Turbocharger Concepts

[ Turbines | Buzz Words | Boost Control | Configurations ]

Description

This page is designed to explain what a turbocharger is and how it works. There are four basic turbocharged engine designs used on Chrysler`s 2.2L and 2.5L engines. Dempsey Bowling's Turbo Engine Designations page gives an overview of how and when these engines were turbocharged over the years. For the REAL dirt on turbochargers in general, visit Garrett's About Charge Air Systems page.

What Is A Turbocharger?

A turbocharger is basically a device that uses exhaust gasses produced by the engine to blow air back into the engine. The additional air is supplemented with fuel by the ECU (engine control unit). This causes the engine to produce much more power since it is being supplied with more air and fuel than it possibly could without it. A naturally aspirated engine (non-turbo, standard engine), or "N/A" enigne, has to "suck" air through the intake manifolds, throttle body, ait filter, etc. With this setup, the most air pressure that can enter the combustion chamber of the engine is a bit less than the current atmoshperic pressure. With the turbo, air is being blown into the chamber with positive pressure so that much more air and fuel can enter. A typical turbocharged engine will generate 7 to 10 psi of maximum positive pressure, or "boost".

The turbocharger, or "turbo", is mounted directly to the exhaust manifold, where exhaust gasses pass over a turbine impeller that is attached to a short shaft. On the other side of this shaft is a compressor turbine, which pulls outside air in through the air filter and blows it into the intake manifold. So basically, the energy from the expelled exhaust gasses, which would normally be wasted on a N/A engine, is being used to pump air back into the engine. The shaft is supported by a bearing housing that is lubricated and cooled by an oil line from the engine. Since engine exhaust has such high temperatures, the exhaust side of the turbo can reach thousands of degrees F. This is why it is so critical that the engine oil be changed religously (every 3,000 miles), because old oil can burn and leave deposits in oil lines and housings, called "coke". Coking can be virtually eliminated by using a synthetic oil and changing it frequently (every 6,000 miles). After 1985, the turbos featured an additional passage for a coolant line, to keep the bearing housing cool. This did little to keep temperatures down while running, but it had a huge effect after the engine was shut off. Without the coolant passage, the oil would drain when the engine was shut off and the turbo bearing housing would reach incredibly high temperatures from the heat transferring out of the exhaust manifold. This took its toll on the life of the bearings. The presence of the water keeps the housing cool. Here is a picture showing a cutaway of a turbochager (this one does not have a coolant passage and features a mechanically-controlled wastegate):

When the engine has been idling or at low speed for a while, the turbo is not spinning or is spinning very slowly because there is very little exhaust leaving the engine. When the throttle is opened, the engine produces more exhaust, which spins the turbo faster. A faster spinning turbo means more air and fuel is being blown into the engine, therefore even more exhaust is being produced, which makes the turbo spin even faster and so on. This cycle is known as turbo "spool-up", which feels like a sudden surge in engine power and appears on your boost guage as a sudden increase in pressure. The time before the surge, when the turbo is spooling up but the engine doesn't have much power yet, is called turbo lag. A large turbocharger can produce more air flow and pressure, but will have more lag because of its increased size. A small turbocharger will have a smaller amount of lag, but will not be able to move as much air. This is explained in more detail is the sections below.

How Turbines Are Designed

Here is an explaination about how a turbine works. Huge thanks go to Garry McKissick for explaining the details of this to me. If you want to see the specifications an details of the Chrysler stock and performance turbochargers, see the Upgrading Your Turbocharger page.

The exhaust turbine's job is to convert the energy in the moving and expanding exhaust gasses into rotating kinetic energy of the shaft and turbines. The compressor turbine's job is to convert that rotating energy into the movement of the air that enters the engine. This air is compressed and (unfortunately) heated. The turbines in a turbocharger are measured by the sizes of each stage of the turbine. A turbine has two stages: the inducer stage and the exducer stage. The size and shape of each stage determines the shape of the turbine's fins and ultimately the characteristics of that turbine.

