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A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine
Applied Thermal Engineering xxx (2014) 1e8
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A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine Pietro Capaldi* Istituto Motori e National Research Council, Largo Barsanti e Matteucci 8, 80125 Naples, Italy
h i g h l i g h t s A new high efficiency microcogenerator based on an Atkinson/Miller cycle engine. Atkinson cycle together with stoichiometric operation deliver better performance. A cost-effective microcogenerator based on widespread elements (automotive engine). The chosen automotive engine has heavy duty characteristics (Diesel derived). A conversion criteria from a Diesel to an Atkinson cycle engine was individuated.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2013 Accepted 14 February 2014 Available online xxx
The paper focuses on the design and the overall performance of a 10 kW electric power microcogeneration plant suitable for local energy production, based on an Atkinson-cycle internal combustion engine prototype and entirely set by Istituto Motori of the Italian National Research Council. The engine was originally a wide-spread Diesel automotive unit, then converted into a methane spark ignition system and finally modified to perform an Atkinson/Miller cycle with an extended expansion, capable of a higher global efficiency and low gaseous emissions. The paper starts by defining the ratio which leaded to this specific choice among many other automotive and industrial engines, in order to obtain a reliable, long endurance, cost effective, high efficiency base, suitable for microcogeneration in residential or commercial applications. The new engine has been coupled with a liquid cooled induction generator, a set of heat exchangers and finally placed in a sealed containing case, to reduce both noise emission and heat losses. Then the plant has been tested as an electricity and heat production system, ready for grid connection thanks to a new designed management/control system. During endurance test a complete description of its functioning behaviour has been given. Ó 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-SA license (http://creativecommons.org/licenses/by-nc-sa/3.0/).
Keywords: Microcogeneration Micro-grids Atkinson and Miller cycle Natural gas
1. Introduction and state of art Micro-cogeneration plants are an interesting solution for energy supplying for single houses and buildings or even for small commercial activities; moreover, they are considered a simple and immediate form to enhance the full utilization of fuel energy and, consequently, to achieve a reduction for CO2 emissions, especially when natural gas is used as fuel. Today there are some plants available on the European market, all based on spark-ignition internal combustion engines, which are able to produce an electric power of about 10 kW, such as Simple-Energie [1], Giese [2], Steinecke [3], EC Power [4], RMB [5], Yanmar [6]. These systems offer
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interesting overall performances in terms of electric efficiency, set between 26% and 32%, and P.E.R. (Primary Energy Ratio, i.e. the utilization rate of fuel) comprised between 86% and 94%; in the following Table 1 the most significant characteristics are reported. All these units are based on small industrial internal combustion engines, resulting quite different among them as regard displacement, number of cylinders and overall dimensions; what they have in common is the lean-burn management of the air-fuel mixture, which, together with oxidative catalyst, can give good performances in terms of gaseous emissions. On the other side this solution can put into effect a significant reduction of power density, this resulting in a bigger displacement for the engine to get the same rated power and consequently proportionally higher frictional losses [7]. Also, bigger displacement engines can express lower efficiencies at partial loads, always due to mechanical losses which result higher; this is the reason for which downsized units
http://dx.doi.org/10.1016/j.applthermaleng.2014.02.035 1359-4311/Ó 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-SA license (http://creativecommons.org/licenses/by-nc-sa/3.0/).
Please cite this article in press as: P. Capaldi, A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.02.035
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P. Capaldi / Applied Thermal Engineering xxx (2014) 1e8
Table 1 The most significant microcogenerators and their characteristics. Model
Engine
Electr. power (kW)
Generator type
Speed (rpm)
Electric efficiency (%)
P.E.R. (%)
Simple Energie Giese
Ford, 4 cyl, 1600 cm3 Kubota, 3 cyl, 1826 cm3 Ford, 4 cyl, 1600 cm3 Toyota, 3 cyl, 1000 cm3 Toyota 4 cyl, 2200 cm3 Yanmar, 3 cyl, 1000 cm3
12.0
Induction
1530
27.5
93
12.0
Induction
1530
27.0
92
10.5
Induction
1525
26.0
93
9.0
Induction
1520
29.0
94
11.0
Induction
1530
29.0
92
10.0
PMSG
1800
32.0
86
Steineke EC Power RMB Yanmar
can express a better performance when throttled. As regards the Yanmar unit, it must be underlined that it is based on an Atkinsoncycle instead of the Otto-cycle adopted by the other competitors, differing from the latter because the first is characterized by an expansion ratio considerably higher than its compression ratio, with the main objective to achieve an over-expansion of the hot gases after combustion and attain a higher global efficiency. Heywood [7] and Yanlin Ge et al. [8] showed that an important increase in engine efficiency could be achieved for over-expanded cycles, while Mikalsen et al. [9] have clearly showed that this kind of thermodynamic cycle could be very attractive for small CHP applications, where power density is not the most important issue, with a good potential in respect of fuel efficiency and reduction of temperature-dependent emission such NOx. In the Yanmar unit the reduction of power density deriving from both the adoption of an Atkinson cycle and a lean-burn combustion management is compensated by the higher rotational speed (1800 rpm instead of 1500 rpm of the other units). As regards electric generators, the most adopted solution is the widespread asynchronous 4-poles machine, (liquid or air cooled) which can meet many different requirements, such as an easy starting and grid parallel insertion, low cost, durability; all these aspects are mandatory in order to reduce global plant cost. As regards the Yanmar unit, it adopts a permanent magnet synchronous generator (PMSG) together with an inverter for grid parallel insertion. This solution can give an important benefit in terms of electric efficiency in respect of the induction machine, but it results more expensive and with a lesser reliability. This last technical solution, together with the adoption of the Atkinson cycle, no doubt validate the outstanding value of the electric efficiency declared for this unit. However all these plants still have some disadvantages if compared to a conventional electricity and heat supplies (such as electric grid distribution and conventional gas heater), in terms of high specific cost (referred to power) and bulk, the first aspect making very difficult the Break Event Point of the investment. For these reasons micro-cogeneration could still be far away from high volume production, waiting for new solutions which can reduce the effect of the above mentioned disadvantages, but still taking benefits deriving from the full utilization of energy. 2. Experimentals: a new engine for small microcogeneration units The previous state of art showed clearly that, in order to get a higher thermal efficiency, an Atkinson cycle should be adopted for a microcogeneration unit, which could make the system really competitive in terms of electric efficiency. Also, this solution could
permit a better electric output even without the adoption of an expensive PMSG mated with an inverter system, but just using a reliable, cost effective 4-poles induction generator. As no engines of this kind were available on the market, the Author realized that the only way to get this result was to tranform an Otto-cycle engine to an Atkinson cycle unit. In order to reach this result a new engine with specific characteristics must be selected; of course just gasfueled units have been considered (because of the strict European limits regarding gaseous emissions) controlled by an accurate stoichiometric management, this to permit the adoption of a three way catalyst and assure better performances in the reduction of gaseous emissions if compared to a lean-burn engine as reported by Cho et al., [10]. Moreover, the efficiency benefit deriving from the adoption of an Atkinson cycle becomes more significant when air/ fuel mixtures of increasing energy are used [9], so making stoichiometric mixture more attractive than any lean mixtures. After some considerations it was clear that the same unit should be characterized by a bore to stroke ratio <1, this in order to obtain an expansion stroke longer than the compression stroke while maintaining a good effective compression ratio (i.e. with closed valves) and a compact combustion chamber. This architectural characteristic is the only which could assure, at the same time, a good compression ratio even with a partial compression stroke (fundamental aspect to obtain a high thermal efficiency [8,11] and just limited by knocking and NOx production), while keeping a compact design of combustion chamber in order to reduce heat losses. Among the industrial Otto units available today, none showed the above mentioned characteristics, because of their bore to stroke ratios typical of spark ignition engines, (usually around 1 or higher) and because of their limited compression ratio (normally lower than 10), which inhibit the possibility to reduce just the compression stroke to obtain an Atkinson cycle while maintaining a compact combustion chamber. Then, automotive gas engines have been considered, usually resulting less expensive than the industrial units (thanks their mass production) and showing very good performances in terms of global efficiency, low noise and vibration. On the other hand, they cannot be compared to the industrial units in terms of durability and reliability, as they have not been designed for heavy-duty service, but