Been thinking about this one for a while now. Once plug-in hybrids come on line, engine torque and driveability will be much less significant, as the electric buffer will be so big the only thing that will be wanted from the engine will be as high outright thermal efficiency as possible.
At the moment the best petrol engine efficiency you can get is about 35% under ideal conditions. However, VW's best TDi engines are now at about 46% efficient. This means that drop for drop, the TDi engine can allow a vehicle to travel 31% further (ie it is able to extract 31% more energy out of every drop of fuel than a spark ignition engine).
As I see it, most of this improvement is down to lack of throttle, and the diesel engine's high compression ratio (20-24:1). Surely, therefore, it would be trivial to make a petrol fuelled compression ignition engine? I read once that old Peugeot diesels were meant to take up to 25% petrol in their diesel tanks during the winter - and on the farm I have seen in the past diesel engines run (accidentally) on about 50% petrol with no ill effects at all (other than much easier starting).
Is there some reason why spark ignition must be used with petrol? Certainly stochiometry must be maintained in these engines (ie amount of air and fuel must match), but this is not the case with diesel engines. In fact, petrol engine efficiency plummets when not used at full throttle, and this loss isn't nearly as pronounced in diesel engines.
Final question - what are peoples opinions on the feasibility of an Atkinson (or Miller) cycle diesel engine? Would likely not be able to rev more than about 3,000rpm, but a potential expansion ratio of 30:1 could mean an efficiency maybe in the 60%s?

At the moment the best petrol engine efficiency you can get is about 35% under ideal conditions. However, VW's best TDi engines are now at about 46% efficient. This means that drop for drop, the TDi engine can allow a vehicle to travel 31% further (ie it is able to extract 31% more energy out of every drop of fuel than a spark ignition engine).
As I see it, most of this improvement is down to lack of throttle, and the diesel engine's high compression ratio (20-24:1). Surely, therefore, it would be trivial to make a petrol fuelled compression ignition engine? I read once that old Peugeot diesels were meant to take up to 25% petrol in their diesel tanks during the winter - and on the farm I have seen in the past diesel engines run (accidentally) on about 50% petrol with no ill effects at all (other than much easier starting).
Is there some reason why spark ignition must be used with petrol? Certainly stochiometry must be maintained in these engines (ie amount of air and fuel must match), but this is not the case with diesel engines. In fact, petrol engine efficiency plummets when not used at full throttle, and this loss isn't nearly as pronounced in diesel engines.
Final question - what are peoples opinions on the feasibility of an Atkinson (or Miller) cycle diesel engine? Would likely not be able to rev more than about 3,000rpm, but a potential expansion ratio of 30:1 could mean an efficiency maybe in the 60%s?