Turbochargers are standard fare on modern diesel engines. At some point, you will probably need (or want) to replace/upgrade the turbocharger(s) in your truck. Some of the turbocharger inner-workings and specifications can be a little hard to comprehend. In this tutorial, we examine the major components of the turbocharger and some of the common terminology.
Origin of the Turbocharger
The original idea for the turbocharger can be traced all the way back to 1905. Alfred Büchi, a Swiss engineer, first patented the idea of capturing exhaust and using an axial compressor and axial turbine on a common shaft. In 1915 he created his first prototype which unfortunately failed, but this did not stop him. He continued to work on his idea for ten more years until he got it to work. In 1925 for the first time, Büchi succeeded in combining his technology with a diesel engine.
The primary function of a turbocharger or “turbo” is to boost the intake air pressure of an internal combustion engine. A turbocharger consists of a compressor wheel and a turbine wheel coupled together by a solid shaft. The turbine wheel extracts energy from the exhaust gas and uses it to drive the compressor wheel. This compressed air is pushed out, allowing the engine to burn more fuel and ultimately produce more power.
Turbocharger vs Supercharger
So now you might be wondering, how is a turbocharger different than a supercharger? A turbocharger is powered by exhaust gas, while a supercharger is driven by the crankshaft (via a belt, gear, or chain). There are different styles of superchargers such as centrifugal, roots, and twin screw. Regardless of the supercharger style they are all engine-driven.
Turbochargers and superchargers both have their pros and cons. One benefit of a supercharger is that they provide boost almost instantly. Although turbo technology in recent years has really helped to offset some of the traditional turbo lag associated with turbochargers in previous years.
One big advantage of turbochargers is that it capitalizes on some of the “free” energy that would otherwise be completely lost in the exhaust. Driving the turbine will increase exhaust back-pressure slightly, which does exert some load on the engine. The net loss still tends to be less by comparison with the direct mechanical load that is required to drive a supercharger.
MAJOR COMPONENTS OF A TURBOCHARGER
Now let’s examine the major components of a turbocharger. The turbo can be split into three main components.
- Center (Bearing) Housing
The turbine consists of the turbine housing and the turbine wheel. In many ways, the turbine can be considered the heart of the turbocharger. The turbine housing guides the exhaust gas into the turbine wheel. The energy from the exhaust gas, in turn, spins the turbine wheel. The gas then exits the turbine housing through an exhaust outlet. The turbine extracts this energy from the exhaust to turn the compressor wheel, so the turbine is basically where it all starts.
The compressor also consists of two parts: the compressor cover and the compressor wheel. The compressor wheel is attached to the turbine by a forged steel shaft. As the turbine turns the compressor wheel spins in relation. This high-velocity spinning draws in air and compresses it. The compressed air is pushed out, which in turn allows the engine to burn more fuel and ultimately produce more power.
Center (Bearing) Housing
The center (bearing) housing is located between the turbine and compressor and supports the rotating assembly. Located inside here is the shaft connecting the two wheels, bearings, as well as oil inlet/outlets for cooling.
When shopping for a turbocharger for your truck many people can get confused by all the different terms. Let’s examine some of the most commonly talked about turbocharger terminology.
A/R (Area/Radius) is a term used to define a geometric characteristic of all turbine and compressor housings.
A measurement of how much air/exhaust can flow through the turbo. Airflow is measured in cubic feet per minute (CFM).
Steel or ceramic bearing cartridge used in place of less expensive brass-sleeve bearings (journal bearings). Ball bearings can provide faster turbo spool-up and require less oiling pressure than stock journal bearings.
The center of the turbo that houses the turbine piston ring seal and the bearings. Utilizes pressurized oil from the engine.
A valve between the turbo and the intake manifold that vents air to avoid turbo surge when a preset pressure limit (boost) is surpassed.
The intake pressure created by the spinning of the compressor wheel inside the housing. Measured in pounds per square inch (PSI) over the normal atmospheric pressure (14.7:1).
A mechanical or electrical device that changes the boost-pressure signal sent to actuate the wastegate, allowing higher boost pressures than would normally be permitted.
When boost rises past a set limit. This is often caused by a wastegate that cannot handle the exhaust flow.
A period of uncontrolled boost when the wastegate and/or blow-off valve cannot act fast enough because of sudden changes in the engine load.
When engine conditions provide enough exhaust pressure to create boost in the intake manifold.
Boreless Turbo / Wheel
A turbo that uses a compressor wheel that does not have a hole drilled through it. This increases the strength of the compressor wheel in the highest stress area.
The right side of a compressor map that indicates the flow limit.
CHRA (Center Housing Rotating Assembly)
The CHRA is the center section of a turbocharger in between the compressor and turbine housings. It incorporates the complete rotating assembly, shaft & wheel, and bearings.
Two or more turbos that feed into each other in series to build high boost pressures.
