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Turbo Tech: Compression Ratio with Boost

Friday, May 4th, 2012

Courtesy of TurbobyGarrett.com ( http://turbobygarrett.com/turbobygarrett/compression_ratio_with_boost )

Before discussing compression ratio and boost, it is important to understand engine knock, also known as detonation. Knock is a dangerous condition caused by uncontrolled combustion of the air/fuel mixture. This abnormal combustion causes rapid spikes in cylinder pressure which can result in engine damage.

Three primary factors that influence engine knock are:
  • Knock resistance characteristics (knock limit) of the engine: Since every engine is vastly different when it comes to knock resistance, there is no single answer to “how much.” Design features such as combustion chamber geometry, spark plug location, bore size and compression ratio all affect the knock characteristics of an engine.
  • Ambient air conditions:   For the turbocharger application, both ambient air conditions and engine inlet conditions affect maximum boost. Hot air and high cylinder pressure increases the tendency of an engine to knock. When an engine is boosted, the intake air temperature increases, thus increasing the tendency to knock. Charge air cooling (e.g. an intercooler) addresses this concern by cooling the compressed air produced by the turbocharger
  • Octane rating of the fuel being used: octane is a measure of a fuel’s ability to resist knock. The octane rating for pump gas ranges from 85 to 94, while racing fuel would be well above 100. The higher the octane rating of the fuel, the more resistant to knock. Since knock can be damaging to an engine, it is important to use fuel of sufficient octane for the application. Generally speaking, the more boost run, the higher the octane requirement.

This cannot be overstated: engine calibration of fuel and spark plays an enormous role in dictating knock behavior of an engine. See Section 5 below for more details.

Now that we have introduced knock/detonation, contributing factors and ways to decrease the likelihood of detonation, let’s talk about compression ratio. Compression ratio is defined as:

or


where                               CR  = compression ratio                               Vd  = displacement volume                               Vcv = clearance volume

The compression ratio from the factory will be different for naturally aspirated engines and boosted engines. For example, a stock Honda S2000 has a compression ratio of 11.1:1, whereas a turbocharged Subaru Impreza WRX has a compression ratio of 8.0:1.

There are numerous factors that affect the maximum allowable compression ratio. There is no single correct answer for every application. Generally, compression ratio should be set as high as feasible without encountering detonation at the maximum load condition. Compression ratio that is too low will result in an engine that is a bit sluggish in off-boost operation. However, if it is too high this can lead to serious knock-related engine problems.

Factors that influence the compression ratio include: fuel anti-knock properties (octane rating), boost pressure, intake air temperature, combustion chamber design, ignition timing, valve events, and exhaust backpressure. Many modern normally-aspirated engines have well-designed combustion chambers that, with appropriate tuning, will allow modest boost levels with no change to compression ratio. For higher power targets with more boost, compression ratio should be adjusted to compensate.

There are a handful of ways to reduce compression ratio, some better than others. Least desirable is adding a spacer between the block and the head. These spacers reduce the amount a “quench” designed into an engine’s combustion chambers, and can alter cam timing as well. Spacers are, however, relatively simple and inexpensive.

A better option, if more expensive and time-consuming to install, is to use lower-compression pistons. These will have no adverse effects on cam timing or the head’s ability to seal, and allow proper quench regions in the combustion chambers.

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OEM Acura RSX K20 Shop Manual — Pass it On.

Thursday, April 19th, 2012

The OEM Shop Manual is always an excellent resource to have around.  For everyone elses good fortune someone has graciously uploaded the entire 2002-2003 RSX Shop Manual Online.

This is very handy for torque specs, fastening sequences and a heck of a lot more.

Here is a direct link to open or download it at your discretion FREE of charge.

Feel free to pass it on and let others know about this great tool as well.

Right Click, Save As — http://IPGParts.com/AcuraRSXShopManualK20.pdf

 

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Turbo Tech: Log Manifold vs Tubular Manifold

Thursday, March 22nd, 2012

More Great Turbo Tech from www.TurbobyGarrett.com

There are two different types of turbocharger manifolds; cast log style (see Figure 3.) and welded tubular style (see Figure 4.).

Figure 3. Cast log style turbocharger manifold

Figure 4. Welded tubular turbocharger manifold

 

Manifold design on turbocharged applications is deceptively complex as there many factors to take into account and trade off .

