Turbochargers at High Altitude
The Volkswagen TDI engine with it's Garrett Turbocharger is a work of art in more ways than one. It's turbocharger is able to operate under a multitude of conditions ranging from sea level to two miles high and from desert to blizzard conditions. All of these various conditions cause drastic changes in air density caused by differing ambient temperatures and atomspheric pressures. Volkswagen designed this engine to function efficiently under these difficult conditions and this is one of the reasons it is one of the most advanced passenger car motors in the world with over 2 million units in operation.
Turbocharger design and matching is relatively straightforward for most industrial and marine applications due to the limited speed and load ranges required. Matching a turbocharger to an automotive application is much more difficult due to the wide speeds and load variations encountered.
The easiest engine to design a diesel turbocharger for is a stationary one such as for a power plant or in a ship at sea level, its altitude is known and it is selected to operate at a constant rpm under a constant load.
The next most easiest diesel engine for design a turbocharger for is a truck in a mobile application. The compressor is selected to operate under a mostly constant load and at a specific rpm at normal altitudes and normal ambient temperatures. The turbocharger is designed to match the normal operating environment of the truck with sufficient margins designed for surge protection, turbine inlet temperature, turbo speed and boost levels.
Turbocharger operation under changing ambient conditions (extreme barometric pressure and/or temperature environments) introduces additional complications for the manufacturer of both the turbocharger and the engine. Foremost is a reduction of the surge margin (the left hand portion of the compressor map) while still offering partial compensation for reduced mass air inlet density at higher altitude levels. As air density and air mass flow rate are reduced, the turbine inlet temperature will rise due to the richer air-fuel mixture resulting. This means that the ratio of compressor to turbine pressure ratios will increase. This pressure ratio increase offsets the reduction in air inlet density at higher altitudes in a turbocharged diesel engine. Also, as the ambient temperature falls, the expansion ratio of the turbine increases, raising the compressor ratio as long as the turbine inlet pressure does not fall at the same rate as the ambient pressure. An increase in ambient temperature has an undesirable effect on the turbine to compressor energy balance to the effect that the turbine will amplify this effect to a change in air flow rate. Low ambient temperatures reduce the required compressor power, so boost rises, which may result in compressor surge.
Selection of a turbocharger compressor wheel is mostly a matter of ensuring a sufficient surge margin while maintaining that the engines operation at both maximum torque and maximum power occur
at reasonable compressor speeds and efficiencies. A good compressor wheel with the correct trim will function clear of the surge line and lie in an area of high operational efficiency.
Wide variations in ambient conditions of a turbocharged diesel engine can lead to problems due to compressor surge, excessive cylinder pressures, turbine inlet temperatures, turbocharger speed, and smoke emissions. Here a couple of examples:
If air mass flow rate and compressor pressure ratio changes, movement across the operation map of the compressor will be accompanied by changes in efficiency. It follows that a similar engine at sea-level will not necessarily perform comparatively at high altitudes. Detailed and rigorous turbine and compressor maps are required to predict the effect of high altitudes and low temperatures on turbochargers in contrast to sea-level operation. A simpler approach is to simply compare the turbocharged engines operation at a constant altitude in either very hot or very cold conditions as this approach emulates the effects of high altitude operation.
At high ambient temperature, the limiting factor in the operation of a turbocharged diesel engine is smoke emission, due to the reduced air flow. The second and third limiting factors are turbine inlet and exhaust valve temperatures or the thermal loading of the engine in its entirety. Conversely, at low ambient temperatures, compressor surge (due to a very high pressure ratio) or maximum cylinder pressure will be the limiting factor. The limitation of the turbocharged diesel engine will be to what extent fueling must be reduced to permit reliable operation or excess smoke emission. Compared to a naturally aspirated diesel engine, the turbocharged diesel engine offers partial compensation of the air inlet density reduction at altitude for the above mentioned factors and therefore is not affected to the extent that a naturally aspirated engine is.
The effect of high altitude on a turbocharged diesel at full power shows that although the absolute inlet manifold pressure reduces with high altitude, the fall-off is slower than that of ambient temperature. Turbocharger speed increases due to the increase in turbine inlet temperature and expansion ratio up to a point. This point is limited by the thermal limits and the maximum permissible turbocharger speed, especially the latter. The movement toward surge on the compressor map will be greatest for a non-intercooled engine, since the post boost air mass density will be reduced considerably due to higher temperatures.
If a turbocharged diesel engine is designed for operation at sea-level and moved to higher altitudes without being rematched for operation at higher levels, then initially smoke emission, then turbocharger speed, and then inlet temperatures will be the factors governing the reduction in fuel input.
The problem of overspeeding the turbocharger and coping with excessively high cylinder pressures becomes more prominent when the turbocharged diesel engines operates over a very wide speed range (1,000 to 5,000 rpm instead of 1,000 to 2,200 rpm) as is the
case in the Volkswagen TDI passenger car. This has been taken into effect in the A3 TDI's Garrett GT15 turbo by the use of wastegate to shunt part of the exhaust gas flow past the turbine and simultaneously increase the exhaust flow area to prevent build up of exhaust back pressure into the cylinders. This reduces both turbine work and effective compressor boost pressures, while maintaining good engine power and fuel economy without deterioration at higher rpm operation.
