The Otto Cycle, which is considered to be a constant volume process, is one in which the fuel mixture reacts so quickly that the piston moves very little during the combustion process. Theoretically, the Diesel cycle is considered to be a constant pressure cycle which means that, as fuel is being sprayed into the combustion chamber, the fuel combusts about as fast as it is sprayed-in. which results in a much slower rise in cylinder pressure. In practice, the fuel injection duration is very short and the combustion is quite fast and is more closely approximated theoretically by the thermodynamic Dual Cycle. The characteristic diesel clatter (violent explosion?) is due to the very fast rise in pressure during combustion due to ignition delay (the time between injection and ignition).
For more information about diesel dual fuel combustion, see the following paper:
Because Diesels do not have throttle valves and rely on the amount of fuel injected into the combustion chamber to control the power produced, Diesels often operate at very lean fuel mixtures. Adding a gaseous fuel to a Diesel's intake charge allows the engine to make more power because it reacts with the excess oxygen already in the engine. However, Stewart reports that the gaseous fuel combustion is dependent upon its equivalence ratio, the characteristics of the pilot injection, and load. Due to its high Octane Rating, LPG & CNG allow the Diesel engine to perform more like an Otto cycle engine under some conditions, thereby slightly increasing its thermodynamic efficiency. The co-combustion of a gaseous fuel with diesel fuel generally reduces diesel particulate (soot) emissions, which reduces Regen Cycle time thereby further improving fuel economy.
Although a higher thermodynamic efficiency tends to reduce exhaust temperatures, additional fuel can only increase combustion temperatures because the system introduces more energy to and reduces the diluting air from the combustion chamber. In order to produce more power, more fuel must be consumed which must result in higher combustion chamber temperatures. Because the heat losses from the engine have not decreased significantly, there is no way for there to be a huge increase in fuel economy. The gain in fuel efficiency is due to the additional amount of work (see Thermodynamic Cycle) possible by the increase in thermodynamic efficiency. Keep in mind that, although you will be buying less diesel fuel, you will be buying more LPG/CNG, and LPG contains less energy per unit of volume than diesel. Only if your local propane or CNG price is significantly lower than diesel will you be substantially reducing your driving costs. Typically, natural gas obtained at a public CNG station run by a natural gas utility will much less expensive (on a BTU basis) than any other fuel.
Obviously, an engine will produce more power when it consumes more energy and this requires putting more fuel into a cylinder per cycle. In practical terms, diesel engines are designed to operate with excess air and adding more fuel will ultimately make combustion hotter, thereby making more power from the resulting higher temperatures and pressures. It is safe to increase the power output of a diesel engine by adding small amounts of propane or natural gas to the air stream. Too much gaseous fuel will cause it to self-destruct. Generally, low loads allow proportionally more propane/CNG to be used relative to diesel fuel. As loads increase, the proportion must be reduced to prevent the onset of highly destructive detonation. CNG's higher CCR allow more fuel substitution than LPG.
Because a gaseous fuel will displace some of the combustion air (as it would in a gasoline engine as well), a turbocharger is the best way to obtain the full benefits from the addition of a gaseous fuel. Ideally, the amount of propane/CNG added to the fuel mixture must be carefully controlled by an electronic fuel metering system, which takes into account many engine variables.