General design tips for best overall performance are to:

  •   Maximize the radius of the bends that make up the exhaust primaries to maintain pulse energy
  •   Make the exhaust primaries equal length to balance exhaust reversion across all cylinders
  •   Avoid rapid area changes to maintain pulse energy to the turbine
  •    At the collector, introduce flow from all runners at a narrow angle to minimize “turning” of the flow in the collector
  •    For better boost response, minimize the exhaust volume between the exhaust ports and the turbine inlet
  •    For best power, tuned primary lengths can be used

Cast manifolds are commonly found on OEM applications, whereas welded tubular manifolds are found almost exclusively on aftermarket and race applications. Both manifold types have their advantages and disadvantages. Cast manifolds are generally very durable and are usually dedicated to one application. They require special tooling for the casting and machining of specific features on the manifold. This tooling can be expensive.

On the other hand, welded tubular manifolds can be custom-made for a specific application without special tooling requirements. The manufacturer typically cuts pre-bent steel U-bends into the desired geometry and then welds all of the components together. Welded tubular manifolds are a very effective solution. One item of note is durability of this design. Because of the welded joints, thinner wall sections, and reduced stiffness, these types of manifolds are often susceptible to cracking due to thermal expansion/contraction and vibration. Properly constructed tubular manifolds can last a long time, however. In addition, tubular manifolds can offer a substantial performance advantage over a log-type manifold.

A design feature that can be common to both manifold types is a ” DIVIDED MANIFOLD” , typically employed with ” DIVIDED ” or “twin-scroll” turbine housings. Divided exhaust manifolds can be incorporated into either a cast or welded tubular manifolds (see Figure 5. and Figure 6.).

The concept is to DIVIDE or separate the cylinders whose cycles interfere with one another to best utilize the engine’s exhaust pulse energy.

For example, on a four-cylinder engine with firing order 1-3-4-2, cylinder #1 is ending its expansion stroke and opening its exhaust valve while cylinder #2 still has its exhaust valve open (cylinder #2 is in its overlap period). In an undivided exhaust manifold, this pressure pulse from cylinder #1′s exhaust blowdown event is much more likely to contaminate cylinder #2 with high pressure exhaust gas. Not only does this hurt cylinder #2′s ability to breathe properly, but this pulse energy would have been better utilized in the turbine.

The proper grouping for this engine is to keep complementary cylinders grouped together– #1 and #4 are complementary; as are cylinders #2 and #3. Because of the better utilization of the exhaust pulse energy, the turbine’s performance is improved and boost increases more quickly.

Figure 5. Cast manifold with a divided turbine inlet design feature

Figure 6. Welded tubular manifold with a divided turbine inlet design feature

Figure 7. Illustration of divided turbine housing

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Turbo Tech — Turbine Housing A/R and Sizing

Monday, March 5th, 2012

Courtesy of www.TurbobyGarrett.com

A/R (Area/Radius) describes a geometric characteristic of all compressor and turbine housings. Technically, it is defined as:

the inlet (or, for compressor housings, the discharge) cross-sectional area divided by the radius from the turbo centerline to the centroid of that area (see Figure 2.).

compressor housing showing A/R characteristic
Figure 2. Illustration of compressor housing showing
A/R characteristic

The A/R parameter has different effects on the compressor and turbine performance, as outlined below.

Compressor A/R – Compressor performance is comparatively insensitive to changes in A/R. Larger A/R housings are sometimes used to optimize performance of low boost applications, and smaller A/R are used for high boost applications. However, as this influence of A/R on compressor performance is minor, there are not A/R options available for compressor housings.

Turbine A/R – Turbine performance is greatly affected by changing the A/R of the housing, as it is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel. This provides increased turbine power at lower engine speeds, resulting in a quicker boost rise. However, a small A/R also causes the flow to enter the wheel more tangentially, which reduces the ultimate flow capacity of the turbine wheel. This will tend to increase exhaust backpressure and hence reduce the engine’s ability to “breathe” effectively at high RPM, adversely affecting peak engine power.

Conversely, using a larger A/R will lower exhaust gas velocity, and delay boost rise. The flow in a larger A/R housing enters the wheel in a more radial fashion, increasing the wheel’s effective flow capacity, resulting in lower backpressure and better power at higher engine speeds.

When deciding between A/R options, be realistic with the intended vehicle use and use the A/R to bias the performance toward the desired powerband characteristic.

Here’s a simplistic look at comparing turbine housing geometry with different applications. By comparing different turbine housing A/R, it is often possible to determine the intended use of the system.

Imagine two 3.5L engines both using GT30R turbochargers. The only difference between the two engines is a different turbine housing A/R; otherwise the two engines are identical:
1. Engine #1 has turbine housing with an A/R of 0.63
2. Engine #2 has a turbine housing with an A/R of 1.06.

