Carburetor Operation

Carburetors, contrary to popular opinion, are a very basic device similar in relative design since Henry Ford and his Model T. The same basic things make them tick no matter who designed it. People over the years have heard horror stories about carburetors or maybe even had a bad experience firsthand (who hasn’t been at the race track and witnessed a carburetor fire in their lifetime?) but that should not keep you from learning how one works. So without further ado, it’s time to dispel the myths, lies, and black magic.
The first thing required for a carburetor to function properly is atmospheric pressure. Pressure is the most important variable tied to a carburetor’s performance, and without it one simply will not run! Most carburetors will have a vent tube that acts as a “port” to the fuel bowl; this “port” provides the carburetor with pressure from it’s ambient surroundings and forces the fuel to move through the metering passages as required based on engine demand. Manipulation or changing the length of the fuel bowl port can have dramatic affects on the fuel curve of a carburetor and should only be done with the assistance of a professional engine dyno. Many people think that fuel pressure is what moves fuel through a carburetor and they are which is incorrect. Fuel pressure simply pushes the fuel to the carburetor, atmospheric pressure takes over from there.
Another phenomenon a carburetor requires is something we refer to as draw. Draw is essentially what the engine wants from the carburetor in terms of air and fuel. When an engine starts going through the RPM range, draw will increase (naturally as engine speed increases air and fuel demand will as well). As draw increases a carburetor must react to properly mix the air and fuel together. Air and fuel mixture is very important and varies depending on the type of fuel you use as well the elevation you are racing at. The word carburetor gurus use for the process of mixing air with fuel is “atomization”. Atomization is where things get tricky and some black magic comes to play; carburetor manufacturers, modifiers and other fuel system related companies are always searching for more efficient ways to atomize fuel and air.
So how does the atomized fuel get to where it needs to be? This is a question that many might know the answer to - the Venturi effect - named after the Italian physicist Giovanni Venturi, the Venturi effect is a phenomenon where pressure is reduced after air flows through a constricted area. The constricted area in questions is easy to spot as it is the skinniest part of the “barrel” on a carburetor. To explain a little further, air rushes past the area with the smallest circumference causing it to speed up and form an area of low pressure right below the venturi, this low pressure will in turn pull (remember our term draw) atomized fuel from the booster venturi and send it along to the intake runners. One thing nearly all carburetors have in common no matter how many barrels is the venturi.
To sum it up; a carburetor will not run without atmospheric pressure, something to mix the fuel and a means of getting the fuel to combustion chamber - these are the basics and the details are where performance is found. Carburetor development has come a long way since Henry Ford’s Model T and each new season of racing brings some innovation giving racers an edge. Still foggy on the principals or want to know more? Give us a call, we love talking about these contraptions and helping people make the most of their race car.

knocking of engine

When engine knocking is detected the knock sensor sends electrical signal to the ECU. Directionally as the compression ratio of the engine increases so does the required octane number of the gasoline if engine knocking is to be avoided. Engine knocking is in fact a pulse detonation and we all known what that can do an engine. Thus, the likelihood to engine knocking is reduced and the engine runs more smoothly. Engine knocking is the premature fuel combustion that can result in power loss of the engine. Engine knocking is compression detonation or pre-ignition of fuel in the power stroke of the engine. Engine knocking is normal for 4-stroke bike.

If you have an older car or a high performance car, you may need a higher octane gasoline to help prevent engine knocking and improve engine performance. In order to prevent engine knocking at high rpm's, NGK's high-spark #7 platinums are used. Before leaded gasolines were removed from the market, bromine was used in an additive to help prevent engine "knocking". Motor mount also help prevent or at least minimize engine knocking. There was a need for improvement in the refining process for fuels that would prevent engine knocking and increase engine efficiency. This prevent engine knocking which is very common at the time of a transmission kick down. MMT is a fuel additive, which is mixed with petrol in order to prevent engine knocking. Has been used in gasoline to prevent engine knocking. Lead was originally added to prevent engine knocking. The lead compound TETRAETHEL LEAD was added to gasoline to prevent engine "knocking".

