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Many current aftermarket natural gas conversions of gasoline fuelled spark ignited engines use an air-valve type mixer with closed loop control of the gas pressure. This control is often provided by an electronic integral controller that uses the output from an exhaust gas oxygen (EGO) sensor to control the duty cycle of a solenoid valve. By varying the duty cycle of this fuel control valve (FCV), the average pressure in the low pressure regulator (LPR) reference chamber and thus the gas pressure can be varied. The transient behaviour of these fuel systems is affected mainly by the mechanical response of the gas mixer and the LPR. The electronic controller can provide compensation only after the EGO sensor has detected an air-fuel ratio excursion. The main weaknesses of this type of fuel system seems to be associated with the finite response of the mixer and the LPR and by the use of an airflow dependent vacuum signal strength for control.

A study was performed to determine the effects of engine operating variables and piston and ring parameters on the crevice hydrocarbon emissions from a spark-ignition engine. Natural gas was used as the test fuel in an effort to isolate crevice mechanisms as the only major source of unburned hydrocarbons in the test engine's exhaust. The largest of the in-cylinder crevices, the piston ring pack crevices, were modified, both in size and accessibility, by altering the piston top land height and the number of piston rings and their end gaps. Each piston and ring configuration was subjected to a series of test sweeps of engine operating variables known to affect exhaust hydrocarbon emissions. None of the physical crevice modifications had any significant effect on the level of the exhaust hydrocarbon emissions, although the cycle-to-cycle repeatability of these emissions, measured with a fast hydrocarbon analyzer, was found to vary between the different configurations.

An optical probe for measuring the motion and rate of growth of the early flame kernel in spark ignition engines is described. The probe consists of a standard spark plug with eight optical fibers installed in a ring at the base of the threaded region of the plug. The fibers collect the light emitted from the flame as it crosses the field of view of the fibers, and transmit the light to photomultiplier tubes. The time from ignition until detection of the flame is used to compute the average flame velocity in the direction of each fiber relative to the spark location. The real-time data acquisition system permits statistical analysis of cycle-by-cycle variations in the combustion rate. Because the probe was built using a standard 14 mm spark plug, it can be used in unmodified production automotive engines.

Crevices in the combustion chamber are the main source of hydrocarbon (HC) emissions from spark ignition (SI) engines fuelled by natural gas (NG). Instantaneous in-cylinder and engine exhaust port HC concentrations were measured simultaneously using a Cambustion HFR400 fast response flame ionization detector (FRFID) concentrated on the post-flame period. The raw data was reconstructed to account for variation in the FFRID sample transit time and time constant due to fluctuating in-cylinder pressure. HC concentration development during the post-flame period is discussed. Comparison is made of the post-flame in-cylinder and exhaust port HC concentrations under different engine operating conditions, which gives a better understanding of the mechanism by which HC emissions form from crevices in SI engines.

Propane is one alternative fuel which is already widely available. Its use as a transporation fuel has largely been confined to spark ignition engines, however. This paper reports on an investigation into liquid propane injection as a means for fueling diesel engines. A single-cylinder CFR cetane rating engine was used to carry out the experimental work. The fuel system was revised to ensure that propane remained liquified in the fuel injection pump and injector. Since propane has a very low cetane number, some means of ignition must be provided. Two means of ignition were evaluated in this investigation, diesel pilot injection and a continuously operating glow plug. Tests were carried out at compression ratios of 19:1, 22:1 and 25:1 and at engine speeds of 1200, 1500 and 1800 rpm for each of the two ignition methods. The results are similar for the same test conditions.

The increasing use of natural gas as a vehicle fuel has generated considerable research activity to characterize the performance and emissions of engines utilizing this fuel. However, virtually all of the results reported have been for pushrod OHV spark ignition engines or SI conversions of heavy-duty diesel engines. Because of the pressure to improve fuel economy imposed by CAFE requirements, passenger cars are increasingly tending toward high specific output, small displacement engines. These engines employ such features as four valves per cylinder and centrally located spark plugs which give them a different dependence on operating variables than traditional pushrod OHV engines. In this study, experiments were carried out with a two-liter four-cylinder Nissan SR20DE engine representative of modern design practice. The engine was operated on gasoline and natural gas at six different loads and three different speeds. Some tests were also done with isooctane.

The light emissions from a spark discharge were observed by inserting a fibre optic cable through the centre electrode of a spark plug, to investigate the possibility of determining the fuel-air ratio in the spark gap at ignition with spectroscopy. The total broadband light emission from the spark and the light emission centred at 385 nm from the cyanogen radical (chemical formula CN), were observed for varied ϕ and residual gas concentrations. Additionally, the spark breakdown voltage, Vs, was monitored for the experiments. All light emissions were observed to be dependent on Vs, which is influenced by mixture composition, temperature and pressure. With a spark gap size of 0.7 mm, the CN emission shows promise for evaluation of the cyclical variation of ϕ for 0.9 < ϕ <1.1.

Methanol-fueled SI engines have proven to be difficult to start at ambient temperatures below approximately 10°C. The use of dimethyl ether (DME) is proposed to improve the cold starting performance of methanol-fueled SI engines. Tests to evaluate this idea were carried out with a modified single-cylinder CFR research engine having a compression ratio of 12:1. The engine was fueled with combinations of gaseous dimethyl ether and liquid methanol having DME mass fractions of 30%, 40%, 60% and 70%. For comparison, tests were also carried out with 100% methanol and with winter grade premium unleaded gasoline. Overall stoichiometric mixtures were used in all tests. All DME/methanol combinations provided good cold starting behavior down to -15°C, the lowest temperature attainable by the refrigeration system. The engine required approximately 50% fewer cranking cycles to produce the first fired cycle when fueled by the DME/methanol combinations compared to fueling with gasoline.