simply as spark-ignited automotive units. Also, most of the considered engines showed the same similar characteristics found for the above seen industrial units (such as bore to stroke ratio 1), with strong limitations regarding the attainable compression ratio when the compression stroke is reduced. For these reasons, the Author concluded that, in order to achieve a higher efficiency and endurance, another engine group should be considered. From this point of view, a modern automotive Diesel engine converted in a spark ignition Otto unit could result more similarly designed to an industrial engine, because more robustly constructed as it has to undertake to much higher pressures than a spark ignition unit. At the same time this engine class usually shows a lower bore to stroke ratio (made to achieve the typical Diesel high compression ratio) and a compact combustion chamber. In this way a so obtained Otto-cycle engine has all these architectural characteristics to attain an expansion stroke longer than the compression stroke still maintaining a compact combustion chamber, so making possible and feasible the conversion to the Atkinson-cycle. To get a first definition of displacement it can be observed that, in order to reach an electric power of 10.0 kW at the rated speed of 1500 rpm, a mechanical power of about 12.0 kW is required (having considered a generator with a mean electric efficiency of 0.85); consequently, a torque of about 75 Nm is needed from the engine. On the other side, a modern gas-fueled Otto cycle engine can express a specific torque of 80 Nm/dm3 around its highest volumetric
Please cite this article in press as: P. Capaldi, A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.02.035
P. Capaldi / Applied Thermal Engineering xxx (2014) 1e8
efficiency; having fixed a conservative value of 60 Nm/dm3 (taking into account the power reduction of about 25% deriving from the adoption of an Atkinson cycle [9]) a 1300 cm3 displacement would be necessary for the engine at 1500 rpm to produce the required power. So, several automotive Diesel engines have been considered during this analysis; at the end the chosen unit was a Fiat-GM 1.3 liters of displacement, also known as 1.3 Multi-Jet, a turbocharged unit in the original Diesel version widely adopted by many car makers (such as Fiat, GM, Suzuki and Ford). This engine represents one of the most advanced unit today available on the market, owing to its double overhead cams with hydraulic lifters and integrated rocker-arms, four valve head with high turbulence intake design and a low bore to stroke ratio (¼0.84), this in order to obtain a high compression ratio, a very compact combustion chamber and consequently a high thermal efficiency. This unit has been transformed first in a stoichiometric Ottocycle engine, atmospheric pressure charged and natural gas fueled, representing the very first 1.3 Fiat Multi-Jet Diesel unit to be modified into a spark ignited methane prototype. As regards the combustion chamber design it can be observed that, being the engine head the same of the original Diesel unit (flat type), the chamber is entirely contained inside the piston. The previous Diesel Saurer chamber (a toroidal enclosed bowl) has been replaced by an open bowl chamber with a very limited squish area, in order to minimize the surface/volume ratio and improve thermal efficiency; the compression ratio achievable with this design is 11.3:1, which could be considered a good compromise between a good thermal efficiency and limited NOx formation. The picture of the modified piston can be seen in the following Fig. 1 while the cutaway of the same is reported in Fig. 2. Also, this design can reduce the HC formation in the crevices and minimize squish which could enhance the swirl motion caused by the intake ducts design (typical of the original Diesel unit) and sustain too much the turbulence which increases heat transfer to the walls. This last aspect was considered to be an important issue by the Author, because the kinetic content of the intake fluxes is fundamental to sustain the flame propagation and avoid knocking, especially with a high compression ratio and when the engine is running at full load and at such a low speed of 1500 rpm; on the other hand turbulence has to be carefully controlled, in order to limit the heat transfer to the combustion chamber walls and
3
Fig. 2. Cutaway of the modified piston with open bowl combustion chamber.
cylinder, so reducing the heat losses and keeping a high global efficiency. As regards camshaft the engine was first provided with the standard component of the original Diesel turbocharged engine, with a valve timing (at zero backlash) reported in the following Table 2. Being derived from a Diesel unit, the engine was not originally provided with any ignition equipment; so an automotive based component was selected among many other commercial systems and then it was slightly modified in order to improve its life cycle. The system is a conventional single coil with a high voltage distributor, provided with an internal hall sensor and it’s characterized by a high price/performance ratio if compared to similar apparatuses commonly mounted on other microcogeneration systems. The same was finally placed on one camshaft with the aim of getting a one-spark per cycle strategy, instead of a one-spark per round (as most of the automotive engines) for a better spark durability and to avoid dangerous back-fire phenomena in the intake manifold. As regards the management system, the engine was provided with an integrated electronic platform, based on a commercial genset control system; the same has been modified and reconceived as a global control system for both electric and thermal generation. The system can also control the air to fuel ratio in a straight stoichiometric mode (even in fast transient load conditions thanks to a linear broad-band oxygen sensor) and drive a fast response valve to deliver the natural gas to the intake manifold. The same system can also manage other sub-systems, such as electric pumps for engine and plant cooling, auxiliary lubricating device, electric power factor regulator, etc.
Table 2 The valve timing diagram of the original diesel engine. Valve
Fig. 1. Modified piston with open bowl combustion chamber.
Exhaust Intake
Opening
54 BTDC 11 ATDC
Closing
2 ABDC 29 BBDC
Max. lift 6.6 mm 6.2 mm
Please cite this article in press as: P. Capaldi, A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.02.035
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P. Capaldi / Applied Thermal Engineering xxx (2014) 1e8
It must be underlined that the modification from a Diesel engine to a spark ignition unit resulted very simple and economical, with few elements to be machined and a reduced number of new designed parts; most of the elements needed for the conversion were, in fact, already available as spare parts from other automotive units, or directly derived from domestic heating systems. As regards the engine primary cost, it must be underlined that the unit itself has been chosen because it resulted one of the most widespread today available on the market, especially for small cars and other small commercial vehicles, being cost-effective in spite of its sophisticated design, high quality and performance. So, in case of mass production it would be absolutely competitive as regards global cost, while, in case of low-volume production, remanufactured engines would be another suitable solution, being this unit one of the most low-priced if compared to other automotive engines and even more if compared to remanufactured industrial engines of the same class.
Fig. 3. The prototype with catalyst and oxygen sensor.
Table 3 Brake performance of engine prototype at full load in the standard configuration. Power [kW]
Speed [rpm]
BMEP [bar]
Power in. [kW]
Global eff.
THC [p.p.m.]
NOx [p.p.m.]
CO [%]
Tex [K]
14.60
1550
9.05
43.0
0.340
250
230
0.01
820
In the following Fig. 3 a first engine version (i.e. the Diesel unit just converted in a spark ignition unit) can be seen, with the three way catalytic converter and part of the heat exchangers. The first has been selected (after testing) among other mass produced catalysts specifically designed for methane and it results a costeffective solution for exhaust gas treatment being an automotive spare part. As regards heat exchangers, just off-the-shelf elements have been selected from the domestic heating systems; as regards the heat recuperating component from the high temperature exhaust gas, it was selected a copper/steel element arranged with a bended finned tube, characterized by good compactness and lower cost if compared to conventional shell and tube systems normally adopted for microcogeneration systems; the prototype is also provided with a condensation stage, in order to recover as much heat as possible from exhaust gas and improve the P.E.R. of the whole plant.
2.1. The engine performance in the reference configuration After having reached a first definition of the engine as regards its displacement, the following step was to carry out some brake test on the system to know the real output in terms of global output in the Otto-cycle reference configuration (i.e. when provided with the large bowl piston configuration and standard valve timing of the original Diesel unit) at full load. As regards laboratory setup, the apparatuses which have been used to characterize the behavior of the prototype were an asynchronous machine by API-COM (as regards brake), a hot wire flow meter (by VSE as regards the air and fuel flow metering), together with a Coriolis fuel flow meter (by Emerson MicroMotion). As regards emissions, raw exhaust gas has been analyzed with an API-COM measurement system (Mod. S5000), consisting of the following analyzers: NDIR (Non-Dispersive Infrared Detector), CLD (Chemiluminescence Detector) and FID (Flame Ionization Detector) all by Emerson. In this standard arrangement the prototype showed interesting overall performances, summarized in Table 3, in respect of global power and emissions after catalyst. The system has been provided with an EGR apparatus optimized just for full load conditions, in order to reduce NOx formation and also fuel consumption [10]. This system is fed with cooled gases spilled after the heat recuperation
Fig. 4. The scheme of the engine on the 1-D simulation program.
