Tuning Wiki

Forced induction

Supercharger and turbocharger types, parallel and sequential twins, twin-scroll and variable-geometry turbines, compound charging and twincharging.

Updated by llandowtuninguk@gmail.com   Revision 1

What forced induction changes

Forced induction raises the pressure of the air entering the engine so that each induction stroke can trap more air mass. More air mass can support more fuel. The power does not come from the boost gauge. It comes from the extra fuel that the extra oxygen can burn, for as long as knock, exhaust temperature and the mechanical parts allow it.

There are two ways to drive the compressor. A supercharger is driven mechanically, almost always from the crankshaft. A turbocharger is driven by the exhaust. Garrett's description of a turbo system is the clearest short account of the gas path: ambient air is compressed, which raises its density; a charge-air cooler often cools it and raises the density further; the engine can then burn more fuel; the exhaust leaves with enough energy for the turbine to drive the compressor. The turbine takes that energy from a drop in exhaust pressure and temperature, and it raises the exhaust pressure the engine has to push against.

Both drives heat the air. What that heat does to density, knock and the rest of the system is covered in charge cooling, intercoolers and water injection. The fuel and ignition changes that usually follow are covered in fuel system upgrades, ignition timing and ECU remapping.

Superchargers

A supercharger can make boost without waiting for exhaust energy. The cost is that the crankshaft has to drive it. Volkswagen's self-study programme for the 1.4-litre dual-charged TSI lists that trade directly: the mechanical charger builds boost quickly and can be clutched out when it is not required, and it consumes drive power from the engine.

Roots-type

A Roots-type supercharger is a positive-displacement air mover. Lobed rotors carry packets of air from the inlet side to the manifold side. Eaton's TVS family is a modern development of that layout, with helical rotors rather than straight ones. Eaton describes the airflow as metered and not reliant on exhaust energy, which is why the response does not wait for a turbine to accelerate.

Eaton rates the current TVS range at a pressure ratio of 2.5 and quotes a speed capability of 18,000 to 24,000 rpm, wider than its earlier generations. The R-series uses four lobes and Eaton aims it at peak power, with the best efficiency at high pressure ratio and high speed. The V-series uses three lobes and Eaton aims it at efficiency at lower speed and moderate pressure ratio, including installations where the supercharger and a turbocharger work together. Those pressure-ratio and speed figures are Eaton's ratings for TVS hardware, not a boost target for every engine the blower can be fitted to.

Twin-screw

A twin-screw supercharger is also positive displacement, but the rotor pair is designed so that the trapped volume gets smaller as the air moves through the machine. Compression therefore starts inside the supercharger rather than only in the manifold downstream. Volkswagen's SSP 359 identifies the mechanical charger on the 1.4-litre dual-charged TSI as a twin-screw unit because of the shape of its two rotors. On that engine the charger is clutched, belt-driven through the coolant-pump module, and geared so that it turns at five times crankshaft speed, with a stated maximum of 17,500 rpm. SSP 359 gives the charger's own maximum as about 1.75 bar absolute. Those speeds and pressures belong to the engine in that document.

Centrifugal

A centrifugal supercharger uses an impeller of the same kind as a turbocharger compressor. Rotrex drives that impeller through a planetary traction drive: a belt pulley turns the input, the traction drive multiplies the speed, and the impeller draws air in, accelerates it and delivers it to the compressor housing. The traction fluid is the drive mechanism as well as the lubricant, and it has its own cooler and reservoir.

Because the pressure rise comes from impeller tip speed, boost on a fixed pulley ratio rises strongly as engine speed rises. A centrifugal unit is typically modest at low engine speed and strongest at the top of the range. A positive-displacement unit moves a volume related to its own speed, so its boost arrives earlier and more evenly. Neither shape is universally better. The shape has to match the speed range the engine is used in, and either one still heats the air.

Turbochargers

A turbocharger puts a turbine and a compressor on one shaft. Exhaust expands through the turbine and the compressor raises the inlet pressure. There is no belt load of the sort a supercharger has. There is exhaust backpressure, heat, and a delay while the shaft accelerates. The housing and wheel sizes decide where in the rev range that delay sits.

