A 2-stroke like the TPi of KTM is hurting at partial loads because it cannot help running with a high quantity of residual gas in the cylinder.
at the beginning of the compression,
the pressure in the cylinder of the 2-stroke is near atmospheric,
while the pressure in the cylinder of the 4-stroke is less than 1/5 of the atmospheric.
The quantity of residual gas in the 2-stroke at idling is five times more than in the 4-stroke.
With a small quantity of air-fuel mixture “lost” into a big quantity of residual gas, the combustion in the 2-stroke cannot be complete / clean.
Reasonably, the triple 750cc TPi, which will run at substantially lighter load than the single cylinder KTM250EXC TPi (which barely passes the euro-4 emissions regulations), cannot but being worse.
The direct injection 2-strokes (like the E-TEC of Rotax / Evinrude / Bombardier) can use stratified charge (direct injection towards the spark plug) to improve the partial load operation of the 2-stroke. However at the areas of the cylinder wherein the mixture gets too lean, the flame outas out and the combustion cannot be complete / clean.
A way better solution for the 2-strokes seems the lean burn with HCCI combustion, like, say, the PatBam HCCI.
The lean mixture is not a limitation, any longer.
The combustion is not progressive, any longer, but instantaneous, or too fast:
At partial loads or idling the cylinder fills with air (no pumping loss during the intake, no sub-pressure into the crankcase).
Mazda claims for their SkyActiv-X HCCI a 20% fuel consumption reduction as compared to their current high-tech SkyActiv-G engines, and 40% fuel consumption reduction as compared to their MZR engines:
If the SkyActiv-X SP-CCI / HCCI and the PatBam HCCI can improve one time the 4-stroke engines,
the PatBam HCCI can improve the 2-strokes several times because, besides the combustion, it also addresses the worst problem of the 2-strokes: the partial load operation (sub-pressure in the crankcase, resulting in high quantity of residual gas in the cylinder, resulting in incomplete combustion etc).
If Mazda is right about the 20% fuel consumption reduction with their lean burn HCCI, the guess is that for the 2-strokes (characterized by mechanical simplicity and lower friction losses) the improvement could be more than 40%.