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turbo charger

 

basics

The energy used to drive the turbo compressor is extracted from waste exhaust gasses. As exhaust gasses leave the engine they are directed through a wheel placed in the exhaust flow. The gasses drive the turbine wheel around, which is directly connected via a shaft, to the compressor wheel. Increased exhaust gas drives the turbine wheel faster, this provides the engine more air, producing more power. A limit is met once a pre-determined boost pressure is achieved. At this point the exhaust gas is redirected away from the turbine wheel, thus slowing it down and limiting the maximum boost pressure. This redirection valve is known as the wastegate. Air entering the engine first passes through an exhaust driven compressor. Compressed air results in a larger quantity of air being forced into the engine, creating more power.

Turbochargers are usually seen as power enhancements on performance cars, but today, turbochargers are becoming more regularly used to provide greater torque on small capacity engines. The advantages of using a turbo engine include improved fuel efficiency and reduced exhaust emissions.

A turbo will accelerate from 20,000 to over 150,000 rev/min in less that one second.

A blade's circumference will travel in the region of 1.300 km/h at average engine speed, and the exhaust gas entering it will be supersonic and the air entering a turbo's compressor impeller can be travelling at a speed close to mach 1.

Turbo shaft balance is crucial - imbalance at maximum revs equivalent to a 20 Newtons is acceptable.

Newer generations turbo impellers rotate up to 3666 rev/s. The impellers on a Boeing 747 engine rotate at about 116 rev/s in comparison.

The average temperature of the exhaust gas, at the entry point to a diesel turbo, is 800 C. A petrol engine can reach 1000 C, glowing bright yellow. Hot enough to melt window glass.

Turbo Charger - Base Theory
A turbo charger is basically an exhaust gas driven air compressor and can be best understood if it is divided into its two basic parts, the exhaust gas driven turbine and its housing, and the air compressor and its housing. I did say divided didn't I. Well I should have said like a set of Siamese twins because each of them perform different functions but, because they are joined together at the hip via a common shaft, the function of one impacts the function of the other. How? Take a perfectly set up compressor section and mate it with an incorrect turbine section, or visa versa, and you end up with with our Siamese twins trying to go in different directions. The result is that our Siamese twins end up wasting all of their energy fighting each other and go nowhere.

When considering a turbo charger most folks tend to look at the maximum CFM rating of the compressor and ignore everything else under the assumption that the compressor and the exhaust turbine are perfectly matched out of the box. I will grant you that in stock factory applications that is probably close to the truth but, in all out performance applications, nothing could be further from the truth because of the extremes of operation in a performance application.

The goal in a performance application is to get the exhaust turbine up to speed as quickly as possible however, it must be mated to a compressor wheel that will generate as much pressure as it can as soon as possible. This is a contradiction because the exhaust turbine generates the drive power and the compressor consumes that power. The larger the compressor and the higher the pressure (boost) we want, the quicker the power from the exhaust turbine is used up. Put in a larger exhaust turbine and it will take the engine longer to develop enough hot expanding exhaust gas to spin it, slowing down the compressor and causing turbo lag. At this point I am going to repeat something stated earlier, do not think of a turbo charger as a bolt on piece of equipment, think of it as a system.

The turbine is powered by hot expanding exhaust gas, a lot of hot expanding exhaust gas, the more and the hotter the expanding exhaust gas the better. I am sure many of you have seen pictures of turbo charged engines with cherry red hot exhaust systems and turbo housings. The captions under most of these types of pictures proclaim outstanding horse power numbers. What most of the articles related to these pictures do not tell you is that the engine was under an extreme load. A load so heavy that the engine was almost at its stall point for a prolonged period of time. A condition that most turbo charged engines will never see.

The real point I am trying to make is that the exhaust turbine will not generate enough power to turn the air compressor fast enough for it to work properly unless the engine is feeding the exhaust turbine a lot of hot expanding exhaust gas, a condition that can only be created when the engine is under a load. There is where the selection of transmission gear ratios and the ring and pinion ratio play a critical part. The fact that the engine must be under a load is the reason why, no matter how high you rev a turbo charged engine with no load on it, you will not see the boost gauge move.

This is also where the term 'turbo lag' came from. Turbo lag is basically the amount of time it takes from the time you place a load on the engine (stomp the gas peddle to the floor and dump the clutch or, get full converter lock up with your automatic trans) until the time the engine develops enough hot expanding exhaust gas to spin the turbine fast enough for the compressor to do its job.

Effectively, a turbo charged engine is a normally aspirated engine until the turbine and compressor spin up. To minimize turbo lag, it is imperative that the turbine and the compressor are properly matched to the engine as well as the engine being properly matched to the transmission gears, the ring and pinion gears, and the tires.

