What is a turbocharger?
How does it work?
Basic parts of a turbocharger
Air Intake comparison
Twin Passage Turbine
Why use Turbochargers?
Different ways of multiple turbocharging
Holset VG Turbocharging explained
First developed for the comercial sector in 1924, the turbocharger uses the engine's exhaust gases to power a turbine, which drives a compressor, and pushes more air into the engine. This allows the engine to produce more power without any increase in size. Turbochargers have provided such significant improvements in engine efficiency that they are now fitted to almost all new diesel engines.
Operation of basic turbocharger. A turbocharger is an exhaust-driven turbine which drives a centrifugal compressor wheel.
The compressor is usually located between the air cleaner and the engine intake manifold, while the turbine is located between the exhaust manifold and the tail pipe of the exhaust system.The prime job of the turbocharger is, by compressing air, to force more air into the engine cylinders. This allows the engine to efficiently burn more fuel, thereby producing more horsepower.
All of the engine exhaust gases pass through the exhaust manifold and into the turbine housing (unless diverted through the wastegate). The expansion of these gases, acting on the turbine wheel, causes it to turn. After passing through the turbine, the exhaust gases are routed to the atmosphere. In many cases, the turbine muffles the exhaust sound, so lillte to no noise muffling is needed.
The turbine also functions as a spark arrester. For example, it is recognised by the U.S. Department of Agriculture as providing a spark arrester function adequate for forestry operations. The compressor is directly connected to the turbine by a shaft. The only power loss from the turbine to the compressor is the slight friction of the journal bearings.
Air is drawn in through a filtered air intake system, compressed by the wheel, and discharged into the engine intake manifold usually via an intercooler.
The extra air provided by the turbocharger allows more fuel to be burned, which increases horsepower output. Lack of air is one factor limiting horsepower of naturally-aspirated engines.
As engine speed increases, the length of time the intake valves are open decreases, giving the air less time to fill the cylinders. On an engine running at 2500 rpm, the intake valves are open less than 0.017 second. The air drawn into a naturally-aspirated engine cylinder is at less than atmospheric pressure.
A turbocharger packs the air into the cylinder at greater than atmospheric pressure. The flow of exhaust gas from each cylinder occurs intermittently as the exhaust valve opens. This results in fluctuating gas pressures (pulse energy) at the turbine inlet. With a conventional turbine housing, only a small amount of the pulse energy is needed.
Also called Twin Flow, Twin Scroll Twin Port or Dual Port Turbochargers, this refers to the turbine (exhaust) hous design with 2 exhaust entry ports. To better utilise these impulses, one design has an internal division in the turbine housing and the exhaust manifold which directs these exhaust gases to the turbine wheel. There is a separate passage for each half of the engine cylinder exhaust.
On a six-cylinder engine, there is a separate passage for the front three cylinders and another passage for the rear three cylinders.
By using a fully divided exhaust system combined with a dual scroll turbine housing, the result is a highly effective nozzle velocity. This produces higher turbine speeds and manifold pressures than can be obtained with an undivided exhaust system.
Also called Twin Flow, Twin Scroll Twin Port or Dual Port Turbochargers, this refers to the turbine (exhaust) hous design with 2 exhaust entry ports. This type of turbine housing design matched to a split pulse exhaust manifold design lifts low engine speed performance but making the turbine side of the turbo up to approx 15% more efficient. The benefit of this is that you can still use a relitivly unrestrictive large A/R housing but still maintain good boost responce. This consept has been used in Diesel applications for a sevceral decades but is only begining to gain popularity in the performance tuning scene.
The turbocharger offers a distinct advantage to an engine operating at high altitudes. The turbocharger automatically compensates for the normal loss of air density and power as the altitude increases.The appearance, construction, and operation of the altitude compensator is the same as that of a turbocharger. However, the purpose is different.
The purpose of a turbocharger is to increase the power output of an engine by supplying compressed air to the engine intake manifold so increased fuel can be utilised for combustion.
The purpose of the altitude compensator is to maintain consistent power output and efficiency of an engine operating at all altitudes. This is done by supplying compressed air to the engine intake manifold at a pressure about equal to that at sea level.
There is no increase of fuel for combustion and consequently no increase in basic horsepower of the engine. However, the extra air provided by the altitude compensator normally increases combustion efficiency, which generally will improve fuel economy and reduce smoke level.
With a naturally aspirated engine, horsepower drops off 3 percent per 1000 ft (300m) because of the 3 percent decrease in air density per 1000 ft (300 m). If fuel delivery is not reduced, smoke level and fuel dilution will increase with altitude.
With a turbocharged engine, an increase in altitude also increases the pressure drop across the turbine. Inlet turbine pressure remains the same, but the outlet pressure decreases as the altitude increases. Turbine speed also increases as the pressure differential increases. The compressor wheel turns faster, providing approximately the same inlet manifold pressure as at sea level, even though the incoming air is less dense.
However, there are limitations to the actual amount of altitude compensation a turbocharged engine has. This is primarily determined by the amount of turbocharger boost and the turbocharger-to-engine match.
All turbochargers operate at a very high speed. This can range from 40,000 to over 300,000 rpm.
There are five basic reasons for using an engine turbocharger:
1. To increase horsepower output of a given displacement engine: Where the engine compartment of a machine is of a given size, a turbocharged engine can be used to provide increased horsepower without having to enlarge the engine compartment for a larger displacement engine.
2. To reduce weight: Turbocharged engines have more horsepower per Kilogram than non-turbocharged engines.
3. To keep down costs: Initial cost of turbocharged engines, on a dollar per horsepower basis, is less than for a naturally aspirated (N.A.) engine, and the differential increases with the rate of turbocharging. It all adds up to more horsepower per dollar.
4. To maintain power at higher altitudes: The altitude compensator also falls in this category, giving vital machine productivity at high altitudes.
5. To reduce smoke: Turbocharging can be an effective way to reduce exhaust density by providing excess air. However, using a turbocharger does not ensure this, as many other components also affect exhaust density and these must be properly designed and matched to provide an acceptable smoke level.
There are four basic ways of using multiple turbochargers on an engine:
1. Twin Turbo. This is where you have 2 turbos being fed exhaust gas from half of the engine each. This is popular in a "V" type engine where one banks exhaust gas feeds each turbo. This is still single stage turbocharging, but using more than one turbo to achieve the desired boost pressure. Common on petrol/gasoline engines.
2. 2 Stage compounding Turbochargers. This is where you have a small High pressure (HP) turbo mounted close to the engines exhaust ports and a larger Low Pressure (LP) turbo being fed from the waste gasses the are coming from the HP turbo without any valving. Sometimes this system uses a wastegate to control the peak pressure. Usually found on hip powered diesel engines where high boost pressures are required. This system offers good throttle response while still achieving high engine performance and very high boost levels by way of compounding the boost pressure.
3. Staged Turbocharging. This is where you have 2 turbos, one larger than the other in a staged setup using valving to bring the 2nd (usually larger) turbo into play at a desired engine speed or load. This system offers good throttle responce as well as high volumens of air flow. Typically yhr boost pressure is not compounded in this setup.
4. Regulated 2 Stage Turbocharging. This system is complex with valving used to control both the airflow and the exhaust gasses. it provides a seamless transition into high boost with amazing response and can offer very high performance. This system can be used with 2, 3 or more turbos for great effect. Also commonly used with VNT style variable geometry turbochargers.
Here is a link to a short animated video clip by Holset (Cummins Turbo Technologies) explaining the their Electronically controlled Veriable Geomertry nozzle opperation. Click here