Please cite this article in press as: P. Capaldi, A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.02.035
P. Capaldi / Applied Thermal Engineering xxx (2014) 1e8
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Table 4 Simulated performance of engine prototype at full load in standard configuration. Power [kW]
Speed [rpm]
IMEP [bar]
BMEP [bar]
Tex. [K]
Power in. [kW]
Global eff.
14.85
1550
10.4
9.15
832
43.4
0.342
at a maximum temperature of 55 C, being this condition very important in a spark ignited engine to avoid knocking and obtain a higher global efficiency. Then, the same unit was analyzed with a 1-D program (GT-Power 7.3 by Gamma Technologies) and compared with the real engine as regards the running parameters, this in order to get a well tuned simulation and with the aim of achieving a more accurate predictions of its behavior when functioning for different configurations, especially as regards valve timing and spark advance parameters. In this way it was possible to know predict, with acceptable accuracy, the prototype output when operated in the Atkinson mode and define the relative valve timing configuration which could give the required mechanical output needed to produce the target value for the electric power (10 kW). As regards the simulation scheme of the engine the same is reported in the following Fig. 4. As reported above, the simulation has been performed with the standard cams of the original Diesel engine, with the valve timing reported in Table 2: the reference speed was set at 1550 rpm (corresponding at about the 3% of the slip factor in a 4-poles asynchronous generator), being this value the expected speed at full load condition. The program simulated the engine at full load, calculating the performances in the following Table 4: After a tuning process, the simulation program produced results in good accordance with the real engine global output (such as produced power, fuel consumption, exhaust temperature etc.), this being also the consequence of a good accuracy, during simulation set-up, in the definition of combustion process and internal friction phenomena. This result made possible to take a second step forward, i.e. the simulation of a different valve closing angle to obtain the needed mechanical power. As already reported before, if a mean efficiency of 85% performed by the electric generator is considered, the required mechanical power should be around 12.0 kW. In order to compensate the reduction in the achievable compression ratio due
Fig. 6. Cutaway of the modified piston with “W” shape combustion chamber.
to a shorter compression stroke, a piston with a different combustion chamber (defined “W” shape) has been designed, with a higher compression ratio of 12:1 instead of the previous 11.3:1. In the following Figs. 5 and 6 are reported the image of this new piston and its cutaway. After that, the program simulated different configuration of valve timing, especially regarding the closing delay of inlet valve; the target mechanical power reported in the following Table 5 needed to obtain the required electric power of 10 kW) was achieved for the following valve timing reported in Table 6. For this configuration the calculated effective compression ratio (with the inlet valve closed) resulted 8.3:1, well below the real possibilities of the engine as regards achievable efficiency and natural gas knock resistance. For this reason a different mass produced piston has been selected and slightly modified in its shape, with a final geometrical compression ratio of 14.6:1 and an effective compression ratio of 10:1. The program showed a clear difference regarding global efficiency when operated with the new piston, this making also possible to fix a longer delay for inlet valve closing to reach the same power. In the following Tables 7 and 8 a complete pattern of engine performance and valve timing are reported. In the following Figs. 7 and 8 are reported respectively the image of the new high compression piston and its cutaway. After having simulated the engine prototype in the Atkinson configuration, the real engine was modified with the above seen high compression pistons and a new designed camshaft with the above seen valve timing of Table 8; then the unit was brake-tested in order to validate the prediction analysis. The results, reported in the following Table 9, are given just for full load condition and they reflect the predicted performances with good accuracy. The obtained results are no doubt interesting, with a net global efficiency gain of nearly 2.5% if compared to the reference Ottocycle. As regards gaseous emissions, an important reduction of NOx production can be underlined, due to a lower peak pressure and temperature during combustion, while no significant reduction of
Table 5 Simulated performance of the Atkinson engine prototype at full load.
Fig. 5. Modified piston with “W” shape combustion chamber.
Compr. ratio
Power [kW]
Speed [rpm]
IMEP [bar]
BMEP [bar]
Tex. [K]
Power in. [kW]
Global eff.
12:1
11.95
1550
8.60
7.40
725
32.7
0.365
Please cite this article in press as: P. Capaldi, A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.02.035
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P. Capaldi / Applied Thermal Engineering xxx (2014) 1e8
Table 6 Valve timing for the simulated Atkinson engine prototype at full load. Valve
Opening
Closing
Max. lift
Exhaust Intake
33 BBDC 5 ATDC
7 ATDC 78 BBDC
5.5 mm 6.0 mm
hydrocarbons can be recorded. The improved global efficiency can be also be confirmed by a lower exhaust gas temperature, as part of thermal energy normally lost at the escape has been converted in mechanical power. 2.1. The electric generator As reminded before, the choice of the electric generator for this kind of microcogeneration plant has to consider many different requirements, such as easy starting and grid parallel insertion, costeffectiveness, durability, etc. In order to meet all these necessities the technical solution widely adopted (as seen in the previous state of art) is represented by the induction machine directly connected to the grid. The most important aspect is that no expensive parallel system (or inverter) is required as no current phase has to be strictly respected; at the same time, system speed can vary through the slip factor d (with respect to grid frequency) just depending on the engine produced torque, as the same directly affect the electric power (See Fig. 9). This last aspect makes possible an easy control in closed loop of energy production by regulate power through engine torque management (throttle control). A three phase 400 VAC induction generator is, no doubt, the simplest solution for a 10 kWe powerplant, as the three phases could serve at least three different 230 VAC utility groups (houses or commercial activities). This particular power management can create some current difference among the coils inside the machine, so that some auxiliary elements are needed by the system to reach the required power quality, such as power-factor control systems together with some line filters [12]. For other specific applications, when a more flexible energy management is needed, a power inverter must be used, especially if electric energy storage (battery stack) has been planned. The final choice of the electric machine was made among different units available on the European market, each of them capable of producing the required power at the rated voltage, torque and speed. After some analysis it was clear that the required performances could only be assured by a water cooled unit, because of its high specific power and efficiency, but also because of its intrinsic low noise, (due to the absence of cooling fan and fins), with the possibility to be placed, together with the internal combustion engine, into a completely closed containing case. Moreover,
Fig. 7. Upper view of the high compression piston (14.6:1).
compactness was taken into account because a significant volume and weight reduction could be obtained with this solution. So it was selected a water cooled machine previously employed as a traction motor and modified as regards windings (number, wire section and copper/iron weight ratio) in order to optimize it as a generator. The final aim of this transformation was to modify an existing element without re-designing the same (avoiding in this way an extra cost), so arranging not only a prototype, but a preindustrial component ready for mass production. This unit has been examined on a test bench and characterized by varying the external torque, as reported in the following Fig. 10. As regards cooling system, the generator is considered to be the first element to be cooled inside the microcogeneration plant, because its temperature should be kept as low as possible in order to protect wire electric isolation. As the reference incoming water temperature from a generic thermal utility (such as a domestic heating system) is normally fixed at 50 C, this last value will be considered as the reference inlet temperature for the cooling of the plant and for the generator itself. Then, a set of experimental data was produced and reported in the following Table 10.
Table 7 Simulated performance of the Atkinson engine prototype at full load with 14.6:1 compression ratio. Compr. ratio
Power [kW]
Speed [rpm]
IMEP [bar]
BMEP [bar]
Tex. [K]
Power in. [kW]
Global eff.
14.6:1
11.95
1550
8.60
7.40
725
32.2
0.371
Table 8 Valve timing for an Atkinson engine prototype at full load with 14.6:1 compression ratio. Valve Exhaust Intake
Opening
33 BBDC 5 ATDC
Closing
7 ATDC 80 BBDC
Max. lift 5.5 mm 6.0 mm
Fig. 8. Cutaway of the high compression piston (14.6:1).
