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).

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.

SWISS MOTOR. Modification d’un moteur diesel pour le fonctionnement au gaz naturel en cogénération. Partie 1: fonctionnement avec mélange stoechiométrique (l = 1).

Daniel Favrat, Roger Röthlisberger

In the frame of the decentralised electrical power and heat cogeneration with stationary I. C. engines operating on natural gas, the Swiss limit for exhaust gas emissions, with 80 mg/mN3 (5% O2), is the most severe in force on an international level. On the other hand, the only technology able to meet this requirement, with the use of a stoechiometric mixture and a three way catalyst, allows a relatively low global engine efficiency and is characterised by a limited reliability of the exhaust gas treatment system. In this context, the Laboratory of Industrial Energy Systems of the EPFL has undertaken, on the basis of a converted diesel engine for natural gas operation, to develop new technologies (mixture, ignition, ...) and to evaluate their potential to reduce exhaust gas emission, particilarly NOx, and reliability. The objective of the first part of this project is to study the behaviour of a naturally aspirated version of the engine “Liebherr G926”, fuelled with a stoechiometric mixture. The main engine characteristics and perfomances have been used to reduce noxious emissions. The main results obtained are: A variation of the spark timing at wide open throttle has allowed to determine the best operating condition with a mixture of l=1.001. The maximal mechanical power output of 106.3 ± 0.5 kW at 1502 ± 5 rpm and the maximal global efficiency of 35.9 + 1,7/-0.6% have been achieved at a spark timing of 20 °CA BTDC. The lowest cycle to cycle variability, with a coefficitent of variability of the mean effective pressure smaller than 1.5%, is reached at the same spark timing (Figure 5.11). No occurrence of knock has been noticed on the spark timing range between 5 and 30 °CA BTDC. An increase of the spark gap from 0.2 up to 1.2 mm reduced the combustion cycle by cycle variability (Figure 5.21) without modifying the engine performances and emissions (Figures 5.15 and 5.17), at full as well as at half load. A increase, respectively a decrease of two units of the spark thermal index from a basis value of 5 has no significant influence on engine performances and emission (Figures 5.23 to 5.26). Similarly, an increase of the central electrode length from 1 up to 3 mm has no detectable effect on the engine behaviour (Figures 5.23 5o 5.26). The use of a three way catalyst for the exhaust gas treatment allows to reduce the NOx and CO emissions well under the Swiss limits (Figure 5.35).

1996

Comparison between direct and indirect (prechamber) spark ignition in the case of a cogeneration natural gas engine, part II : engine operating parameters and turbocharger characteristics

Daniel Favrat, Roger Röthlisberger

In a first paper (part I), prechamber ignition in cogeneration natural gas engines has been shown to significantly intensify and accelerate the combustion process, offering a further potential to reduce the exhaust gas emissions while keeping efficiency at a high level. This second part discusses the influence of the engine operating parameters (spark timing and load) and the turbocharger characteristics with the objective of evaluating the potential to reduce the exhaust gas emissions, particularity the CO emissions, below the Swiss limits (NOx and CO emissions: 250 and 650 mg/m3N, 5% O2, respectively), without exhaust gas after treatment. The advantage of using an unscavenged prechamber is conditioned by a significant delay of the spark timing in order to generate substantial gas jets. This results in a large decrease in peak cylinder pressure and in an important reduction of NOx, CO and THC emissions. Minimum emissions are achieved at a spark timing of about 8°CABTDC . In comparison with the direct ignition, the prechamber ignition yields approximately 40% and 55% less CO and THC emissions, respectively. However, this also leads to about 2%-point lower fuel conversion efficiency. The optimisation of the turbocharger results in a recovery of about 1%-point in fuel conversion efficiency, but a consequent change in the exhaust manifold gas dynamics attenuates the reduction in THC emissions. At the rated power output (150kW), the prechamber ignition operation fulfils the Swiss requirements for exhaust gas emissions and still achieves a fuel conversion efficiency higher than 36.5%.

2002

Prechamber autoignition in natural gas engines

Daniel Favrat, Stefan Heyne, Jan-Bernhard Vos

Unscavanged prechamber ignition has been developed at the Laboratory of Industrial Energy Systems to reduce the emissions from cogeneration spark ignition gas engines, operating with both natural gas and biogas. Demonstration has been made that a lean burn engine unit equipped with optimized prechambers allows efficient operation, while remaining below the Swiss limits [1,2]. Further development aims at using prechambers operating in autoignition mode similar to HCCI (Homogeneous charge compression ignition). The advantages are on the one hand that there is no need for maintenance and replacement of the spark plugs which usually have a limited life time while on the other hand we are expecting to benefit from the HCCI characteristics, e.g. low NOx emission levels. The prechamber configuration though should be a system easier to control than an engine operating completely in HCCI mode. The investigation of the applicability of this technique for engines used in the transport sector with rapidly changing operating conditions might be a further step in the development. In order to better understand the phenomena, simulations are done taking into account both fluid dynamics and chemical kinetics. Ignition delay measurements with mixtures of methane, ethane and propane have been conducted at the Rapid compression machine facility at the University of Lille at varying stoichiometric ratios. These measurements are used as a basis to choose an appropriate chemical mechanism for combustion simulation. The kinetic solver CHEMKIN has been used for zero-dimensional simulations at TDC conditions of the rapid compression machine to reproduce the ignition delays. Several detailed combustion mechanisms have been applied for these simulations, e.g. GRI 3.0 + RAMEC, n-Heptane and PRF mechanisms (LLNL) and Konnov’s mechanism. The aim is to obtain a reduced mechanism that will be used in combination with computational fluid dynamics. In order to do this, sensitivity and reaction pathway analysis is performed, as well as multi- objective optimization of the kinetic parameters. In cooperation with CFS engineering a coupling between the CFD code NSMB (Navier Stokes Multi Block) and the chemical solver CHEMKIN has been established enabling a simulation of the experiments conducted with the rapid compression machine. Further steps will be the design of a prechamber to be used with an experimental mono-cylinder engine and the simulation of the in-cylinder flow with prechamber to determine the influencing factors for controlling the autoignition event. Behaviour at part load as well as in transient conditions will also be investigated.

2005

Prechamber autoignition in natural gas engines

Daniel Favrat, Stefan Heyne, Jan-Bernhard Vos

2005

SWISS MOTOR. Modification d’un moteur diesel pour le fonctionnement au gaz naturel en cogénération. Partie 1: fonctionnement avec mélange stoechiométrique (l = 1).

Daniel Favrat, Roger Röthlisberger

1996