What can we infer about the intended use and the turbocharger matching for each engine?

Engine#1: This engine is using a smaller A/R turbine housing (0.63) thus biased more towards low-end torque and optimal boost response. Many would describe this as being more “fun” to drive on the street, as normal daily driving habits tend to favor transient response. However, at higher engine speeds, this smaller A/R housing will result in high back pressure, which can result in a loss of top end power. This type of engine performance is desirable for street applications where the low speed boost response and transient conditions are more important than top end power.

Engine #2: This engine is using a larger A/R turbine housing (1.06) and is biased towards peak horsepower, while sacrificing transient response and torque at very low engine speeds. The larger A/R turbine housing will continue to minimize backpressure at high rpm, to the benefit of engine peak power. On the other hand, this will also raise the engine speed at which the turbo can provide boost, increasing time to boost. The performance of Engine #2 is more desirable for racing applications than Engine #1 since Engine #2 will be operating at high engine speeds most of the time.

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Turbo Tech – Wheel Trim

Thursday, January 5th, 2012

Courtesy of www.TurbobyGarrett.com

Getting a little more complex here.

Trim is a common term used when talking about or describing turbochargers. For example, you may hear someone say “I have a GT2871R 56 Trim turbocharger”. What is ‘Trim?’ Trim is a term to express the relationship between the inducer* and exducer* of both turbine and compressor wheels. More accurately, it is an area ratio.

* The inducer diameter is defined as the diameter where the air enters the wheel, whereas the exducer diameter is defined as the diameter where the air exits the wheel.

Based on aerodynamics and air entry paths, the inducer for a compressor wheel is the smaller diameter. For turbine wheels, the inducer it is the larger diameter (see Figure 1.)

inducer and exducer diameter of compressor and turbine wheels

Figure 1. Illustration of the inducer and exducer diameter of compressor and turbine wheels

Example #1:
GT2871R turbocharger (Garrett part number 743347-2) has a compressor wheel with the below dimensions. What is the trim of the compressor wheel?

Inducer diameter = 53.1mm
Exducer diameter = 71.0mm

Example #2:
GT2871R turbocharger (part # 743347-1) has a compressor wheel with an exducer diameter of 71.0mm and a trim of 48. What is the inducer diameter of the compressor wheel?

Exducer diameter = 71.0mm
Trim = 48

The trim of a wheel, whether compressor or turbine, affects performance by shifting the airflow capacity. All other factors held constant, a higher trim wheel will flow more than a smaller trim wheel. However, it is important to note that very often all other factors are not held constant. So just because a wheel is a larger trim does not necessarily mean that it will flow more.

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Turbo Tech Part 3 — Wastegates

Monday, January 2nd, 2012

Courtesy of www.TurbobyGarrett.com

On the exhaust side, a Wastegate provides us a means to control the boost pressure of the engine. Some commercial diesel applications do not use Wastegates at all. This type of system is called a free-floating turbocharger.

However, the vast majority of gasoline performance applications require a Wastegate. There are two (2) configurations of Wastegates, internal or external. Both internal and external Wastegates provide a means to bypass exhaust flow from the turbine wheel. Bypassing this energy (e.g. exhaust flow) reduces the power driving the turbine wheel to match the power required for a given boost level. Similar to the BOV, the Wastegate uses boost pressure and spring force to regulate the flow bypassing the turbine.

Internal Wastegate

Internal

Wastegates are built into the turbine housing and consist of a “flapper” valve, crank arm, rod end, and pneumatic actuator. It is important to connect this actuator only to boost pressure; i.e. it is not designed to handle vacuum and as such should not be referenced to an intake manifold.
External Wastegate

External

Wastegates are added to the exhaust plumbing on the exhaust manifold or header. The advantage of an external Wastegates is that the bypassed flow can be reintroduced into the exhaust stream further downstream of the turbine. This tends to improve the turbine’s performance. On racing applications, this Wastegated exhaust flow

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Turbo Tech — Journal versus Ball Bearings

Thursday, December 29th, 2011

Courtesy of www.TurbobyGarrett.com.

The journal bearing has long been the brawn of the turbocharger, however a ball-bearing cartridge is now an affordable technology advancement that provides significant performance improvements to the turbocharger.

Ball bearing innovation began as a result of work with the Garrett Motorsports group for several racing series where it received the term the ‘cartridge ball bearing’. The cartridge is a single sleeve system that contains a set of angular contact ball bearings on either end, whereas the traditional bearing system contains a set of journal bearings and a thrust bearing

Journal Bearings
Ball Bearings

 

Turbo Response – When driving a vehicle with the cartridge ball bearing turbocharger, you will find exceptionally crisp and strong throttle response. Garrett Ball Bearing turbochargers spool up 15% faster than traditional journal bearings. This produces an improved response that can be converted to quicker 0-60 mph speed. In fact, some professional drivers of Garrett ball-bearing turbocharged engines report that they feel like they are driving a big, normally aspirated engine.

