Turbocharger

01

Match the compressor map to the engine

Garrett's compressor-mapping guide defines pressure ratio as absolute compressor outlet pressure divided by absolute inlet pressure. At sea level (14.7 psia), 12 psig of boost gives 26.7 ÷ 14.7 = 1.82; allowing for a 1 psig depression across the air filter it becomes 1.95, and at 5000 ft (about 12.4 psia) it becomes 2.14. The map plots pressure ratio against mass flow; its left boundary is the surge line, where flow becomes unstable and heavy thrust loading can lead to premature failure, and its right boundary is the choke line, which Garrett defines where efficiency drops below 58%, with turbo speed rising quickly towards over-speed past it. [4]

In Garrett's worked example a 400 hp target needs about 44 lb/min of air; a 2.0 L engine at 7200 rpm then needs a pressure ratio of 3.14 and a 5.0 L engine at 6000 rpm 1.72, so the same airflow lands at different points on the map. A ported shroud in the compressor housing moves the surge line to the left. [4]

Garrett's G25-550 product page shows how a performance turbocharger is listed: 300–550 hp for 1.4–3.0 L engines, a compressor wheel with a 48 mm inducer, 60 mm exducer, trim 65 and housing A/R 0.70, up to 80% compressor efficiency, and a warning that maximum allowable turbocharger speed is 185 krpm, with a speed sensor port to monitor it. [1]

Editorial calculation: 44 lb/min at Garrett's sea-level air density of 0.076 lb/ft³ is about 579 ft³/min, or 579 × 0.0283 ÷ 60 ≈ 0.273 m³/s. At 185,000 rpm a 60 mm compressor exducer has a tip speed of π × 0.060 m × 185,000 ÷ 60 ≈ 581 m/s. [4] [1]

Give the supplier the engine type, displacement and rated speed, target power or airflow, boost and pressure ratio at peak power and peak torque, altitude and ambient range, air filter and charge-air cooler losses, fuel, the compressor map you require the operating points to fall on, maximum shaft speed, and whether a speed sensor port is needed.

02

Choose the turbine housing, boost control and bearings

Garrett's Turbo Tech 102 explains that A/R is the inlet cross-sectional area of the housing divided by the radius from the turbo centreline to the centroid of that area. Turbine performance is strongly affected by A/R: a smaller A/R raises exhaust velocity into the wheel for quicker boost rise but reduces flow capacity and raises backpressure at high engine speed, while a larger A/R lowers backpressure and favours peak power at the cost of slower boost rise. Compressor performance is comparatively insensitive to A/R. [3] The G25-550 is offered with turbine housing A/R of 0.49, 0.72 or 0.92, with a 54 mm turbine inducer, a Mar-M alloy turbine wheel and stainless steel turbine housings each rated up to 1050 °C, and a turbine peak efficiency of 74%. [1]

Boost is controlled on the turbine side. Garrett describes internal wastegates built into the turbine housing (a flapper valve, crank arm, rod end and pneumatic actuator) and external wastegates on the exhaust manifold, both bypassing exhaust around the turbine wheel; some commercial diesel applications use no wastegate, a free-floating turbocharger. [2] Cummins' Holset VGT brochure describes a variable geometry turbine with a patented nozzle ring working with a shroud plate, plus an actuator, to maximise boost over the widest field of engine operation, with stated benefits in engine braking capacity, exhaust gas recirculation control and throttle response. [5]

Turbine housing material follows exhaust temperature. A Wescast patent states that exhaust gas reaches 800 to 900 °C in diesel engines and 950 to 1050 °C in gasoline engines of current passenger vehicles, that this can exceed the working limit of low-cost ferritic cast irons, and that Ni-Resist (D5S) or austenitic stainless steels such as 1.4848 and 1.4849 are then required, at substantial cost. [6]

Garrett contrasts journal bearings, a set of journal bearings plus a thrust bearing, with its ball-bearing cartridge, a single sleeve with angular contact ball bearings at each end. It states that its ball-bearing turbochargers spool up 15% faster, that CART tests showed up to half the bearing power consumption, and that they need less oil, so an oil inlet restrictor is recommended above about 60 psig oil pressure. Water-cooled centre housings limit oil coking from heat soak after a hot shutdown. [2]

Editorial calculation: 60 psig is about 60 × 0.00689 ≈ 0.41 MPa. Multiplying the G25-550's peak compressor (80%) and turbine (74%) efficiencies gives about 0.80 × 0.74 ≈ 59%, an upper bound because the two peaks need not coincide and bearing losses are excluded. [2] [1]

03

Replacement, cartridge and ball-bearing upgrade turbos on offer

Listings range from replacement turbochargers and cartridges for truck, construction-machine and passenger-car engines to ball-bearing upgrade turbochargers for performance cars.

04

Wheels, housings, boost control and the bearing cartridge

05

Map points, speed limit, materials and oil supply for the enquiry

Air flow and pressure ratio only mean something as points on the compressor map, and the speed and temperature limits come from the wheel and housing materials, so state them together.

06

Send the engine, map and turbine requirements together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Engine: [type; displacement; rated speed; fuel]
Target: [power or airflow; boost; altitude]
Compressor: [pressure ratio; map points]
Turbine: [A/R; flanges; wastegate or VGT]
Bearings: [journal; ball; oil and water cooling]
Housing: [material; exhaust temperature]
Quantity: [units]

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