turbocharger

A turbocharger is a turbine-driven, forced induction unit that increases an engine’s efficiency by forcing extra compressed air into the combustion chamber, liberating more power and torque. The key difference between a naturally aspirated engine is the presence and functionality of a compressor. This forces more air, mixing with more fuel, facilitating a greater pressure and hence generating more power.

How does a turbocharger work?

In naturally aspirated (N/A) engines, intake gases are sucked into the engine via atmospheric pressure, filling the void of the downward stroke of the piston. The volume of air, compared with the theoretical volume, if the engine could maintain atmospheric pressure, is called volumetric efficiency. A turbocharger optimises an engine’s volumetric efficiency by increasing density of the intake gas facilitating an increase of power on each engine cycle.

The turbocharger compresses ambient air from which it enters the intake manifold at a heightened pressure. This results in a greater mass of air entering the cylinders, per intake stroke. The energy required to spin the centrifugal compressor is derived from the kinetic energy of the recycled exhaust gases.

 

Turboboost

Boost is the term by which the manifold pressure exceeds atmospheric pressure. The control of a turbocharger can be by the use of a waste-gate, blow-off valves and variable geometry.

 

Can a turbocharger damage the engine?

In petrol engine turbocharger applications, boost pressure is limited to keep the engine system within its thermal and mechanical design. Over-boosting an engine causes damage through pre-ignition, overheating, and over-stressing the engine’s internals. For example, to avoid engine knocking or detonation, the intake manifold pressure must not get too high, thus the pressure at the intake manifold of the engine must be controlled. Opening the wastegate allows excess energy  to bypass it directly to the exhaust pipe, thus moderating boost pressure. The wastegate can be either controlled manually or by an actuator, via the ECU.

 

What is turbo lag?

Lag is the time required to change power output in response to a throttle change, noticed as a hesitation when accelerating. The exhaust system and turbocharger has to generate the required boost, spooling, which is not instantaneous. Inertia, friction, and compressor load are the primary contributors to turbocharger lag.

 

Do superchargers suffer from lag?

Superchargers do not suffer from turbo lag, as the compressor is being driven by the engine.

 

Turbocharger applications

Turbocharger applications can be categorised into those that require changes in output power (such as automotive) and those that do not (such as marine, aircraft, commercial automotive, industrial, engine-generators, and locomotives). While important to varying degrees, turbocharger lag is most problematic in applications that require rapid changes in power output. Engine designs reduce lag in a number of ways:

  • Lowering the rotational inertia of the turbocharger by using lower radius parts and lighter materials
  • Changing the turbine’s aspect ratio
  • Increasing upper-deck air pressure and improving wastegate response
  • Reducing bearing frictional losses, e.g., using a foil bearing rather than a conventional oil bearing
  • Using variable-nozzle or twin-scroll turbochargers
  • Using multiple turbochargers sequentially or in parallel
  • Using an anti-lag system

 

Petrol powered turbocharged cars

The first turbocharged passenger car was the Oldsmobile Jetfire option on the 1962–1963 F85/Cutlass. Also in 1962, Chevrolet introduced a special run of turbocharged Corvairs, initially called the Monza Spyder (1962–1964). Later, Porsche added the technology to the 911/930 from 1975 and Saab followed with the famous 1978–1984 Saab 99 Turbo.

Today, turbocharging is common on both diesel and petrol-powered cars. Turbocharging can increase fuel efficiency by allowing a smaller displacement engine.

 

 

The first turbocharged cars in F1

In F1 a 1,500cc turbo engine was equivalent to 3,000cc N/A engine. Jean-Pierre Jabouille gave the turbo-engined Renault RS01 its debut at Silverstone in 1977, retiring with turbo failure. 7 races later, he achieved his first finish. The twin-turbo setup came into play in 1979 and within 2 years, Jabouille gained the first victory in the French GP. BMW, Alfa Romeo and McLaren /TAG/Porsche, in 1983, experimented with the forced induction route and applied various configurations, aimed at gaining a competitive advantage. The BMW’s turbo, a truck turbo, was gigantic and the delivery was explosive. The lag was also significant. In later years, this was addressed but always remained a feature with the combination of huge turbo units and relatively tiny engine capacities.

 

BMW turbo F1 cars – extreme qualifying specification

In 1985, 1,300 bhp was extracted from as little as 1,500cc in qualifying specification. Porsche/TAG’s engine powered McLaren to titles in 1984, 1985 and 1986. The most extreme turbocharged F1 car was arguably the Benetton BMW. In 1986 the entire grid had utilised turbochargers. In 1986, a rule restricted to 195 litres of fuel on race day, but in 1987, when a pop-off valve – set to 4 bar, limited turbocharger pressures. The demise culminated in 1989 when the FIA announced a move to a 3.5 litre normally aspirated engine.

