Forced Induction for Dummies (Turbo, Supercharger, Nitrous)

[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.

[Kane]

Turbocharger / Turbo / Snail – this raises the pressure of the air (boost) by using the exhaust gases exiting the engine to power a compressor that pressurizes the incoming air.
Supercharger / Blower – this raises the pressure of the air (boost) by using the eccentric shaft / crankshaft to power a compressor that pressurizes the incoming air.
Nitrous / Spray / Bottle – this raises the oxygen CONTENT or partial pressure by injecting nitrous oxide into the incoming air. A partial pressure of oxygen (pp O2) of .42 or 42% would be equal to the partial pressure of oxygen in air at 2 ATA, or roughly 14.7 PSI of boost. Oxygen at sea level is pp O2 = .21 or 21% of the total volume of air.

Turbo Advantages: can be installed anywhere in the exhaust stream. Usually smaller than a supercharger. Uses energy that is already being wasted anyway; low parasitic loss. Modern turbos can be highly reliable and provide boost at a wide range of engine speeds (RPM). Modular, you can build or buy some or all the parts to make your own system or get a kit. Boost can be increased to the system without mechanical changes.

Turbo Disadvantages: Can be difficult to install. Compressor is not linear meaning it does not provide a specific pressure at a certain RPM, may cause tuning difficulty. It is an obstruction in the exhaust stream lowering the flow of the exhaust gases and therefore lowering overall VE. Adds heat to the oil and or coolant (in order to cool the central housing of the turbo.

Supercharger Advantages: Can provide pressurized air as soon as the engine is running. May also provide a linear increase in pressure due to it being tied to the engine RPM. Tuning may be easier than a turbo.

Supercharger Disadvantages: Limited location area since it is physically powered by the eccentric / crankshaft of the engine. Uses some engine power to power compressor causing parasitic loss. Typically not modular. Boost cannot be increased without mechanical changes (decreasing the size of the pulley).

Nitrous Oxide Advantages: Inexpensive to purchase and easy to install. Does not cause any additional heat of the intake air (intake charge). Can be installed in numerous locations depending on the size of the bottle. Easier to maintain.

Nitrous Oxide Disadvantages: Must refill bottle. Typically only on during wide open throttle and or a button is pushed. To reach any significant power levels, some expense must be added for engine management.

Special note about temperature and air – when air is compressed regardless of how, it creates heat due to the friction of the air molecules being squished together. Intercoolers and other modifications have been invented to cool this charge air, but the reality is that 1 cubic foot of air compressed into 1⁄2 cubic foot area will become hotter, thus is the nature of gas. So both a supercharger and a turbo will create more heat than a naturally aspirated engine of the same size and design. This does not account for heating of the exhaust gas due higher fuel volume in the chamber, all FI will cause a hotter “burn”, this is a good thing when properly managed. See General Gas Law ^^^


Special note about exhaust temperature- all FI systems will raise exhaust temperatures due to the increase of air and fuel being burned in the combustion chamber. In most cases this is not a great concern. If you get an EGT or exhaust gas temperature monitor you can monitor changes from normal (assuming you know what your normal EGT is). You can also use EGT as a tuning aid.

Special note about fuel systems- Once you go FI, your max ability to boost will likely be limited by your ability to deliver fuel. You must work your FI system and fuel system together. If your FI application requires 30 lb/hr of fuel but you can only deliver 25 lb/hr, you will run lean and cause engine damage. Our injectors are typically rated in cc/min or lb/hr. The time it takes to deliver the rated fuel (the injector is open) is called duration. If a .44 lb/min injector needs to deliver .44 lb/min of fuel, it will be at 100% Duty Cycle AKA Maxed Out. Based on RPM, the duration available will be shorter in high RPMs and longer in low RPMs, all of this affects the total volume of fuel you can flow.


Special note about tuning - your non-factory FI-ed engine is designed to provide fuel in a variety of engine situations, none of which cover forced induction. With all of the extra air the engines “brain” will not add enough fuel causing you to run lean and destroy your engine. Therefore aftermarket forced induction kits have or will need an aftermarket computer to address this shortcoming. Setting this aftermarket computer up and refining the air fuel mixtures and timing is called tuning. All forced induction systems need to be tuned to your engine. If you car came factory with a turbocharger or supercharger; then tuning while still required if you make major VE changes - is usually better supported by the factory PCM.



Recommended Reading:
Street Turbocharging - Mark Warner
How to Tune and Modify Engine Management Systems - Jeff Hartmen
Turbocharging - Corky Bell
Engine Management: Advanced Tuning - Greg Banish

[Kane]

Reliability of Forced Induction Intro - Any FI will place more heat and more pressure on an engine than its naturally aspirated twin. However, Rotary engines are as FI-able as piston engines, with some special considerations. All FI-ed engines need more maintenance than a NA engine, but with proper maintenance can provide many trouble free miles. Ultimately, FI will shorten the life of your engine to some degree.

