A New Pulsed Multipoint Injection System to Reduce Natural Gas Vehicle Emissions.

Nowadays, most of the natural gas conversion systems available on the market for bifuel vehicles are based on the venturi principle or on the continuous injection principle for the carburation of the engine. Even if the driveability of these systems is satisfactory, their emission performance does often not reflect the full environmental potential offered by natural gas. This is mainly due to the low response of these systems to transient conditions. During the past years, EPFL has been involved in a co-operative program with the industry to develop a new natural gas injection system intended for multipoint sequenced and phased indirect gaseous fuel injection. Mounted on a four cylinder 1.4-liter engine, the system includes specifically developed injectors and a PC based electronic control unit. Emission results on the engine test vench are shown for typical transient sequences as well as steady state conditions. Results on the chassis dynamometer are given for the New European Driving Cycle (94/12/EC directive). Keeping the standard vehicle components (like catalytic converter and lambda sensor), the current emissions results from our novel injection system are well below the legal emissions limits. Those results are compared with the performance of a state of the art multipoint continuous injection system from the market, measured on the same engine. In order to control accurately the narrower lambda window when working with natural gas, the system, among others, accounts for the sensitivity of the lambda sensor to the composition of the exhaust gases. While maintaining a good driveability, this system has shown significant emissions improvements compared to actual natural gas systems. New engine control strategies currently under evaluations are likely to still further reduce emissions.

Swiss Motor. Modification d’un moteur diesel pour le fonctionnement au gaz naturel en cogénération. Fonctionnement avec mélange stoechiométrique (l=1) et pauvre (l>>1).

Daniel Favrat, Geoffrey Basil Leyland, Roger Röthlisberger

The Swiss limit for NOx exhaust gas emissions from decentralised electrical power and heat cogeneration units using stationary I.C. engines operating on natural gas, despite a recent increase from 80 to 250 mg/mN3 (5% O2), remains the most severe in force in the world. The only technology currently able to meet this limit - the use of a stoichiometric mixture and a three-way catalyst - results in a relatively low global engine efficiency and suffers from a limited reliability of the exhaust gas treatment system. In this context, the Laboratory for Energy Systems of the EPFL has undertaken to develop new technologies (mixture, ignition,...) on the basis of a converted diesel engine for natural gas operation and to evaluate their reliability as well as their potential to reduce exhaust gas emissions, particularly NOx. The objective of this project is to perform an experimental study of the influence of the operating mode, either a stoichiometric mixture (l = 1) and a 3-way catalyst for exhaust gas treatment or a lean mixture (l >> 1) without catalyst, on the performance and emissions of a Liebherr G 926 engine, in order to establish a basis of comparison for future evaluation of new technologies. The realisation of this project required the construction of a new test bed especially developed for the study of gas engines. In the case of the stoichiometric operating mode, the results show that at an MBT spark timing of 20 °CABTDC, a mechanical power output of 106.3 ± 0.5 kW at 1502 ± 5 rpm and a global efficiency of 35.9 +1,7/-0.6% is achieved. A series of tests with different sparkplugs showed only a weak influence on the engine behaviour. The use of a 3-way catalyst for exhaust gas treatment reduces the NOX and CO emissions comfortably below the limits prescribed by the Swiss Federal Clean Air Act. In the case of the naturally aspirated engine, lean operation with an air to fuel ratio over 1.6 keeps the NOX emissions under the prescribed limit without a catalyst. However, the power output falls below 69 kW and the global efficiency below 35%. By turbocharging and intercooling the intake air and gas mixture the power output was increased from this low level to 150 kW (pme = 12 bar). As with stoichiometric operation, changing the spark plugs has only a weak influence on the engine behaviour. However, an increase of the ignition coil discharge time led to a perceptible reduction of the cycle-by-cycle variability. The results show that the best method of reducing the amount of energy dissipated by the exhaust manifold is to insulate the manifold, rather than water cool it. The insulation preserves more energy for the turbocharger and a high temperature for a possible catalytic exhaust gas after treatment. The reduction of the dead volume located between the cylinder liner, the cylinder head and the cylinder gasket led to a 40 % decrease of the THC emissions. Further, the use of a new piston, generating a higher level of turbulence during the combustion, raised the lean limit and thus reduced NOX emissions below 150 mg/mN3, 5% O2, while still keeping the global efficiency over 37%; these results are valid for a intake mixture temperatures of both 50 and 80 °C, however in the second case at a somewhat lower power output than 150 kW. Despite these improvements, and because of an extremely low CO raw emissions limit, the use of a oxidation catalyst is needed to fulfil the requirements of the Swiss Federal Clean Air Act. In order to reduce the raw CO emissions and to increase the engine global efficiency, this project will continue with a third part aiming to study the potential of an ignition system based on a prechamber.

1998

Unscavenged Combustion Prechamber as a way to Improve Efficiency and Emissions of a Lean-Burn Cogeneration Biogas Engine: Comparison with Natural Gas

Daniel Favrat, Roger Röthlisberger

Gaseous fuels produced, for example, by waste or agricultural by-products fermentation (biogas) can be burned in-situ by cogeneration systems like spark-ignition internal combustion engines. However, the more and more stringent legislation for exhaust gas emissions requires improvement of the combustion process particularly when catalytic after treatment is not reliable as in the case of sewage or landfill biogas. The system proposed in this paper is the use of an unscavenged combustion prechamber instead of direct ignition on a turbocharged 6 cylinders 150 kW gas engine. This prechamber is used for operation with a simulated biogas (40% CO2 in natural gas). The results show, that compared to natural gas operation for the same rated power ouput of 150kW and the same NOx emissions, the CO emissions are reduced by 15% and the HC emissions at least by 8%. Efficiencies higher than 36% are achieved which is very promising and the lower CO emissions give a margin to consider an increase of compression ratio.

2002

Lean-Burn Cogeneration Biogas Engine with Unscavenged Combustion Prechamber: Comparison with Natural Gas

Daniel Favrat, Roger Röthlisberger

Gaseous fuels produced, for example, by waste or agricultural by-products fermentation (biogas) can be burned in-situ by cogeneration systems like spark- ignition internal combustion engines. However, the more and more stringent legislation for exhaust gas emissions requires improvement of the combustion process particularly when catalytic after treatment is not reliable as in the case of sewage or landfill biogas. The system proposed in this paper is the use of an unscavenged combustion prechamber instead of direct ignition on a turbocharged 6 cylinders 150 kW gas engine. This prechamber is used for operation with a simulated biogas (40% CO2 in natural gas). The results show, that compared to natural gas operation for the same rated power ouput of 150kW and the same NOx emissions, the CO emissions are reduced by 15% and the HC emissions at least by 8%. Efficiencies higher than 36% are achieved which is very promising and the lower CO emissions give a margin to consider an increase of compression ratio.

2002

Swiss Motor. Modification d’un moteur diesel pour le fonctionnement au gaz naturel en cogénération. Fonctionnement avec mélange stoechiométrique (l=1) et pauvre (l>>1).

Daniel Favrat, Geoffrey Basil Leyland, Roger Röthlisberger

1998