 

Diesel-powered turbocharged cars

The first production turbocharger diesel passenger car was the Mercedes 300SD, produced from 1978. Most diesels are now turbocharged, due to improved efficiency and performance of diesel engines. The concept Audi R10, complete with a diesel engine, famously won the 24 hours race of Le Mans, for three consecutive years, in 2006, 2007 and 2008.

 

Turbocharged motorcycles

The first example of a turbocharged bike is the 1978 Kawasaki Z1R TC. Several Japanese companies produced turbocharged high-performance motorcycles in the early 1980s, such as the CX500 Turbo from Honda- a transversely mounted, liquid cooled V-Twin also available in naturally aspirated form. The Dutch manufacturer EVA motorcycles builds a small series of turbocharged diesel motorcycle with an 800cc smart CDI engine.

 

Aircraft

Experimental installations within aircraft started in the 1920’s. As an aircraft climbs to higher altitudes the pressure of the surrounding air quickly falls off. At 5,486 m,  the air is at half the pressure of sea level and the airframe experiences only half the aerodynamic drag. Consequently, since the charge rely’s on air pressure, the engine normally produces only half-power at this altitude. The development of the Merlin engines, powering the RAF Spitfires in the 1940’s, led to supercharging, to obtain greater power and performance advantages.

 

Turbochargers vs superchargers (which is better?)

Turbochargers were originally called turbosuperchargers. A supercharger is actually only referred to a mechanically driven forced induction unit. The difference between a turbocharger and a supercharger is that it is driven by a belt attached to the crankshaft, whereas a turbocharger is powered by a turbine driven by exhaust gases.

Can you turbocharge and supercharge a car?

A Twin-charger refers to an engine with both a supercharger and a turbocharger. Twin-charging has been demonstrated in Gruppe B rallying cars such as the Lancia Delta S4. In commercial, mass produced vehicles, Volkswagon manufactured a variation of the popular Golf model. The 1400cc engine was both supercharged and turbocharged.

 

Disadvantages of supercharges

Belts, chains, shafts, and gears are methods of powering a supercharger, placing a mechanical load on the engine. An example is the single-speed supercharged Rolls-Royce Merlin engine, where the supercharger uses 150 hp. The benefits are staggering as the engine will now generate an additional 400 hp. This is still a disadvantage of a supercharger as the engine must withstand the net power output plus the power to drive the supercharger.

Superchargers have a lower adiabatic efficiency, especially Roots-type superchargers. Adiabatic efficiency is a measure of a compressor’s ability to compress air without adding excess heat. The compression process always results in high temperatures, however, more efficient compressors produce less excess heat. Roots superchargers impart significantly more heat to the air than turbochargers. Turbocharged air is cooler and denser and therefore more potential power than the supercharged air.

Turbocharger’s do not place a direct mechanical load on the engine, although they place exhaust back pressure on engines, increasing pumping losses. This is more efficient because while the increased back pressure taxes the piston exhaust stroke, much of the energy driving the turbine is provided by the still-expanding exhaust gas that would otherwise be wasted as heat through the tailpipe.

 

Disadvantages of turbochargers

The primary disadvantage of turbocharging is what is referred to as turbo lag. This is the time between opening the throttle and onset of increased intake pressure and power.

 

Variable output turbochargers

In variable output systems exhaust gas pressure at idle or low engine speeds is unable to drive the turbine. When the engine reaches sufficient speed the turbine section spools up enough to produce sufficient intake pressure.

 

What are the main components of a turbocharger?

The turbocharger has three main components:

  1. The turbine, a radial inflow turbine
  2. The compressor, generally a centrifugal compressor
  3. The centre housing rotating assembly

Many turbocharger installations use additional technologies, such as wastegates, inter-cooling and blow-off valves.

 

Turbine unit – turbocharger

Energy provided for the turbine is converted from the enthalpy and kinetic energy of the gas. The central housing directs the gas flow through the turbine. The size and orientation offers alternative performance characteristics. The turbine and impeller wheel sizes also dictate the amount of air or exhaust that can flow and the relative efficiency. In general, the larger the turbine wheel and compressor wheel the larger the flow capacity.

Large turbochargers take more heat and pressure to spin the turbine, creating noticeable lag. Small turbochargers spin rapidly, but are less effective at higher engine speeds. Combinations, broadening the overall effectiveness across an engines range are popular, such as twin-turbochargers or variable-geometry turbochargers.

How to choose a turbocharger?

*Note to read more please visit the original superb article, a thorough reference to a variety of configurations, applications and bespoke to your vehicle and modification targets – click here.

Garrett turbos support an engine displacement from 1.4L up to 12.0L and horsepower at the crank from 140 up to 3000. Selecting the right size turbocharger for your application is critical.

