While the topic of turbos is up, can anyone explain why there hasn't been an initiative into turbo compounding (Ala BMW 320d) in gasoline engines in conjunction with (very) aggressive downsizing? I'm envisioning a 1 liter engine with twin sequential turbos making ~250hp and 300lb/ft from 1500ish rpms.

Cost vs. benefit.

... and detonation. Gasoline engines that are expected to run on ordinary pump gasoline don't like high boost pressure. If you want to run high boost pressure, you have to back the compression way off. Bye-bye thermal efficiency.

Diesels are a whole different ball game due to not having pre-mixed air and fuel.

By the way, GreenPlease, you may wish to consider that (in Imperial traditional units) HP = TQ x RPM / 5252. For example, if you want 250 hp at 1500 rpm, the required torque will be 1827 lb.ft!

Another rule of thumb is that for a normally aspirated 4 stroke gasoline engine with good "breathing", you are going to get somewhere near 75 lb.ft of torque for each 1 litre of displacement. A naturally aspirated 1 litre 4 stroke 4 cyl (example, high performance motorcycle engine) will make 160+ horsepower, but does so at 11,000 rpm or thereabouts (and around 75 lb.ft of torque). This is from an engine that is very highly developed to make power ... not necessarily high efficiency at part load. Another thing to keep in mind is that if you want very high torque at low revs, and you don't want the engine to feel like it's delivering power in a series of sledgehammer-blows, you need lots of cylinders, and that's usually not good for efficiency.

A small single-cylinder engine with balance shafts can be relatively smooth, but only by letting it rev, so that you can't feel the spaces between power pulses.

You might wish to re-state your objectives.

5000 psi CNG, turbo charged Natural gas Octane 130, high compression 15/1, ,One C to four H's (CH4) for low carbon output per Joule, direct injection, high rpm driving 400 Hz generator for serial hybrid, no fancy valves or waste gate or other nonsense since it can be tuned for full load and speed or off. No torque problem on hybrid.
When tank is down to 10% full at about 500 psi, a boost compressor can pump the Methane up to 500 for injection.
Small Compressor in your house, cheap NG stations along HP pipelines (900psig) since they compresssion costs are lower anlong with normal stations.
Please correct my logic.
What kind of milage would a SERIAL HYBRID Honda insight have gotten with this system?

I did not mean to say that the engine would make 250hp at 1500 rpm. That figure I assumend would come at around 6000rpm. Instead I meant to say that the peak torque of 300lb/ft would start around 1500rpm.

I do like the 130 octane in NG. I would prefer ANG at 500 psi to CNG at 5000 psi. The home compressors and vehicle tanks are simpler and less expensive.

I would say that a high compression engine with smaller displacement running direct injection with NG could be an efficient genset prime mover. Let's say you could get 70 mpg best case out of a series hybrid Insight with that 1L 3 cylinder running DI and NG.

You still might have to contend with a buying public that would rather have a hybrid Camry that seats five and gets 35 mpg than an Insight that seats two and gets 70 mpg.

For example, if you want 250 hp at 1500 rpm, the required torque will be 1827 lb.ft!

A 1-liter turbodiesel running at 320 PSI BMEP would yield roughly 135 ft-lb of torque.  You aren't going to get 300 ft-lb out of 1 liter without something like a hyperbar diesel, and what's the point, anyway?

Busted ... you're right. Still, even the original 300 lb.ft from 1 litre represents a BMEP that is well out of range of any practical turbocharged Otto-cycle engine running on any known practical fuel.

I was obviously in over my head with that one. Sorry gang.

No problem...we all learn from good valid information here. I like the idea of people researching topics and bringing back the information. That is a form of parallel learning that can make us all a little bit wiser in engineering, science and even... policy. The name of this site does include the word Congress, so policy discussions are allowed here as well.