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Demonstration of Air-Fuel Ratio Role in One-Stage Turbocompound Diesel Engines
KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
2013 (English)In: SAE Technical Papers, 2013, Vol. 11Conference paper (Refereed)
Abstract [en]

A large portion of fuel energy is wasted through the exhaust of internal combustion engines. Turbocompound can, however, recover part of this wasted heat. The energy recovery depends on the turbine efficiency and mass flow as well as the exhaust gas state and properties such as pressure, temperature and specific heat capacity.

The main parameter influencing the turbocompound energy recovery is the exhaust gas pressure which leads to higher pumping loss of the engine and consequently lower engine crankshaft power. Each air-fuel equivalence ratio (λ) gives different engine power, exhaust gas temperature and pressure. Decreasing λ toward 1 in a Diesel engine results in higher exhaust gas temperatures of the engine.  λ can be varied by changing the intake air pressure or the amount of injected fuel which changes the available energy into the turbine. Thus, there is a compromise between gross engine power, created pumping power, recovered turbocompound power and consumed compressor power.

In this study, the effects of different λ values and exhaust back-pressure have been investigated on the efficiency of a heavy-duty Diesel engine equipped with a single-stage electric turbocompounding. A one-dimensional gas dynamics model of a turbocharged engine was utilized that was validated against measurements at different load points. Two configurations of turbocompound engine were made. In one configuration an electric turbocharger was used and the amount of fuel was varied with constant intake air pressure. In another configuration the turbocharger turbine and compressor were disconnected to be able to control the turbine speed and the compressor speed independently; then the compressor pressure ratio was varied with constant engine fuelling and the exhaust back-pressure was optimized for each compressor pressure ratio.

At each constant turbine efficiency there is a linear relation between the optimum exhaust back-pressure and ideally expanded cylinder pressure until bottom dead center with closed exhaust valves. There is an optimum λ for the turbocharged engine with regard to the fuel consumption. In the turbocompound engine, this will be moved to a richer λ that gives the best total specific fuel consumption; however, the results of this study indicates that turbocompound engine efficiency is relatively insensitive to the air-fuel ratio.

Place, publisher, year, edition, pages
2013. Vol. 11
, SAE Technical Papers, ISSN 0148-7191 ; Vol. 11
Keyword [en]
Turbocompound, Diesel engine, Waste heat recovery, Air-fuel ratio
National Category
Vehicle Engineering Energy Engineering Applied Mechanics Fluid Mechanics and Acoustics
URN: urn:nbn:se:kth:diva-139145DOI: 10.4271/2013-01-2703ScopusID: 2-s2.0-84890377517OAI: diva2:684860
SAE/KSAE 2013 International Powertrains, Fuels and Lubricants Meeting, FFL 2013; Seoul, South Korea, 21-23 October 2013
Swedish Energy Agency

QC 20140108

Available from: 2014-01-08 Created: 2014-01-07 Last updated: 2014-10-01Bibliographically approved
In thesis
1. Exhaust Heat Utilisation and Losses in Internal Combustion Engines with Focus on the Gas Exchange System
Open this publication in new window or tab >>Exhaust Heat Utilisation and Losses in Internal Combustion Engines with Focus on the Gas Exchange System
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Exhaust gas energy recovery should be considered in improving fuel economy of internal combustion engines. A large portion of fuel energy is wasted through the exhaust of internal combustion engines. Turbocharger and turbocompound can, however, recover part of this wasted heat. The energy recovery depends on the efficiency and mass flow of the turbine(s) as well as the exhaust gas state and properties such as pressure, temperature and specific heat capacity. The exhaust gas pressure is the principal parameter which is required for the turbine energy recovery, but higher exhaust back-pressures on the engines create higher pumping losses. This is in addition to the heat losses in the turbochargers what makes any measurement and simulation of the engines more complex.

This thesis consists of two major parts. First of all, the importance of heat losses in turbochargers has been shown theoretically and experimentally with the aim of including heat transfer of the turbochargers in engine simulations. Secondly, different concepts have been examined to extract exhaust heat energy including turbocompounding and divided exhaust period (DEP) with the aim of improved exhaust heat utilisation and reduced pumping losses.