Tests run on CART turbos have shown that ball-bearings have up to half of the power consumption of traditional bearings. The result is faster time to boost which translates into better drivability and acceleration.

On-engine performance is also better in the steady-state for the Garrett Cartridge Ball Bearing

Reduced Oil Flow – The ball bearing design reduces the required amount of oil required to provide adequate lubrication. This lower oil volume reduces the chance for seal leakage. Also, the ball bearing is more tolerant of marginal lube conditions, and diminishes the possibility of turbocharger failure on engine shut down.

Improved Rotordynamics and Durability – The ball bearing cartridge gives better damping and control over shaft motion, allowing enhanced reliability for both everyday and extreme driving conditions. In addition, the opposed angular contact bearing cartridge eliminates the need for the thrust bearing commonly a weak link in the turbo bearing system.

Competitor Ball Bearing Options – Another option one will find is a hybrid ball bearing. This consists of replacing only the compressor side journal bearing with a single angular contact ball bearing. Since the single bearing can only take thrust in one direction, a thrust bearing is still necessary and drag in the turbine side journal bearing is unchanged. With the Garrett ball bearing cartridge the rotor-group is entirely supported by the ball bearings, maximizing efficiency, performance, and durability.

Ball Bearings in Original Equipment – Pumping up the MAZDASPEED Protegé’s heart rate is a Garrett T25 turbocharger system. With Garrett technology on board, the vehicle gains increased acceleration without sacrificing overall efficiency and it has received many rave reviews from the world’s top automotive press for it’s unprecedented performance.

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Basic Turbo Tech — Gotta Start Somewhere

Tuesday, December 27th, 2011

We are big fans of the Garrett Turbochargers. Over the years they continue to offer a great product with great value. Reliability is very important to us when it comes to the products we sell and use and the Garrett Turbo Product line are some of the most reliable turbochargers on the market.

This excerpt is taken from the www.TurbobyGarrett.com website — they are a wealth of information and we will do what we can to get the knowledge to our customers.

http://www.turbobygarrett.com/turbobygarrett/basic

How a Turbo System Works

Engine power is proportional to the amount of air and fuel that can get into the cylinders. All things being equal, larger engines flow more air and as such will produce more power. If we want our small engine to perform like a big engine, or simply make our bigger engine produce more power, our ultimate objective is to draw more air into the cylinder. By installing a Garrett turbocharger, the power and performance of an engine can be dramatically increased.

So how does a turbocharger get more air into the engine? Let us first look at the schematic below:

 1 Compressor Inlet
2 Compressor Discharge
3 Charge air cooler (CAC)
4 Intake Valve
5 Exhaust Valve
6 Turbine Inlet
7 Turbine Discharge

The components that make up a typical turbocharger system are:
  • The air filter (not shown) through which ambient air passes before entering the compressor (1)
  • The air is then compressed which raises the air’s density (mass / unit volume) (2)
  • Many turbocharged engines have a charge air cooler (aka intercooler) (3) that cools the compressed air to further increase its density and to increase resistance to detonation
  • After passing through the intake manifold (4), the air enters the engine’s cylinders, which contain a fixed volume. Since the air is at elevated density, each cylinder can draw in an increased mass flow rate of air. Higher air mass flow rate allows a higher fuel flow rate (with similar air/fuel ratio). Combusting more fuel results in more power being produced for a given size or displacement
  • After the fuel is burned in the cylinder it is exhausted during the cylinder’s exhaust stroke in to the exhaust manifold (5)
  • The high temperature gas then continues on to the turbine (6). The turbine creates backpressure on the engine which means engine exhaust pressure is higher than atmospheric pressure
  • A pressure and temperature drop occurs (expansion) across the turbine (7), which harnesses the exhaust gas’ energy to provide the power necessary to drive the compressor

1 Ball Bearings (support and control the rotating group)
2 Oil Inlet
3 Turbine Housing (collects exhaust gases from the engine and directs it to the turbine wheel
4Turbine Wheel (converts exhaust energy into shaft power to drive the compressor)
5 Center Housing (supports the rotating group)
6 Oil Outlet
7 Compressor Housing (collects compressed air and directs it to the engine)
8 Compressor Wheel (pumps air into the engine)
9 Backplate (supports the compressor housing provides aero surface)

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Brake Ducts for ITA Integra

Tuesday, October 11th, 2011

Since we started racing the ITA Integra we have consistently had braking issues. A cooling setup was on the to do list for quite some time and we finally got around to getting a nice setup done on it. All design and fabrication was done by Russell McMullen for us who also happens to be co-driving the car.

