...do some research on the
Continental AVCR-1360 "Variable-Compression-Ratio" diesel engine that
GM used in their XM1 Tank prototype, the experimental tank that lost out to the
Chrysler/General Dynamics "Turbine" XM1-B during the Jimmy Carter presidential years.
...brief SAE info on the AVCR-1360:
http://papers.sae.org/760051/