Why are Superchargers not preferred for boosting power of a diesel engine whereas a turbocharger is preferred for boosting power of a diesel engine? Both of them increases the pressure inside the combustion chamber albeit using different approach. The only application where you can find a supercharged diesel is in a 2 stroke diesel engine which is also only for scavenging purpose.
There are several reasons. A supercharger has much less impact on the overall efficiency of a reciprocating IC engine than a turbocharger. The SC divides the work up into different parts (equipment), while the TC changes the processes. A TC changes the efficiency (better or worse) all over the range of operation. The SC adds power on demand, which is why it is preferred in drag racing. The TC adds power over a prolonged period of time, which is why it is preferred for fleet truck application. While it wasn't true 60 years ago, now TCs are cheaper than SCs and last longer in service so that they are a better investment. As for a 2-stroke engine (such as was commonly found in busses 60 years ago with the 6-71 engine), you must not restrict the exhaust (by attaching a TC), as this will kill scavenging of the cylinders. A valved 4-stroke engine has a "powered" exhaust stroke.
Adding a supercharger changes the performance of an engine in a manner similar to increasing the displacement. Adding a turbocharger changes many more characteristics. One may be preferable in an application, while not in another or vice versa. It depends on what you need.
The disadvantages of a supercharger include:
- Increased gas loading on the engine.
- Higher heat loss due to increased turbulence.
- Increased cooling requirements for the engine.
- Elevated thermal stress on engine components.
- Greater likelihood of detonation in spark ignition (SI) engines.
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