For the compressor turbine (or "compressor wheel"), the inducer part of of turbine is at the end of the shaft and can be seen by looking into the intake of the turbo compressor housing (looks like a fan). The blades that you see there extend into a larger diameter at the other end of the turbine. This is the exducer stage. The inducer on the compressor turbine is responsible for generating the vacuum at the compressor housing inlet that pulls air into the compressor. The air then "rides" the fins towards the exducer stage, which is a larger diameter, and gets sling-shot towards the ouside of the compressor housing. The housing collects this moving air an expells it through the housing's outlet. The way that the air leaves the fins causes it to swirl as it leaves the housing. Since the intake manifolds on the early Turbo I engines were so close to the turbo, an anti-swirl fin had to be installed in the turbo outlet duct to stop this air motion from effecting the flow characteristics of the intake manifold.

The exhaust turbine also has an inducer and exducer, but because the exhaust turbine has the opposite function of the compressor turbine, the two are switched. The exhaust gasses are directed towards the outside of turbine through a nozzle. This is the inducer stage because it is the part of the turbine that collects the gasses. As the energy from the gasses is transferred into the turbine, the gasses slow down and exit the turbine through the exducer stage.

Turbocharger Buzz Words

There are several factors that determine the performance of a turbocharger. The three most important ones are the type of exhaust turbine, the A/R ratio of the exhaust housing, and the size of the compressor turbine. Usually, it seems that the exhaust turbine is just referred to as the "turbine" and the compressor turbine is referred to as the "compressor wheel".

The Exhaust Turbine

The exhaust turbine design is a balance between absorbing as much energy from the exhaust gasses as possible and allowing the gasses to flow as easily as possible. This is closely related to the size of the exhaust housing. A larger turbine can absorb more energy from the gasses and spin the shaft with more torque and speed, but too large a turbine will restrict the flow of exhaust such that engine performance is greatly reduced. Typically, the inducer is only slightly larger than the exducer on the exhaust turbine. There are many turbine sizes available, but only one type of turbine was used on Garrett turbochargers for Chrysler. Generally, you would want to stick with the stock turbine because it's size is not nearly as relavent as the compressor turbine's size. If you want to reduce restriction through a smaller housing, you can have the tubine "clipped", which reduces the size of the fins and allows more air to flow around the turbine.

The Turbine Housings

The exhaust and compressor housings on Chrysler's turbochargers use a "scroll" design. For example, the exhaust housing's scroll is where the exhaust gasses enter the housing and are directed at the turbine. It's basically a smooth, tubular chamber that surrounds the turbine with a slot all the way around that acts as a nozzle to direct the exhaust gasses at the turbine. It's called a scroll because it slowly gets smaller in diameter as a goes around the turbine. This pressurizes the gasses, forcing them out of the slot/nozzle at a fast rate. In turbo-terms, the scroll is measured by the cross-sectional area of the scroll's "tube" (A) and the distance from the center of the "tube" to the turbine shaft (R). The values by themselves are not meaningful to the user and for the most part, R does not change much for different housings, but by dividing R into A, you get the A/R ratio. So, the A/R ratio of the exhaust housing refers to the size and shape of the scroll that is cast into the housing. It basically determines how restrictive the housing will be, versus how quickly the turbine will spin up. A lower A/R ratio (smaller scroll area, A) results in a more restrictive housing. This restriction speeds up the exhaust gasses and increases the amount that the gasses will expand. It's the speed and expansion of the gasses that causes the turbine to spin. So with a low A/R ratio, the turbine will spin up quicker, but as engine output and rpms increase, the restriction of the housing begins to build up too much back pressure on the engine, which reduces performance. A good rule of thumb for when thereis too much backpressure is when the pressure in the exhaust manifold is more the half of the pressure in the cylinder. A good A/R ratio seems to be about 0.48, which just happens to be the ratio used on stock 2.2L and 2.5L turbocharged engines. A popular turbo upgrade, the Super 60 turbo, has an A/R ratio of 0.63. This higher ratio removes some of the restriction of the stock unit at high engine speeds, but it also causes significant turbo lag because of the low exhaust gas speed and expansion at lower engine speeds. So basically, a larger A/R ratio will improve your engine's top end, while losing some midrange power and increasing turbo lag. A smaller A/R ratio will help the bottem and mid-range, but may effect the top end.

