All heat engines run on pressurised gas (steam turbine/gas turbine/steam engine/diesel/petrol engine). In the case of steam/air engines this is directly observable. In the case of internal combustion, the combustion (increase in temperature/molality) acts to increase the pressure. I propose an engine that uses coal, the most abundant and cheapest fuel available.
Internal combustion engines using coal have often been proposed and in fact when Rudolf Diesel was designing his engine he initially tried to run it on coal dust (again because it was cheap). This engine is different in that it operates similarly to a steam engine where you have a constant high pressure supply being intermittantly admitted to the cylinder.
The idea consists of a combustor operating under pressure (analogous to the combustor in a gas turbine or the boiler in a steam engine/turbine) where compressed air enters and coal is burned. The coal exists as a stationary bed that burns while being continually replenished and the flue gas leaves passing through cyclonic separators and/or electrostatic precipitators before entering the engine to be expanded.
A similar idea has been tried with a coal turbine (see link) where the fly ash was prohibitively errosive. This idea trys to sidestep this problem by running an engine instead of a turbine (preferably rotary like the Tower engine) so therefore runs at slower speed, doesn't maintain a high-speed flow of ashy gas over its surfaces and has some separation steps between combustion and expansion.
Criticism go!-- sneakythumbs, Jan 12 2010 Coal Turbine http://en.wikipedia...oal_Burning_Turbine [sneakythumbs, Jan 12 2010] coal powder burning Diesel engines http://www.freepate...ne.com/5109666.htmlWidely known to exist. [8th of 7, Jan 16 2010] I can't imagine the amount of ash it would produce.-- RayfordSteele, Jan 12 2010 Waste heat engines work this way.-- reensure, Jan 16 2010 You want us to try to disprove the function of direct turbine generation? Honestly I think that due to the issues with coking and high temp deposits that are present with coal as an unprocessed fuel you are better off with a liquefaction or gas process, possibly using waste heat, than a direct combustion process. So IMHO this is an idea that is baked. [Marked-For-Deletion]-- WcW, Jan 16 2010 Baked. <link>-- 8th of 7, Jan 16 2010 random, halfbakery