A turbo-compound engine uses the exhaust gases through a downstream turbine. This can be installed in addition to a turbocharger.
While the first exhaust gas turbocharger provides more boost pressure in the combustion chamber due to the exhaust gas energy and thus higher engine power, the second turbocharger converts the remaining energy into movement power. The generated energy is passed on to the crankshaft via a mechanical or hydraulic transmission.
The overall complexity of a turbo-compound system and especially for road vehicles quite low saving effect has prevented widespread introduction of the turbo-compound engine.
A turbocharger (also exhaust gas turbocharger) uses the energy that is lost by the exhaust gas of a reciprocating engine, profitable for driving the cylinder. It compresses the combustion air supplied to the engine, providing more torque and an increase in engine power.
In a normal reciprocating engine, the amount of air needed for combustion in the intake stroke is drawn by the piston into the cylinder. This type of engines are therefore also called ‘naturally aspirated’. For high performance engines, the movement of the piston alone is not enough to get enough air into the cylinder. Therefore turbochargers are used, which provide for a higher oxygen content in the combustion chamber of the engine.
In essence, a turbocharger consists of a centripetal turbine connected by a fixed shaft to a centrifugal compressor. The exhaust gas stream sets the turbine wheel in rotation. Its torque and speed is transmitted via the common shaft to the compressor wheel in the intake system.
How a Turbocharger works
When using a turbocharger, the thermal energy of the exhaust gases is converted into kinetic energy. The exhaust gases drive the turbine, which in turn chases fresh air through the compressor. The compressor compresses the fresh air, which is then passed through the intercooler back into the cylinder. This results in a better filling of the cylinder compared to pure naturally aspirated engines. The result is higher engine performance with lower fuel consumption and better emissions.
As long as enough exhaust gas flows, the speed is sufficient to generate an overpressure on the intake side. But this condition is reached only at higher gas flow rates from engine speeds of about 1500 to 2000 rpm, so that turbo engines operate in the lower speed range as naturally aspirated and delayed response to sudden acceleration.
The intercooler is installed between the turbocharger and the intake valve. It cools the air heated by the charge. Since cold air takes up less space, more oxygen fits into the combustion chamber. Cooling also thermally relieves components of the engine (cylinders, valves, pistons, spark plugs, etc.). Intercoolers themselves are either air or water cooled.
The Turbo Lag
In older turbochargers, a certain speed was required for the charge. In the low speed range, the amount of exhaust gas is insufficient to produce the required boost pressure for the turbocharger. Even with more modern engines, the onset of the turbocharger requires some time in stationary accelerations, since the turbine must first be sufficiently accelerated by the exhaust gas flow. As the size of the turbocharger increases, it takes longer for the compression to take place. This delay until the action of the turbocharger is called turbo lag.
Bi-turbochargers (also: twin-turbo) are two parallel turbochargers. They are powered by two cylinders of the engine, which allows them to work more efficiently.
Sequential bi-turbochargers do not charge both chargers continuously. As soon as higher engine power is demanded, the second turbocharger is activated. Typically, bi-turbochargers are installed in larger engines, where manufacturers want to keep fuel consumption low.
In addition to a regular turbocharger, the exhaust gas energy can also be utilized by the turbo-compound technology. For this purpose, an additional turbine is switched, whereby the remaining energy is transmitted in the form of kinetic energy to the crankshaft.
Turbochargers with variable turbine geometry (also VTG superchargers) have adjustable vanes on the input side of conventional turbochargers. These become steeper with increasing exhaust gas quantity, which increases the cross section.
With a VTG charger, the turbo lag can be reduced or even avoided. A large engine torque is possible in the lower and upper speed range. However, if the engine is adjusted incorrectly or the car used for sort distances only, the VTG-adjustment can be contaminated by exhaust gas residues.
VTG Turbocharger of the Porsche 911 Turbo (997). With the variable turbine geometry, the cross sections of the optimum load size are simulated by vanes that are located in the exhaust gas flow.
In addition to cars with petrol or diesel engines, turbochargers are also used in aircraft and ships. In planes, where the power of the engine decreases with increasing altitude, it can be counteracted with a turbocharger. In addition, turbochargers allow for downsizing in engine size, which saves additional weight through a smaller unit.
While in the early days the increase in performance was predominantly the reason for the engine’s charging, nowadays the main focus is on reducing emissions. For this purpose, more and more car manufacturers rely on smaller engine sizes with correspondingly optimized turbocharging.
Turbocharger: Pros / Cons
A turbocharger allows a higher maximum mean pressure and thus an increase in torque and maximum power at a given displacement. This increase either results in a more powerful engine of approximately the same size and weight as the original, uncharged engine, or allows the same power to be obtained from a smaller unit.
In addition to the many advantages, turbochargers also bring some disadvantages. The use of a turbocharger leads to higher mechanical and thermal loads as well as higher mean pressures. Therefore, many components in the engine such as e.g. Engine block and cylinder are designed to be more resistant. These measures generally increase vehicle weight.