You have probably heard or read that an engine is nothing more than an air pump and that the more air you can get into it the more power it will make. This is, in reality, true. However, there is a point at which, although the engine can handle more air, some of the component parts that make up the engine cannot take the stresses of the additional power generated. Using this concept one may get the false impression that air is 'cool' and unlimited amounts can be ingested by the engine without harm to it as long as you watch the RPM.

I prefer to consider an engine a heat and pressure generator and apply the same rule, the more heat and pressure generated the more power it will make. Using this concept tends to bring to mind that if the engine generates too much heat something is going to melt (like pistons, heads, and valves), likewise, too much pressure something is going to blow (like bearings, rods, and head gaskets) regardless of the RPM.

Turbos provide enhanced fuel economy and performance. A turbo is a basic "air pump" that pushes a volume of air into the engine, which increases the power output. This turbo "air pump" is driven by a fan located in the exhaust by a direct shaft. The more exhaust that flows, the more air is pumped into the engine. In most automotive and some other applications, a wastegate is provided which opens as pressure is increased by the "air pump". This device prevents an overboost from damaging the engine.

As air is pumped and compressed into the engine by the turbo, it rises in temperature. To reduce this problem and make the turbo more efficient, vehicle manufacturers have been adding intercoolers. An intercooler is a radiator for air and is usually located in front of, or behind the main radiator itself. To add to the life of the turbo unit, some turbos are also water-cooled by coolant system connections. This feature limits the operating temperature of the turbo to the temperature of the cooling system, thus, protecting the bearing assemble from excessive exhaust temperature.

Turbo units may obtain speeds up to 100,000 RPMs, depending on the application, so it is extremely important that a sufficient supply of clean oil always be entering the turbo while the engine is running. If for any reason, whenever the oil supply is interrupted or becomes contaminated, "good-bye turbo".

Turbo failures are mostly caused by lack of lubrication or abrasive material in the oil. Other failures occur when heavy particles enter the air stream on the suction side. Therefore, a clean air filter and ducting is necessary. Another type of failure may be caused by objects from within the engine leaving via the exhaust. This could be hard carbon, broken engine parts, manifold rust, etc.

maintenance

  • Contaminated oil
  • Dirty oil
  • Lack of lubrication
  • Low oil pressure
  • Kinks in the oil inlet lines
  • Clogs in the oil inlet line
  • Plugged air cleaners
  • Collapsing hose connections
  • Undersized air pipes
  • Prolonged engine idling
  • Over-fueling
  • Hot engine shut-down
  • Improperly installed gaskets
  • Nuts & washers dropped into exhaust system
  • Keep clean oil in engine
  • Keep the air filter clean and unrestricted
  • The duct work from the air cleaner should be free of holes
  • The duct work connections should be tight to prevent leaks
  • Warm up the engine for two to five minutes prior to throttling up
  • Let engine idle for approximately 2 minutes prior to engine shut down
 

modification of stock engine for turbo charger

  Turbo charger can be installed on a stock engine, but some modifications must be done to achive optimal performance and engine endurance. It is also very important to know what is the purpose of turbine installation, better torque curve or tons of power. In case a first possibility only little additional work must be done, but to achieve high performance severe interventions must be done to other parts of engine, since turbo engine is not combined of indepentent subsystems, it must be seen as a summ of interaffecting parts.

Head Preparation
Setting up heads, whether aluminum or cast iron, for a turbo charged engine is similar to building any high compression (12.5:1 to 15.0:1) engine. Trued surfaces on the heads are absolutely critical if you expect other parts being installed in or attached to the heads to mate up and function properly. The first reason is the limitation of the intake valve springs. The intake valve springs in a self respirating engine were designed to pull the valves back to their seats in a vacuum to low pressure environment. A turbo charger pressurizes the intake system. As the pressure starts to build up in the intake system, the valve springs are not strong enough to close properly the intake valves. The result is a tremendous loss of power and possible damage to the valves and the piston tops. This is the same condition that develops when you over rev an engine and the valves 'float' because the valve springs are not strong enough to pull them back to their closed position quickly enough.

To minimize stress cracks, all edges on the heads should be radiused. All moving parts should be balanced, including the valves, lash caps, retainers, and push rods. There is no reason to spend a lot of time and effort putting together a set of matched valve springs only to assemble them with unmatched parts and then expect to get matching horse power from cylinder to cylinder.