In the study of heat transfer in turbochargers, the turbocharged engine simulation was improved by including heat transfer of the turbocharger in the simulation. Next, the heat transfer modelling of the turbochargers was improved by introducing a new method for convection heat transfer calculation with the support of on-engine turbocharger measurements under different heat transfer conditions. Then, two different turbocharger performance maps were assessed concerning the heat transfer conditions in the engine simulation. Finally, the temperatures of turbocharger’s surfaces were predicted according to the measurements under different heat transfer conditions and their effects are studied on the turbocharger performance. The present study shows that the heat transfer in the turbochargers is very crucial to take into account in the engine simulations, especially in transient operations.

In the study of exhaust heat utilisation, important parameters concerning turbine and gas exchange system that can influence the waste heat recovery were discussed. In addition to exhaust back-pressure, turbine speed and turbine efficiency, the role of the air-fuel equivalence ratio was demonstrated in details, because lower air-fuel equivalence ratio in a Diesel engine can provide higher exhaust gas temperature. The results of this study indicate that turbocompound engine efficiency is relatively insensitive to the air-fuel equivalence ratio.

To decrease the influence of the increased exhaust back-pressure of a turbocompound engine, a new architecture was developed by combining the turbocompound engine with DEP. The aim of this study was to utilise the earlier phase (blowdown) of the exhaust stroke in the turbine(s) and let the later phase (scavenging) of the exhaust stroke bypass the turbine(s). To decouple the blowdown phase from the scavenging phase, the exhaust flow was divided between two different exhaust manifolds with different valve timing.

According to this study, this combination improves the fuel consumption in low engine speeds and deteriorates it at high engine speeds. This is mainly due to long duration of choked flow in the exhaust valves because this approach is using only one of the two exhaust valves on each cylinder at a time.

Therefore, the effects of enlarged effective flow areas of the exhaust valves were studied. Two methods were used to enlarge the effective flow area i.e. increasing the diameters of the blowdown and scavenging valves by 4 mm; and modifying the valve lift curves of the exhaust valves to fast opening and closing. Both methods improved BSFC in the same order even though they were different in nature. Fast opening and closing of the exhaust valves required shorter blowdown duration and longer scavenging duration. The modified lift curves provided less pumping losses, less available energy into the turbine and larger amplitude of the pulsating flow through the turbine.

In order for defining a set of important parameters that should be examined in experimental studies, a sensitivity analysis was performed on the turbocompound DEP engine in terms of break specific fuel consumption to different parameters concerning the gas exchange such as blowdown valve timing, scavenging valve timing, blowdown valve size, scavenging valve size, discharge coefficients of blowdown and scavenging ports, turbine efficiency, turbine size and power transmission efficiency.

Finally, to overcome the restriction in the effective flow areas of the exhaust valves, DEP was implemented externally on the exhaust manifold instead of engine exhaust valves, which is called externally DEP (ExDEP). This innovative engine architecture, which benefits from supercharging, turbocharging and turbocompounding, has a great fuel-saving potential in almost all load points up to 4%.

Abstract [sv]

Avgasenergiåtervinning bör övervägas för att förbättra förbränningsmotorers bränsleekonomi. En stor del av bränslets energi förloras via förbränningsmotorernas avgaser. Dock kan turbo och turbocompound återvinna en del av denna värmeförlust. Energiåtervinningen beror på verkningsgraden och massflödet genom turbinen (erna), liksom avgasernas tillstånd och egenskaper, såsom tryck, temperatur och specifik värmekapacitet. Avgastrycket är den viktigaste parametern som påverkar turbinenergiåtervinningen, men högre avgasmottryck skapar högre pumpförluster. Detta utöver värmeförlusterna i turboaggregator gör mätning och simulering av motorer komplexa.

Avhandlingen består av två huvuddelar. Först har vikten av värmeförluster i turboladdare visats både teoretiskt och experimentellt, med syfte att införa värmeöverföring av turboladdare i motorsimuleringar. För det andra, har olika koncept undersökts för att utvinna avgasvärmeenergi med turbocompound och delad avgasperiod (DEP), i syfte att förbättra avgasvärmeutnyttjande och minska pumpförluster.

I studien av värmeöverföring i turboladdare förbättrades turbomotorsimulering genom att inkludera värmeöverföring av turboladdaren i simuleringen. Härnäst förbättrades värmeöverföringsmodelleringen av turboladdarna genom att införa en ny metod för konvektiv värmeöverföringsberäkning, med stöd av mätningar på turbon på motorn, under olika förutsättningar för värmeöverföring. Därefter bedömdes två olika turboaggregats prestandamappar för värmeöverföringsförhållandena i motorsimulering. Slutligen beräknades temperaturerna på turbons ytor, baserat på mätningarna, under olika värmeöver­föringsförhållanden och effekterna studerades på turboprestanda. Den aktuella studien visar att värmeöverföringen i turboladdarna är mycket viktigt att ta hänsyn till i motor simuleringarna, speciellt i transienter.

