Benefits and importance of the turbocharger and intercooler in diesel engines

The correct air and fuel ratio is an essential factor in making better use of the calorific value of the fuels used by Otto and Diesel cycle internal combustion engines, which are the main ones used in the agricultural sector. Through an exothermic reaction inside the cylinder, the fuel is burned in the presence of oxygen and the expansion movement of the gases during burning moves the piston from top to bottom dead center, transmitting force to the connecting rod, which, in turn, transmits torque. to the crankshaft shaft.

The atmospheric air, after passing through one or more types of air filter, is directed to the cylinder to supply oxygen in the exact proportion that each fuel requires, so that the burning process occurs fully. In the case of gasoline and diesel, the proportion is approximately 15 parts of air to 1 part of fuel by weight, while ethanol requires 9 to 1.

With the increased demand for force and torque to carry out heavier agricultural activities, engineers seek to increase engine power to meet this requirement. Therefore, one of the solutions is to optimize the fuel burning process, providing the appropriate air/fuel ratio so that there is no waste of fuel due to lack of oxygen, especially when unburned fuel is thrown into the exhaust pipe.

One of the first strategies is to improve air intake into the cylinders. Normally, the air intakes for engines with several cylinders are grouped in a single intake manifold, right after the air passes through the air filter – this design allows only one filter to be sufficient to supply all cylinders, and The design of the collector must give the air a swirling movement when it enters the cylinders.

In gasoline engines, a system of multiple valves per cylinder is normally used, such as the famous 16-valve engines in traditional four-cylinder engines, with two intake valves and two exhaust valves for each cylinder – making the entry and exit of gases better at high temperatures. working rotations (rpm), thus increasing power. In diesel engines, there is no interest in improving the intake and exhaust system, as the rotations are lower – in these cases, the use of several valves per cylinder aims to improve fuel burning, which makes the engines more efficient. clean.

TURBOCHARGER

To further increase engine power and performance, the industry developed the turbocharger, or just turbo. This system, patented in 1905 by the Swiss Alfred Büchi, aims to supercharge the engines with air, since the filling rate of the cylinders, caused by the depression created by the movement of the piston towards its bottom dead center, does not represent more than 80% of 90% of the unit cylinder capacity. In aspirated engines (without turbo), the opening and closing of the intake valves leads to the formation of pressure waves that cause a slight increase in pressure in the intake manifold, which makes it difficult for air to enter the cylinders.

The turbocharger then serves to offer more power without having to increase the size of the engine. As the efficiency is directly related to the mass of air that the engine can suck in per intake cycle, the turbo's mission is to compress the air before it is admitted, resulting in more air mass in the combustion chamber, and more air means more fuel can be added. Therefore, more power is obtained from the explosions in each cylinder. A turbocharged engine produces more power than the same engine without the device. This can significantly improve the engine's power-to-weight ratio.

The turbocharger, previously used only in large diesel vehicles and sports cars, is now used in all types, especially those that require lower consumption. It is part of the “downsizing” concept, with smaller engines capable of delivering the same power as larger ones, with benefits in fuel consumption. And this is what we see in the car industry today – automakers are replacing 2.0 L engines with 1.3 L or even 1.0 L turbocharged engines. The engines are getting the same power but with lower fuel consumption.

The use of turbines is especially important in diesel engines, as increasing the pressure and temperature inside the cylinders reduces the risk of detonation, while their application in gasoline engines increases this risk, which requires more attention in their application in these engines.

OPERATION

The turbocharger is made up of a system composed of a compressor with two turbines, one of which, called the exhaust turbine, is driven by the exhaust gases, thus functioning as a motor element, and the other, called the intake turbine, functions as the engine. a pump, that is, causes the suction of air which is then conveyed, under pressure, through the intake manifold, to the cylinders.

The discharge piping is connected on one side to the engine discharge manifold and on the other to the compressor body – the discharge gases enter tangentially to the turbine, with the exit in an axial direction relative to the propeller. In some engines, the exhaust manifold is divided into two parts, which allows gases from the front and rear cylinders to pass through separate ducts, thus avoiding the clash of air currents, which allows for a more regular supply. of the turbine.

The discharge turbines, made up of several vanes, whose profile allows them to optimize their performance, are subject to very high temperatures, from 700°C to 900°C, and are manufactured in special refractory steel or ceramic, and separated from the central body by thermal insulation, made of stainless steel.

The intake turbine, which is connected to the discharge turbine, also has vanes, but is made of lightweight material, and the air movement is opposite to that for the discharge, as the entry is axial and the exit is tangential. . As the two turbines are connected by a shaft, the amount of air sucked in depends on the rotation of the discharge turbine, which is very high and can even exceed 100.000rpm, which can double the engine's power.

The intake pipe has an axial air inlet, which is then directed tangentially towards the intake manifold, is made of lightweight material and is fixed at the other end to the compressor body.

The compressor body, in addition to supporting the pipes, features bearings to support the shaft that connects the two turbines and an oil circuit for lubricating and cooling the turbocharger. The bearings have bronze or alloy bushings, which, together with the turbine shafts, have rotational movement, allowing very high rotations.

Due to these high speeds, lubrication between the bearings and the bushings, and between the bushings and the shaft, occurs under very difficult conditions, and the oil pressure must be sufficient to permanently maintain a liquid layer between the parts. Therefore, the engine speed should never be reduced abruptly in turbocharged engines, as the turbocharger, due to its high speed, still remains in rotation for a certain period, and the decrease in pressure resulting from the reduction in speed may not be sufficient. to ensure effective lubrication, potentially damaging the system prematurely.

Considering the increase in temperature resulting from the operation of the turbochargers, the aspirated air can reach values ​​of 100°C to 150°C, which causes its density to decrease, also reducing the system's performance. So, in order to minimize the effects resulting from increased temperature, an air-to-air or air-to-water radiator associated with the turbocharger can be used. This system defined by intercooler allows the air temperature to be lowered to approximately 70°C.

INTERCOOLER

The turbo uses exhaust gases, causing them to spin a turbine at up to 100.000rpm, pumping fresh air into the engine. When compressed, the air heats up and this makes it take up more space than it should. It can also cause the fuel to combust prematurely, causing the famous “knocking” in gasoline engines. Therefore, the turbocharger is almost always associated with an intake air cooling system – the intercooler.

This system has a type of radiator that cools the air admitted to the engine after passing through the turbocharger. In addition to keeping the mixture at the right temperature, this allows more air to be put into the cylinders.

The main benefits of using the intercooler are to increase engine torque and power, provide greater durability of the engine and its components and fuel economy.

As the intercooler is a heat exchanger, it requires a system that exchanges heat from the air compressed by the turbo with the environment, and thus there are air-to-air and air-to-water models. The difference between them is that the first exchanges heat with the ambient air and the second uses a pressurized water system to exchange heat. The air-to-air system is much simpler, however the second tends to be more efficient.

The air-to-air model must be located in a position that receives a lot of ambient air, otherwise it will be of no use. Therefore, its location is generally at the front of the vehicle, in front of the engine radiator so as not to receive hot air and compromise its function. A fan can be installed next to the intercooler heat exchanger to improve air flow between the fins, providing better heat exchange.

The air-water model does not require a specific position to receive ambient air, as it uses an independent water circuit for cooling, so it can be located wherever is most convenient. However, you need a system so that the water loses heat.

Ricardo Ferreira Garcia, Uenf (Campos dos Goytacazes, RJ)

Read more in issue 203 of Cultivar Máquinas.