A graph that diagrams the performance of a turbo by showing turbo speed, efficiency, flow rate, and boost pressure. The left border represents the surge line, the right side shows the choke line, and the patterns in between are efficiency islands.
Radial multi-bladed compressor in a turbocharger that draws in ambient filtered air, compresses it within housing, and then blows it out into the intake of the engine.
Corrected Air Flow
To plot the air flow data on a compressor map, you must ensure the flow is corrected to account for differences that affect air density, such as atmospheric conditions.
Divided Turbine Inlet
A split inside the exhaust manifold and turbine housing that separates exhaust output from cylinders to prevent turbulence based on the firing order. Also known as twin scroll.
The exhaust pipe that leads from the turbine outlet to the exhaust system under the vehicle.
Exhaust-gas recirculation routes some exhaust gas back into the intake manifold after it passes through a water-cooled heat exchanger. EGR reduces emissions of nitrogen oxides.
The Exhaust Gas Temperature, should be monitored to prevent the turbo from overheating, which can lead to failure (1,250 degrees F maximum for extended periods).
Where flow exits the turbine or compressor wheel.
A wastegate that is not built into the turbo.
This is a turbo without a Wastegate. Therefore, the turbo is unable to regulate its own boost levels.
Another name for a compressor wheel.
Where flow enters the turbine or compressor wheel.
Air-to-air or air-to-water radiators used between the turbo and the intake manifold to reduce intake temperature, which is heated by the pressurization inside the turbo (also called aftercharger or charge air cooler).
A built-in valve that diverts exhaust gases away from the turbine when a certain boost level is reached on the compressor side of the turbo. Can be mechanically or electrically controlled.
Fits inside the bore of the bearing housing and prevents radial movement of the shaft & wheel.
Mass Flow Rate
The mass of air flowing through a compressor over a given period of time and is commonly expressed as lb/min (pounds per minute).
A device that reduces the amount of oil delivered to a ball-bearing turbo because it requires less oil pressure than a stock journal-bearing unit.
Routes engine oil to and from the turbo to lubricate the bearings in the center section.
Open Turbine Inlet
An exhaust manifold and turbine housing with a single opening (no divider). Also known as a single scroll.
On-Center Turbine Housings
On-center turbine housings is a turbine housing with a centered turbine inlet pad.
The term used when a turbo is operating well above its normal operating limits.
Absolute outlet pressure divided by absolute inlet pressure.
The component, usually made from aluminum, has a bore housing the compressor end piston ring seal that runs in the groove of the oil flinger or thrust collar. On some models, this component doubles up as being the supporting flange that the compressor housing fixes to.
Shaft & Wheels
The complete assembly that describes the turbine wheel and shaft as one component – (Also referred to as a ‘Turbine Wheel’). It uses the exhaust flow from the engine to spin a turbine, which in turn spins an air pump.
Holds the compressor wheel and other rotor group parts onto the shaft. This normally has a left-hand thread on modern turbos, this high precision machined nut must be tightened to a specific torque.
Another term for turbo boost. A turbo is spooled up when it is creating boost in the intake manifold.
When boost pressure builds up to the point that it causes the compressor wheel to stall. This can be prevented with the use of bypass valves. Also referred to as Bark.
The left boundary of a compressor map that represents a region of flow instability.
A single flat bearing that supports the positive and negative axial loads generated when the turbo rotor accelerates and decelerates. The bearing is fixed, and the thrust collar runs within it. A pressurized oil film separates the ramped faces of the bearing from the journal surfaces of the collar.
The half of a complete thrust collar that runs on the underside of the thrust bearing. The thrust collar is sometimes in two pieces for applications like a 360-degree thrust bearing.
Located on the turbine shaft, this sits through the hole in the seal plate and acts as a separator/seal between the thrust bearing, seal plate and compressor wheel.
Fits between the journal bearing and thrust bearing, acting as a cushion to prevent the parts form touching.
The area ratio used to define the turbine and compressor wheels. To calculate the trim, you use the inducer and exducer diameters. For Example: Inducer2/Exducer2 = Trim
A wheel that is spun by exhaust gases that pass through the fins and into the housing before dumping into the exhaust pipe.
The time it takes for a turbo to spool up after the throttle has been increased.
Turbo Speed Lines
Lines on a compressor map that represent the rotational speed of the compressor wheel.
A system using two turbos mounted in parallel.
The exhaust pipe that connects the exhaust manifold to the turbo.
A turbo that uses variable vanes or a sliding nozzle to alter the volume inside the exhaust housing to maximize turbo speed at low engine rpm. Also known as variable-turbine geometry (VTG).
Adjustable blades that route exhaust gases directly into the turbine wheel at low engine rpm to increase spool on tap at low speeds.
Bypass that diverts excess exhaust gases away from the turbine once a preset boost level is reached in the compressor side of the turbo. It can be built into the exhaust-turbine housing (internal) or can be separate from the turbo housing (external).
Stay tuned for upcoming articles where we will discuss everything else turbo related!