TORQUE AND PRESSURE CURVES
Please click the graph below to view an enlarged version (opens in a new window).
The graph above shows a typical curve of force A (red) on the piston crown. Turning arm length D (orange) and the product of those two equals torque E (turquoise). The curves assume a normal compression ratio of, say, 9:1. The vertical axis is scaled arbitrarily for convenience. The relevant curve period is after top dead centre TDC and the area under the torque curve equates to power output.
Energy Storage Piston –The ESP with compression ratio doubling.
The spring will compress an amount decided by the compression pressure but after full load combustion has taken place it will, by design, fully compress such that the combustion chamber is restored to its original volume of that of the standard piston. The force exerted by the spring will be equal and opposite to the combustion gas force which has now halved, and both will be acting on pushing the piston downwards. The force on the ESP therefore will be the same as that of the standard piston as shown on the graph, but the releasing of the spring energy will maintain the pressure constant until the spring is totally inactive.
Clearly the torque arm will have increased during this gas pressure equalisation as shown in C (light green) on the graph compared to normal combustion pressure B (dark green) therefore greatly increasing the torque available. Heat losses in both cases have been assumed to be equal. The engine in this arrangement will attempt to race off at high speed and have to be “throttled back” to maintain a normal tick over. It is this “throttling back” to achieve tick over which reduces the overall fuel consumption.
Clearly the graph shows that the ESP arrangement is considerably more efficient than the standard piston, and will not incur heavy peak loads on the crank shaft.