How do turbochargers work? Theory. Before we begin, we need to briefly review one of the laws of physics - the ideal gas law. In a nutshell, the essence of the law is this: the temperature, pressure and volume of a gas are interrelated. Compress a gas (reduce its volume) and the temperature will rise. Allow the gas to expand and the temperature and pressure will decrease. Increase the temperature of the gas and the pressure (in a confined space) or the volume (if the gas is allowed to expand) will increase. Finally, the gas flows from the higher pressure zone to the lower pressure zone.
The greater the pressure difference, the more forcefully the gas is forced into the lower pressure zone. For example, explode an inflatable rubber balloon - small boom, explode an oxygen tank used for welding - big boom.
As we know, a four-stroke engine does its work by expanding the gas in a confined space when high gas pressure pushes on the piston. In addition, the gas heats up during combustion, resulting in even higher pressures and therefore more power. Unfortunately, most of the heat is dumped out the exhaust pipe before it can be used. Even though heat is energy. Heat is not used because the cylinder is too short to convert all the heat into mechanical energy. It is not practical to make cylinders long enough to squeeze all the energy out of the expanding gas.
So what can we do? We can point the exhaust pipe in the opposite direction to the direction of motion and try to get some jet thrust, but except in very rare cases the amount of gas is not enough to get any useful impetus. A few older IndyCar cars did generate a few kilograms of thrust this way, but not enough to be of any real use. OK, but what about the possibility of attaching some additional engine to the exhaust stream? Steam engines have been using this approach for years...
Turbines. So, a turbocharger. This is a turbine driven by exhaust gases and connected by a shaft to a compressor that pumps air into the engine. More air in the cylinder means that more fuel can be burned per engine cycle. More fuel burned means more exhaust gases, more exhaust gases means more power and expansion. It is simply a "free brunch" in engineering, so that power that would otherwise be lost is used at almost no cost. The equipment becomes a little more complex, with some additional manufacturing costs, but the turbine hardly reduces the engine power.
Surely the turbine does not reduce the power of the engine? Doesn't the turbine increase exhaust backpressure? When a turbocharger increases engine power, it does not. When the exhaust valve opens, the gas pressure in the cylinder is much higher than at the turbine inlet. The pressure in the cylinder is very quickly 'blown off' and the cylinder volume decreases rapidly during the exhaust stroke of the engine and the ideal gas law keeps the pressure higher than the turbine inlet pressure. Finally, at the end of the exhaust stroke, when the pressures are almost equal, the intake valve opens and the air is "pumped" into the engine cylinder as it is compressed by the compressor. Hooray! We have high cylinder pressure again.
A little more about how the turbine works. In the last chapter, we said that a turbine is a device that uses energy that is otherwise lost. Now it will be examined in more detail. What energy drives a turbine? It is often mistakenly thought that an exhaust gas turbine is driven only by the kinetic energy of the gas as it strikes the turbine wings. Like a child's wing attached to the end of an exhaust pipe. Yes, the kinetic energy of the exhaust stream does contribute to the turbine's operation, but most of the energy extracted comes from elsewhere. Remember the relationship between temperature, volume and pressure when we were talking about gases. High temperature, high pressure and low volume are high-energy states, while low temperature, low pressure and high volume are low-energy states. Our exhaust gas wave exits the cylinder at high temperature and pressure.
The wave combines with exhaust waves from other cylinders and enters the turbine inlet, where space is limited. At this point we have very high pressures and temperatures, so our gases have a lot of energy. When the gas enters the turbine through the diffuser, it goes from a small cavity to a large one. Consequently, the gas expands, cools, decreases in velocity and gives up all its energy to the turbine wings. We cleverly insert the wings inside the turbine so that the expanding gas puts pressure on the wings and makes the turbine turn. Hooray! We have just gained energy from the heat of the exhaust gas that would otherwise have been lost. The energy loss from the gas can be measured: put thermometers before and after the turbine and you can see the huge difference in temperature.
How can I get more energy out of the gas? So what does this mean for our world? All else being constant, the work done by a turbine is defined by the pressure difference between the turbine's inlet and outlet. Increase the inlet pressure, decrease the outlet pressure, or change both, and you get more power. Pressure is heat, heat is pressure. Raising the inlet pressure of a turbine is possible but difficult. Lowering the output pressure is easy- just screw on a bigger exhaust pipe with less resistance to gas flow. I have seen several posts from people who have done exhaust tuning. They report that "now my turbo cranks much faster".
