Forced Induction for Dummies (Turbo, Supercharger, Nitrous)

[Whizbang]

First note: This is the tip of a VERY VERY VERY VERY VERY big iceberg. If you are thinking of going FI, DO YOUR HOMEWORK! Laziness now can cost you an engine.

I noticed that there is no easy to find single document that clearly and easily describes this set of mods in plain English for people who have no idea other than “make more power, me want”. So, I thought I would make one.


Terms you Should Know
General Gas Law – In any gas (liquid is a gas or gas is a liquid and they are both fluids; you pick), the combination of volume, pressure and temperature are related. A change in any one will affect a corresponding change in the others. Pressure and Volume are inversely related (volume up, pressure down), Volume and Temperature are inversely related (Volume Up, Temperature Down) and Pressure and Temperature are proportional (Pressure Up, Temperature Up).


Volumetric Efficiency – The amount of air the engine is ingesting divided by the amount of air the engine is capable of ingesting at Atmospheric Pressure (14.7 PSIA). More simply air volume ingested/engine displacement.


Engine Cycle – One power stroke / engine revolution. RPM is (Revolutions per Minute). This is when the piston / rotor moves from Top Dead Center to Bottom Dead Center and back to Top Dead Center. For each Engine Rotation (RPM), the rotor/ piston leaves Top Dead Center (TDC) and moves to Bottom Dead Center (BDC), this movement creates vacuum as the volume of the space increases, this vacuum causes the Atmospheric Pressure to push air from the intake track into the engine in order to equalize the pressure differential. The perfect volume of air ingested by the engine is equal to the volume of the engine (displacement). Once the air is ingested, it is compressed by the reduction of volume as the rotor / piston travels from BDC to TDC. Along with fuel, this compressed mixture is ignited by the spark plug(s) creating a pressure expansion which drives the rotor / piston from TDC back to BDC rotating the eccentric shaft / crankshaft. Then exhaust is forced out the engine by movement from BDC back to TDC and the whole thing starts over again. This cycle is not perfect as restrictions on the intake, overlapping valves, mixtures of the outgoing / incoming gases and time to equalize the pressure differential all affect how much air the engine actually gets. The driving force of engine air movement is pressure gradients, the difference in pressure.

Tuning – Engine Tuning is the process of specifying the proper amount of fuel and the proper time to induce ignition based on the specific spot (load point, or fuel map cell) your engine is currently in. The engines Brain (or aftermarket Brain; or both) read from the sensors what your engine is doing. Based on the air, temperature, RPM, pressure, air density, throttle position, RPM change…yada yada yada; put in this amount of fuel and ignite the spark at this time. There are so many specific tuning “things” to deal with and know about, for now just know that each engine is different and tuning for FI will almost always have to be done.


Flame Speed / Brisance – Fuel is an explosive (kinda) and an engine produces a controlled explosion (kinda). The term Brisance is used to reference the shattering effect of an explosion. This can also be thought of as the speed in which the potential energy is converted to kinetic energy. In demolitions, we convert Brisance into Relative Effectiveness (RE) of an explosive, with TNT = 1. So C4, RDX and other High Explosives have an RE greater than 1, and Ammonium Nitrate and other Low Explosives have an RE less than 1. Low Explosives PUSH and High Explosives SHATTER. While TNT has a detonation velocity of 6,940 Meters Per Second a Stoichiometric Air Fuel Mixture has a flame speed of .34 Meters Per Second or an RE less than .0001. So Fuel is a LOW LOW EXPLOSIVE which PUSHES. The point to this is the higher the oxygen content the faster the flame speed (more push), this is also why timing is retarded during FI applications.


Partial Pressure of Oxygen (PPO2) - The master of all things “power” in your engine. In the end, what really matters is the number of oxygen molecules in the combustion chamber (provided you have enough fuel to use it.) With an increase in pressure, the number of molecules goes up due to squeezing more air in a smaller space. The term Partial Pressure is in reference to the pressure of the Oxygen as a percentage of total volume. So, in air at sea level the Partial Pressure is .21 or 21% of total volume at 1 Atmosphere. If you boost to 14.7 PSI, or 2 Atmospheres, the partial pressure is going to double to .42, since you have twice as many oxygen molecules per volume. This “extra” oxygen is what provides the extra power and why Forced Induction can make a small engine perform like a much larger one. You see, you do not need extra PRESSURE to make more power, you need more PPO2. You can get it from pressurizing the intake, or from increasing the Oxygen content in the engine (vis a vis Nitrous). Remember the General Gas Law, since you will heat the mixture while you pressurize it, 14.7 PSI of boost is not exactly .42 PPO2 since it would heat up and become less dense. The fine details are more complicated, but the theory is - plan for, fuel for, tune for, EVERYTHING for PPO2, that is absolute unlike other strategies.