On the compressor side, the housing also features a scroll design, but it has the opposite function. The air leaving the compressor turbine has a lot of speed, but not much pressure. The scroll on the compressor housing starts small and gets larger as it approaches the compressor outlet. This collects the air and builds up air pressure. So, the compressor housing is designed to convert the speed-energy of the air coming off of the compressor turbine into pressure-energy, which is much more useful to an engine.

The Compressor Turbine

The size of the compressor turbine determines the maximum amount of boost that the turbocharger can produce. It also effects the spool-up time of the turbo. The type of compressor wheel is usually designated as its "trim", which is a value that describes the inducer and exducer sizes. Typically, the exducer is significantly larger than the indicer on a compressor turbine. All Garrett units used by Chrysler as stock and performace turbochargers seem to use a 60mm exducer stage. The inducer stage is varied, based on the desired performance of the turbocharger. Stock Garrett turbochargers used by Chrysler on the Turbo I (early) and turbo II featured a 40mm inducer. For more information about other inducer and exducer sizes, see the Upgrading Your Turbocharger page.

So in conclusion, a turbocharger's design becomes a balancing act between these three factors (whew!). The rest of this page won't give you a headache like these sections did. :)

How Boost Is Controlled

The amount of boost produced by the turbo is controlled with another device called a wastegate. The wastagate is a large valve that sits at the exhaust inlet to the turbo that, when opened, causes the exhaust gasses to bypass the exhaust turbine instead of through it. The further the wastegate is opened, the more exhaust is bypassed and less boost is produced. 1984 2.2L turbo engines used a mechanical wastegate actuator (the device that controls the wastegate) calibrated to produce a maximum of 7 psi. All later models had the wastegate actuator controlled by a vacuum solenoid, which was controlled by the logic module. A spring in the actuator closes the wastegate (by pulling on the rod). The back-side of the actuator diaphram is connected to the intake manifold. As pressure builds up in the manifold, the actuator rod pushes out and the wastegate opens. This pressure is bled off by a solenoid that is modulated (switched on and off quickly) by the logic module. The longer the duty cycle (amount of time it spends turned on) of the solenoid, the higher the boost pressure that is produced. There were two different configurations used to accomplish this, depending which turbocharger was installed on the vehicle. The function of both is the same and the only real difference is where the manifold pressure comes from.

Different Turbo Configurations

As stated in the beginning, Dempsey Bowling's Turbo Engine Designations page gives most of the details on the four different types of turbo engines produced by Chrysler. Here, I will give a basic, technical explaination of the turbo types, while Dempsey's site gives more information about the chronology of these engines.

Turbo Types

There are four types of turbo configurations, each given a roman numeral. Most of this information came from Dempsey's site. The most basic type is called the Turbo I. It simply has a turbocharger (as described above) installed on the 2.2L engine with multi-port fuel injection to produce about 146 HP at 5200 rpm and 168 lb-ft at 3600 rpm. The intake configuration was changed in 1988 as well as a smaller turbo, but the engine output was the same. In 1989, the engine displacement was changed to 2.5L and the engine output was increased to 150 hp at 4800rpm and 180 ft-lbs at 2000rpm. Some of the computers were reprogrammed in 1991 with a more aggresive response, which increased engine output torque to 210 ft-lbs.

One step up from this is the Turbo II, which features an air-to-air intercooler installed next to the radiator. The intercooler's purpose is to cool down the extremely hot air coming out of the turbo. This serves two purposes. One is to decrease the inital temperature of the air/fuel mixture entering the engine, which serves to reduce detonation and permit higher boost levels. The second reason is that cooler air is denser, therefore more air can fit into the combustion chamber. So you can see that denser air at higher pressure allows the engine to produce more power. This is illustrated by the improved performance of 175 HP at 5200 rpm and 175 ft-lbs from 2200-4800 rpm.

