Turbo Talk: Definitions

  1. A/R

    Describes a geometric property of all compressor and turbine housings. Increasing compressor A/R optimizes the performance for low boost applications. Changing turbine A/R has many effects. By going to a larger turbine A/R, the turbo comes up on boost at a higher engine speed, the flow capacity of the turbine is increased and less flow is wastegated, there is less engine back pressure, and engine volumetric efficiency is increased resulting in more overall power.
  2. Choke Line

    The choke line is on the right hand side of a compressor map and represents the flow limit. When a turbocharger is run deep into choke, turbo speeds will increase dramatically while compressor efficiency will plunge (very high compressor outlet temps), and turbo durability will be compromised.
  3. CHRA

    Center housing rotating assembly – The CHRA includes a complete turbocharger minus the compressor, turbine housing, and actuator.
  4. Clipped Turbine Wheels (Cut Back Turbine Blades)

    When an angle is machined on the turbine wheel exducer (outlet side), the wheel is referred to as being ‘clipped’. Clipping causes an increase in the turbine wheel’s flow capability as it reduces backpressure in the turbine housing. However, it dramatically lowers the turbo efficiency at low speeds. This reduction causes the turbo to come up on boost at a later engine speed (increased turbo lag). High performance applications should only use a clipped turbine wheel where outright power is the prerequisite. All Garrett GT turbos use modern unclipped wheels.
  5. Corrected Airflow

    Represents the corrected mass flow rate of air, taking into account air density (ambient temperature and pressure) Example: Air Temperature (Air Temp) – 60°F Barometric Pressure (Baro) – 14.7 psi Engine air consumption (Actual Flow) = 50 lb/min Corrected Flow= Actual Flow Ö([Air Temp+460]/545) Baro/13.95 Corrected Flow= 50*Ö([60+460]/545) = 46.3 lb/min 14.7/13.95
  6. Efficiency Contours

    The efficiency contours depict the regional efficiency of the compressor set. This efficiency is simply the percentage of turbo shaft power that converts to actual air compression. When sizing a turbo, it is important to maintain the proposed lugline with a high efficiency range on the map.
  7. Free-Float

    A free floating turbocharger has no wastegate device. This turbocharger can’t control its own boost levels. For performance applications, the user must install an external wastegate.
  8. GT

    The GT designation refers to Garrett’s state-of-the-art turbocharger line. All GT turbos use modern compressor and turbine aerodynamics which represent huge efficiency improvements over the old T2, T3, T3/T4, T04 products. The net result is increased durability, higher boost, and more engine power over the old product line.
  9. On-Center Turbine Housings

    On-center turbine housings refer to an outdated style of turbine housing with a centered turbine inlet pad. The inlet pad is centered on the turbo’s axis of rotation instead of being tangentially located. Using an on-center housing will significantly lower the turbine’s efficiency. This results in increased turbo lag, more back pressure, lower engine volumetric efficiency, and less overall engine power. No Garrett OEM’s use on-center housings.
  10. Pressure Ratio

    Ratio of absolute outlet pressure divided by absolute inlet pressure Example: Intake manifold pressure (Boost) = 12 psi Pressure drop, intercooler (DPIntercooler) = 2 psi Pressure drop, air filter (DPAir Filter) = 0.5 psi Atmosphere (Atmos) = 14.7 psi at sea level PR= Boost +DPIntercooler+ Atmos Atmos-DPAir Filter PR= 12+2+14.7 = 2.02 14.7-.5
  11. Surge Line

    The surge region, located on the left hand side of the compressor map, is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur.
  12. Wastegate

    A wastegated turbocharger includes an integral device to limit turbo boost. This consists of a pneumatic actuator connected to a valve assembly mounted inside the turbine housing. By connecting the pneumatic actuator to boost pressure, the turbo is able to limit its maximum boost output. The net result is increased durability, quicker time to boost, and adjustability of boost.

Useful and important Turbocharger Facts

  • Fully synthetic oils and mineral oils do not mix. In severe cases they can coagulate.
  • If the engine’s oil pressure is low, the turbocharger will be the first engine component to fail.
  • Piston blow-by and the resulting crankcase/sump pressure if not vented off adequately is the most common cause of turbocharger oil leakage.
  • The most common contaminants found in the oil are ‘free floating’ carbon deposits, fuel and the by-products of combustion.
  • A typical diesel turbocharger with rotor speeds in excess of 200,000 R. P.M. will have a blade speed on the compressor wheel of 850 miles per hour.
  • Many operators assume, quite wrongly, that if they run an engine with dirty or contaminated oil, the oil filter will remove any foreign matter before it reaches the engine and more importantly the turbocharger.
  • Oil delay of a mere four seconds will start to cause journal and thrust bearing wear. Delay of just eight seconds can cause irreparable damage. This damage will not necessarily manifest itself immediately, the final failure may occur after several days.
  • The average temperature of the exhaust gas, at the entry point to a diesel turbo, is 800 degrees centigrade. A petrol engine can reach 1000 degrees, glowing bright yellow. Hot enough to melt window glass.
  • New generation turbocharger impellers rotate at up to 220,000 revs per minute. The impellers on a Boeing 747 engine rotate at about 7,000 revs in comparison.
  • The air entering the compressor impeller of the turbocharger can be traveling at a speed close to mach 1.
  • At average engine revs, a medium size turbocharger will swallow 130 cubic feet of air per minute, equivalent to the interior volume of a transit van.
  • Turbo shaft balance is crucial – imbalance at maximum revs equivalent to a 2 kilogram force is acceptable. We often find turbos supplied for service with 6 kilos of imbalance. This is equivalent to driving along with a house brick attached to your wheel rim.
  • The “hot end” turbine blades in a turbocharger, are made from a high nickel content alloy, as used in jet aircraft engines. A blade will travel in the region of 820 mph at average engine speed, and the exhaust gas entering it will be supersonic.
  • The turbocharger rotor will accelerate from 20,000 revs per minute to over 150,000 revs per minute in less than one second.
Turbine Wheel