Static vs Dynamic Compression

"Hey, I need these lower compression pistons so I can make REAL power; 26 PSI here I come." - ORLY?

Ok one of the most common misconceptions about engines is that higher compression motors are somehow weaker and more prone to detonation than lower compression motors. The fact of the matter is that they are; when given the same intake air pressure. But that overlooks the differences in static and dynamic compression.

Static compression is the fixed pressure change of the combustion chamber by the movement of the piston up to top dead center (fixed displacement). Based on the volume at Bottom Dead Center and Top Dead Center - you have a fixed compression ratio.

Dynamic compression refers to the total change of pressure from intake through Top Dead Center; or some people just think of it in terms of the Compression of the Air Intake. For this article I am using it to describe the total pressure change (intake and static compression).

So if my 10:1 compression motor is ingesting air at 10 PSI; then at TDC I am housing 100 PSI of air molecules (all things being equal); if my 9:1 motor is ingesting 11 PSI of air; then at TDC I am housing 100 PSI of air molecules. So which one will make more power?

If you say both are equal (in our simplified example); then you are mostly right. The bigger differences are timing / thermoefficiency and the physical constraints of the motor. IE the higher the compression ratio the more flame speed you will lose to crevice flow (the air around the spark plug threads and rings etc); since it is a higher percentage of the combustion chamber volume than on a lower compression motor. And in extreme builds - the physical limitations of the engine size, turbo and intake tract will come to bear; and finally the thermoefficiency changes as you are bringing in cool - HP air (assuming a properly sized intercooler) vs compressing a larger volume of air in a fixed compression (makes air nice and warm)... but I leave those items at the higher purpose built race car level.

But in a Street Build - flow is king! So don't worry so much about pressure - worry about flow; and tune for safety around pressure as required.

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Timing
Timing is the single most important element of a properly tuned engine. Since the hot rod days; performance enthusiasts and automakers alike have been in pursuit of the perfect instance to initiate the spark event in order to achieve the maximum cylinder pressure at a known and specified point to make the most torque - aka Mechanical Advantage (typically 12 degrees After Top Dead Center for a piston motor) or to be a little safer/later in order to avoid detonation (retarding timing). This is of paramount importance to safe and effective power.

The proposed method of today’s dyno tuning (think Jeff Hartmen - How to Tune and Modify EFI Engines) is to over-retard timing and slowly advance it until you begin to see small signs of detonation; and then back it off a small bit for safety. In simple terms think of it this way; the MOST advanced timing (max cylinder pressure closest to TDC) that you can obtain without detonation will be the most amount of power that can be obtained with that particular fuel. Obviously, pre-ignition and detonation are serious concerns with drastic consequences but the idea is to be as advanced as you can.

Another method is to use a pyrometer and work the timing until you see the coolest EGT's while still making good power. If you start out over reterded the EGT will be very high as most of the burn happens outside of the motor; as you advance it will cool the EGT's until you start to get some detonation - which will super heat the motor and cause the EGT's to rise again.

Here is how the science works

In order to determine the proper timing for an engine; you have to know how fast the fuel propagates in the combustion chamber. In Explosives; this is referred to as Brisance; and the value is a known entity - so TNT always burns at the same rate. In the automotive realm; our little controlled fuel-air explosion is often measured by referring to the flame speed, and there are two separate values; turbulent and laminar. Once the spark ignites there is a flame front that moves very quickly and purns the skin f the fuel drop (I think of it as a shockwave) this is the Laminar flame speed. Then the molecules that are unburned inside the Laminar bubble begin to be consumed; this slower but more powerful burn is the Turbulent flame (and the one more responsible for power). So, lets simplify and think of it as two separate events (two different Brisance numbers).


In theory, if you were to know these two flame speeds, and you knew the volume in the chamber that was available along with the cylinder geometry (rotors are not round after all) and spark plug location; you can determine quite readily how long from ignition until max cylinder pressure is achieved - then subtract that time from your goal of say… 12 Degrees ATDC and bang; your done.

In reality, the volume of the cylinder is always changing; so we have to think of it as a phased event:
Phase 1: After spark, the mixture is still being compressed (since ignition is often initiated BTDC. So flame speeds are X1 and Y1. This speed actually changes for each change in volume or each rotation of the crank. Remember, the engine is moving all the time.
Phase 2: At TDC; the flame speed is X2 and Y2 based on our maximum mechanical compression.
Phase 3: After TDC; the flame speed is X3 and Y3; as the turbulent flame increases exponentially; and the volume increases for each rotation of the crank.
The combination of these three sets of numbers are computed into derivatives in order see the effect of our time-lapse on our flame propagation / heat / volume changes. Based on this information we can calculate the number of milliseconds from spark to maximum cylinder pressure. With the RPM and our time-lapse information we can then calculate the number of degrees of rotation required as Z; subtract that from your goal timing of (in our example) 12 Degrees ATDC and therefore initiate timing 12-Z degrees.

This is ROUGHLY the same models that are used in closed loop driving on your OEM engine; coupled with a knock sensor check - in order to verify that the math is meeting real world expectations.