Turbine housing A/R

Garrett defines turbine A/R as the inlet area divided by the radius from the turbo centre to the centroid of that area. A smaller A/R speeds the exhaust into the wheel, so the turbine makes more power at lower engine speed and boost rises sooner. The same small passage meets the wheel more tangentially, limits the wheel's flow and raises exhaust backpressure, which works against the engine at high speed. A larger A/R does the opposite: slower response, more high-speed flow and less backpressure. Garrett's point is to choose the A/R for the speed range the vehicle actually uses.

Wastegate

A wastegate lets exhaust bypass the turbine so the shaft does not keep accelerating once the required boost is reached. Volkswagen's dual-charged TSI uses a wastegated turbocharger, and BorgWarner builds wastegated stages into its regulated two-stage systems. The valve may sit in the turbine housing or in a separate body. The job is the same. The control signal, whether pneumatic or electric, is part of the calibration: changing the compressor without changing the wastegate control does not by itself produce a stable new boost level.

Bearings

Garrett describes two bearing arrangements. The older one uses journal bearings and a separate thrust bearing. The cartridge ball-bearing arrangement uses angular-contact ball bearings at either end of a single sleeve. A ball-bearing cartridge is a mechanical choice for friction and thrust, not a power figure. The shaft still needs clean oil. BorgWarner's description of its later two-stage unit for the Mercedes-Benz OM 656 also includes water-cooled bearing and compressor housings, which is one way to limit heat in the shaft after the engine stops. Do not assume every turbocharger has a water jacket.

Single, twin and sequential turbos

A single turbocharger carries the whole exhaust flow through one turbine and one compressor. It is the simplest layout. The wheel and housing have to cover both the bottom of the speed range and the top, which is the compromise A/R describes.

Parallel twins

In a parallel layout, two turbochargers operate at the same time. Each turbine takes the exhaust from a group of cylinders, often one bank of a V engine or one half of a divided inline manifold, and the two compressors discharge together into the inlet. Each unit can be smaller than a single turbo sized for the whole engine, so each shaft has less inertia. The two units need a sensible split of the exhaust pulses. An uneven split, or a balance pipe that mixes the groups too early, gives away the reason for having two.

Sequential twins

A sequential layout changes which turbo is doing the work as flow rises. One turbo, or one stage, carries the low-speed work. A second path opens later. The name is used loosely. Garrett's own "two-stage parallel" description says the two turbos work in sequence, on engines it associates with about 2.0 litres and larger. That is a product description, not the workshop meaning of two equal turbos in parallel. Look at the pipework. If both compressors are fed from the air filter and both blow into the same inlet all the time, the layout is parallel. If a valve brings the second turbine or the second compressor in only after a set flow, the layout is sequential.

Twin-scroll

A twin-scroll, or divided, turbine housing has two volutes. Garrett pairs it with a divided manifold whose purpose is to keep interfering exhaust pulses apart. On a four-cylinder with firing order 1-3-4-2, cylinder 1 opens its exhaust valve at the end of the expansion stroke while cylinder 2 can still be on overlap. In one shared manifold, the blowdown pulse from cylinder 1 can push exhaust back into cylinder 2. Separating those cylinders preserves the pulse energy that helps the turbine respond, and it reduces that contamination. A twin-scroll housing on an undivided manifold does not create the separation. The manifold and the housing have to be divided together, and the pairs have to be the cylinders that would otherwise disturb each other.

Variable geometry

A variable-geometry turbo changes the turbine inlet instead of relying on one fixed A/R. Garrett's diesel VNT uses a ring of vanes. At low engine speed the vanes close, the exhaust is forced through a smaller passage, turbine power rises and boost rises. At high engine speed the vanes open, so the turbine can pass the flow without overspeeding, and the boost can be held where it is wanted. Garrett also describes this as a way to support exhaust-gas recirculation, because the turbine can be used to set exhaust pressure as well as boost.

