Rotary Car Club - View Single Post - Forced Induction for Dummies (Turbo, Supercharger, Nitrous)

Kane · 10-17-2008, 04:29 PM

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.

-----
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.

10-17-2008, 04:29 PM	#4
Kane All about the O2 Join Date: Sep 2008 Posts: 295 Rep Power: 17	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. ----- 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. __________________ My 2007 Turbocharged RX8