Fuel Vaporization and Droplet Size

In the automotive industry, extensive research has been devoted to improving fuel atomization and vaporization by carburetor and induction tract modifications that use nozzles, heated manifolds, vibrating plates, exhaust gas recirculation, swirl and other methods. Results show that fully vaporized fuel may not ignite quite as readily as ideally atomized tiny fuel droplets. Droplets of less than 15 micron (.00059")diameter have been found to best follow the airflow path of induction manifolds.

The design of the Smart Carby main jet allows for altering the droplet size to suit individual applications or experiments. We have found that controlling the droplet size influences the engines ability to make more power at high revs. If an engine has a slightly inefficient cylinder head flow or lower than optimum compression ratio for the choice of fuel, the result is not enough effective compression at the top end of the air flow range. When this occurs the heat built in the chamber during the compression stroke is not enough to achieve sufficient vaporization of the fuel droplets. Droplets are vaporized by heat transfer from the surrounding atmosphere through the surface area of the droplet. The same volume of fuel spread into many small drops has a greater surface area than one large drop hence fine atomization improves the vaporization and burn ability at high revs.

The emitter orifices of the Smart Carby main jet deliver a finely atomized fuel fog to the induction system. This fog distributes more evenly and ignites more readily than larger fuel droplets. Mixture distribution is evident in exhaust gas temperature measurements.

Exhaust Gas Temperature And Cylinder Burn

Exhaust Gas Temperature (EGT) reflects the leanness or richness of the fuel mixture. EGT is also an important diagnostic parameter. It rises significantly during misfiring due to a fouled spark plug or other causes, and it falls during knock or detonation. The EGT probe measures the average gas temperature just outside a cylinder's exhaust port and this reflects the relative temperature of the gases that bathe the head of the exhaust valve when the valve is open. At a fixed RPM and manifold pressure, the EGT spread observed across a range of mixture settings reflects the evenness of the mixture distribution among all cylinders and can be used as a guide to modify the induction system to improve that distribution. Such modifications should only use EGT data obtained at wide open throttle settings due to the highly variable induction flows at part throttle.

There is a general rule that a leaner cylinder will show a higher exhaust temperature but this is not always the case. This is a useful rule to follow if you are dealing with a standard type low power output production engine but don't always follow it for high output race engines. FACT- 'The temperature in the exhaust pipe is the left over energy from the combustion.' Basically the exception to the general rule is when the temperature is high because the fuel is still burning in the pipe. Slow burning of the fuel in the chamber leaves more heat in the exhaust pipe. Lean or rich mixtures both cause slow burning, basically any time the cylinder output efficiency is less than optimum it means the energy input to the piston is less, therefore more energy is converted to heat when the exhaust valve opens. The temperature in the exhaust can go up as the cylinder is run leaner and it can also go up as the cylinder is run richer. It is important to assess the other factors like sparkplugs and timing and exhaust analysis before making a decision on tune.

Droplet size also influences exhaust temperature, especially when the mixture is not fully vaporized and/or correctly distributed throughout the chamber. A large drop stuck out by the cold walls of the chamber and jammed in the squish may have trouble evaporating the heavy components of the fuel leading to a gas environment of light hydrocarbons that may become overheated and start exploding. This is one of the largely overlooked causes of detonation. Detonation that occurs firstly in the gap between the piston and cylinder wall is usually because of this problem. Remember that gasses burn, liquids don't. This means that you have to convert liquid fuel into a gas before it will burn. Petrol and methanol are quite different in their vaporization. Methanol is a single molecule and all the molecules require the same energy input to convert them into a gas. Petrol is a blend of various chemicals and they all boil into a gas at different temperatures, i.e. energy input. As the pressure of the atmosphere around the droplet increases the temperature necessary to convert the liquid chemicals into a gas increases too. This makes it difficult to convert large drops to gas in the short time available at high rpm, so the general solution is to lean the mixture slightly so that there is less liquid to be converted by the same amount of heat ( a high speed lean out). The cylinder then may make more power but it has less fuel than the most powerful correctly vaporized mixture. It is important to not band-aid a good feature to prop up inadequacy. Testing so far has shown that by using the droplet size of the Quick Change Smart Carby correctly, high rpm power may be increased.

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