A diesel engine pumps 100% of it's air capacity 100% of the time while a gasser pumps 100% of its capacity only at wide open throttle.  Reason out your question based on that tip.

rmw

Isn't the turbocharger and its turbine mainly 'engaged' at full throttle anyway, since there's not much need for boost a partial or low throttle settings (as far as the gas-engine is concerned)? Thus, I'd argue that the reason for the larger turbine on the diesel engine is rather based on the higher compression of the diesel engine. The higher the compression ratio the lower the enthalpy left in the exhaust gas, thus a larger turbine is needed to generate sufficient power to drive the compressor wheel.

Just a quickie---We work, on a daily basis,  w/ diesels that average 60-70+HP per liter at rated outputs. Turbo charging and low temp aftercooling has played an enormous part in allowing these types of outputs to become realistic and reliable..

 As for "low-end" power/torque, again the turbo has been the one of the biggest contributors as to having a power curve with this low end power--With engines rated at 1800 to 3000 RPM max ( 25-30+++ PSI at WOT) , we see boost starting to rise rapidly from 1000 rpm and up with 15-20+ PSI in the 1500-1700 RPM ranges on many engines..

 Newer waste gate technology, new or variable shaped air  turbo inlets, and new ideas in turbo air mapping has allowed new designs to give this low-end boost while controlling max boost pressures at high speeds---

In a nutshell, turbos do not just engage or "turn on"--They come up to speed based on design and the power that the engine needs to deliver whether that is at a low RPM below peak torque, in mid-RPM's at peak torque,  or at WOT..


Tony  

Tony Athens
http://www.sbmar.com

High efficiencies, high boost numbers, after cooling and fast response is important with gasoline as well as diesel engines (comparing 2 similarly sized gasoline and diesel engines). So this wouldn't necessarily explain why the turbine wheels of turbos on diesel engines are larger than on gasoline engines.
However, due to the lower compression ratio a gasoline engine has more enthalpy left in the exhaust gas (more pressure and temperature). This would at least partially explain why the turbine can be downsized accordingly on gasoline engines.    

And as far throttled operation is concerned: Turbos shouldn't come up to speed when the throttle is closed or partially closed on a gasoline engine regardless of turbos used. Otherwise a turbocharged gasoline engine would not be very efficient. Or why 'boosting' a throttled air flow unless perhabs for some race application?

I would like to mention an observation, then explain my theory.  I have a Duramax diesel with propane injection.  This truck runs 4 lbs of boost going 70 mph when running just #2.  If I turn on the propane, boost drops to around 1.5 psi.  When running lp, apparently it is pumping less air through the engine at the same power output.  I believe this is because you are introducing a homogeneous lp/air mixture that burns more oxygen in the combustion chamber when the #2 is injected.  When #2 is injected, it burns so fast that it doesn't have time to mix well with all the air in the chamber.  So, even when the engine is loaded to the point of starting to produce smoke, there is still unburned oxygen in the exhaust.  The lp helps it burn more of that.  I believe this is why it enables the engine to make more power than it would by just adding more #2 and making alot of smoke.