Pressure Gradient - Picture that old Jr. High Science Class Exp. No, not the girl who first let you touch her boob, focus! The one where you have two cups of liquid and a hose between them, lift one cup and the liquid will flow into the other (lower) cup. This is a good demonstration of a pressure gradient. Simply put, air is going to flow from the higher pressure, into the lower pressure. The higher the “high” pressure / the lower the “low” pressure are, affects the velocity of that air movement. So in your engine cycle, the intake pressure is higher which moves the air into the cylinder. After combustion the expanded exhaust gas pressure is higher than the pressure in the exhaust so the air moves into the exhaust. In boosted applications, this air is moving faster due to an increased pressure gradient.


Mixed Gas - Technically “Air” is the mixture of 21% oxygen, and 79% nitrogen (I know, but for simplicity). When you alter that ratio, it is no longer air; it is classified as a MIXED GAS. This applies to storage and handling requirements as well as HAZMAT classification (more oxygen the more flammable the gas). Plus this is an important distinction for divers. So while Turbo and Superchargers deal with air, Nitrous deals with a mixed gas.


Volumetric Efficiency - the amount of power the engine can produce based on the amount of air it can get for each engine cycle. Technically, volumetric efficiency only relates to air flow, however for the sake of simplicity assume; more air + more fuel = more power, therefore volumetric efficiency can be directly related to power. Called VE for short, the goal of most engine mods is to increase VE, with the rest being designed to reduce loss of power by drag, heat or other variables. At ambient air pressure VE can approach 100%, it can even be slightly over 100% in some highly designed racing engines; but for all intensive purposes the goal is a VE of (1 or 100%); and naturally aspirated motors can’t get there. An engine with manifold pressures greater than atmospheric will put a larger amount of air into the chamber than what atmospheric pressure would do and, therefore, have volumetric efficiencies greater than 100%.


Forced Induction - this is the term that applies to adding pressurized *air to make it contain more oxygen molecules per space in the engine. This means you can burn more fuel. By adding more air and more fuel you raise the combustion pressure on the eccentric shaft/ crankshaft which makes it turn harder (torque) and makes more horsepower (torque x engine RPM). Since all engine modifications (mods for short) attempt to increase the volumetric efficiency of the engine; forced induction is attractive as it can achieve a VE in excess of 1 quite readily. The saying “there is no replacement for displacement” should be “there is no replacement for VE”.

* Special note about nitrous, NO2, NOS, ZEX, whatever. It raises the partial pressure of O2 (higher oxygen content, not the pressure) so it is not technically air anymore. But they all do the same thing; allow more fuel to be burned.

[Kane]

First note: This is the tip of a VERY VERY VERY VERY VERY big iceberg. If you are thinking of going FI, DO YOUR HOMEWORK! Laziness now can cost you an engine.

I noticed that there is no easy to find single document that clearly and easily describes this set of mods in plain English for people who have no idea other than “make more power, me want”. So, I thought I would make one.


Terms you Should Know
General Gas Law – In any gas (liquid is a gas or gas is a liquid and they are both fluids; you pick), the combination of volume, pressure and temperature are related. A change in any one will affect a corresponding change in the others. Pressure and Volume are inversely related (volume up, pressure down), Volume and Temperature are inversely related (Volume Up, Temperature Down) and Pressure and Temperature are proportional (Pressure Up, Temperature Up).


Volumetric Efficiency – The amount of air the engine is ingesting divided by the amount of air the engine is capable of ingesting at Atmospheric Pressure (14.7 PSIA). More simply air volume ingested/engine displacement.


Engine Cycle – One power stroke / engine revolution. RPM is (Revolutions per Minute). This is when the piston / rotor moves from Top Dead Center to Bottom Dead Center and back to Top Dead Center. For each Engine Rotation (RPM), the rotor/ piston leaves Top Dead Center (TDC) and moves to Bottom Dead Center (BDC), this movement creates vacuum as the volume of the space increases, this vacuum causes the Atmospheric Pressure to push air from the intake track into the engine in order to equalize the pressure differential. The perfect volume of air ingested by the engine is equal to the volume of the engine (displacement). Once the air is ingested, it is compressed by the reduction of volume as the rotor / piston travels from BDC to TDC. Along with fuel, this compressed mixture is ignited by the spark plug(s) creating a pressure expansion which drives the rotor / piston from TDC back to BDC rotating the eccentric shaft / crankshaft. Then exhaust is forced out the engine by movement from BDC back to TDC and the whole thing starts over again. This cycle is not perfect as restrictions on the intake, overlapping valves, mixtures of the outgoing / incoming gases and time to equalize the pressure differential all affect how much air the engine actually gets. The driving force of engine air movement is pressure gradients, the difference in pressure.

Tuning – Engine Tuning is the process of specifying the proper amount of fuel and the proper time to induce ignition based on the specific spot (load point, or fuel map cell) your engine is currently in. The engines Brain (or aftermarket Brain; or both) read from the sensors what your engine is doing. Based on the air, temperature, RPM, pressure, air density, throttle position, RPM change…yada yada yada; put in this amount of fuel and ignite the spark at this time. There are so many specific tuning “things” to deal with and know about, for now just know that each engine is different and tuning for FI will almost always have to be done.


