Davis and Elliott [2] in 1951, observed low efficiency and low output in dual-fuel engines at part loads due to the weak propagation of flame in a lean mixture of air and gas. They pointed out that a portion of gas fuel leaves the combustion chamber without participating in the oxidation reaction.
In 1952, Lewis and Moore [3] stated that in very lean mixtures only the regions in direct contact with the firing fuel spray react and a portion of gaseous fuel always remains unburned up to the discharge process.
In 1955, experiments on the performance of dual fuel engines at part loads were done by Moore and Mitchell [4]. Increasing the amount of firing fuel and fuel injection advance were suggested as a solution to improve the combustion process.
In 1964, Karim [5] tried to predict the output of an engine that is restricted by knock in pilot injection engines. He assumed that the knock related to the auto-ignition of a fresh gas fuel charge at the same temperature as reached at the end of the compression process. He obtained a simple relationship between the inlet charge temperature and the engine output, for an engine that is restricted by knock.
Karim et al. [6] in 1966 showed that the observed knock in pilot injection engines is basically due to the auto-ignition of the gas-air mixture which is dependent upon existing ignition centres of diesel fuel spray. In this year (1966), Azouz [7] investigated experimentally the initiation of knock in a wide range of performance conditions of inlet temperature, the amount of firing fuel, fuel injection timing and the pressure of injected fuel. He concluded that the initiation of knock is very sensitive and dependent on the variation of inlet temperature, but increasing the diesel fuel for firing does not have significant effects on the knock limit. He also analysed the exhaust gases over a wide range of injection timing for the firing fuel versus the inlet temperature and injected pressure. He found that most of CO and unburned methane is produced for fuel equivalence ratios from 0.4 to 0.5. Increasing or decreasing this ratio can reduce CO and unburned methane in the exhaust gases.
In 1980, Karim and Burn [8] experimented with the combustion phenomenon in dual fuel engines with different gas fuels. They found that the increase of ignition delay depends strongly on the inlet temperature. The low temperature of inlet mixtures with low amounts of firing fuel causes irregularities in the function of the engine and also causes an increase of ignition delay.
In 1986, Acker [9], observed that temperature, pressure and the concentration of fuel have a significant role in an engine in determining the ignition temperature and ignition delay.
In 1989, Karim et al. [10] stated that the development of the combustion process in dual-fuel engines is independent of the characteristics of spray and the ignition of diesel fuel, but it depends on the type of gaseous fuel and its concentration levels in the cylinder. They showed that in very lean mixtures and with very low pilot ignition, the initial flames cannot propagate in all parts of the cylinder. Any process that increases the region of pilot combustion or improves the capability of ignition around the ignited fuel can also decrease the unburned hydrocarbons remarkably.
In 1992, Tao [13] experimentally found that emissions of NOX in dual fuel engines are lower than in conventional diesel engines. He showed that the ignition delay and the low temperature of gas combustion are the means of producing low NOX in dual-fuel engines.
In 1993, Karim et al. [14] observed the decrease of methane concentration in compression of natural gas and air before the spray of igniting fuel. Although this decrease of concentration is insignificant, these results show the
importance of chemical kinetics at low temperature, and the effects of the increase of temperature caused by compression of natural gas fuel and air.
In 1997, Liu, Karim [15] simulated the combustion phenomenon in dual fuel engines by using a multi-region model of thermodynamic and chemical kinetics.
They found that the increase of the temperature and pressure caused by compression processes on the mixture of natural gas and air causes certain reactions, and produces intermediate species like radicals, carbon monoxide and formaldyde.
In 2000, Kusaka, et al. [19] investigated the effects of using a heat exchanger and EGR on combustion and emissions in a four-cylinder engine with direct spray. Increasing the inlet temperature improves the combustion at part load, but increasing the combustion temperature causes higher NOX emissions.
In 2003, Selim [26] studied the effects of cold EGR on the performance and noise output of dual-fuel engines. The results showed that increasing the EGR percentage decreases the noise of output.
In 2000, Abd Alla et al. [18] did research on the effects of the amount of igniting fuel on the performance of dual-fuel engines. The results show that an increase of diesel igniting fuel improves the performance of dual-fuel engines at all loads.