If anything, high octane gas will help reduce engine knocking in most cars (assuming your car's manual says it's okay to use such a gas). Replace Air Filter - Dirty filter can reduce fuel economy by 10% or more. Ethanol in unleaded gasoline helps reduce carbon monoxide emissions by as much as 30 percent. It is oxygen-bearing additive used to reduce engine knocking and assist gasoline burn more cleanly. An antiknock agent is a gasoline additive used to reduce engine knocking and increase the fuel's octane rating. Thereafter, the engine performance will peak and emission will reduce. Lead has also been added to gasoline to reduce engine knocking. Standard Oil began adding ethanol to gasoline to increase octane and reduce engine knocking. Lead, in the form of tetra-methyl lead or tetra-ethyl lead, is added to petrol to increase its octane rating to reduce engine knocking. Some manganese compounds have been added to gasoline to boost octane rating and reduce engine knocking.

When engine knocking is detected the knock sensor sends electrical signal to the ECU. Engine knocking is in fact a pulse detonation and we all known what that can do an engine. Thus, the likelihood to engine knocking is reduced and the engine runs more smoothly. Directionally as the compression ratio of the engine increases so does the required octane number of the gasoline if engine knocking is to be avoided.

Twin spark ignition engine

Twin Spark name usually refers to the engines installed in Alfa Romeo cars. The 8 valve engine was fitted initially to the Alfa Romeo 75 but also the Alfa Romeo 164 and Alfa Romeo 155. The 16 valve engines appeared in the Alfa Romeo 145, Alfa Romeo 146, Alfa Romeo 156, Alfa Romeo 147, Alfa Romeo 166, Alfa Romeo GTV & Spider and even Alfa Romeo GT models.
The TS series engines are all '4 cylinder inline' with twin cam (DOHC) shafts. The original 8 valve engine featured a light alloy (Si enhanced alu alloy) block + head, wet cooled iron cylinder liners and the camshafts were driven by double row timing chains (long and short). Similar design to the earlier and famous Alfa Romeo DOHC engines, but with narrower valve angle on this model.
The later 16 valve engines had a heavier cast iron block engine, with an alloy head, and the camshafts were belt driven. The Twin Spark name comes from the fact that there are two spark plugs per cylinder. The block was Fiat sourced. It was cast iron for its higher beam strength, less complexity and hence lower production costs. When new, these engines were notable for their high efficiency as demonstrated by the BMEP (brake mean effective pressures) exerted upon the piston crowns.
The two sparks on the 8V Alfa Twin Spark engines do not fire at exactly the same time (true at least of the 75 which uses traditional ignition coils with king leads and distributors). This was deliberate to avoid the most powerful part of the flame fronts meeting at the centre (weakest part) of the piston. As there are 2 symmetrically placed sparks plugs, the flame front must travel less distance which allows less ignition advance to be used and it is accepted ignition timing can be closer to optimal than a single spark plug would allow. Also, leaner mixtures can also be tolerated for better fuel economy. The 8V engine also has 8 identical spark plugs. There is no room for a centrally positioned spark plug due to the 2 valve design which uses a 44mm diameter inlet valve on the 2.0 engine.
On 16V engines there is room for a spark plug in the centre of the cylinder as in all 4-valve configuration engines, but also a second smaller plug (off to the side on the axis) is installed. Both of the plugs fire at exactly the same time on compression and exhaust stroke, due to the way in which the coils are paired (1&4 and 2&3). This production economy allows the use of 4 coils, instead of eight, which would normally be required to fire eight plugs, and is common practice called "wasted spark" system. (used also in Ford EDIS system as well as in some Alfa Romeo V6 engines 3 coils for the 6 cylinders). The main reason for the wasted spark system is cost. As both plugs are connected to the same coil the spark one of them operates with reversed polarity and requires decreased breakdown voltage. On the later CF3 (2001 on Euro 3 emissions standard) 16v TS the four coils fire the same cylinder spark plugs (so not 1 and 4 and 2 and 3 as pairs but coil #1 fires the big and small plug of cylinder 1, and so on). The TS 16V engines, 1.6, 1.8 and 2.0, all use a 10 mm diameter and a 14 mm diameter long life platinum electrode spark plug per cylinder. The spark plugs have a replacement interval of 100,000 kilometres (62,000 mi). The operation of the Twin spark engine has been subject to much debate but this is the general theory of operation, as described by Auto Italia magazine.

16 valve Twin Spark with older cover.

