Looks like Diesel engines have a thermal efficiency of up to 51.7% [1] (with a theoretical maximum of 56% [2], and Toyota claiming a practical 38% [3]) and Gasoline engines ~25%. Based on that, a Diesel powered phone would last 3.8-5.6 days.<p>[1] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Thermal_efficiency" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Thermal_efficiency</a><p>[2] <a href="http:&#x2F;&#x2F;hyperphysics.phy-astr.gsu.edu&#x2F;hbase&#x2F;thermo&#x2F;diesel.html" rel="nofollow">http:&#x2F;&#x2F;hyperphysics.phy-astr.gsu.edu&#x2F;hbase&#x2F;thermo&#x2F;diesel.htm...</a><p>[3] <a href="http:&#x2F;&#x2F;www.greencarreports.com&#x2F;news&#x2F;1091436_toyota-gasoline-engine-achieves-thermal-efficiency-of-38-percent" rel="nofollow">http:&#x2F;&#x2F;www.greencarreports.com&#x2F;news&#x2F;1091436_toyota-gasoline-...</a>