Turbocompound: what it is and how it works.
The turbocompound is a system used on some heavy-duty trucks (like some Scania and Volvo trucks) that allows an additional amount of energy to be recovered from the exhaust gases at the outlet of the conventional turbocharging coil. As you may know, no naturally aspirated diesel engines are produced for automotive use today because the performance benefits would not make it reasonable to produce larger displacement engines without supercharging. Therefore, all heavy-duty trucks on the road (and virtually all large earthmoving machines, called MMTs) are powered by turbocharged diesel engines. In another article, we will address why turbochargers are used instead of positive displacement compressors, but today we want to focus on the turbocompound system for increased efficiency.
Turbocompound – what's inside.
Let's start with the turbocharging system, which uses an exhaust gas turbine connected to a centrifugal compressor used to supercharge the engine, whether diesel or gasoline-powered. As this system is only used on some heavy-duty vehicles, we want to specify that we are talking about large diesel engines with displacements that can be as large as 10,000cc or 15,000cc.
These engines usually use one turbocharger (or two in some cases) with an air-cooling system (intercooler). If you have read any of our other technical articles on the subject, you will know more or less how a turbocharger works, but we will continue on. In normal turbodiesel engines without the turbocompound system, at the outlet of the hot coil, the one containing the turbine, there is a duct leading to the first section of the exhaust system. This may consist of particulate filters, catalytic converters, or mufflers, depending on the model.
In vehicles that use the turbocompound system, before the exhaust gases reach the first part of the exhaust system, they are routed to a second turbine. This turbine is mechanically connected to a set of gears whose purpose is to demultiply the turbine's rotational speed and multiply its torque. This torque is transferred to the crankshaft, thus recovering some of the energy from the exhaust gas velocity and temperature that would otherwise be wasted after leaving the turbocharger.
This particular system closely resembles the final stage of turboshaft engines used chiefly in helicopters. These engines contain groups consisting of compressors and turbines that are multi-stage and connected by a shaft. In aviation, this configuration is usually called a "turbine" and provides thrust to the aircraft through the escape of large masses of hot exhaust gases. In turboshaft engines, the thrust from the exhaust gases is routed to a second turbine connected to a gear reduction system to provide torque to the helicopter rotor.
With obvious differences related to flow rate, temperature, and rotational speed, the turbocompound system can be considered to operate very similarly to the final part of a helicopter engine. According to manufacturer data, this system (meaning the turbocompounds used on heavy-duty trucks) can save about 5 to 6 percent in diesel fuel consumption.
We wanted to shed a little bit of light on the operation of this particular device with this article. This device is often confused with multi-stage turbocharging or variable geometry turbines. The purpose of the article was to clarify the turbocompound system is not part of the turbocharging system but is a device for recovering energy from the exhaust gases of turbodiesel engines mainly used in road transport.
If you liked this article, don't miss the next one in which we will talk more about turbochargers, specifically a particular electric turbocharger created by Mercedes.
Until next time!
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