The engines also incorporate two other devices to enhance the performance under operation, the Camshaft Phase Variator and the Variable Intake Length Control (or Modular Inlet Manifold in Alfaspeak) on the later (plastic cam cover) 1.8 and 2.0 litre versions. Where both these variable systems are deployed they are controlled in tandem by the Bosch Motronic Engine Management ECU in response to rpm, load, and throttle position. According Fiat Auto S.p.A DTE electronic service documentation for the Alfa Romeo 156 Twinspark (1.8/2.0)...

"To optimise the quantity of air drawn into the engine the control unit checks: inlet timing on two angle positions (and) geometry of inlet ducts at two lengths (only 1.8/2.0 TS). At maximum torque speed the control unit sets the "open" phase: cam advanced by 25°, inlet casing long ducts (Only 1.8/2.0 TS). At the maximum power speed the control unit sets the "closed" phase: cam in normal position, inlet box short ducts. At idle speed the control unit sets the "closed" phase: cam in normal position and inlet box short ducts. In the other engine operating conditions, the control unit selects the most suitable configuration to optimise performance - consumption - emissions. During overrunning, the inlet ducts of the box are always short."^[2]

The advancing of the inlet camshaft by 25 crankshaft degrees (or 12.5 camshaft degrees) opens and closes the intake valves earlier in the inlet cycle. This allows the filling of the cylinders with air/fuel mix to begin earlier, and by closing sooner trap more of the fuel/air mix for the compression phase of the combustion process. On 1.8 and 2.0 16V Twinsparks the longer inlet ducts deployed in conjunction with the advanced inlet cam, are of a tuned length that assists cylinder filling by harnessing the harmonic pressure wave present in the lnlet duct. In this way the dynamic effective compression ratio is increased which produces more torque at the given engine speed. As the intake valve is also opened earlier in relation to the exhaust valves, the valve overlap (the period both inlet and exhaust valves are simultaneously open) is also increased at this mode. This promotes the scavenging effect of the exiting exhaust which causes a partial vacuum in the cylinder to further assist in filling the cylinder with a fresh charge.
As with similar inlet cam phasing systems like BMW VANOS the phasing is returned to the retarded state at higher rpm to improve power and effiency as inlet gas dynamics change with rpm. The short inlet ducts being tuned to the higher frequency and thus shorter inlet duct pressure wave.
On 8V engines the valve overlap and intake open duration are quite big. These engines hardly idle with the variator at On position so on these models it had the meaning also to enhance lower speed operation. On the newer 16V engines the camshaft variator is used to enhance the performance/emissions but also might be the source to the common 'diesel noise' problem often seen on high mileage used models which used the earlier internals of the variator. The same variator system is also used in many Fiat/Lancia engines like one used in Lancia Kappa 5-cylinder engine, some Fiat Bravo/Fiat Marea engines, Fiat Barchetta, Fiat Coupe, Fiat Stilo etc. models.

Variable Inlet Control

16 valve Twin Spark

The Variable Inlet Control causes the air intake (plenum) to shorten its path from the intake trumpet to the valve bores, again when the engine reaches a pre-defined RPM. This works on the principle that the air intake length is a tuned pipe that when operating at its ideal frequency-in tune with the valves, will allow a smooth and even air flow, and will assist with filling the cylinders efficiently. This works in a similar method to the tuned exhaust system on all modern cars, where the exhaust system creates back pressure to pull spent gasses from the cylinder bores. The variable intake system uses the cone inside the air box to reflect negative pressure waves back down the inlet. These waves are used to assist in filling of the cylinders. the variable inlet allows this to take place at different engine speeds.
The notable effect that these two devices have is that the engine will have a linear power delivery from low RPM up until the red line, without the lack of torque at low RPM, and a kick in power at higher RPM, that is typical of multivalve engines as they come "On cam".
It is a type of Variable Length Intake Manifold

Engine of car

  

The engine is the heart of your car, but instead of pumping blood, the engine pumps air and fuel. The engines main function is to convert air and fuel into rotary motion so it can drive the wheels of the car. How does it do that ??

Pistons: Most common engines have 4, 6, or 8 pistons which move up and down in the cylinders. On the upper side of the piston is what is called the combustion chamber where the fuel and air mix before ignited. On the other side is the crankcase which is full of oil. Pistons have rings which serve to keep the oil out of the combustion chamber and the fuel and air out of the oil. Pistons are made from lightweight aluminum alloy and are designed to float in the cylinder without contacting the cylinder walls. They float on a thin layer of oil which is below the rings. If the rings fail, oil can leak into the combustion chamber and you will see grey smoke coming from the exhaust. If the rings wear or you lose oil to the engine, the pistons can score the cylinder walls damaging the engine and requiring a rebuild

Crankshaft: The crankshaft is connected to the pistons via a connecting rod. As the piston moves up and down in the cylinder it rotates the crankshaft and converts the straight line motion into rotary motion.




