"since the first mass produced engines, there has been little progress in the design." John

I'm sure you have a valid point intended here John. I'll paraphrase an engineer's comment about something he may have called -horsepower inflation-.

- A ~2.5 liter engine used to get about 100 hp. Now, with numerous refinements, not turbo-charging or anything like that, just refinements in fuel management (fuel injection instead of carburation), lubricants, etc.; the same displacement engine now gets about 170 hp, and pollutes less. -

I agree John, we're using reciprocating engines, and not the wankel, or the jet engines, or external combustion engines.

But at this point I suspect we're up against the law of diminishing returns. I think getting nearly 30 MPG out of a passenger car is impressive; an artifact of the economies of scale, and distributed costs.
"Tractors, harvesters, heavy machinery and public transport should, in my opinion be prioritised for oil, whilst cars, recreational vehicles and non-essential business vehicles should be modified as soon as possible to use other forms of fuel" John

That's essential, but insufficient.
It will only postpone the inevitable.

"Whenever you start posting dictionary quotes, we know you've been bested." captainkbt