In 1991, the Turbo III engine was breifly produced by Chrysler. This engine was mainly identical to the Turbo II setup except that it featured a new head designed by Lotus, which was a DOHC (dual overhead cam), 16 valve engine. It also had a different turbocharger. The extra air flow gave this engine 224 hp at 6000 rpm and 217 ft-lbs at 2800 rpm, but the head is known to have a high failure rate.

The most elaborate turbo setup was the Turbo IV VNT (variable nozzle turbo). It was a breakthrough in turbo design because it featured a type of turbo that reduced turbo lag to almost nothing. It has a larger exhaust housing, so it can produce higher boost with ower back pressure, but it has moveable fins located around the exhaust turbine. When the vents are mostly closed, the exhaust gasses are sped up (since they are passing through a smaller oriface) and the turbine is spun faster. When they are opened, the gasses slow down but there is a larger amount of gasses to spin the turbine and give it more pumping power. When the throttle is opened at idle, the computer-controlled vents momentarily pinch off the exhaust to quickly spin the turbo, but open them right away to keep from bogging the engine with the restricted exhaust. The quick spin-up gives the engine more power right away and the vents will close and open for the maximum amount of power build up. With this setup, no wastegate is necessary because the wide-open vent can slow the turbo down, even when the engine is at maximum output. Since this engine was intercooled, but did not feature the Lotus head, power output is still 175 HP but the torque is up to 200 ft-lbs at 3700 rpm.

Pull-Through vs Blow-Through

This section refers to how the intake components were configured, relative to the throttle body. The original Turbo I engine had a pull-through design. This meant that air is pulled through the throttle body by the turbo. Following the flow of air from the outside, the air travels through this path: air filter, to the throttle body, to the turbo, to the intake manifold. The Turbo II, III, and IV engines, as well as the Turbo I engine after 1987, had the blow-through setup. Here, the air followed this path: air filter, to the turbo, to the throttle body, to the intake manifold. These setups are diagrammed below:

The reason for the change in the Turbo I in 1988 was probably more a matter of convenience than of performance, proven by the fact that there was no performance gain by the change. This way, Chrysler didn't have to have two completely different intake setups for its Turbo I and Turbo II engines. In fact, the blow-through design is actually more complicated than the pull-through, because of what happens when the throttle is closed. One problem with the pull-through intake is that if a hose comes off of the turbo, it would be like having a wide open throttle to the engine because the throttle body is being bypassed. I have never heard of this happening to anyone, but it is possible.

On a pull-through intake, when the turbo is boosting and the throttle is suddnely shut (while shifting, for instance), a huge vacuum is generated between the engine and the throttle body, and the turbo is spinning in a low-pressure environment. This is good because then there is less air friction to slow the turbine, and so there will be less lag when the throttle is opened again. However, it takes a moment for the compressor turbine to get the air flowing fast again, since the air was at a dead stop. On the blow-through intake, when the throttle is closed, a huge pressure spike is produced between the turbo and the throttle body, because the turbo is blowing against a closed throttle plate. This slows the turbo down very quickly and there is a lot of lag when the throttle is opened again. The pressure spike is also hard on the turbo and intake hoses. To remedy this, the production blow-through setup features a blow-off valve (BOV) on the output hose of the turbo. This is a large, vacuum actuated valve that opens when there is vacuum in the intake manifold (closed-throttle). This valve vents off the pressure spike, avoiding the quick spool-down of the turbo and potentially damaging pressure spikes. Although the turbo still slows down more than the pull-through setup does during the time that the throttle is shut, pressure is brought back up just as quickly because the air is still moving. When the throttle plate is opened again, the BOV immediately shuts and the air flow is directed back into the engine. Also, the 1988 and later Turbo I engines had a smaller turbo, which further reduced the lag. So all in all, the two setups are about even in performance. For more details on BOVs, see the Blow-Off Valves page.

If you have a Turbo II engine (including the Shelby engines), your engine will not have a BOV installed. To see a significant decrease in inter-shift lag, install a BOV on one of the pressure hoses. See the Blow-Off Valves page for instruction on how to do this.

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This page is maintained by Russell W. Knize and was last updated 01/19/99. Comments? Questions? Email minimopar@myrealbox.com.