Diesels are dinosaurs. Diesels became popular in cars in Europe because of incentives / subsidies. They were regarded as less polluting only because they used to score well on the emissions tests designed for petrol vehicles. As emissions standards have been adopted for diesel engines we see makers going to extreme measures like particulate filters and fuel additives (AdBlue) to meet the standards.
We are entering an age with direct injection / compression ignition petrol engines where fuel economy / efficiency will increase dramatically. Maybe double from current levels.
Aircraft engines have exceptional specific torque (ie torque for engine capacity) and exceptional reliability. Most car engines spend their life (say 250,000km @ 50 km/h average = 5,000 hours) typically at 15-20% of its rated power. Aircraft engines spend their life (maybe 2,200 hours) at 75% power. Try running a car engine at 75% power continuously and see how it goes.
Aircraft engine development is hampered by government regulation / certification. If we relaxed this I think we'd see a dramatic improvement. Government restrictions are the only reason we don't have cheap fuel injection / electronic ignition systems that can be retrofitted. Fuel injection and a revised fuel system that allows GA aircraft (under say 10,000ft) to run on Mogas and the aircraft world would change.
Multipoint, closed loop electronic injection with knock sensors and lambda sensors, conversion to dual alternators / dual batteries with a modern EFI fuel pump ought to be possible for around AUD$5k installed. This ought to then handle 98 Octane Mogas up to at least 10,000ft.
Look at the time it is taking the US to test the leaded AVGAS replacement candidates. And the FAA is the speed of greased lightning compared with CASA. We need to stop regulators blocking innovation.