulsed Multipoint Aftermarket Injection for distribution due to its price, simplicity and still giving me all the features I need. There are many good points on EFI, the bad ones only tend to raise their heads from Incorrect Installation or later manipulation, of course there are also dudís but veeeeeeeery seldom.
To finish of my article, I want to edge your mind on a bit from an Argument 2 Friends were having while I turned the meat: Dís theory " A inefficient engine requires advanced timing to run". This was borne out by a fact that 99 % of all Engines will run on 10-15 Deg. Pís counter argument "A slow burning engine requires advanced timing. Efficiency has nothing to do with it. Efficiency means for me fuel consumption, and a engine with advanced timing can be efficient" Here I had to quip in my own theory, since my lot is not to Braai alone
"There are a variety of theories on when to light the flame, and before I start, let me throw an Interesting point at you. Buick 3.8 Liter- makes most Power at 32 Deg. mechanical Advance. This is Normally Aspirated at an 8:1 Compression ratio and also whenTurbocharged at 8 Psi. Up the Compression to 9.5: 1 normally Aspirated and you could go to 34 Deg.- not that 2 Deg make much Difference." Thatís when I could see my friends get Edgy, so I carried on "Now when I did Emissions work overseas, we realized the following; The more fuel/Air the Engine Burns the more Power. The more efficient the Engine is, the less of both, it needs to make the same power. Smaller Fuel Droplets and fewer variations thereof, provide a smoother (less detonation) and faster burn. Less timing Advance was needed giving us higher Combustion pressures." By now rude remarks were coming my way, about why I did not accept that professorate at Harvard. So I roughshod carried on "Racing Small Block Chevy's though start of at 20 Deg. initial and top out at 38-40 Deg at 3000 Rpm though the Advance Curve only starts at 1500. I guess that because the Bore is bigger it needs more Spark propagation." This brought about a hefty debate which I wanted to cap of with that I personally believe far to little work has been done on Spark propagation & Ionization with regards to Part throttle operation (everyone is in at WOT). Never mind how few people think about HOW the spark happens, not WHEN. Also I think the shape of the combustion Chamber and Squish Band has a lot to do with this. Compare a Hemispherical head (XR 600) to a Wedge, Bathtub or Heron (Ford V6). And for a Nightcap before I bore you to death. Swirl and Tumble in the Intake port/ barrel also Contribute to this. But in an Arse about Face way. Take the 22R in the Cressida and now Hilux. Initially 12 Deg. Advance was called for by the factory and now it is specified at 5. If you have our 95 Octane most workshops set it at 10 Deg for best performance. Now I port the head (no increase in compression) for more swirl etc. I can lean the Engine out to use 7.5 k/L at 140 KM/H (before it was 4.5 K/L) and bringing the initial timing (and therefore the whole curve) to 5 Deg giving me best performance and efficiency of 100 - 250 ppm HC.