This is due to a reduction in pressure at the turbine outlet. The pressure differential has increased, the exhaust gases in the turbine expand more and do more work. There should also be a noticeable drop in torque at maximum engine speed, caused by the exhaust system's limited capacity to pass the gas flow. If the maximum capacity of the exhaust system is exceeded, any extra gas you try to push through the exhaust system will only increase the turbine's output pressure. Higher outlet pressure, lower pressure differential, less work, less torque. Incidentally, please note that in this case, the compressor is also affected.
Compressor. Since you can extract work from the gas expanding in the turbine, you can also successfully compress the gas by turning the turbine shaft with the help of an external power source. In other words, a compressor is a turbine working in reverse. The same laws of physics are at work, but in the other direction: we take a low-pressure gas, work on it by compressing it with the compressor's wings, and get a high-pressure, high-temperature gas at the compressor's outlet. The temperature rise that occurs during compression is undesirable and we will have problems with it, but more on that later. Although the turbine and compressor parts are essentially the same, they are not exactly the same because of the processes that take place during combustion.
A given volume of air is sufficient to burn a well-defined amount of fuel. The ratio of air to fuel is approximately 14:1. The volume of exhaust gas is much greater than the volume of air consumed during combustion and the pressure of the exhaust gas is much greater than that of the supply air, which results in a very different internal design of the shaft and turbine. So we have a turbine/compressor design.
The main disadvantages of the turbine and the turbo lag. Turbines are amazing machines. They are lightweight, very efficient, but have a limited operating speed range. A turbocharger is very efficient at a certain speed and with a certain amount of gas, but if you vary the shaft speed over a large range, the efficiency drops dramatically. If the speed is too high, cavitation and other undesirable aerodynamic phenomena kick in and the airflow is reduced. If the speed is too low, the wings do not get enough "push" and the gas flow is also reduced. Example. An M1A1 Abrams tank weighs about 55 tonnes, most of the weight is made up of armour steel and depleted uranium. The tank is powered by a gas turbine that generates 1800AJ of power in the wheels... that is, the tracks. That's enough power to propel this beast to a speed of 110km/h.
The turbine is incredibly small, I don't remember the exact weight, but it looks like it weighs about 130-220kg. Compared to the weight of a tank, you could say that there is no engine. However, the turbine is designed for WOT operation. At WOT, the turbine consumes less fuel (gas mileage) than a diesel engine at the same power level. But at idle, the turbine's efficiency drops to the point where fuel consumption is higher than at WOT (vert: maybe I'm misunderstanding something here?). Turbines are perfect for vehicles that move at a constant speed all the time - tanks, ships, planes, IndyCars. Vehicles that change engine speed as they go along are less suited to turbines. True, if someone develops a good transmission with continuously variable gear ratios, then that's another matter.
So what does all this have to do with turbochargers? A turbine is suitable for constant speed operation. We only have enough exhaust for the turbine at WOT, and we need a limiting device to maintain constant turbine speeds (when we reach them). We know what air pressure we want to create at the inlet of the engine, and we know how much air the engine consumes when running at maximum power. We can then select the turbine, and more specifically the compressor and the turbine casing, to achieve maximum turbine efficiency at the required operating point. What are the benefits? A smaller turbine. A smaller turbine has less inertia, gets up to WOT speed faster and achieves the required efficiency. The time between throttle opening and the turbine's application of maximum engine power is commonly referred to as turbo lag.
Turbo lag is the only major disadvantage of turbines. Ever wondered why a 2G turbine is so small? It is precisely tailored to the engine's air consumption when driven by Joe Public, which rarely, if ever, exceeds 4500 rpm. Reducing lag has another side effect. If you draw an acceleration curve for a car, the area under the curve will show the transitional power band. A little calculation shows that increasing that band, even without increasing the maximum power, greatly increases the torque required to accelerate the machine (vert: I didn't get it). One of our tuning engineers measures the exhaust flow, the dynamics, calculates the engine's air consumption in a given operating mode, and selects the compressor and casing to maximise the efficiency of the turbine for that mode. How the turbine is selected is beyond the scope of this post, but in short, pressure graphs are compared. After all that, the ride is really fast.
Intercooler. In the last chapter we finished with the compressed air coming out of the compressor side of the turbocharger. Unfortunately, the laws of physics are working against us this time, and the work we've done on the air has caused it to heat up. This is bad. High-temperature gases have a lower density. In addition, the likelihood of detonation increases.
