Let’s just start with a statement of Jeremy Clarkson, which shows the basic understanding of most people about turbochargers:
“A turbo: exhaust gasses go into the turbocharger and spin it, witchcraft happens and you go faster.”
So before we show our turbo setup, which is currently still in the making, we thought it would make sense to go into the basics of turbocharging an engine.
As stated in a previous episode, making more power out of your engine is just possible when you are able to burn more fuel. But you can just burn more fuel, when you get more air into your engine. Usually an engine is fed with air by means of the atmospheric pressure, which pushes the air into the combustion chamber(s). The amount of air for combustion is theoretically your engine’s total displacement for every two engine revolutions on a 4-stroke engine. This amount is limited trough airflow losses while the air flows into the combustion chamber(s) and usually further decreases with engine speed, since the air has less time to get into your combustion chamber.
To get more air into your engine, as trough atmospheric pressure, you have to force it in by increasing the pressure, or creating boost as tuners would call it . This is usually done by adding an air pump in form of a compressor. This compressor needs to flow large amounts of air without having to dramatically increase the pressure. Centrifugal compressors are a good choice for this task, working on a basic principle without a huge number of moving parts or needing complicated sealing.
To drive such a compressor, you need mechanical energy. This mechanical energy can come directly from your engine via a special gearbox, stepping up the engine speed, to drive the impeller. Another option is to use the energy out of the exhaust gasses to drive the compressor, as it is done in a turbocharger.
In a turbocharger, the centrifugal compressor is directly coupled (on a common shaft) to a turbine wheel. The turbine wheel is driven trough the energy still available in the exhaust gasses and transferring this energy to the compressor wheel and you can force more air into your engine.
In theory, this is pretty simple, but when it comes to choosing the right turbocharger, it gets a little more complicated, since the offer in turbochargers is huge. Basically you can make the choice for a compressor, based on horsepower numbers you “want” to make. Turbo manufacturers give you the numbers for the maximum amount of air, their compressors will flow. This is nice, but will you be able to get all that air through you engine? This is, where pressure an turbo speed come into the game. Two basic theories:
- Your engine can not flow the air, the pressure increases, the compressor wheel slows down, less air is moved, …
- Your engine can not flow the air, the pressure increases, the turbine delivers enough energy to keep the compressor spinning, the air heats up, boost pressure rises but you are not making more power, …
The two reflections lead to two extremes:
- The compressor wheel slows down to a point where it won’t make any gain in pressure.
- The compressor wheel over-spins and will not be able to make any gain in pressure neither.
Therefore, turbo manufacturers provide compressor maps for compressors.
- The horizontal axis shows the mass-flow of air, thus somehow proportional to the power you can make.
- The vertical axis is the pressure ratio, compared to atmospheric pressure.
- The islands in the map show the compressor efficiency.
- The dotted lines show compressor speed in rounds per minute.
- Everything on the left of the islands is above the surge limit of your compressor wheel. Basically your compressor wheel is to huge and you cannot keep it spinning with the energy you provide trough the exhaust turbine.
- On the right of the islands, your compressor spins but you are not able to get the air through your engine and compressor efficiency drops. You are just producing hot air
As an optimum, you want to operate your compressor in the center islands. When you try to follow the basic thoughts, you come to the conclusion, that the compressor operation is in direct relation with the exhaust turbine, providing the energy to the compressor. Therefore, you can combine a given compressor with different exhaust turbines and exhaust turbine housings to make best use of the energy in your exhaust gasses for you application. There is a huge difference between low end torque for street driven vehicles and WOT (wide-open-throttle) as used in tractor-pulling.
Our class is limited with air restrictors, which every competitor has to use. The aim is to limit the maximum power for everyone to about the same amount. Running two turbos, we have to use two 76mm (~3 inch) restrictors. Our first thoughts were to just go with compressors of exact this size. But since we have to use the restrictors anyways, we decided to go with slightly bigger compressor wheels an target for high efficiency. Based on availability and budget, the choice was made to go with S400 clones equipped with 88mm billet compressor wheels.
