Diesels do not have to be “bad polluters”. As most of the pollution is attributed to various oxides of nitrogen, simply remove the nitrogen from the atmosphere and replace with argon.
Of course, this may not be practical. There is also the option of using various kinds of urea injection systems into the exhaust stream, which reduces the various NOx compounds to carbon dioxide and and free nitrogen. This system is available on most newer European Diesel vehicles.
Also, there is the further option of using an auxiliary alternative fuel, compressed natural gas, as the means by which high power demand is put on the power unit, such as acceleration or heavy load conditions. All internal combustion engines accelerate by overfueling, resulting in incomplete combustion in the instance of Diesel fuel, with the production of much soot. If the overfueling is provided by injection of compressed natural gas, thee is far less soot production, and consequently much less need for the urea injection system referred to above.
Diesel can work, and the technology has progressed to the point that the inherent efficiencies as compared to a spark ignition engine are a sufficiently large advantage to eventually displace the spark-ignition engine altogether.
Personally, I would go to a regenerative and recycling steam external combustion engine, much as powered the Doble steam car of the 1920’s. Double-acting pistons on the high-pressure side of the motor, coupled with a secondary low-pressure set of double-acting pistons, driving a common output shaft, connected directly to the rear axle, eliminated a vast number of multiple gearing systems. And it was all done nearly a hundred years ago.
I have always thought Doble was a great steam car that was killed by the great depression and wondered why the technology was never taken up again.