Please cite this article in press as: P. Capaldi, A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.02.035
P. Capaldi / Applied Thermal Engineering xxx (2014) 1e8
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Table 9 Global brake performance of the Atkinson engine prototype at full load. Power [kW]
Speed [rpm]
BMEP [bar]
Power in. [kW]
Global eff.
THC [p.p.m.]
NOx [[p.p.m.]
CO [%]
Tex [K]
11.8
1550
7.30
32.4
0.365
230
140
0.01
734
The recorded data shown above are very good for a prototype induction generator of such limited power, with an efficiency hgen around 89% over most of its functioning curve. However the Author considers that some diminution in the generation efficiency could occur when the unit would be coupled with the internal combustion engine, because of the intrinsic torque and speed fluctuations of the latter [12]. 3. Results and discussion The two sub-systems (the Atkinson internal combustion engine and the asynchronous generator) have been finally coupled and tested together as a complete electric and heat generator, in order to acquire the global plant performance. As electric efficiency in induction generators is deeply influenced by torque and speed variability, the two systems were coupled with a special elastic joint and a high inertia flywheel, which limit the rotational speed fluctuation of the whole system. Also, the system was put inside a new designed containing case, just constructed in two rigid structures and expressly conceived to avoid any kind of resonance (so reducing noise emission) and limit heat losses; plant compactness was an important issue for the Author, so that the system is contained in a case which occupy a volume of just 0.80 m3. In the following Fig. 11 the plant can be seen in its external view, while in Fig. 12 the inside of the same is shown, with the four cylinder engine and the intake manifold. As regard the global performances the unit ran with three different loads (i.e. full load, 75% and 50%) by means of a passive electric load which could simulate a generic utility; in this way the control system could command the throttle openings and regulate the produced electric power. All tests have been performed with natural gas from the Italian distribution network, with a measured LHV (Lower Heating Value) of 46,600 kJ/kg. The whole set of power performance data are reported in the following Table 11, while regarding emissions the complete data set is reported in Table 12.
Fig. 10. Water cooled asynchronous generator on the test bench.
The obtained results are very interesting if referred to an experimental unit; as regards the electric efficiency hE.E, the system showed a remarkable 31.5% (with a net power generation of 10.1 kW and a thermal power generation of 21.7 kW), corresponding to a thermal efficiency hT.E of 66.3%, this leading to a P.E.R. of 97.8% at full load. This result is no doubt consequence of the adoption of the Atkinson cycle which permits a better utilization of the primary energy if compared to the Otto cycle; moreover, the exhaust temperature result very low for an internal combustion engine, this confirming the better utilization of heat through an extended expansion. The thermal efficiency is also remarkable, this because of the condensation of water in the exhaust but also thanks to the containing case which efficiently isolated the generation system. Interesting, as well, is the output at 75% of load, where just a reasonable drop of electric efficiency has been recorded, this confirming that smaller displacement engine with a good specific power can express a better global efficiency even at partial loads, if compared to other units of bigger displacement and same electric power. As regards the electric efficiency, it can be observed that generator probably results still affected by torque variability from the internal combustion engine, this in spite of the adoption of a special elastic coupling and large flywheel; in fact, if the two different working conditions are compared for the same produced power (i.e. when the generator is brake-tested and when it is coupled to the internal combustion engine) higher currents and slip factor were recorded, with an important decrease for the power factor value [12]; however the same can be easily corrected to the required value with cost-effective condenser stages. As regards emissions the same are quite interesting and are consequence of the specific thermodynamic cycle adopted for this prototype. The Atkinson cycle, thanks to lower peak temperatures and, together with the adoption of an EGR system, can inhibit NOx formation. As regards THC formation the behavior is just acceptable with a standard automotive catalyst, but it must be carefully kept under control, because the lower exhaust gas temperature can Table 10 Asynchronous generator experimental data characteristics.
Fig. 9. Electrical torque/slip factor diagram for three phase induction machine.
Load [%] Power [kW] Current [A] Power factor Efficiency [%] Speed [rpm] Voltage [V] Mech. pow. [kW] Losses [kW]
100 10.0 19.2 0.82 88.9 1541 388 11.25 1.125
75 7.5 16.0 0.75 89.5 1530 388 9.14 1.076
50 5.0 13.1 0.62 88.1 1522 388 6.30 0.746
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P. Capaldi / Applied Thermal Engineering xxx (2014) 1e8 Table 11 Microcogenerator experimental data characteristics. Load [%] Electric power [kW] Speed [rpm] Current [A] Power factor Primary power [kW] Electric efficiency [%] Thermal power [kW] Thermal efficiency [%] P.E R. [%]
100 10.1 1543 20.1 0.75 32.1 31.5 24.7 66.3 97.8
75 7.5 1532 16.6 0.70 26.7 28.1 20.8 69.9 98.0
50 5.0 1523 13.5 0.54 20.8 24.0 17.8 74.0 98.0
Table 12 Microcogenerator emissions before and after the three way catalyst.
Fig. 11. The external view of the microcogeneration plant.
reduce the efficiency of the same, especially for the oxidation of methane gas. This aspect makes advisable the adoption of dedicated catalyst optimized for lower gas temperatures (which can also assure better global performances), as standard catalyst normally adopted by natural gas fueled cars could not assure a satisfactory gas treatment after a long running period. 4. Conclusions The 10 kW power unit developed by Istituto Motori-CNR showed very interesting global performances, better than the
Load [%] Electric power [kW] THC (b.c.) [p.p.m.] NOx (b.c.) [p.p.m.] CO (b.c.) [%] Ex. Temp. (b.c.) THC (a.c.) [p.p.m.] NOx (a.c.) [p.p.m.] CO (a.c.) [%] Ex. Temp. (a.c.)
100 10.1 1330 1080 0.07 734 230 140 0.01 740
75 7.5 1210 810 0.06 698 210 105 0.01 688
50 5.0 1090 535 0.06 652 195 75 0.01 645
best commercial competitors provided with asynchronous generator and competitive with models fitted with expensive power systems based on permanent magnet generator and inverter. The adoption of an Atkinson cycle, with its extended expansion stroke, can give a significant benefit in terms of global efficiency and in terms of gaseous emissions especially when a dedicated catalyst is adopted. The system has been obtained through a selection of “off the shelf” high quality elements, in order to keep low the global cost, but is fundamentally based on the choice of a widespread automotive unit characterized by heavy duty characteristics (being originally a Diesel unit) and which can easily converted first in a spark ignition engine and then in an Atkinson-cycle unit. Nomenclature I.C.E. internal combustion engine P.E.R. primary energy ratio E.G.R exhaust gas recirculation THC total hydrocarbons (gaseous emission) NOx nitrous oxides (gaseous emission) hE.E electric efficiency of microcogenerator hT.E thermal efficiency of microcogenerator hgen electric efficiency of generator VAC voltage in alternate current References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] Fig. 12. The internal view of the microcogeneration unit.
www.simple-energie.de. www.energator.de. www.bhkw-steinecke.de. www.ecpower.dk. www.rmbenergie.de. www.yanmar.co.jp. J.B. Heywood, Internal Combustion Engine Fundamentals, Mc. Graw Hill Inc., 1998. Yanlin Ge, Lingen Chen, Fengrui Sun, Chih Wu, Reciprocating heat-engine cycles, Appl. Energy 81 (2005) 397e408. R. Mikalsen, Y.D. Wang, A.P. Roskily, A comparison of Miller and Otto cycle natural gas engines for small scale CHP applications, Appl. Energy 86 (2009) 922e927. Haeng Muk Cho, Bang-Quan He, Spark ignition natural gas engines e a review, Energy convers. Manage. 48, 2007, 608e618. M Hitomi, N Iwata, M Yamakawa, T Nishimoto, Spark ignition gasoline engine, US Patent 7.484.498 B2, 2009. P. Capaldi, A. Del Pizzo, L. Piegari, R. Rizzo, A low cost microcogeneration system for domestic and industrial applications, in: Proc. of IEEE ICIT Conf., Hammamet, vol. 1, Dec. 2004, pp. 514e518.
