Rapport final BiogasMotor/ programme de recherche chaleur ambiante et rejets thermiques; installations chaleur-force.

Daniel Favrat
2002
Report or working paper

Abstract

The object of this work is to test the unscavenged prechamber ignition system on a 150 kW cogeneration engine feed with biogas. The engine performances in term of fuel conversion efficiency are observed and the reduction potential f the exhaust emissions below the Swiss limit are verified (NOx CO : 400 and 650 mg/Nm3, 5 % O2 and 0 % relative humidity) ; and the non-limited HC emissions too. The interesting variables are the air-fuel ratio, the fuel composition, the compression ratio and the spark-timing. The aim of this work is to reach at least 36 % in fuel conversion efficiency with exhaust gas emissions lower than the regulations limits without exhaust gas after-treatment. The engine used is a turbocharged spark ignited Liebherr G926 TI, 10 L, 6 cylinders in line engine fitted with combustion prechambers (with a volume of 3 % of the cylinder volume). As a first step, the engine fuel supply has been modified to allow the use of natural gas / CO2 mixtures to simulate biogas. Natural gas tests have been performed with direct ignition and prechamber ignition as references. The same experimental conditions have been applied for the simulated biogas tests. Different spark timing advances have been tested in the configuration with the initial compression ratio of 12,0. Afterwards new pistons have been developped to reach a compression ratio of 13,3. Two different CO2 proportions (20 % and 40 %) have been used to observe the influence of CO2 in fuel on the performances. With a compression ratio of 12,0, 40 % of CO2 inducee a CO reducion of 15 % and an HC reduction of 8 % for the same NOx emissions with a slight reduction of 1 % in fuel conversion efficiency, compared to pure natural gas. The results for the compression ratio of 13,3, a spark timing of 8 caBTDC, of 1,63 and 39,5 % CO2 are a fuel conversion efficiency of 38 % and NOx and CO emissions below the swiss limit. Those performances and emissions are compared to the ones obtained for the natural gas with a compression ratio of 12,0 and prechamber ignition. The use of a compression ratio of 13,3 instead of 12,0 allows to balance the loss in fuel conversion efficiency due to the 40 % of CO2 . If the percentage of CO2 is lower then the efficiency is higher but the CO and HC emissions too. Moreover, it seems better to use a higher compression ratio than a sooner spark timing (see fig. 29, 30, 31). Those tests demonstate that the CO2 does not modify the combustion process nor the prechamber ignition mode. This study shows that gas engines with unscavenged prechambers could allow a significant boost in energy conversion from biogas while keeping the emissions within the tough Swiss limits.

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https://infoscience.epfl.ch/record/53447?ln=en

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Daniel Favrat
2002
Report or working paper

Abstract

Official source

https://infoscience.epfl.ch/record/53447?ln=en

About this result

Related concepts (21)

Related concepts

Related publications (18)

Related publications

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

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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

NUMERICAL FLOW SIMULATION OF A NATURAL GAS ENGINE EQUIPPED WITH AN UNSCAVANGED AUTO-IGNITION PRECHAMBER

Daniel Favrat, Stefan Heyne, Jan-Bernhard Vos, Dirk Wunsch

This work is part of ongoing research on stationary natural gas engines equipped with unscavanged prechambers. Spark ignition inside the prechamber has been successfully proven with natural gas as well as biogas, reducing emissions while keeping good efficiencies. The conversion of the prechamber system from spark ignition to auto-ignition will lead to longer maintenance intervals in the case of stationary natural gas engines and to a probable more efficient and cleaner combustion. The latter expectation is based on the combustion mode: HCCI like combustion inside the prechamber and faster combustion in the main chamber due to an earlier arrival of the flame front, compared to prechamber spark ignition. To the authors knowledge no work following this approach has been conducted. From this new approach one of the major difficulties arises. While for standard prechamber systems either the spark plugs or injecting gas directly into the prechamber creating rich conditions guarantee to well know the point of ignition, auto-ignition of a premixed gas does not do so. The basic idea of this prechamber is to create as homogeneous conditions as possible. Then, the reacting mixture streams through the orifices into the main chamber. The difficulty here is to create a state that triggers auto-ignition only inside the prechamber. As temperature is the main influencing factor on the reaction chemistry the prechamber walls are heated, aiming at higher gas temperatures inside the prechamber. However, auto-ignition will take place wherever the energy level is sufficiently elevated. In particular, this causes difficulties in the region of the exhaust valve. In this work the three- dimensional flow conditions in a mono-cylinder test engine were numerically simulated in order to better understand the conditions inside the engine, with the new prechamber configuration. Therefore, a geometric model was created, disregarding intake and exhaust valves to simplify matters. A suitable multi-block mesh for parallel computing was generated and models for the initial and boundary conditions derived. The chemical reactions were incorporated to the flow field, using a 54 species mechanism developed for natural gas combustion by two approaches. In a first step the flow field was superposed with zero-dimensional Chemkin calculations whereas in a second step the species conservation equations were solved together with the Navier Stokes equations, which is much more expensive in calculation time. Finally, the numerical results were compared to experimentally gained data. Results show the sensitivity of the computations to varied initial and boundary conditions and provide a guideline for modifications in order to enhance the prechamber configuration. Numerical results show the risk of auto-ignition in the main chamber. Experimental results could be reproduced numerically in regard to the time dependency of first ignition.

2007