A comparison of Miller and Otto cycle natural gas …

1 A comparison of Miller and Otto cycle natural gas engines for smallscale CHP applications. R. Mikalsen , Wang, RoskillySir Joseph Swan Institute for Energy Research, Newcastle University, Newcastle upon Tyne, NE1 7RU, United paper presents an investigation into the feasibility and potential advantages of a small scale Miller cycle naturalgas engine for applications such as domestic combined heat and power systems. The Miller cycle engine is comparedto a standard Otto cycle engine using cycle analyses and multidimensional simulation, and basic engine designimplications are discussed. It is found that the Miller cycle engine has a potential for improved fuel efficiency, but atthe cost of a reduced power to weight ratio.

2 A fuel efficiency advantage of 5 10 per cent compared to a standardOtto cycle engine appears possible, however it is stated that further investigations, in particular into the topic ofengine friction, are required in order to validate the words: Miller cycle , natural gas engine, CHP, IntroductionDomestic energy use, heating and electricpower consumption, is the largest contributor tothe average UK citizen s CO2footprint [1], as themajority of this energy is generated using fossilfuels. As evidence of the environmental impacts ofcarbon dioxide emissions from fossil fuels becomesclearer and fuel costs increase, the interest in smallscale, domestic combined heat and power (CHP)systems has increased.

3 By utilising a part of thefuel energy to produce electricity locally, where thewaste energy (typically two thirds of the fuel heatenergy) is needed for heating, electric power con-sumption from large, fossil fuel based power plants,in which most of the excess heat energy is lost, canbe reduced. This is a preprint version. This paper was published as:Applied Energy, Volume 86, Issue 6, June 2009, Pages 922 927. Corresponding author. Tel. +44 191 246 Mikalsen). heat and power systems for smallscale applicationsThe yearly energy consumption for a typical UKhousehold consists of around 15 25 per cent elec-tric energy and 75 85 per cent energy for these numbers clearly have seasonal vari-ations, this scales well with the efficiency one wouldexpect from a combustion engine and electric gener-ator set, which typically lies in the range 20 35 percent, depending on size and application.

4 Effectivemethods of coupling small CHP units to the electricgrid exist, in order to account for variations in thesupply from the unit and household 1 shows two possible configurations fora small natural gas engine in domestic applica-tions. Figure 1a shows a standard combined heatand power system, in which the mechanical outputfrom the engine drives an electric generator. Theelectric power can be supplied to the grid, therebyreducing the household s electricity costs, and theexhaust heat from the engine is utilised for heatingPreprint submitted to Elsevier25 February 2009 BoostcombustorExhaustCentralheatingAirNa tural gasGenerator~(a) Combined heat and electric power gasAirExhaustOutdoorevaporatorheating(b) natural gas engine and heat pump 1.

5 Possible configurations for a natural gas engine in domestic Figure 1b shows an alternative configu-ration of a heating-only system based on a naturalgas engine driving a heat pump. Depending on theconfiguration, modern domestic heat pumps typi-cally achieve a coefficient of performance (COP) of3 5, for one unit of input energy 3 5 unitsof heat can be supplied. With an engine efficiencyof 25% and a heat pump COP of 3 (a conservativeestimate), a reduction in fuel use for heating in theorder of 30% is possible. As an alternative the heatpump could be used to provide cooling, which couldbe an attractive solution in applications such as of-fice buildings with rooms for computing equipmentor convenience stores which require cooled storagefor pre-requisite for such a system is, however, thata suitable natural gas engine can be movers for small scale CHP systemsSeveral different designs have been proposed forsmall scale CHP systems, however there is currentlyonly a limited number of units commercially avail-able.

6 The smallest units, such as the WhisperGenmicro-CHP system manufactured by WhisperGenLtd. [2], are mainly based on the Stirling engine, andhave electric power output down to around 1 the Stirling engine is capable of electricgeneration efficiencies of up to 25 per cent, unitscurrently available in this size range typically havelower ratios of electric to heating power et al. [3] stated that there are currently around1000 micro-CHP units installed in the UK, and thatsuch units are financially viable with payback timeof 3 5 combustion engines are generally consid-ered unsuitable for micro-scale CHP systems due tofactors such as high levels of vibration and noise,higher exhaust gas emissions levels compared to con-tinuous combustion systems, less fuel flexibility, andshorter lifetime.

7 Some examples do, however, exist,such as the kWeDachs micro-CHP unit manu-factured by SenerTec GmbH [4], and such engineshave some clear advantages over existing external2combustion technologies, including high power den-sity, simple control, and high fuel efficiency. In addi-tion, internal combustion engine technology is well-proven and highly developed. With the low price,high availability, and well established distributionnetworks for natural gas existing in many countries,including the United Kingdom, natural gas inter-nal combustion engines present a potential optionfor use in a combined heat and power system. Ifthe challenges of noise and vibration can be over-come, for example through acoustic insulation in themounting of the unit, there is no reason why inter-nal combustion engines should not be feasible formicro-scale CHP , small scale internal combustion enginesare mainly used in applications such as small mo-torbikes and portable electric generators, in whichthe requirement of a high power to weight ratio isthat of highest importance.

8 In the design of a com-bustion engine there is a clear trade-off betweenpower density and efficiency, and, since the powerdensity requirement is relaxed in stationary appli-cations such as a CHP system, the engine can beoptimised for high efficiency and long life. Engineoptimisation may include factors such as: the useof a high stroke to bore ratio to reduce in-cylinderheat transfer losses; reducing engine mean pistonspeed to minimise frictional and gas flow losses; re-ducing the compression ratio to reduce sealing re-quirements; lean burn operation to reduce peak in-cylinder gas temperatures and pressures; and the useof an over-expanded cycle to maximise the mechan-ical work extracted from the cylinder gases.

9 Sincemost of these result in a reduction in engine powerto weight ratio, they are normally not used in exist-ing small scale internal combustion engines. Hence,there is a significant potential for enhancing enginefuel efficiency if a penalty in engine size and weightcan be Miller cycle engineThe Miller cycle was proposed by Miller [5], withthe main objective of improving engine is an over-expanded cycle , a cycle with anexpansion ratio higher than its compression , the Miller cycle has also been proposed asa means of reducing harmful NOxemissions whilemaintaining a high engine efficiency, by reducing theengine compression ratio and thereby also peak in-cylinder gas temperatures and number of reports have described the Miller cy-cle engine concept and investigated various aspectsof its design and operation.

10 Al-Sarkhi et al. [6] andZhao and Chen [7] presented theoretical investiga-tions into Miller cycle engine performance, studyingthe influence of the main engine design variables andsystem irreversibilities. Endo et al. [8] described thedesign of a commercially available large scale (280 1100 kW) gas engine using the Miller cycle principle,claiming a fuel efficiency advantage of more than5 % over comparable conventional technology. Ghe-orghiu and Uebersch ar [9] studied an overexpandedengine for use in hybrid vehicles and investigatedsources of efficiency loss in the conventional imple-mentation of such cycles. Wang and Ruxton [10] andWang et al. [11] investigated the application of theMiller cycle concept to reduce engine exhaust gasemissions and found that significant NOxreductionscould be achieved, albeit with a penalty in enginefuel (a) Air-standard Otto (b) Air-standard Miller 2.