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Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine Pietro Capaldi* Istituto Motori e National Research Council, Largo Barsanti e Matteucci 8, 80125 Naples, Italy
h i g h l i g h t s A new high efficiency microcogenerator based on an Atkinson/Miller cycle engine. Atkinson cycle together with stoichiometric operation deliver better performance. A cost-effective microcogenerator based on widespread elements (automotive engine). The chosen automotive engine has heavy duty characteristics (Diesel derived). A conversion criteria from a Diesel to an Atkinson cycle engine was individuated.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2013 Accepted 14 February 2014 Available online xxx
The paper focuses on the design and the overall performance of a 10 kW electric power microcogeneration plant suitable for local energy production, based on an Atkinson-cycle internal combustion engine prototype and entirely set by Istituto Motori of the Italian National Research Council. The engine was originally a wide-spread Diesel automotive unit, then converted into a methane spark ignition system and finally modified to perform an Atkinson/Miller cycle with an extended expansion, capable of a higher global efficiency and low gaseous emissions. The paper starts by defining the ratio which leaded to this specific choice among many other automotive and industrial engines, in order to obtain a reliable, long endurance, cost effective, high efficiency base, suitable for microcogeneration in residential or commercial applications. The new engine has been coupled with a liquid cooled induction generator, a set of heat exchangers and finally placed in a sealed containing case, to reduce both noise emission and heat losses. Then the plant has been tested as an electricity and heat production system, ready for grid connection thanks to a new designed management/control system. During endurance test a complete description of its functioning behaviour has been given. Ó 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-SA license (http://creativecommons.org/licenses/by-nc-sa/3.0/).
Keywords: Microcogeneration Micro-grids Atkinson and Miller cycle Natural gas
1. Introduction and state of art Micro-cogeneration plants are an interesting solution for energy supplying for single houses and buildings or even for small commercial activities; moreover, they are considered a simple and immediate form to enhance the full utilization of fuel energy and, consequently, to achieve a reduction for CO2 emissions, especially when natural gas is used as fuel. Today there are some plants available on the European market, all based on spark-ignition internal combustion engines, which are able to produce an electric power of about 10 kW, such as Simple-Energie [1], Giese [2], Steinecke [3], EC Power [4], RMB [5], Yanmar [6]. These systems offer
* Tel.: þ39 817177150. E-mail address: [email protected].
interesting overall performances in terms of electric efficiency, set between 26% and 32%, and P.E.R. (Primary Energy Ratio, i.e. the utilization rate of fuel) comprised between 86% and 94%; in the following Table 1 the most significant characteristics are reported. All these units are based on small industrial internal combustion engines, resulting quite different among them as regard displacement, number of cylinders and overall dimensions; what they have in common is the lean-burn management of the air-fuel mixture, which, together with oxidative catalyst, can give good performances in terms of gaseous emissions. On the other side this solution can put into effect a significant reduction of power density, this resulting in a bigger displacement for the engine to get the same rated power and consequently proportionally higher frictional losses [7]. Also, bigger displacement engines can express lower efficiencies at partial loads, always due to mechanical losses which result higher; this is the reason for which downsized units
http://dx.doi.org/10.1016/j.applthermaleng.2014.02.035 1359-4311/Ó 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-SA license (http://creativecommons.org/licenses/by-nc-sa/3.0/).
Please cite this article in press as: P. Capaldi, A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.02.035
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P. Capaldi / Applied Thermal Engineering xxx (2014) 1e8
Table 1 The most significant microcogenerators and their characteristics. Model
Engine
Electr. power (kW)
Generator type
Speed (rpm)
Electric efficiency (%)
P.E.R. (%)
Simple Energie Giese
Ford, 4 cyl, 1600 cm3 Kubota, 3 cyl, 1826 cm3 Ford, 4 cyl, 1600 cm3 Toyota, 3 cyl, 1000 cm3 Toyota 4 cyl, 2200 cm3 Yanmar, 3 cyl, 1000 cm3
12.0
Induction
1530
27.5
93
12.0
Induction
1530
27.0
92
10.5
Induction
1525
26.0
93
9.0
Induction
1520
29.0
94
11.0
Induction
1530
29.0
92
10.0
PMSG
1800
32.0
86
Steineke EC Power RMB Yanmar
can express a better performance when throttled. As regards the Yanmar unit, it must be underlined that it is based on an Atkinsoncycle instead of the Otto-cycle adopted by the other competitors, differing from the latter because the first is characterized by an expansion ratio considerably higher than its compression ratio, with the main objective to achieve an over-expansion of the hot gases after combustion and attain a higher global efficiency. Heywood [7] and Yanlin Ge et al. [8] showed that an important increase in engine efficiency could be achieved for over-expanded cycles, while Mikalsen et al. [9] have clearly showed that this kind of thermodynamic cycle could be very attractive for small CHP applications, where power density is not the most important issue, with a good potential in respect of fuel efficiency and reduction of temperature-dependent emission such NOx. In the Yanmar unit the reduction of power density deriving from both the adoption of an Atkinson cycle and a lean-burn combustion management is compensated by the higher rotational speed (1800 rpm instead of 1500 rpm of the other units). As regards electric generators, the most adopted solution is the widespread asynchronous 4-poles machine, (liquid or air cooled) which can meet many different requirements, such as an easy starting and grid parallel insertion, low cost, durability; all these aspects are mandatory in order to reduce global plant cost. As regards the Yanmar unit, it adopts a permanent magnet synchronous generator (PMSG) together with an inverter for grid parallel insertion. This solution can give an important benefit in terms of electric efficiency in respect of the induction machine, but it results more expensive and with a lesser reliability. This last technical solution, together with the adoption of the Atkinson cycle, no doubt validate the outstanding value of the electric efficiency declared for this unit. However all these plants still have some disadvantages if compared to a conventional electricity and heat supplies (such as electric grid distribution and conventional gas heater), in terms of high specific cost (referred to power) and bulk, the first aspect making very difficult the Break Event Point of the investment. For these reasons micro-cogeneration could still be far away from high volume production, waiting for new solutions which can reduce the effect of the above mentioned disadvantages, but still taking benefits deriving from the full utilization of energy. 2. Experimentals: a new engine for small microcogeneration units The previous state of art showed clearly that, in order to get a higher thermal efficiency, an Atkinson cycle should be adopted for a microcogeneration unit, which could make the system really competitive in terms of electric efficiency. Also, this solution could
permit a better electric output even without the adoption of an expensive PMSG mated with an inverter system, but just using a reliable, cost effective 4-poles induction generator. As no engines of this kind were available on the market, the Author realized that the only way to get this result was to tranform an Otto-cycle engine to an Atkinson cycle unit. In order to reach this result a new engine with specific characteristics must be selected; of course just gasfueled units have been considered (because of the strict European limits regarding gaseous emissions) controlled by an accurate stoichiometric management, this to permit the adoption of a three way catalyst and assure better performances in the reduction of gaseous emissions if compared to a lean-burn engine as reported by Cho et al., [10]. Moreover, the efficiency benefit deriving from the adoption of an Atkinson cycle becomes more significant when air/ fuel mixtures of increasing energy are used [9], so making stoichiometric mixture more attractive than any lean mixtures. After some considerations it was clear that the same unit should be characterized by a bore to stroke ratio <1, this in order to obtain an expansion stroke longer than the compression stroke while maintaining a good effective compression ratio (i.e. with closed valves) and a compact combustion chamber. This architectural characteristic is the only which could assure, at the same time, a good compression ratio even with a partial compression stroke (fundamental aspect to obtain a high thermal efficiency [8,11] and just limited by knocking and NOx production), while keeping a compact design of combustion chamber in order to reduce heat losses. Among the industrial Otto units available today, none showed the above mentioned characteristics, because of their bore to stroke ratios typical of spark ignition engines, (usually around 1 or higher) and because of their limited compression ratio (normally lower than 10), which inhibit the possibility to reduce just the compression stroke to obtain an Atkinson cycle while maintaining a compact combustion chamber. Then, automotive gas engines have been considered, usually resulting less expensive than the industrial units (thanks their mass production) and showing very good performances in terms of global efficiency, low noise and vibration. On the other hand, they cannot be compared to the industrial units in terms of durability and reliability, as they have not been designed for heavy-duty