That vane account is Garrett's diesel design. A petrol engine's exhaust is hotter, so a variable-geometry petrol turbo is a specific high-temperature design. It is not a diesel unit moved across. The control is still a calibration problem: the vane position is an actuator, and the boost and exhaust-pressure results belong to that engine's software and hardware together.

Compound and two-stage charging

Compound charging, in turbocharger terms, means the air is compressed more than once. A larger low-pressure turbocharger and a smaller high-pressure turbocharger are placed in series. The low-pressure compressor feeds the high-pressure compressor. On the exhaust side the high-pressure turbine usually sees the gas first, and the low-pressure turbine sees it afterwards. Garrett calls this a two-stage serial arrangement and describes it as a way to improve transient response and low-speed torque, on passenger and commercial diesels.

BorgWarner's regulated two-stage system, R2S, is this series layout with bypass valves so that the handoff can change. BorgWarner's 2017 description of the unit on the Mercedes-Benz 2.9-litre OM 656 is specific: at low engine speed the small high-pressure turbo, with variable turbine geometry, provides most of the boost; as engine speed rises a bypass opens and nearly all of the exhaust goes to the larger wastegated low-pressure turbo, which then does the main compression. The 2019 OM 654 description uses the same idea, with variable geometry on the high-pressure stage. BorgWarner notes that this is the common arrangement, and also that the 2.0-litre BMW diesel application announced in 2019 was built the other way round: variable geometry on the low-pressure turbo and a wastegate on the high-pressure turbo. The stage that carries the variable geometry is an application choice. Copying the actuator from one R2S engine to another is not valid.

A 2004 BorgWarner paper on the same regulated two-stage idea shows why the bypass plumbing matters. With only a high-pressure turbine bypass, both stages can still compress at high engine speed, and the high-pressure compressor remains a flow limit. Adding a bypass around the high-pressure compressor lets the low-pressure turbo work alone at high speed, which is the passenger-car pattern. The paper treats these as different matches, not as one generic "compound kit".

Supercharger and turbocharger together

Twincharging uses both a mechanical supercharger and a turbocharger. Eaton's V-series TVS is explicitly aimed at that combination, because its efficiency is biased toward lower speed, where the turbocharger has least exhaust energy. Volkswagen's SSP 359 is a production example with a clutched twin-screw supercharger and a wastegated turbocharger in series.

On the engine SSP 359 describes, the supercharger is brought in when response is required, up to about 3,500 rpm. A regulating flap decides whether intake air is sent through the supercharger or onward toward the turbocharger. From about 3,500 rpm the document says the turbocharger can hold the required boost alone, the flap opens fully, and the air goes to the turbocharger without the supercharger. Manifold pressure in that description reaches up to 2.0 bar absolute. The 103 kW and 125 kW versions are both in the scope of the programme. Use the figures as that engine's description, not as the boost a different supercharger-and-turbo pairing will make.

The advantage is the one each device already has. The supercharger covers the delay while the turbo shaft speeds up. The turbocharger covers the higher engine speeds without keeping the supercharger clutched in. The costs add up as well: belt drive, a clutch, the flap, the intercooler, and a control system that has to decide which path is open. Heat still has to be removed. SSP 359 includes an intercooler in the engine specification for that reason.

What has to be matched

  • Compressor map. Pressure ratio and mass flow have to stay inside the map the wheel was measured on. High pressure at low flow is where a centrifugal compressor surges. A compressor bypass, or blow-off valve, exists to move the operating point when the throttle shuts and the flow collapses.
  • Exhaust energy. A turbine only produces what the exhaust supplies. Late ignition, a large leak before the turbine, or an oversized housing all delay boost. A housing that is too small brings boost early and then costs high-speed flow.
  • Fuel and knock. Extra air without extra fuel, or extra boost without a knock margin, is not a completed upgrade. See the fuel, ignition and remap pages.
  • Charge temperature. Pressure without cooling is only part of the density gain. See charge cooling.
  • The driven parts. Cylinder pressure and torque rise with the fuel that is burned. The head gasket, pistons, rods and transmission are part of the limit even though they are not part of the compressor.

Sources