[phkahler]

There is a fundamental limit to ICE efficiency dictated by the laws of thermodynamics. You can push that theoretical limit up, but it is dependent on having a higher compression ratio. Diesel engines use higher compression ratios for ignition, but they are dirty (in comparison). Higher compression ratios tend to result in more NOx emissions too, so for the regular car makers there is a direct trade between efficiency/emissions/reliability. But even if you aim for highest efficiency at all cost, you'll never get close to 100 percent, as the theoretical max never goes there (or does it at infinite compression ratio?).