Reprint from Precision Auto Research

Fuel tuning a race engine 

Any attempt to optimize engine performance is entirely dependant on having enough air, correct ignition and proper fueling. Easier said than done.

Engine Air Flow 

The volume of air flowing into any engine is determined by the inlet cross sectional area and the differential pressure between the chamber and ambient atmosphere. The inlet effective area is a function of induction tube diameter, flow control valves, valve curtain area, camshaft events and the system flow co-efficient.

As a piston moves downward, a low pressure zone is created in the cylinder. The differential pressure is the difference between ambient pressure and cylinder pressure.

The greater the pressure differential is, the greater the airflow. The square root of the pressure differential times the inlet minimum area times the flow co-efficient will produce a flow of air into the cylinder.

But air is a compressible fluid. It can also be stretched.  A rapidly moving piston displaces more air than can flow through the induction inlet, causing the air to stretch and become less dense. If the ambient pressure is low, less air will flow. If we pressurize or boost inlet Pressure, more air will flow.
The ability to breathe, called volumetric efficiency, is normally determined by engine design, applicable rules or a tuners cleverness.

Since all combustion processes are governed by the weight (not volume), of the air and the weight of the fuel, the density of air ingested and trapped in the cylinder become critical to performance and power. A change in inlet air density will necessitate a change in mixture and will result in a change in the amount of power that can be produced.

While the density of the air will determine the amount of fuel that will be required, air density will not accurately predict the change in engine power that is produced. Engine power is a function of both inlet air temperature and pressure. A power correction factor must be used which is not the same as a fuel correction curve.

The amount of water in the air is determined by relative humidity. Warm air can hold more water than cold air. The actual percentage of water vapor has little effect on the air/fuel ratio as long as it remains between .9 and 1.5%. Water vapor does, however have a significant effect on ignition timing which must be retarded in very dry air and advanced in very wet air.

Spark Ignition Process 

When a coil (or transformer or capacitor) is electrically charged, then suddenly discharged, a large voltage surge is formed, usually on the order of 10,000-15,000 volts (10-15kV). When a high voltage is sent to the gap of a spark plug, a spark will jump across the gap heating the air space to a temperature of approx 60,000 oF. An increase in firing voltage will not increase spark plug gap temperature but will increase spark duration and the gap size that is possible to fire.

The exact moment at which the ignition spark fires is critical to maximum performance. If  it fires too late, the peak combustion pressure will occur at an angle that is also too late, resulting in lower cylinder pressure and increased exhaust temperature. If it fires too early, the peak combustion pressure will occur before top center and cause pre-ignition and destruction of the piston, wrist pin, connecting rod or rod bearings, whichever comes first. And then the engine will lose power!

The ignition advance curve must be determined by engine speed, throttle position and load. As an engine increases in speed, the ignition timing must be advanced (retarded for two strokes). Partially closed throttles require more ignition advance. High load conditions require the advance to be retarded. Richer or leaner mixtures require advanced ignition timing. The actual ignition curve required is not a smooth curve but rather resembles the profile of a mountain range.                                                                                                                                                                                                                         At very high temperatures the spark plug air gap becomes ionized or electrically charged.

The O2 an N2 gas molecules are broken apart (called disassociation) to form O and N ions. Similarly, any carbon and hydrogen (HC) fuel is disassociated into carbon and hydrogen atoms and the combustion process begins. Free atoms of carbon and hydrogen combine with oxygen to form CO2 and H20 and give off heat. But the actual process is convoluted and non-linear, involving a series of complex chemical chain reactions.

The flame kernel is a tiny ball of flame equal in size to the spark plug gap and begins to grow as a mis-shapened wrinkled sphere. The flame kernel will lose combustion heat to the spark plug electrode tips. When the electrode tips are very hot, the flame kernel moves away from the spark plug gap and moves toward any available heat source.

This early ignition process takes some amount of time after the spark has fired. This time period is called the ignition delay period. It may last for as little as 3 crank degrees or much as 10 crank degrees. The ignition delay period can be modified by fuel preparation, fuel composition, spark plug gap and heat range.

The flame front is actually the surface area of a growing sphere and is very wrinkled in  appearance and is about 2-4mm thick. As the fireball grows, it leaves behind a zone of hot gases which expands outward and causes the fireball to grow larger. The expanding hot gases in the burned zone will increase the pressure in the unburned fuel zone. When the fireball touches or reaches the wall of the combustion chamber, piston or head surface, the flame will go out and combustion will cease at that point.
The flame travel speed (called laminar flame speed) is quite slow and would barely reach the edges of the combustion chamber before the exhaust valve opens. To speed up the burn rate and allow the engine to operate at higher speeds, a squish velocity is produced by the piston and cylinder head. As the piston approaches the head (approx. 10oBTC), the chamber volume becomes smaller, forcing the mixture to evacuate the region, pushing the unburned mixture towards the flame fireball.  This mixture velocity, combined with the  laminar flame speed, produces a much higher combined flame speed called turbulent velocity. Without high turbulent velocity, an engine could only run slowly and would not completely burn the air/fuel charge.

A problem with a slow burning charge is that the unburned mixture is prone to detonation. As the flame front expands, it heats and pressurizes the unburned fuel mixture ahead of it. When a volume of mixture reaches auto ignition temperature, the entire volume (called the end gas) will ignite spontaneously, much like an impatient mixture. Some fuels can resist the increased pressure and temperature of the end gas zone for a longer time. These fuels are referred to as having high octane. A high octane fuel does not burn slower but resists auto ignition for a longer period of time.

Fueling the flame  

Proper combustion requires the correct amount of fuel, the correct type of fuel and proper preparation of the fuel/air mixture. Ideally, there is a chemically correct ratio of air and fuel which will result in complete combustion without residual air or fuel. This ratio is called the stoichiometric ratio and is different for each fuel. The theoretical stoichiometric ratio for pump gas is aproximately 14.7. Remember that all chemical reactions are controlled by the weight of the reactants.

In this case, it means we will need 14.7 lbs of air in order to burn 1 lb of gasoline. Each fuel has its own stoichiometric ratio which, for gasoline, may range from 14.1 to 15.3.

All fuels have both a lean limit and a rich limit, beyond which the fuel will not burn. In actual engines, combustion is never ideal and some unburned fuel and unused oxygen always escape into the exhaust stream along with various emissions hydrocarbons and water.

An engine cannot burn a liquid; a fuel must be fully evaporated in order to burn. A poorly prepared fuel will present lean air to the ignition spark as well as contain liquid droplets.

Fuel liquids will pyrolyze or form carbon deposits without complete combustion. Carbon deposits on the piston crown can glow with an orange color inside a running engine since the carbon build-up prevents the proper dissipation of combustion heat. This extraneous heat source may easily cause pre-ignition to occur or at the very least, will accelerate the flame propagation process resulting in a combustion peak pressure too close to TDC.                                                                                                                                                                                                                                \

We hope the read will help you with the tuning process.