The turbine is incredibly small, I don't remember the exact weight, but it looks like it weighs about 130-220kg. Compared to the weight of a tank, you could say that there is no engine. However, the turbine is designed for WOT operation. At WOT, the turbine consumes less fuel (gas mileage) than a diesel engine at the same power level. But at idle, the turbine's efficiency drops to the point where fuel consumption is higher than at WOT (vert: maybe I'm misunderstanding something here?). Turbines are perfect for vehicles that move at a constant speed all the time - tanks, ships, planes, IndyCars. Vehicles that change engine speed as they go along are less suited to turbines. True, if someone develops a good transmission with continuously variable gear ratios, then that's another matter.
So what does all this have to do with turbochargers? A turbine is suitable for constant speed operation. We only have enough exhaust for the turbine at WOT, and we need a limiting device to maintain constant turbine speeds (when we reach them). We know what air pressure we want to create at the inlet of the engine, and we know how much air the engine consumes when running at maximum power. We can then select the turbine, and more specifically the compressor and the turbine casing, to achieve maximum turbine efficiency at the required operating point. What are the benefits? A smaller turbine. A smaller turbine has less inertia, gets up to WOT speed faster and achieves the required efficiency. The time between throttle opening and the turbine's application of maximum engine power is commonly referred to as turbo lag.
Turbo lag is the only major disadvantage of turbines. Ever wondered why a 2G turbine is so small? It is precisely tailored to the engine's air consumption when driven by Joe Public, which rarely, if ever, exceeds 4500 rpm. Reducing lag has another side effect. If you draw an acceleration curve for a car, the area under the curve will show the transitional power band. A little calculation shows that increasing that band, even without increasing the maximum power, greatly increases the torque required to accelerate the machine (vert: I didn't get it). One of our tuning engineers measures the exhaust flow, the dynamics, calculates the engine's air consumption in a given operating mode, and selects the compressor and casing to maximise the efficiency of the turbine for that mode. How the turbine is selected is beyond the scope of this post, but in short, pressure graphs are compared. After all that, the ride is really fast.
Intercooler. In the last chapter we finished with the compressed air coming out of the compressor side of the turbocharger. Unfortunately, the laws of physics are working against us this time, and the work we've done on the air has caused it to heat up. This is bad. High-temperature gases have a lower density. In addition, the likelihood of detonation increases.
Specifically, the result is a high pressure wave that travels from the throttle back to the compressor and hits the compressor wings. The effect is similar to that of a pillow in the spokes of a bicycle. The compressor blades and shaft bearings suffer from the repetitive impact, as well as the reduced turbine speed and the subsequent time wasted in re-starting. The BOV shall be mounted between the compressor and the throttle valve. If the BOV detects a shock wave, it discharges it somewhere, either back into the atmosphere or into the compressor inlet. In this way, we lose pressure, but retain turbine speed. It's hard to confirm how effective this is in a sports car without measurements. However, for a normal car, the BOV is undoubtedly a good idea because it protects the turbine from mechanical shock.
Overview. So, a brief summary of the previous sections with some additions.
1.The turbine conserves energy that would otherwise be wasted as heat. The exhaust gases drive a turbine, which drives a compressor, which compresses the air fed into the engine.
2.The compressed air at the inlet of the engine makes it possible to increase the power of the engine by burning more fuel per stroke. Compressed air also provides better removal of waste gases, which are blown out of the cylinder by the compressed air.
3.The work done by a turbine depends on the exhaust flow and the pressure difference between the turbine inlet and outlet.
4.The pressure difference between the turbine inlet and outlet can be increased by installing a new exhaust system with a higher gas flow. The exhaust system capacity is never too high.
5.A compressor works best when it is specifically adapted to the particular airflow and power output of the engine.
6.The best way to select the right compressor is to call the turbine manufacturers and ask them all your questions.
7.The intercooler is your friend. It lowers the temperature of the air coming out of the compressor, which increases during compression. This reduces the risk of detonation and the cold air has a higher density.
8.The intercooler is better when: it has a lower pressure drop; it has a higher flow of air cooling the intercooler.
9.The wastegate limits power boost by reducing turbine speed.
10.Unlimited boost creep means that the wastegate is too low.
11.Engines with high power creep require a good combustion system. The most common reason for engine stalling is combustion system problems.
12.There is currently no alternative to engine power take-off
Remember that this is just a brief summary of 80 years of turbocharger development history and theory. Not everything has been described and much has been simplified to avoid distractions. The theory of the processes between the casing and the turbine alone would make a book in itself. If you want to know more about turbines, there are various excellent books on the subject.