Another problem with installing a turbine on a stock engine is gaskets. Actually it is not totally a gasket problem, it is a combination of the gaskets and the number of bolts used to apply the clamping pressure needed to keep the gaskets from blowing out. This wider bolt spacing allows parts to move under high pressure. The movement is normally away from each other. Once that happens, clamping force is lost and the gasket itself cannot hold back the high pressure. The result is that either the gasket warps and leaks, or it completely blows out.

Besides checking for correct head seating surfaces, all bores and seats should be checked for roundness and proper clearances. It is necessary to keep in mind that modifications of this kind will build up pressure that was never attended for a stock aspirated engine. All of the bores fit and finish are crucial as too loose will cause excessive blow-by and/or scuffing and too tight will cause seizure.

That being said, forget the use of cast iron parts if you want to produce enormous horse powers on a consistent basis. While cast parts will allow you to do it some of the time, quality forged chromemoly and aluminum parts will allow you to do it all of the time.

To minimize stress cracks, all edges on the block, crank, rods, pistons, flywheel, clutch, and timing pulley should be radiused. To cut down on windage in the oil sump area, 'knife edging' the counter weights on the crank is also beneficial. All of the mentioned moving parts should be balanced, including the valves, lifters and push rods. Valves, lifters and pushrods you say? Absolutely! Why spend a lot of time and effort putting together a set of matched valve springs only to assemble them with unmatched parts and then expect to get matching horse power from cylinder to cylinder.

A common practice on VW and Porsche opposed cylinder blocks is to machine the block and install threaded inserts to allow the use of larger 10mm head studs in place of the stock 8mm head studs. This operation is needed for the additional clamping power required to keep the heads in place when running upwards of 15:1 compression ratios. On all out competition VW and Porsche blocks, two additional studs per cylinder are added for a total of six studs per cylinder instead of four.

OK, OK, I will have my block and heads machined for bolt girdles and additional bolts to make sure there is enough clamping force to stop parts from moving and gaskets from blowing, then I can create tons of horse power right?
Well, yyyyeeeeeesssss, sort of, for a short while. The next problem is your cast crankshaft, rods, and pistons as well as the type of bearings in your engine. None of these were designed to operate at the pressures that your engine will encounter once the turbo is installed. The stock cast parts will take the beating for a short while but they will quickly fatigue and, when one of them decides it can't take any more, it usually takes something else out with it, another 'not a good thing'.

What I am trying to point out is that you should not think of a turbo charger as a bolt on piece of equipment, think of it as a system. A system that includes the turbo, a special intake and exhaust system, modifications to your fuel system, carb or injectors, heads, crank, rods, pistons, transmission gears, rear end gears, and tires. If you do not want to make all of the modifications, you will end up with something you will not be pleased with.

   
  Larger Turbocharger

Advantage

Properly sized for the application, larger turbos produce a cooler denser air charge due to less "Beating of the Air" and more compressing. Also lower pressure between exhaust valve and turbine wheel.

Disadvantage

Increased lag due to heavier moving parts and less low-speed efficiency.

Larger Intercooler

Advantage

Reduces air temperature to increase threshold of detonation. Increases air charge density, delivering more oxygen per stroke. Also less pressure drop across intercooler.

Disadvantage

Slight lag increase due to larger volume and therefore elasticity of air

On turbo charged cars without a dump valve fitted, pressure is forced in the opposite direction into the turbo when the throttle is released.  This causes compressor stall and   the turbo stops spinningwhen changing gear, resulting in a "lag" during initial pickup.  This stall has also been known to shatter the turbo compressor on some cars ruinning high boost.

By fitting a dump valve after the intercooler outlet and as close to the throttle body as possible, excess compressed ait in the intake system and intercooler is relieved when the throttle is released.

One other area you need to pay particular attention to is ventilation. The pistons and crank are generating the equivalent of hurricane force winds within the block that are changing direction in fractions of a second. To minimize the force that the crank and pistons have to work against, make sure that your blocks breathing and scavenge lines are large enough to avoid any restriction.

To do things right we need to look at:

Intake and exhaust port size - while larger means more power at the high end it also means the loss power at the low end.

Port matching - exhaust and intake - do you really want 'speed bumps' in those high speed tunnels? - laminar flow

Valve spring seats - keep them absolutely perpendicular to the valve stems and watch their thickness.

Valve springs - single and multiple springs, open and closed pressure, installed height, and valve stem length, coil bind.

Valve spring retainers and valve locks - lock them down tight or allow them to rotate?

Valve guides - what material and how much lubrication?

Stainless or titanium valves - lash caps

Valve seats

Combustion chamber size and material left in the area

Rocker arm geometry, rocker stands, shafts and shaft support

Rocker arms and ratio - cheap horse power - cam bearing eater?

Roller rockers and oiling -

Pushrods - tapered vs straight - what does that have to do with harmonics?