Sunday, 1 May 2016

A very basic Turbo 101

Now this is meant to be a very basic Turbo 101, mainly to explain the ways I moved along to decide to move from a blow-through to a draw-through setup.

Below you see a very simple schematic on how a turbo works including a dimensionally incorrect depiction of a wastegate.

In essence a turbo is an exhaust gas driven pump, which will push in air (blow-through) or air-fuel-mix (draw-through) into the engine. The wastegate is meant to limit the turbine speed and hence the created pressure inside the combustion chamber (aka "boost"), by venting a part of the exhaust gas straight into the exhaust.

The most simple design to build is the so-called draw-through setup. This is pretty much where the upsides end unfortunately. With the turbo compressing the air-fuel-mix there is no room for a plenum, which would buffer the alternating demand for fresh air(-fuel-mix), which makes the engine react relatively sluggish on throttle changes. Additionally due to the explosive nature of the air-fuel-mix it is impossible to use an intercooler for charge-cooling thereby limiting the amount of boost an engine can take as intake temperature and pinging are directly related. Another issue is part-throttle response. With the carburettor being placed on the inlet-side of the turbo it will never actually see boost and therefore can only be tuned according to the vacuum created on the inlet-side. This works quite well off-boost and in wide-open-throttle situations, but not so well anywhere else. One can get around this to a certain extent by using a CV carb, which is also a reason, why one can see SU-carbs on so many draw-through setups as the throttle slide lift is directly related to the vacuum. One final let down has to be mentioned though, in order to build a draw-through setup a carbon-sealed turbo has to be used or a conventional turbo has to be retrofitted with a carbon-seal. This basically leaves you with two choices: Get an old Rajay-Turbo (where it is quite to find parts for) or buy a China T3-Turbo as I did and live with the fact, that the overall quality may leave a bit to be desired.

Nevertheless, the system is easy to build and in 1/4-mile applications (or the like) there's still room for this setup. The lack of intercooler could at least be partially offset by using water-methanol-injection for charge cooling.

The blow-through setup is way more sophisticated. Here the carb is placed between turbo and engine. With the turbo only pumping air, the usage of intercoolers for efficient charge cooling is possible. The tricky part with this setup lies in the fact, that the carb actually sees pressure. This subsequently leads to several other requirements: First the fuel has to be pressurized to a pressure-level slightly over boost, which means a fuel pump and a rising rate fuel pressure regulator are necessary to take care of that. Secondly the float bowl has to be pressurized to slightly over boost-pressure as otherwise fuel may be squirted out of the jets, but it would really only be fuel in depending on the boost level. This is where the so called pitot-tube comes into effect (there will be a separate post on this subject as it is HUGE). Now bearing in mind that a blow-through setup requires all this extra tech, there must be a benefit for both the manufacturers and consumers in the end? As the carb's fuelling is directly coupled to boost-level overall fuel consumption is drastically reduced (and therefore also meeting emission requirements is made a lot easier!) Additionally with the usage of a plenum a reservoir for pressurized air is available making meeting the engine's air requirements a lot easier and as a result smaller turbos can be used to achieve similar power levels as before. Thereby reducing turbo lag and speeding up spool-up. And if that weren't enough unlike on draw-through turbos, where a piston-ring type dynamic seal can be used, which incorporates a lot less drag adding to the efficiency of the turbo.
The last part mentioned in the schematic below is the blow off valve (B.O.V.), which vents boost pressure into the atmosphere to keep turbine speeds up for a longer time, even when the throttle is already shut.

That said my original setup is/was a blow-through setup. It is in my book the perfect choice for a well refined turbo setup, ideal for road use.
Unfortunately as mentioned before the setup is very complex and the amount of space available on the TR1 is rather limited.

As such, I decided to take a u-turn and restart the build with a draw-through setup. Especially bearing in mind, that the main usage of the Turbo TR1 will be on the dragstrip.

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