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Why can a gasoline engine not be operated without a throttle plante as a diesel?
And, why can a Diesel engine not be operated the same way as a gasoline engine?
Not something I'll claim to know a massive amount about, but have a google of mazda's SkyActiv engine development. They're using a combination of spark ignition and compression ignition on a gasoline engine. I believe the fuel injection still happens before the compression though, in order to get as homogeneous a mix as possible.
With a diesel engine the torque is controlled via fuel delivery. A diesel engine will also operate very happily at incredibly lean air fuel ratios where as a gasoline engine will only run happily over a relatively narrow range of AFR. For this reason we can't control torque sufficiently with a gasoline engine by fuel management alone and a throttle is required. In a diesel engine a throttle body isn't necessary and this gives one of the diesel's advantages in that they have less pumping losses. Despite this you will find that many diesel engines are fitted with a throttle but they aren't there for the same reason. The throttle is used to help the engine shut down quickly and it can also be used to improve the effectiveness of the EGR valve.
All the "throttle less" gasoline engines out there, such as BMW Valvetronic engines, still use the throttle. That's simply a misunderstanding combined with bad marketing.
For example, on a Valvetronic engine the stock computer (DME as they call it) basically targets 90 kPa of absolute pressure in the manifold during cruising range, and adjusts the valve lift and valve phasing (VANOS) to achieve the target. "Dethrottling" on a gasoline engine just means reducing the pressure drop in the intake manifold (running less vacuum). Some of the vacuum is used for emission components like evaporative purge. You will see similar behavior on systems like the Fiat MultiAir and the Jaguar version of that (which they just call electrohydraulic valvetrain).
On a diesel, a throttle isn't needed to control load, but it is used to help control emissions systems such as exhaust gas recirculation (EGR). With regards to the Mazda Skyactive HCCI style engine, if you read the patent info it has bunch of different zones of operation, some of which use a throttle like a normal engine.