Turbo drive question

[RDBuck]

Pardon my ignorance but why do most diesels use the exhaust to drive the turbo? Is it because it is the most economical way to do it? Would't the engine run better with a crank driven turbo and no restriction in the exhaust stream?Wouldn't there be a lot more options about turbo size if it were removed from the exhaust? I think Detroits were rigged this way but I am not sure.

 

[jwgary]

Using the exhaust is a more efficient use of waste power than using the crank to turn a belt. I don't have any of my engines books here, but I seem to remember that for the same pressure increase and flow rate, a turbo is something like 30% more efficient than a crank driven supercharger. The old Detroits were two cycle diesels and they needed a supercharger to force intake air into the cylinders (and to force the exhaust out). Some Detroits also had turbos that took advantage of the power left in the hot exhaust gasses. Turbos do suffer from spool-up lag which is why the top fuelers use those huge blowers. After all, they only have 4. 5 seconds to go 1/4 mi.

 

[DieselB59]

The basic idea behind the turbo diesel vs. belt driven blower diesel comes down to engine efficiency and parasitic drag. The diesel engine, by design, is very efficient because of the usage of an exhaust driven turbocharger that uses the force of exhaust (wasted energy) to propel a turbine that compresses the intake air. A supercharger does the same principle, but uses a belt drive and, henceforth produces parasitic drag..... meaning that you actually have to use some horsepower just to make more. One more thing about the Detroits is that they all the valves on the cylinder head are exhaust valves. Interesting. Some guys just look at you crosseyed when you say that one to them... . especially the die hard DOHC Honda import gurus.