Comparison with a standard turbo out of a 1.9TDI engine 😉
Based on the thought, that burning methanol in a high displacement engine will generate huge amounts of exhaust gasses, we went for the biggest exhaust wheel and housing combination available for this compressor.
Speaking about exhaust gas volume. What do you do with excess exhaust gasses, or can you even control the amount of exhaust gasses to the turbine of your turbocharger? Of course you can 😉
Controlling the amount of exhaust gasses to your turbo allows you to control the energy you provide to the compressor, and thus you can control the amount of air the compressor flows. Basically you are using a mechanical valve to bypass exhaust gasses before the turbine, which will not drive the turbine. This valve is called the wastegate.
Wastegates are eather integrated in the turbochargers turbine housing, called an internal wastegate or, as shown in the image above, they are positioned on the exhaust manifold before the turbocharger, called an external wastegate.
Basic wastegate control is done trough the wastgate spring, which counteracts against the exhaust gas pressure. The wastegate opens progressively when the exhaust gas pressure rises, reducing the energy delivered to the turbocharger and thus regulating the boost pressure generated by the compressor. This is just a very basic control and most wastegates are build with an additional pneumatic control, having a membrane under the spring, so that you can add force to the spring or counteract against it by applying pressures trough the two additional control ports. These pressures can either be directly your boost pressure or controlled trough your engine management system.
But what about the “choo-chooo-chooo” in this episodes title. This describes the distinctive sound, lots of people directly relate to a turbocharged engine. But it is neither generated by the turbocharger, nor the wastegate. This sound is generated, and often ridiculously amplified, in the so called blow-off valve. A turbocharged system does not necessarily need this valve and one usually does not find it on Diesel powered engines and low boost gasoline applications. But why is it needed and extensively used on tuned, mostly spark ignited engines?
The need comes from the fact, how the engines power delivery is controlled. Generally speaking this is throttle control. On a spark ignited engine, you have a throttle, which controls the airflow to the engine. This throttle is placed between the compressor and the engine’s intake. A Diesel powered engine is controlled through the amount of fuel, the fuel injection pump delivers, thus it does not need a throttle controlling the airflow.
Now the blow-off valve’s main purpose is to prevent pressure peaks and avoid excessive wear of a turbo charger. The pressure peaks are generated, in a spark ignited engine, when you shut your throttle after high boost operation. With the throttle shut, the exhaust gasses are still delivering energy to the turbine and the compressor pumps air. The throttle allows just a marginal amount of air to travel to the engine and the pressure between compressor and throttle body rises, well over the surge pressure of the turbocharger and excess air would push back out of the compressor intake. This results in high thrust bearing wear or failure of your turbocharger.
Basically the function of a blow-off valve is identical to that of the wastegate. The blow-off valve is positioned, as a spring loaded valve, in the intake manifold, between the compressor and throttle body. To help the opening of the valve, the pressure chamber in the valve containing the spring, is connected to the intake manifold, between the throttle body and the engine. Shutting the throttle results in a vacuum between the throttle body and the engine and thus helps in the opening of the blow-off valve. Releasing pressure through the blow-off valve sometimes results in an oscillation of the blow-off valve, resulting in the distinctive “choo-chooo-chooo” sound 😉
A basic turbocharger setup then looks as in the following picture. The picture shows no intercooler. We will do intercooling with additional fuel or water injection (topic for another episode 😉 )
Our system will be setup exactly like this. Well, we have 8 cylinders, 2 turbos, 2 throttles, 2 wastegates, 2 blow-off valves …
The plan is to build a separate turbo setup for each side of the engine. Time to figure out, where we will fit all that stuff …
A mock-up is a good idea to find out if everything works as expected, but after all we still imported everything into CAD to design the needed flanges and supports. Oh, and you need some piping 😉
Time to get all the parts build and we will keep you updated in the following episodes.