service, but simply as spark-ignited automotive units. Also, most of the considered engines showed the same similar characteristics found for the above seen industrial units (such as bore to stroke ratio 1), with strong limitations regarding the attainable compression ratio when the compression stroke is reduced. For these reasons, the Author concluded that, in order to achieve a higher efficiency and endurance, another engine group should be considered. From this point of view, a modern automotive Diesel engine converted in a spark ignition Otto unit could result more similarly designed to an industrial engine, because more robustly constructed as it has to undertake to much higher pressures than a spark ignition unit. At the same time this engine class usually shows a lower bore to stroke ratio (made to achieve the typical Diesel high compression ratio) and a compact combustion chamber. In this way a so obtained Otto-cycle engine has all these architectural characteristics to attain an expansion stroke longer than the compression stroke still maintaining a compact combustion chamber, so making possible and feasible the conversion to the Atkinson-cycle. To get a first definition of displacement it can be observed that, in order to reach an electric power of 10.0 kW at the rated speed of 1500 rpm, a mechanical power of about 12.0 kW is required (having considered a generator with a mean electric efficiency of 0.85); consequently, a torque of about 75 Nm is needed from the engine. On the other side, a modern gas-fueled Otto cycle engine can express a specific torque of 80 Nm/dm3 around its highest volumetric
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P. Capaldi / Applied Thermal Engineering xxx (2014) 1e8
efficiency; having fixed a conservative value of 60 Nm/dm3 (taking into account the power reduction of about 25% deriving from the adoption of an Atkinson cycle [9]) a 1300 cm3 displacement would be necessary for the engine at 1500 rpm to produce the required power. So, several automotive Diesel engines have been considered during this analysis; at the end the chosen unit was a Fiat-GM 1.3 liters of displacement, also known as 1.3 Multi-Jet, a turbocharged unit in the original Diesel version widely adopted by many car makers (such as Fiat, GM, Suzuki and Ford). This engine represents one of the most advanced unit today available on the market, owing to its double overhead cams with hydraulic lifters and integrated rocker-arms, four valve head with high turbulence intake design and a low bore to stroke ratio (¼0.84), this in order to obtain a high compression ratio, a very compact combustion chamber and consequently a high thermal efficiency. This unit has been transformed first in a stoichiometric Ottocycle engine, atmospheric pressure charged and natural gas fueled, representing the very first 1.3 Fiat Multi-Jet Diesel unit to be modified into a spark ignited methane prototype. As regards the combustion chamber design it can be observed that, being the engine head the same of the original Diesel unit (flat type), the chamber is entirely contained inside the piston. The previous Diesel Saurer chamber (a toroidal enclosed bowl) has been replaced by an open bowl chamber with a very limited squish area, in order to minimize the surface/volume ratio and improve thermal efficiency; the compression ratio achievable with this design is 11.3:1, which could be considered a good compromise between a good thermal efficiency and limited NOx formation. The picture of the modified piston can be seen in the following Fig. 1 while the cutaway of the same is reported in Fig. 2. Also, this design can reduce the HC formation in the crevices and minimize squish which could enhance the swirl motion caused by the intake ducts design (typical of the original Diesel unit) and sustain too much the turbulence which increases heat transfer to the walls. This last aspect was considered to be an important issue by the Author, because the kinetic content of the intake fluxes is fundamental to sustain the flame propagation and avoid knocking, especially with a high compression ratio and when the engine is running at full load and at such a low speed of 1500 rpm; on the other hand turbulence has to be carefully controlled, in order to limit the heat transfer to the combustion chamber walls and
3
Fig. 2. Cutaway of the modified piston with open bowl combustion chamber.
cylinder, so reducing the heat losses and keeping a high global efficiency. As regards camshaft the engine was first provided with the standard component of the original Diesel turbocharged engine, with a valve timing (at zero backlash) reported in the following Table 2. Being derived from a Diesel unit, the engine was not originally provided with any ignition equipment; so an automotive based component was selected among many other commercial systems and then it was slightly modified in order to improve its life cycle. The system is a conventional single coil with a high voltage distributor, provided with an internal hall sensor and it’s characterized by a high price/performance ratio if compared to similar apparatuses commonly mounted on other microcogeneration systems. The same was finally placed on one camshaft with the aim of getting a one-spark per cycle strategy, instead of a one-spark per round (as most of the automotive engines) for a better spark durability and to avoid dangerous back-fire phenomena in the intake manifold. As regards the management system, the engine was provided with an integrated electronic platform, based on a commercial genset control system; the same has been modified and reconceived as a global control system for both electric and thermal generation. The system can also control the air to fuel ratio in a straight stoichiometric mode (even in fast transient load conditions thanks to a linear broad-band oxygen sensor) and drive a fast response valve to deliver the natural gas to the intake manifold. The same system can also manage other sub-systems, such as electric pumps for engine and plant cooling, auxiliary lubricating device, electric power factor regulator, etc.
Table 2 The valve timing diagram of the original diesel engine. Valve
Fig. 1. Modified piston with open bowl combustion chamber.
Exhaust Intake
Opening
54 BTDC 11 ATDC
Closing
2 ABDC 29 BBDC
Max. lift 6.6 mm 6.2 mm
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P. Capaldi / Applied Thermal Engineering xxx (2014) 1e8
It must be underlined that the modification from a Diesel engine to a spark ignition unit resulted very simple and economical, with few elements to be machined and a reduced number of new designed parts; most of the elements needed for the conversion were, in fact, already available as spare parts from other automotive units, or directly derived from domestic heating systems. As regards the engine primary cost, it must be underlined that the unit itself has been chosen because it resulted one of the most widespread today available on the market, especially for small cars and other small commercial vehicles, being cost-effective in spite of its sophisticated design, high quality and performance. So, in case of mass production it would be absolutely competitive as regards global cost, while, in case of low-volume production, remanufactured engines would be another suitable solution, being this unit one of the most low-priced if compared to other automotive engines and even more if compared to remanufactured industrial engines of the same class.
Fig. 3. The prototype with catalyst and oxygen sensor.
Table 3 Brake performance of engine prototype at full load in the standard configuration. Power [kW]
Speed [rpm]
BMEP [bar]
Power in. [kW]
Global eff.
THC [p.p.m.]
NOx [p.p.m.]
CO [%]
Tex [K]
14.60
1550
9.05
43.0
0.340
250
230
0.01
820
In the following Fig. 3 a first engine version (i.e. the Diesel unit just converted in a spark ignition unit) can be seen, with the three way catalytic converter and part of the heat exchangers. The first has been selected (after testing) among other mass produced catalysts specifically designed for methane and it results a costeffective solution for exhaust gas treatment being an automotive spare part. As regards heat exchangers, just off-the-shelf elements have been selected from the domestic heating systems; as regards the heat recuperating component from the high temperature exhaust gas, it was selected a copper/steel element arranged with a bended finned tube, characterized by good compactness and lower cost if compared to conventional shell and tube systems normally adopted for microcogeneration systems; the prototype is also provided with a condensation stage, in order to recover as much heat as possible from exhaust gas and improve the P.E.R. of the whole plant.
2.1. The engine performance in the reference configuration After having reached a first definition of the engine as regards its displacement, the following step was to carry out some brake test on the system to know the real output in terms of global output in the Otto-cycle reference configuration (i.e. when provided with the large bowl piston configuration and standard valve timing of the original Diesel unit) at full load. As regards laboratory setup, the apparatuses which have been used to characterize the behavior of the prototype were an asynchronous machine by API-COM (as regards brake), a hot wire flow meter (by VSE as regards the air and fuel flow metering), together with a Coriolis fuel flow meter (by Emerson MicroMotion). As regards emissions, raw exhaust gas has been analyzed with an API-COM measurement system (Mod. S5000), consisting of the following analyzers: NDIR (Non-Dispersive Infrared Detector), CLD (Chemiluminescence Detector) and FID (Flame Ionization Detector) all by Emerson. In this standard arrangement the prototype showed interesting overall performances, summarized in Table 3, in respect of global power and emissions after catalyst. The system has been provided with an EGR apparatus optimized just for full load conditions, in order to reduce NOx formation and also fuel consumption [10]. This system is fed with cooled gases spilled after the heat recuperation
Fig. 4. The scheme of the engine on the 1-D simulation program.