A system was designed to use existing mounting points as much as possible to funnel the air directly into the center of the rotor. All mocked up on a spindle on a workbench here is what was came up with.

Here are all the pieces after the initial fabrication and of course some noticeable powder coating.

Here is a view of the setup with the brake rotor and caliper removed so you can see how it mounts nicely on the spindle and directs the air flow where we want it as much as possible with the given space.

And with the brake rotor and caliper installed.

Lastly, here is a shot of the car from the front with the bumper installed and the entrance of the brake ducts installed.  We still have some fine tuning to do with the setup but we are headed in the right direction to alleviate the ITA Integra braking issues.

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Common Failures for Fasteners — Head Studs, Main Studs, Rod Bolts, etc

Tuesday, November 23rd, 2010

Recognizing Common Failures

There are six types of metallurgical failures that affect fasteners. Each type has unique identifying physical characteristics. The following examples are designed to be used like a spark plug reading chart to help analyze fastener failures.

While few of us have access to sophisticated analysis equipment, a standard Bausch and Lomb three lens magnifying glass will generally show 98% of what we want to see. Several of the photos below have been taken utilizing a Scanning Electron Microscope (SEM) and are presented to simply illustrate typical grain configurations after failure.

1. Typical Tensile Overload

In a tensile overload failure the bolt will stretch and “neck down” prior to rupture. One of the fracture faces will form a cup and the other a cone. This type of failure indicates that either the bolt was inadequate for the installation or it was preloaded beyond the material’s yield point. 

2. Torsional Shear (twisting)

Fasteners are not normally subjected to torsional stress. This sort of failure is usually seen in drive shafts, input shafts and output shafts. However we have seen torsional shear failure when galling takes place between the male and female threads (always due to using the wrong lubricant or no lubricant) or when the male fastener is misaligned with the female thread. The direction of failure is obvious and, in most cases, failure occurs on disassembly.

3. Impact Shear

Fracture from impact shear is similar in appearance to torsional shear failure with flat failure faces and obvious directional traces. Failures due to impact shear occur in bolts loaded in single shear, like flywheel and ring gear bolts. Usually the failed bolts were called upon to locate the device as well as to clamp it and, almost always, the bolts were insufficiently preloaded on installation. Fasteners are designed to clamp parts together, not to locate them. Location is the function of dowels. Another area where impact failures are common is in connecting rod bolts, when a catastrophic failure, elsewhere in the engine (debris from failing camshaft or crankshaft) impacts the connecting rod.

4. Cyclic fatigue failure originated by hydrogen embrittlement.

Some of the high strength “quench and temper” steel alloys used in fastener manufacture are subject to “hydrogen embrittlement.” L-19, H-11, 300M, Aeromet and other similar alloys popular in drag racing, are particularly susceptible and extreme care must be exercised in manufacture. The spot on the first photo is typical of the origin of this type of failure. The second is a SEM photo at 30X magnification.

5. Cyclic fatigue cracks propagated from a rust pit (stress corrosion)

Again, many of the high strength steel alloys are susceptible to stress corrosion. The photos illustrate such a failure. The first picture is a digital photo with an arrow pointing to the double origin of the fatigue cracks. The second photograph at 30X magnification shows a third arrow pointing to the juncture of the cracks propagating from the rust pits. L-19, H-11, 300M and Aeromet, are particularly susceptible to stress corrosion and must be kept well oiled and never exposed to moisture including sweat. Inconel 718, ARP 3.5 and Custom age 625+ are immune to both hydrogen embrittlement and stress corrosion.

6. Cyclic fatigue cracks initiated by improper installation preload

Many connecting rod bolt failures are caused by insufficient preload. When a fastener is insufficiently preloaded during installation the dynamic load may exceed the clamping load resulting in cyclic tensile stress and eventual failure. The first picture is a digital photo of such a failure with the bolt still in the rod. The arrows indicate the location of a cut made to free the bolt. The third arrow shows the origin of the fatigue crack in the second picture – an SEM photo at 30X magnification that clearly shows the origin of the failure (1), and the telltale “thumbprint” or “beach mark” (2). Finally (3) tracks of the outwardly propagating fatigue cracks, and the point where the bolt (unable to carry any further load) breaks-away.

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