The feedback (knock sensor) is important for a lot of reasons; one being fuel makeup, Air Fuel Ratio as well as PPO2 - as these things change this has a dramatic effect on the flame speed. But OEMs cannot put the extra strain on Joe driver to know those values… But we Can!!! Well, some of them anyway.


Some interesting discoveries -
-There are over 50 different fuel blends in the United States alone; while octane is important; the molecular make up of each separate blend has an effect. Additionally, even the fuel manufacturers have variations allowed; we are compiling all of the MSDS sheets for each fuel type and brand - and even a single one has a range of ingredients. We are working the math to see how much variation changes the timing models and the method to add it to the software and or ignore it. Worse case; you will select the State and or Zipcode and Brand/Octane of choice for your timing map needs. Or you can just set your timing map a little more safe (aim for say 16 Degrees ATDC).
-There is NO WAY to properly calculate timing unless you know your Bore and Stroke. This information can be easily found for your engine; but you’ll have to input into your engine profile in order to get the proper timing map out of it.
-If your EMS can read a knock sensor and log that; then we can also use it in the same manner as an OEM closed loop EMS and adjust accordingly.
Lastly, the heat of the cylinder or rotor walls is of importance - we are still working out the best way to approximate this value.


As you can see; we are still working out some of the details and what if any effect this has on timing (some may not even be in the realm of adjustability so we’ll ignore it).


Theoretical investigation of flame propagation process in an SI engine running on
gasoline–ethanol blend
Hakan Bayraktar, Mechanical Engineering Department, Faculty of Engineering, Karadeniz Technical University, Turkey

A Simulation Model for a Single Cylinder Four-Stroke Spark
Ignition Engine Fueled with Alternative Fuels
Maher A. R. Sadiq AL-BAGHDADI
Mechanical and Energy Department, Higher Institute of Mechanical Engineering,
Yefren-LIBYA

Internal Combustion Engine Fundamentals.
Heywood, J.B.
Massachusetts Institute of Technology
McGraw-Hill, 1989.



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

[Kane]

Just thought I would add this here (in case I end up moving here for good) - it is a work in progress; so correct stuff you see and I'll add to it as I get time.

[PercentSevenC]

Originally Posted by Kane

Supercharger Disadvantages: ...Boost cannot be increased without mechanical changes.

I would clarify this part. A increasing boost requires decreasing pulley size.

[Kane]

Dones - thanks for the help.

[Kane]

More information can be found at www.ppo2performance.com

[Signal 2]

Originally Posted by Kane

...... If you car came factory with a turbocharger or supercharger; then tuning while still required if you make major VE changes - is usually better supported by the factory PCM.

^I'm not sure if I'm understanding you correctly...but I think I disagree with this.

Also, I may have missed it...but mention of the differences between a MAP and MAF system might be helpful.

[Kane]

The Factory FI-ed engine PCM already has in it; options for Boost Controller / PSI based enrichment, boost cut, fuel cut etc. As such an option like the PFC for the FD; or a Flash Tuner is available as a plug and play solution. In the case of a non-FI-ed car that was FI-ed - those sensors and therefore the tuning options do not exist. Make better sense?

I will definitely add a MAF vs MAP section. Good Call.

[Signal 2]

edit

[Signal 2]

Originally Posted by Kane

Make better sense?

Yes, thanks. I'm not a tuner, but thinking that might be particularly true if modifying a NA MAF engine to a FI MAF engine since has at least something already in place to help account for the added air.

[Kane]

NA MAF to FI MAF; is one of the harder processes to do IMO.

But I think the primary reason for that is the "newness" of MAF based Tunable EMS's - which are basically Flash Tools like the Cobb AP.

The upside is that since MAF = MASS of air; you don't have to account for heat from FI for fueling purposes a.k.a. Loss of PPO2 due to heat (you do for timing obviously; as a super hot intake charge will be more likely to detonate).

I am pretty darn sure that MOST of the factory boosted EMS's (MS6, MS3, EVO, STI) - have both a MAF and MAP; which would a really good way to tune; hence why the Oem's use them both.

[Signal 2]

Originally Posted by Kane

NA MAF to FI MAF; is one of the harder processes to do IMO.

I'm not arguing, just surprised. That's the reverse of what I would have thought. Since MAF could already (to some extent) account and adjust for the added flow from boost, I figured that an OEM MAF would be easier to mod to FI. I thought a MAP system would be pretty much fixed based on throttle position, temperature and intake pressure but only up to atmospheric. So you'd need to start all over to move to FI. In any event it's been fun to try to think it through.

[Kane]

You are right if you have a 1 BAR MAP - but MAF scaling can be tough too.

You bring up some really good points - I will have to come up with a good write up on MAF vs MAP...like you already mentioned. I gotta stop slackin and get back to work

[CyberPitz]

Pretty awesome write-up! I read through that entire thing, and though I knew a lot of it...still a great read, and I learned a couple of things.

Thanks a ton!