Please cite this article in press as: P. Capaldi, A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.02.035
P. Capaldi / Applied Thermal Engineering xxx (2014) 1e8
5
Table 4 Simulated performance of engine prototype at full load in standard configuration. Power [kW]
Speed [rpm]
IMEP [bar]
BMEP [bar]
Tex. [K]
Power in. [kW]
Global eff.
14.85
1550
10.4
9.15
832
43.4
0.342
at a maximum temperature of 55 C, being this condition very important in a spark ignited engine to avoid knocking and obtain a higher global efficiency. Then, the same unit was analyzed with a 1-D program (GT-Power 7.3 by Gamma Technologies) and compared with the real engine as regards the running parameters, this in order to get a well tuned simulation and with the aim of achieving a more accurate predictions of its behavior when functioning for different configurations, especially as regards valve timing and spark advance parameters. In this way it was possible to know predict, with acceptable accuracy, the prototype output when operated in the Atkinson mode and define the relative valve timing configuration which could give the required mechanical output needed to produce the target value for the electric power (10 kW). As regards the simulation scheme of the engine the same is reported in the following Fig. 4. As reported above, the simulation has been performed with the standard cams of the original Diesel engine, with the valve timing reported in Table 2: the reference speed was set at 1550 rpm (corresponding at about the 3% of the slip factor in a 4-poles asynchronous generator), being this value the expected speed at full load condition. The program simulated the engine at full load, calculating the performances in the following Table 4: After a tuning process, the simulation program produced results in good accordance with the real engine global output (such as produced power, fuel consumption, exhaust temperature etc.), this being also the consequence of a good accuracy, during simulation set-up, in the definition of combustion process and internal friction phenomena. This result made possible to take a second step forward, i.e. the simulation of a different valve closing angle to obtain the needed mechanical power. As already reported before, if a mean efficiency of 85% performed by the electric generator is considered, the required mechanical power should be around 12.0 kW. In order to compensate the reduction in the achievable compression ratio due
Fig. 6. Cutaway of the modified piston with “W” shape combustion chamber.
to a shorter compression stroke, a piston with a different combustion chamber (defined “W” shape) has been designed, with a higher compression ratio of 12:1 instead of the previous 11.3:1. In the following Figs. 5 and 6 are reported the image of this new piston and its cutaway. After that, the program simulated different configuration of valve timing, especially regarding the closing delay of inlet valve; the target mechanical power reported in the following Table 5 needed to obtain the required electric power of 10 kW) was achieved for the following valve timing reported in Table 6. For this configuration the calculated effective compression ratio (with the inlet valve closed) resulted 8.3:1, well below the real possibilities of the engine as regards achievable efficiency and natural gas knock resistance. For this reason a different mass produced piston has been selected and slightly modified in its shape, with a final geometrical compression ratio of 14.6:1 and an effective compression ratio of 10:1. The program showed a clear difference regarding global efficiency when operated with the new piston, this making also possible to fix a longer delay for inlet valve closing to reach the same power. In the following Tables 7 and 8 a complete pattern of engine performance and valve timing are reported. In the following Figs. 7 and 8 are reported respectively the image of the new high compression piston and its cutaway. After having simulated the engine prototype in the Atkinson configuration, the real engine was modified with the above seen high compression pistons and a new designed camshaft with the above seen valve timing of Table 8; then the unit was brake-tested in order to validate the prediction analysis. The results, reported in the following Table 9, are given just for full load condition and they reflect the predicted performances with good accuracy. The obtained results are no doubt interesting, with a net global efficiency gain of nearly 2.5% if compared to the reference Ottocycle. As regards gaseous emissions, an important reduction of NOx production can be underlined, due to a lower peak pressure and temperature during combustion, while no significant reduction of
Table 5 Simulated performance of the Atkinson engine prototype at full load.
Fig. 5. Modified piston with “W” shape combustion chamber.
Compr. ratio
Power [kW]
Speed [rpm]
IMEP [bar]
BMEP [bar]
Tex. [K]
Power in. [kW]
Global eff.
12:1
11.95
1550
8.60
7.40
725
32.7
0.365
Please cite this article in press as: P. Capaldi, A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.02.035
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P. Capaldi / Applied Thermal Engineering xxx (2014) 1e8
Table 6 Valve timing for the simulated Atkinson engine prototype at full load. Valve
Opening
Closing
Max. lift
Exhaust Intake
33 BBDC 5 ATDC
7 ATDC 78 BBDC
5.5 mm 6.0 mm
hydrocarbons can be recorded. The improved global efficiency can be also be confirmed by a lower exhaust gas temperature, as part of thermal energy normally lost at the escape has been converted in mechanical power. 2.1. The electric generator As reminded before, the choice of the electric generator for this kind of microcogeneration plant has to consider many different requirements, such as easy starting and grid parallel insertion, costeffectiveness, durability, etc. In order to meet all these necessities the technical solution widely adopted (as seen in the previous state of art) is represented by the induction machine directly connected to the grid. The most important aspect is that no expensive parallel system (or inverter) is required as no current phase has to be strictly respected; at the same time, system speed can vary through the slip factor d (with respect to grid frequency) just depending on the engine produced torque, as the same directly affect the electric power (See Fig. 9). This last aspect makes possible an easy control in closed loop of energy production by regulate power through engine torque management (throttle control). A three phase 400 VAC induction generator is, no doubt, the simplest solution for a 10 kWe powerplant, as the three phases could serve at least three different 230 VAC utility groups (houses or commercial activities). This particular power management can create some current difference among the coils inside the machine, so that some auxiliary elements are needed by the system to reach the required power quality, such as power-factor control systems together with some line filters [12]. For other specific applications, when a more flexible energy management is needed, a power inverter must be used, especially if electric energy storage (battery stack) has been planned. The final choice of the electric machine was made among different units available on the European market, each of them capable of producing the required power at the rated voltage, torque and speed. After some analysis it was clear that the required performances could only be assured by a water cooled unit, because of its high specific power and efficiency, but also because of its intrinsic low noise, (due to the absence of cooling fan and fins), with the possibility to be placed, together with the internal combustion engine, into a completely closed containing case. Moreover,
Fig. 7. Upper view of the high compression piston (14.6:1).
compactness was taken into account because a significant volume and weight reduction could be obtained with this solution. So it was selected a water cooled machine previously employed as a traction motor and modified as regards windings (number, wire section and copper/iron weight ratio) in order to optimize it as a generator. The final aim of this transformation was to modify an existing element without re-designing the same (avoiding in this way an extra cost), so arranging not only a prototype, but a preindustrial component ready for mass production. This unit has been examined on a test bench and characterized by varying the external torque, as reported in the following Fig. 10. As regards cooling system, the generator is considered to be the first element to be cooled inside the microcogeneration plant, because its temperature should be kept as low as possible in order to protect wire electric isolation. As the reference incoming water temperature from a generic thermal utility (such as a domestic heating system) is normally fixed at 50 C, this last value will be considered as the reference inlet temperature for the cooling of the plant and for the generator itself. Then, a set of experimental data was produced and reported in the following Table 10.
Table 7 Simulated performance of the Atkinson engine prototype at full load with 14.6:1 compression ratio. Compr. ratio
Power [kW]
Speed [rpm]
IMEP [bar]
BMEP [bar]
Tex. [K]
Power in. [kW]
Global eff.
14.6:1
11.95
1550
8.60
7.40
725
32.2
0.371
Table 8 Valve timing for an Atkinson engine prototype at full load with 14.6:1 compression ratio. Valve Exhaust Intake
Opening
33 BBDC 5 ATDC
Closing
7 ATDC 80 BBDC
Max. lift 5.5 mm 6.0 mm
Fig. 8. Cutaway of the high compression piston (14.6:1).
Please cite this article in press as: P. Capaldi, A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.02.035
P. Capaldi / Applied Thermal Engineering xxx (2014) 1e8
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Table 9 Global brake performance of the Atkinson engine prototype at full load. Power [kW]
Speed [rpm]
BMEP [bar]
Power in. [kW]
Global eff.
