The history of transportation engines provides many examples of multi-stage or compounded expansion of heated gases. Several railways operated steam locomotives with high and low-pressure cylinders while the early ships used triple-stage steam expansion. Multi-stage steam turbine engines provided propulsion for trans-Atlantic passenger ships of an earlier period while modern commercial jet engines feature multi-stage power turbines placed downstream of the combustion chambers. Even modern commercial road and railway diesel engines feature turbo-chargers that act as low-pressure engines that drive air pumps. However, there is scope to use compounded gas expansion on some modern non-turbocharged automotive engines.
The 90-degree V8 engine spaces crank throws at 90-degree intervals. One pair of cylinders on each bank lead the other pair of companion cylinders by 90-degrees. Under heavy load, some modern V8 engines fire all cylinders and under part load, may operate either 4- or 6-cylinders. The ‘unloading’ of cylinders provides the basis by which raise efficiency by operating compound gas expansion. A re-designed cylinder head would include pipes with control valves that connect between adjacent pairs of cylinders. The interconnecting pipe would operate when the engine runs under part load.
The basis of compounded gas expansion in a V8 engine is the ‘next-door’ cylinders trailing the lead cylinder by 90-degrees, allowing it to further expand the low-pressure gas. When the lead cylinder is at 90-degrees after top-dead-center (TDC), the trailing cylinder would be at TDC. At that point, the valves in the interconnecting pipe would begin to open for expanding gas to exert pressure on the surface of the trailing piston. As the lead cylinder approaches bottom-dead-center (BDC), the leverage between its connecting rod and crank rapidly diminishes while leverage between the connecting rod and crank of the trailing cylinder would be near maximum.
Optimal leverage between connecting rod and crank on piston engines occurs between 30-degrees after TDC and 30-degrees before BDC. When the lead cylinder is at 30-degrees before BDC, the trailing cylinder would be at 60-degrees after TDC and with the angle between con-rod and crank being in the range of optimal leverage. The interconnecting pipe may remain open until the lead cylinder reaches 30-degrees after BDC, when the valves in the pipe close. While a single cylinder may exert effective torque on the crankshaft over 150-degrees of rotation, compound expansion of decreasing pressure gas would extend this angle of rotation to 240-degrees.
Compound expansion of combustion gas is a variation of the Atkinson cycle spread over 2-cylinders spaced at different crank angles. The crank spacing allows the lower gas pressure in the trailing cylinder to exert torque on the crankshaft using the greater leverage between connecting rod and crank throw. While operating in power cutback mode, a V8 engine running compound expansion may burn the fuel of a 4-cylinder engine while delivering equal or slightly greater power output of a 5-cylinder engine. During motorway cruising, it may have less need to periodically operate 6-cylinders.