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Diesel vs. Gasoline Turbo Design
There is much confusion about turbos and their use on diesel engines compared to their use on gasoline engines. We'll see if we can shed some light on the subject.
The principals of their operation are the same and their plumbing of intake air and exhaust gases are the same, but where the real issues come into play are the combustion pressures and temperatures, operating RPM range, and the driving cycle.Diesel Engines and Turbos
Let's start with the basics. Whether it is a semi truck or a generator set, turbocharged diesel engines are typically large displacement, low operating RPM power plants. They run in a very narrow RPM range. Diesel fuel is atomized as it enters the combustion chamber and upon compression (with only residual heat) the air/fuel mixture lights off. The combustion pressures are very high due to the high compression ratios needed to ignite the mixture. Because of the high compression pressures, mechanical parts must be strengthened to handle such loads: connecting rods are larger, crankshafts are stronger, and pistons are bigger. All of this reinforcement means that there is a much heavier rotating mass, which is more difficult to spin at high RPMs, so diesel engines tend to be low RPM power plants.
As with any internal combustion engine, the more air and fuel that can enter the combustion chamber, the more power it will produce. Diesel engines, because they are spinning at low RPMs, can't really "suck" in enough air to fill the chamber on each cycle, so the addition of a turbocharger helps.
Diesel burns at low temperatures and exhaust temps are typically in the range of 500-800ºF and the volume of exhaust gases is quite high. When sizing a turbo for a diesel engine, the turbine section must have enough flow for the high volume of exhaust gas and minimize back pressure. It also must flow a very large volume of intake air in order to keep the cylinders filled. As mentioned previously, diesel engines already run high compression, so adding a turbo is to increase the volume of air entering. The boost is typically very low: in the 5-8 psi range. (I know there is someone reading this saying "Hey, I've seen tractors that are running 40 psi of boost." You may want to look into that because it is not uncommon to have people mistake "inches of manifold pressure" for "psi of boost." There are a few show trucks and tractors that run outrageous amounts of boost, but remember we are dealing with the drivable vehicles here.)
Now that we have this turbocharged diesel engine with a huge turbo to make the volumes that we need, let's talk about spool-up times and drivability. Spool-up time? Any watch with a second hand can time the spool-up of a diesel turbo. They aren't the fastest in the world, but they don't have to be. Most diesel engines are designed for continuous reliable running, not 0-60 sprints. And even though that huge turbo takes a while to spool up, it also takes a while to spool down. Most diesel engines don't have throttle bodies (they use metered fuel to control engine speed), so there is less back pressure when you lift of the accelerator, and therefore less to spool the turbo down. The only spool down is from the reduced exhaust gases. These larger turbos spin at lower RPMs, typically below 50,000. So, you can size the turbo larger to get the volume you need without the main concerns being spool-up time.Gasoline Engines and Turbos
Gasoline is much more volatile than diesel fuel: it burns hotter, ignites quicker, does not need to be atomized for combustion, and also uses a lower air-to-fuel ratio than diesel fuel. These are smaller displacement engines that need to operate at a wide RPM range for drivability. They already have four valves in the head so they can get a lot of air and fuel into the cylinder and exhaust gas out. So we have an engine that runs very well, but we want more power, so we (or the manufacturers) add a turbo.
Our engine already "breathes" fairly well due to its head design. There are some four-valve heads that achieve 98% efficiency at filling the chambers. What we need to do to get more air in is to pressurize it, so we need more intake air pressure along with the correct volume. If we were to run 5 psi of boost, that would be just enough so that the engine doesn't waste energy "sucking" air in. If we really want to make more power, we have to step up to the 10-15 psi pressure range, along with the correct volume. So, we need high pressure and volume. In order to generate these higher boost pressures, the turbo spins at a much higher rate, from 40,000 all the way up to 125,000 RPM.
We need to have quick spool-up time of the turbo. We tend to wind through gears more than a diesel, so there is more up-down-up in the RPM range. Unlike a diesel, we have throttle plates that close off the intake tract, so whenever the throttle closes, the turbo spools down due to the back pressure and lack of exhaust gases. The closing of the throttle plates can cause a significant surge in the intake tract and can even damage the compressor wheel if it is severe enough. With our broad RPM range we need to make the proper volume of air for the engine speed. Our engine needs twice the volume of air at 7000 RPM than at 3000 RPM and it needs to make it efficiently.
The turbine side of the turbo needs to be addressed differently. Gasoline exhaust gases are HOT, in the range of 1000-1400ºF when under load, but they are of a comparatively lower volume to a diesel. The turbine can be sized to take advantage of this heat. Hot gases will expand, and by keeping a small amount of back pressure to the engine, the cylinders will be filled completely. The hot gases have only one place to go: through the turbine. Any restrictions after the turbo are a waste and only cause slower spool up times.Quick Summary
The information is provided to educate turbo owners. The differences between gasoline and diesel turbos is significant. This is another reason to have your turbo rebuilt by a shop that works with gasoline turbos. Some diesel shops would be impressed by working on a turbo for an exotic car and would be willing to do the work, but they are typically limited with their testing ability. If the shop can only balance your turbo to 30,000 or 40,000 RPM (where diesels operate), it really doesn't do much good because your turbo is operating at double, triple, or even four times that speed. Try to get it balanced to at least 70,000 RPM at minimum. In addition, the oil-to-air pressure differentials of a diesel turbo and a gasoline turbo are significantly different. Pay the extra money and get the work done by an experienced shop.
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