Disabling the field reduces alternator pulley mechanical loading to nearly zero, maximizing crankshaft power. Early engines had compression ratios of 6 to 1. With early induction and ignition systems the compression ratios had to be kept low.
On the first upstroke of the engine there would be no fuel inducted into the cylinder as the crankcase was empty. Most diesels also have a battery and charging system; nevertheless, this system is secondary and is added by manufacturers as a luxury for the ease of starting, turning fuel on and off which can also be done via a switch or mechanical apparatusand for running auxiliary electrical components and accessories.
As the piston is driven downward with power, it first uncovers the exhaust port where the burned fuel is expelled under high pressure and then the intake port where the process has been completed and will keep repeating. The same thermodynamic considerations about the expansion apply.
They must use the transfer port s as a carefully designed and placed nozzle so that a gas current is created in a way that it sweeps the whole cylinder before reaching the exhaust port so as to expel the combustion gases, but minimize the amount of charge exhausted.
Starting the description at TDC, these are:   Intake, induction or suction: The intake valves are open as a result of the cam lobe pressing down on the valve stem. CI engines that use a blower typically use uniflow scavenging. Starting at TDC the cycle consist of: Power: While the piston is descending the combustion gases perform work on it, as in a 4-stroke engine.
At the end of this stroke, the exhaust valve closes, the intake valve opens, and the sequence repeats in the next cycle. With low octane fuel, a problem would occur as the compression ratio increased as the fuel was igniting due to the rise in temperature that resulted.