Engine Tuning Dictionary

[Captain Moai]

Courtesy of Shiv Pathak's Engine Tuning Dictonary
http://www.vishnutuning.com/tuning_dictionary.htm

Read and Learn.


**********************************************

Knock Threshold: (Noun) The discrete point at which controlled combustion/burn is replaced by spurious detonation. Often, modern turbocharged cars, equipped with knock sensors, operate under constantly adapting ignition advance curves which keep the engine operating just below the knock threshold.

MBT: (Noun) Minimum ignition advance for Best Torque output. Any less ignition advance yields less engine torque. Any more ignition advance either reduces or maintains engine torque. While running more ignition advance often yields no performance compromises, it does result in much greater internal engine stress due to a dramatic rise in peak cylinder pressures. Needless to say, engine tuners should aim to operate all engines at MBT. However, in light of common octane limitations, MBT may be impossible to reach safely. In such case, engine tuners should tune engines to operate a few degrees of ignition advance shy of the knock threshold. These few degrees of ignition advance is one component of an engine’s safety margin.

EGT: (Noun) Exhaust Gas Temperature. Monitoring this temperature provides a very good indication of an engine’s thermal efficiency. General rule of thumb indicates that the higher the temperature, the lower the engine’s thermal efficiency. However, low exhaust gas temperatures don’t always indicate high levels of thermal efficiency.

Thermal Efficiency: (Noun) The measurement of the amount of combustion energy used to push downwards upon the pistons versus the amount of energy transferred into the cooling system and into the exhaust. Typical reciprocating, internal combustion (Otto Cycle) engines operate with a Thermal Efficiency rating of 30-40%. This means that only 30-40% of the total combustion energy is used to make the car go fast. Thermal efficiency is highly sensitive and almost directly proportional to ignition advance.

Reversion: (Noun) An exhaust gas behavioral phenomenon that occurs in conditions where localized exhaust back-pressure is equal to or higher than the pressure exerted by the closing piston during the exhaust stroke. During exhaust gas reversion, exhaust gases sneak back into the combustion chamber during valve overlap (the short period in which both intake and exhaust valves are open). This results in elevated in-cylinder temperatures and a dramatic reduction in an engine’s volumetric efficiency.

Knock Hysteresis: (Noun) The tendency for knock to persist stubbornly once induced. This means that quelling existing knock requires more ignition retard than preventing it in the first place.

Knock: (Noun) Also known as detonation, rattle or ping. Represents an uncontrolled combustion event resulting in the creation of ultra-high in-cylinder pressures and temperatures. Whereas control combustion is represented by a nice, smooth burn, knock is represented by a sudden, destructive explosion. Engines are not designed to withstand sustained knock. Even the most heavy-reinforced engines will fail under such conditions. Typical failure modes include cracked pistons lands, impaired piston rings, damaged head gaskets, bent rods and, at times, shattered pistons! Knock is, by far, the leading cause of catastrophic engine failure.

Death Spiral: (Noun) A conditions which usually occurs in race cars that run too lean with too little ignition advance. While the lack of ignition advance means that detonation can’t be induced conventionally, the ultra-high in-cylinder temperatures can induce detonation due to the creation of hot spots. And even if detonation was not induced, the very high temperatures and lack of chemical cooling can actually begin to actually melt aluminum engine components.

Ignition Advance: (Noun) The period of time, measured in degrees of rotation, before the piston reaches TDC (Top Deal Center) of the compression stroke, in which the combustive spark is initiated. The spark must be initiated some time before the TDC so that, by the time the flame front propagates, maximum air/fuel charge expansion occurs during the power stroke. Too little ignition advance means that maximum air/fuel charge expansion occurs too late in the power stroke. This results in low engine torque, poor thermal efficiency and, as a result, high exhaust gas temperatures. Too much ignition advance results in too-high cylinder pressures caused by maximum expansion occurring too early in the power stroke. This can, very quickly, result in destructive engine knock.

Enrichment: (Noun) Additional short-term fueling necessitated by a sudden change of engine load, unusually cool intake charge temperatures, a reduction of gasoline octane, etc,. Fuel enrichments are often triggered by more than an external variable in an attempt to yield safe operating conditions.

Hot Spot: (Noun) Localized areas in the combustion chamber which retain combustion heat. These spots can, under certain conditions, accumulate so much heat that engine knock can be induced. Often the roughest portions of the combustion chamber become hot spots.

Enleanment: (Noun) A short-term reduction of fueling necessitated by a sudden decrease in engine load, an increase in gasoline octane, hotter intake charge temperatures, etc,. Like enrichments, enleanments are often triggered by more than one external variable.

Chemical Cooling: (Noun) The process in which excess fuel is used to cool the hot intake charge through the process of in-cylinder vaporization. In water-injected applications, water can also take part in the chemical cooling process.

Charge Cooling: (Noun) The process in which intake charges are cooled through the use of an intercooler or a water injection system. Charge cooling enhances knock resistance which yields improved engine torque through the use of more ignition advance. Charge cooling also yields a denser air/fuel charge which means that more oxygen can be combusted per each engine cycle. Charge cooling, as a result, improves an engine’s volumetric efficiency.

Lean: (adj) Describes a condition when there is more than 14.6 parts of air to each 1 part of fuel. For example, an air/fuel ratio of 15:1 represents a lean-run condition.

Rich: (adj) Describes a condition when there is less than 14.6 parts of air to each 1 part of fuel. For example, an air/fuel ratio of 14:1 represents a rich-run condition.

Stoichiometric: (adj) Describes a condition when there is exactly 14.6 parts of air to each 1 part of fuel. Also referred to as “stoich”.