With the development of the Garrett VNT15 variable nozzle turbocharger for the Volkswagen TDI turbodiesel engine many of these concerns have been addressed and resolved. However this could not have been completed without the electronic control using modern computerized software alogrithims to keep the turbine and compressor operating in unison. In fact the Garrett VNT turbos were first designed and created over 15 years ago, however now they are first being utilized in modern passenger car turbodiesels specifically due to the simultaneous development of the computer hardware and software to allow the control of these powerful turbochargers under a wide variety of applications. The development of the BOSCH VE VP 37 rotary distributor microprocessor fuel injection pump and its associated equipment also has allowed additional freedom in the fuel delivery over the speed range of the engine to match the VNT turbocharger operation. The VNT's turbocharger operation has been closely linked with the entire fuel system and must maintain this match throughout its entire operation by maintaining optimum fuel injection rates, extremely high fuel pressure rates, extremely precise nozzle sizes, and specific combustion chamber swirl. The factor that is the most restrictive when trying to achieve a desirable torque characteristic with the VNT turbo is the low speed smoke limit. This should be no big surprise since it is normal for boost pressure to rise with engine speed as a result of the exhaust flow rate increase of the turbine. The smoke limit is caused by insufficient boost pressure, and hence air flow, at low engine speeds. In order that the TDI engine is to make an effective torque curve, the fuel delivery (per cycle) is held relativley constant over the speed range while the VNT successfully raises the boost effectively at very low engine speeds. This is achieved by the increased A/R ratio that is a function of the nozzle opening on the VNT turbine inlet so that the thermodynamic availability of exhaust energy delivered to the turbine, ie its specific available energy, has been increased at both low speed and high speed operation. By keeping the fuel delivery relatively unchanged while the turbine energy is increasing and boost pressure is increasing, it weakens the air-fuel ratio and reduces low speed smoke. This has been the dream of turbocharger engineers for decades! Low exhaust smoke, low fuel consumption, and high BMEP are ideal.
Now what does all this mean to us as VW TDI performance addicts? First, a super clean high flow air filter is absolutely essential to keep the turbocharger operating in its maximum effective rate zone on the compressor map. As much as a 20% decrease in air mass flow into the air intake due to an air filter restriction, especially at higher altitudes, would push the compressor towards the surge line and decrease its efficency tremendously. A clean
high flow air filter is a must. A Garrett VNT15 turbocharger on a Volkswagen TDI engine simply cannot tolerate a dirty air filter or otherwise compromised air intake mass.
Second, due to the very effective air-air intercooler that is standard on the VW TDI, a reduction in compressor post-boost air temperature down to 30-40 degrees F. above ambient is outstanding. This intercooler is highly effective in reducing the intake air temp and increasing the intake air density, while at the same time reducing peak combustion chamber temperatures, exhaust valve temperatures, and exhaust gas temperatures. Volkswagen not only did an excellent job designing this air-air turbocharger intercooler for high performance and long engine life, it also decreases NOx production about 30% as well. Only when the turbocharger boost level is raised above the OEM levels is this intercooler capacity diminished. While a boost from OEM 13 PSI to about 16 PSI using a Wetterauer Software EPROM is acceptable at all temperatures and altitudes, a boost above the 16 PSI level begins to raise the compressor air temperature levels straining the capacity of the OEM intercooler. In fact, an intercooler contaminated with oil and sludge build-up from lack of cleaning can easily decrease the efficency of the TDI's OEM intercooler 40-50% with a resultant rise in intake air temperature, lowered fuel economy, higher engine temperatures, and long term thermal stress on the entire engine, all NOT GOOD. In addition, torque and horsepower levels are directly diminished as well.
Third, fuel quality becomes even more essential for good reliable operation at higher levels of either performance or altitude. Therefore, a 50 cetane fuel level should be utilized as a minimum in order to effectively reduce combustion lag, combustion chamber pressure peaks, low speed smoke limits, and operational thermal stress on the VW TDI turbodiesel engine under load.
Fourth, the addition of a Techtonics Tuning turbo exhaust system will increase the turbocharger boost pressure by increasing the exhaust mass air flow and reducing backpressure into the combustion chamber for more effective high rpm operation. Stronger torque levels at lower rpms are due to more heat and air mass passing through the turbine.
The take home message is that the Garrett VNT15 turbine and compressor do an outstanding job under a wide variety of conditions attendent on the fact that the ECU controls the fuel and turbo pressure levels according to ambient temperatures and altitudes up to 10,000 feet via internal MAF fuel maps, MAP boost pressure maps, IAT intake air temperature maps, and ambient barometric determinations . The higher you are above sea level, the more difficult it is to produce the same level of work, with increase attendant risks to asking for higher performance. Pay attention to the four main restrictions to TDI turbo power at altitude listed above to keep ahead of the game and the competition!
