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post #2 of Old 05-01-2012
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Re: Turbo charged diesel

The basic answer is that with a turbo, you can have a much smaller engine which is advantageous for many reasons. You are correct that turbos force more air into the engine. This then means that you can put more fuel in too. So for the same sized engine, you have more power.

Engines are positive displacement, volumetric devices so without a turbo, they will suck in a known volume of air which has a mass which is determined by the volume and the pressure drop associated with sucking the air in. With a turbo, the volume is the same but the pressure is much greater so the mass flow is greater.

If you consider a positive displacement engine, the closer you expand to atmosphere, the more power you will extract from the gas charge. This is a large portion of the reason why a diesel is more efficient than a gas engine because it has a higher compression ratio. Because of practical design concerns, you can't actually expand to atmosphere so a lot of energy goes out your exhaust. Turbos are thermodynamic devices which use this energy to do something useful.

Turbo sizing is a real science and a turbo will have an optimal operating point. This means that your turbo will spool up more as it gets close to the designed operating point. A lot of people talk about your boost pressure as a way to describe how much a turbo is doing. This is really on a measure of restriction in the intake and how much heat is going out the exhaust but it gives a decent indication of what your turbo is doing. You won't see it in your boat because it is very hard to change the engine load at a constant rpm but if you go in a turbocharged car or truck, mash the pedal and watch the boost climb while the rpm stays the same because you are putting more energy out through the exhaust. Just because a turbo isn't singing doesn't mean that it isn't doing anything. There is no point in having too much air.

The other reason for all of this is emissions. It is very hard to pass emissions without one these days.

Personally, I really like turbocharged diesels. The turbos are amazingly reliable considering the abuse that they take and they have huge benefits. It is not uncommon to have a turbocharged engine have a horsepower rating of 3X compared to the same engine in a naturally aspirated version. If your engine is simply too big, that is a different problem and has nothing to do with the turbo, someone sized it wrong.
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