Valvetrain: The valvetrain consists of valves, rocker arms, pushrods, lifters, and the cam shaft. The valvetrain's only job is that of a traffic cop. It lets air and fuel in and out of the engine at the proper time. The timing is controlled by the camshaft which is synchronized to the crankshaft by a chain or belt.
Now that we have a general overview of the parts involved let's talk about what happens during the normal operation of your engine. Most automotive engine today are 4-stroke (or 4-cycle) engines, meaning they have four distinct events which make up the cycle. A 4-stroke engine takes two complete crankshaft revolutions to complete the cycle. Below are the 4 complete parts of the 4-stroke cycle...
* Intake stroke: The camshaft opens the intake valve and the piston moves down the cylinder. This creates vacuum and sucks in air and fuel into the combustion chamber above the piston.
* Compression stroke: As the piston starts moving back up the cylinder the intake valve closes and seals off the combustion chamber. The causes the air and fuel to compress.
* Power stroke: As the fuel is compressed and the piston nears the top of the cylinder the spark plug fires and ignites the fuel and air. This explosion pushes the piston back down the cylinder and drives the crankshaft.
* Exhaust stroke: After the piston reaches the bottom of the cylinder, the exhaust valve opens and the gasses left over from the fuel and air are sent out to the exhaust system.
To get a more indepth look into the engine, take a look at the Road Machines CD free preview.
Put these four events together in the above order and you have a complete cycle. Are you asleep yet? That's enough theory, let's talk about the real world and problems you might encounter with the above mentioned parts.
Pistons: Remember I talked about the rings which seal the combustion chamber from the crankcase. The rings over time tend to wear out. When they wear they allow the fuel and air to enter into the oil and dilute it. This dilution reduces the oils ability to lubricate your engine and can cause premature wear. Also if the rings wear down they can allow oil from the crankcase to enter the combustion chambers. This will result in oil being burned and exiting your tailpipe as grayish/white smoke. If your car spews grayish white smoke and it does not go stop in the first few minutes after start-up you might have warn rings. If the smoke goes away after start-up look to the valvetrain section.
Crankshaft: The crankshaft rides on bearings which can wear down over time. The bearings support the crankshaft and also the rods which connect the pistons to the crankshaft. A loud medium pitched knocking noise in the engine points to warn bearings most of the time. This is usually a costly repair and involves removing the crankshaft and either machining the surface where the bearings ride, or replacing the entire crankshaft. To prevent this type of problem, use a high quality oil, change your oil at suggested intervals (3 months or 3000 miles is a safe number) and always maintain your oil level between oil changes. Many times it is more economical to buy a replacement engine, than to have your engine rebuilt when you have a crankshaft bearing failure. Your mechanic can give you a better idea of costs involved.
Valvetrain: Remember the oil smoke problem mentioned above in the piston sections. If your car only smokes grayish/white smoke at start-up you may have leaking valve seals. Valve seals keep oil from above the valve from leaking into the combustion chamber. When they wear, they can allow oil to seep into the combustion chamber and collect there until your start the engine again. You generally do not get oil leaking past the valve seals while the engine is running since the seals expand with the heat of the engine and plug the leak.
Another common problem is the timing chain or belt will slip or even break causing the camshaft to stop rotating. Remember the camshaft tells the valves when to open and if it stops spinning then the valves stop opening and closing. No valve moving, no engine running :-)
A term you will here when talking about timing chains and belts is "interference engine". When an engine is an "interference engine" the pistons and valves are so close together that if the valves were to stop moving (broken belt or chain) and the crankshaft kept spinning they would crash into the piston. (that's the interference) This crash tends to do bad things to an engine, breaking valve, bending pushrods, and even cracking pistons. This is why most manufacturers recommend changing the timing chain or belt every 60,000 miles. Timing belts dry out, stretch and deteriorate over time so even if you do not have 60,000 miles on the car think about changing the belt after it's 6 years old. If you are wondering if your engine is an interference engine, you can check with Gates, who makes timing belts and has a PDF file which will tell you if your engine is an interference engine and the recommended service interval.