THC [p.p.m.]
NOx [[p.p.m.]
CO [%]
Tex [K]
11.8
1550
7.30
32.4
0.365
230
140
0.01
734
The recorded data shown above are very good for a prototype induction generator of such limited power, with an efficiency hgen around 89% over most of its functioning curve. However the Author considers that some diminution in the generation efficiency could occur when the unit would be coupled with the internal combustion engine, because of the intrinsic torque and speed fluctuations of the latter [12]. 3. Results and discussion The two sub-systems (the Atkinson internal combustion engine and the asynchronous generator) have been finally coupled and tested together as a complete electric and heat generator, in order to acquire the global plant performance. As electric efficiency in induction generators is deeply influenced by torque and speed variability, the two systems were coupled with a special elastic joint and a high inertia flywheel, which limit the rotational speed fluctuation of the whole system. Also, the system was put inside a new designed containing case, just constructed in two rigid structures and expressly conceived to avoid any kind of resonance (so reducing noise emission) and limit heat losses; plant compactness was an important issue for the Author, so that the system is contained in a case which occupy a volume of just 0.80 m3. In the following Fig. 11 the plant can be seen in its external view, while in Fig. 12 the inside of the same is shown, with the four cylinder engine and the intake manifold. As regard the global performances the unit ran with three different loads (i.e. full load, 75% and 50%) by means of a passive electric load which could simulate a generic utility; in this way the control system could command the throttle openings and regulate the produced electric power. All tests have been performed with natural gas from the Italian distribution network, with a measured LHV (Lower Heating Value) of 46,600 kJ/kg. The whole set of power performance data are reported in the following Table 11, while regarding emissions the complete data set is reported in Table 12.
Fig. 10. Water cooled asynchronous generator on the test bench.
The obtained results are very interesting if referred to an experimental unit; as regards the electric efficiency hE.E, the system showed a remarkable 31.5% (with a net power generation of 10.1 kW and a thermal power generation of 21.7 kW), corresponding to a thermal efficiency hT.E of 66.3%, this leading to a P.E.R. of 97.8% at full load. This result is no doubt consequence of the adoption of the Atkinson cycle which permits a better utilization of the primary energy if compared to the Otto cycle; moreover, the exhaust temperature result very low for an internal combustion engine, this confirming the better utilization of heat through an extended expansion. The thermal efficiency is also remarkable, this because of the condensation of water in the exhaust but also thanks to the containing case which efficiently isolated the generation system. Interesting, as well, is the output at 75% of load, where just a reasonable drop of electric efficiency has been recorded, this confirming that smaller displacement engine with a good specific power can express a better global efficiency even at partial loads, if compared to other units of bigger displacement and same electric power. As regards the electric efficiency, it can be observed that generator probably results still affected by torque variability from the internal combustion engine, this in spite of the adoption of a special elastic coupling and large flywheel; in fact, if the two different working conditions are compared for the same produced power (i.e. when the generator is brake-tested and when it is coupled to the internal combustion engine) higher currents and slip factor were recorded, with an important decrease for the power factor value [12]; however the same can be easily corrected to the required value with cost-effective condenser stages. As regards emissions the same are quite interesting and are consequence of the specific thermodynamic cycle adopted for this prototype. The Atkinson cycle, thanks to lower peak temperatures and, together with the adoption of an EGR system, can inhibit NOx formation. As regards THC formation the behavior is just acceptable with a standard automotive catalyst, but it must be carefully kept under control, because the lower exhaust gas temperature can Table 10 Asynchronous generator experimental data characteristics.
Fig. 9. Electrical torque/slip factor diagram for three phase induction machine.
Load [%] Power [kW] Current [A] Power factor Efficiency [%] Speed [rpm] Voltage [V] Mech. pow. [kW] Losses [kW]
100 10.0 19.2 0.82 88.9 1541 388 11.25 1.125
75 7.5 16.0 0.75 89.5 1530 388 9.14 1.076
50 5.0 13.1 0.62 88.1 1522 388 6.30 0.746
Please cite this article in press as: P. Capaldi, A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.02.035
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P. Capaldi / Applied Thermal Engineering xxx (2014) 1e8 Table 11 Microcogenerator experimental data characteristics. Load [%] Electric power [kW] Speed [rpm] Current [A] Power factor Primary power [kW] Electric efficiency [%] Thermal power [kW] Thermal efficiency [%] P.E R. [%]
100 10.1 1543 20.1 0.75 32.1 31.5 24.7 66.3 97.8
75 7.5 1532 16.6 0.70 26.7 28.1 20.8 69.9 98.0
50 5.0 1523 13.5 0.54 20.8 24.0 17.8 74.0 98.0
Table 12 Microcogenerator emissions before and after the three way catalyst.
Fig. 11. The external view of the microcogeneration plant.
reduce the efficiency of the same, especially for the oxidation of methane gas. This aspect makes advisable the adoption of dedicated catalyst optimized for lower gas temperatures (which can also assure better global performances), as standard catalyst normally adopted by natural gas fueled cars could not assure a satisfactory gas treatment after a long running period. 4. Conclusions The 10 kW power unit developed by Istituto Motori-CNR showed very interesting global performances, better than the
Load [%] Electric power [kW] THC (b.c.) [p.p.m.] NOx (b.c.) [p.p.m.] CO (b.c.) [%] Ex. Temp. (b.c.) THC (a.c.) [p.p.m.] NOx (a.c.) [p.p.m.] CO (a.c.) [%] Ex. Temp. (a.c.)
100 10.1 1330 1080 0.07 734 230 140 0.01 740
75 7.5 1210 810 0.06 698 210 105 0.01 688
50 5.0 1090 535 0.06 652 195 75 0.01 645
best commercial competitors provided with asynchronous generator and competitive with models fitted with expensive power systems based on permanent magnet generator and inverter. The adoption of an Atkinson cycle, with its extended expansion stroke, can give a significant benefit in terms of global efficiency and in terms of gaseous emissions especially when a dedicated catalyst is adopted. The system has been obtained through a selection of “off the shelf” high quality elements, in order to keep low the global cost, but is fundamentally based on the choice of a widespread automotive unit characterized by heavy duty characteristics (being originally a Diesel unit) and which can easily converted first in a spark ignition engine and then in an Atkinson-cycle unit. Nomenclature I.C.E. internal combustion engine P.E.R. primary energy ratio E.G.R exhaust gas recirculation THC total hydrocarbons (gaseous emission) NOx nitrous oxides (gaseous emission) hE.E electric efficiency of microcogenerator hT.E thermal efficiency of microcogenerator hgen electric efficiency of generator VAC voltage in alternate current References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] Fig. 12. The internal view of the microcogeneration unit.
www.simple-energie.de. www.energator.de. www.bhkw-steinecke.de. www.ecpower.dk. www.rmbenergie.de. www.yanmar.co.jp. J.B. Heywood, Internal Combustion Engine Fundamentals, Mc. Graw Hill Inc., 1998. Yanlin Ge, Lingen Chen, Fengrui Sun, Chih Wu, Reciprocating heat-engine cycles, Appl. Energy 81 (2005) 397e408. R. Mikalsen, Y.D. Wang, A.P. Roskily, A comparison of Miller and Otto cycle natural gas engines for small scale CHP applications, Appl. Energy 86 (2009) 922e927. Haeng Muk Cho, Bang-Quan He, Spark ignition natural gas engines e a review, Energy convers. Manage. 48, 2007, 608e618. M Hitomi, N Iwata, M Yamakawa, T Nishimoto, Spark ignition gasoline engine, US Patent 7.484.498 B2, 2009. P. Capaldi, A. Del Pizzo, L. Piegari, R. Rizzo, A low cost microcogeneration system for domestic and industrial applications, in: Proc. of IEEE ICIT Conf., Hammamet, vol. 1, Dec. 2004, pp. 514e518.
Please cite this article in press as: P. Capaldi, A high efficiency 10 kWe microcogenerator based on an Atkinson cycle internal combustion engine, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.02.035