 

Wheel Vs Crank Horsepower

Target horsepower refers to the peak horsepower you want the car to make when it is at max engine speed, at the crank. Parasitic loss, is the difference between power at the crank vs the wheels. Drivetrain loss is determined from the time it travels through the transmission to the driveline, and through the axles to the wheels. This is affected by transmission type and automatic transmissions typically suffer greater differences. Brakes, heavy wheels and tyres also affect drivetrain loss.

As an example we are going to start with a wheel horsepower target of 600 (RWD). In order to find a turbo that can support our target power we need to calculate for the drivetrain loss so we must multiply 600 * 1.15 = 690.

  • Front Wheel Drive 10% (multiply HP target by 1.1)  Wheel Horsepower * 1.1 = Crank Horsepower
  • Rear Wheel Drive 15% (multiply HP target by 1.15) Wheel Horsepower * 1.15 = Crank Horsepower
  • All-Wheel Drive 20%  (multiply HP target by 1.2)     Wheel Horsepower * 1.2 = Crank Horsepower

 

The twin-turbo

Twin-turbo designs have separate turbochargers working in sequence or in parallel. In the later, both turbochargers are provided an equal proportion of the exhaust gases. In a sequential setup, one turbocharger runs at low speeds and the second activates at a specified engine speed. Sequential turbochargers further reduce turbo lag, but require an intricate set of pipes to properly feed both turbochargers.

Two-stage variable twin-turbos are connected in series so that boost pressure from one turbocharger is enhanced by the second larger one. The distribution of exhaust gas is continuously variable, so the transition from using the small turbocharger to the large one can be done incrementally. These are used in Diesel engines such as the Opel bi-turbo Diesel, whereby the smaller turbocharger works at low speed, providing high torque at 1,500–1,700 rpm. Both turbochargers operate together in mid range, with the smaller one pre-compressing the air, which the larger one further compresses. At higher speed (2,500 to 3,000 RPM) the larger turbocharger takes over.

 

 

The twin-scroll turbocharger

Twin-scroll turbochargers have two exhaust gas inlets and two nozzles, a smaller sharper angled one for quick response and a larger less angled one for peak performance. In twin-scroll designs, the exhaust manifold physically separates the channels for cylinders that can interfere with each other, so that the pulsating exhaust gasses flow through separate spirals (scrolls). This leads to the pairing of cylinders. This promotes scavenging techniques, leading to improved turbine efficiency.

 

 

A variable-geometry turbocharger

Variable-geometry turbochargers use adaptable vanes to adjust the air-flow, imitating a turbocharger of the optimal size across the engines range.  The vanes are placed just in front of the turbine like a set of slightly overlapping walls. Their angle is adjusted by an actuator to optimise air flow. The result is that the turbocharger improves fuel efficiency without the onset of lag.

Turbo inter-cooling

When the pressure of the engine’s intake air is increased, the temperature increases. Heat soak from the hot exhaust gases, spinning the turbine, will also heat the intake air. The air is therefore less dense, hence reduced oxygen available for combustion, minimising the volumetric efficiency. Engine knock, or detonation is a result of this situation, a common cause of fundamental engine deterioration. An inter-cooler reduces the intake air temperature.

 

Blow off valves and anti-surge valves – turbocharging

At the point when the throttle is closed and the engine is stressed, revolving at its peak rpm, the compressed air flows to the throttle valve without an exit. This rush raises the pressure of air leading to the compressor stalling and this air decompresses back across the impeller. The reverse flow across the turbocharger makes the turbine shaft speed reduce more quickly than it would naturally. To prevent this from happening, a release valve is fitted between the turbocharger and inlet, which vents off the excess air pressure.

The primary aim is to maintain the spool of the turbocharger. The air is usually recycled back into the turbocharger inlet (diverter or bypass valves), but can also be vented to the atmosphere. Valves that recycle the air also shorten the time needed to re-spool the turbocharger after sudden engine deceleration, since load on the turbocharger when the valve is active is much lower than if the air charge vents to atmosphere.

 

 

Turbocharging – going green

In the 2010’s, forced induction and a shift towards smaller capacity engines has been dictated by the ever growing shift to zero carbon. The automotive industry is undergoing another tidal shift in reducing emissions through its products and also process and manufacturing initiatives, to achieve net zero.

The inevitable turn to electrification in the late 2010’s has exponentially increased the focus of automotive manufactures to look at full electric vehicles, heightening the existing technology of car batteries. Additionally, light-weighting of vehicles plays a role in reducing emissions. Variations of composite, wood, novel carbon composites and lighter alloys are being considered for components and the chassis, structural elements of future vehicles.

 

 

 

 

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