I studien av avgasvärmets utnyttjande, undersöktes viktiga parametrar med avseende på turbinen och gasväxlingen, som kan påverka värmeåtervinningen. Förutom avgasmottryck, turbinvarvtal och turbinverkningsgrad, visades påverkan av luft-bränsleförhållandet, eftersom lägre luft-bränsleförhållandet i en dieselmotor kan ge högre avgastemperatur. Resultaten av denna studie indikerar att turbocompoundmotorns verkningsgrad är ganska okänslig för luft-bränsleförhållandet.

För att minska påverkan av det ökade avgasmottrycket hos en turbokompoundmotor, utvecklades en ny arkitektur genom att kombinera turbokompoundmotorn med DEP. Syftet med denna studie var att utnyttja den tidiga fasen (blowdown) av avgastakten i turbinen (erna) och låta den senare fasen (scavenging) av avgastakten gå förbi turbinen (erna). För att frikoppla blowdown fasen från scavenging fasen delades avgasflödet upp mellan två olika avgasgrenrör med olika ventiltider.

Enligt denna studie, förbättrar denna kombination bränsleförbrukningen vid låga varvtal och försämrar den på höga varvtal. Detta är främst på grund av lång varaktighet av kritiskt flöde i avgasventilerna eftersom DEP använder endast en av de två avgasventilerna på varje cylinder i taget.

Därför studerades effekten av förstorade effektiva flödesareor hos avgasventilerna. Två metoder användes för att förstora den effektiva flödesarean, ökning diametrarna av blowdown och scavenging ventilerna med 4 mm och ändring av avgasventillyftkurvorna till snabb öppning och snabb stängning. Båda metoderna förbättrades BSFC i samma storleksordning trots att de var av olika slag. Snabb öppning och stängning av avgasventilerna krävde kortare blowdownvaraktighet och längre scavengingvaraktighet. De modifierade lyftkurvorna gav mindre pumpförluster, mindre tillgänglig energi in i turbinen och större amplitud av pulserande flöde genom turbinen.

För att definiera en uppsättning viktiga parametrar som bör undersökas i experimentella studier, genomfördes en känslighetsanalys på turbocompound DEP motorn i fråga om specifik bränsleförbrukning som funktion av olika parametrar som rör gasväxling såsom blowdown ventiltider, scavenging ventiltider, blowdown ventilstorlek, scavenging ventilstorlek, strömningskoefficienterna hos blowdown och scavenging kanalerna, turbinverkningsgrad, turbinstorlek och kraftöverföringsverkningsgraden.

Slutligen, för att övervinna begränsningar av de effektiva flödesareorna hos avgasventilerna genomfördes DEP externt i avgasgrenröret i stället för att utnyttja avgasventilerna, här kallad externt DEP (ExDEP). Denna innovativa motorarkitektur, som drar nytta av överladdning, turboladdning och turbocompound, har en stor bränslesparande potential, i nästan alla belastningspunkter upp till 4%.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. xiv, 101 p.
TRITA-MMK, ISSN 1400-1179 ; 2014:07
Turbocharger, heat transfer, heat loss, exhaust heat utilisation, waste heat recovery, turbocompound, divided exhaust period, internal combustion engine, gas exchange, pumping loss, variable valve timing, WHR, DEP, Turbo, värmeöverföring, värmeförluster, avgasvärme utnyttjande, förluster vid värmeåtervinning, turbocompound, delad avgasperiod, förbränningsmotor, gasväxling, pumpningsförlust, variabla ventiltider, WHR, DEP
National Category
Applied Mechanics Energy Engineering Vehicle Engineering
Research subject
Vehicle and Maritime Engineering; Energy Technology
urn:nbn:se:kth:diva-152520 (URN)978-91-7595-279-6 (ISBN)
Public defence
2014-10-30, F3, Lindstedsvägen 26, KTH, Stockholm, 14:00 (English)
Swedish Energy Agency, F6432

QC 20141001

Available from: 2014-10-01 Created: 2014-09-26 Last updated: 2015-08-26Bibliographically approved

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