Flame Speed / Brisance – Fuel is an explosive (kinda) and an engine produces a controlled explosion (kinda). The term Brisance is used to reference the shattering effect of an explosion. This can also be thought of as the speed in which the potential energy is converted to kinetic energy. In demolitions, we convert Brisance into Relative Effectiveness (RE) of an explosive, with TNT = 1. So C4, RDX and other High Explosives have an RE greater than 1, and Ammonium Nitrate and other Low Explosives have an RE less than 1. Low Explosives PUSH and High Explosives SHATTER. While TNT has a detonation velocity of 6,940 Meters Per Second a Stoichiometric Air Fuel Mixture has a flame speed of .34 Meters Per Second or an RE less than .0001. So Fuel is a LOW LOW EXPLOSIVE which PUSHES. The point to this is the higher the oxygen content the faster the flame speed (more push), this is also why timing is retarded during FI applications.


Partial Pressure of Oxygen (PPO2) - The master of all things “power” in your engine. In the end, what really matters is the number of oxygen molecules in the combustion chamber (provided you have enough fuel to use it.) With an increase in pressure, the number of molecules goes up due to squeezing more air in a smaller space. The term Partial Pressure is in reference to the pressure of the Oxygen as a percentage of total volume. So, in air at sea level the Partial Pressure is .21 or 21% of total volume at 1 Atmosphere. If you boost to 14.7 PSI, or 2 Atmospheres, the partial pressure is going to double to .42, since you have twice as many oxygen molecules per volume. This “extra” oxygen is what provides the extra power and why Forced Induction can make a small engine perform like a much larger one. You see, you do not need extra PRESSURE to make more power, you need more PPO2. You can get it from pressurizing the intake, or from increasing the Oxygen content in the engine (vis a vis Nitrous). Remember the General Gas Law, since you will heat the mixture while you pressurize it, 14.7 PSI of boost is not exactly .42 PPO2 since it would heat up and become less dense. The fine details are more complicated, but the theory is - plan for, fuel for, tune for, EVERYTHING for PPO2, that is absolute unlike other strategies.


Pressure Gradient - Picture that old Jr. High Science Class Exp. No, not the girl who first let you touch her boob, focus! The one where you have two cups of liquid and a hose between them, lift one cup and the liquid will flow into the other (lower) cup. This is a good demonstration of a pressure gradient. Simply put, air is going to flow from the higher pressure, into the lower pressure. The higher the “high” pressure / the lower the “low” pressure are, affects the velocity of that air movement. So in your engine cycle, the intake pressure is higher which moves the air into the cylinder. After combustion the expanded exhaust gas pressure is higher than the pressure in the exhaust so the air moves into the exhaust. In boosted applications, this air is moving faster due to an increased pressure gradient.


Mixed Gas - Technically “Air” is the mixture of 21% oxygen, and 79% nitrogen (I know, but for simplicity). When you alter that ratio, it is no longer air; it is classified as a MIXED GAS. This applies to storage and handling requirements as well as HAZMAT classification (more oxygen the more flammable the gas). Plus this is an important distinction for divers. So while Turbo and Superchargers deal with air, Nitrous deals with a mixed gas.


Volumetric Efficiency - the amount of power the engine can produce based on the amount of air it can get for each engine cycle. Technically, volumetric efficiency only relates to air flow, however for the sake of simplicity assume; more air + more fuel = more power, therefore volumetric efficiency can be directly related to power. Called VE for short, the goal of most engine mods is to increase VE, with the rest being designed to reduce loss of power by drag, heat or other variables. At ambient air pressure VE can approach 100%, it can even be slightly over 100% in some highly designed racing engines; but for all intensive purposes the goal is a VE of (1 or 100%); and naturally aspirated motors can’t get there. An engine with manifold pressures greater than atmospheric will put a larger amount of air into the chamber than what atmospheric pressure would do and, therefore, have volumetric efficiencies greater than 100%.


Forced Induction - this is the term that applies to adding pressurized *air to make it contain more oxygen molecules per space in the engine. This means you can burn more fuel. By adding more air and more fuel you raise the combustion pressure on the eccentric shaft/ crankshaft which makes it turn harder (torque) and makes more horsepower (torque x engine RPM). Since all engine modifications (mods for short) attempt to increase the volumetric efficiency of the engine; forced induction is attractive as it can achieve a VE in excess of 1 quite readily. The saying “there is no replacement for displacement” should be “there is no replacement for VE”.

* Special note about nitrous, NO2, NOS, ZEX, whatever. It raises the partial pressure of O2 (higher oxygen content, not the pressure) so it is not technically air anymore. But they all do the same thing; allow more fuel to be burned.