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Energy xxx (2010) 1–7

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/energy

A highly efficient six-stroke internal combustion engine cycle with water injection for in-cylinder exhaust heat recovery James C. Conklin, James P. Szybist*
Oak Ridge National Laboratory, 2360 Cherahala Blvd, Knoxville, TN 37932, USA

a r t i c l e i n f o
Article history: Received 9 September 2009 Received in revised form 3 December 2009 Accepted 8 December 2009 Available online xxx Keywords: Engine efficiency Six-stroke cycle Water injection Steam cycle

a b s t r a c t
A concept adding two strokes to the Otto or Diesel engine cycle to increase fuel efficiency is presented here. It can be thought of as a four-stroke Otto or Diesel cycle followed by a two-stroke heat recovery steam cycle. A partial exhaust event coupled with water injection adds an additional power stroke. Waste heat from two sources is effectively converted into usable work: engine coolant and exhaust gas. An ideal thermodynamics model of the exhaust gas compression, water injection and expansion was used to investigate this modification. By changing the exhaust valve closing timing during the exhaust stroke, the optimum amount of exhaust can be recompressed, maximizing the net mean effective pressure of the steam expansion stroke (MEPsteam). The valve closing timing for maximum MEPsteam is limited by either 1 bar or the dew point temperature of the expansion gas/moisture mixture when the exhaust valve opens. The range of MEPsteam calculated for the geometry of a conventional gasoline engine and is from 0.75 to 2.5 bars. Typical combustion mean effective pressures (MEPcombustion) of naturally aspirated gasoline engines are up to 10 bar, thus this concept has the potential to significantly increase the engine efficiency and fuel economy. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction In internal combustion engines, a significant amount of the fuel energy exits the engine in the form of thermal energy in the exhaust. To demonstrate this, Fig. 1 shows previously unpublished experimental data from a turbo-charged 2007 Saab Biopower vehicle during a federal test protocol (FTP)-75 engine cycle, the engine cycle that is largely used to calculate the EPA ‘‘city’’ fuel economy. The FTP-75 cycle is highly transient with numerous stops and starts, and corresponding fluctuations in the engine-out exhaust temperature from 400 to 600  C. The exhaust temperature range of naturally aspirated gasoline engine is higher, typically from 450 to 800  C. The total fuel energy consumed during the course of the driving cycle is approximately 58.5 MJ, or about 1.7 L (0.45 gallons) of unleaded gasoline fuel. The percentage of fuel energy converted to useful work for this driving cycle (i.e. the vehicle thermal efficiency) is 10.4%. A much larger portion of fuel energy, 27.7%, exits the vehicle in the form of thermal energy in the exhaust, while the remaining 61.9% of the energy balance consists of energy losses to friction, coolant, and other. For these

* Corresponding author. Tel.: þ1 865 946 1514; fax: þ1 865 946 1248. E-mail address: szybistjp@ornl.gov (J.P. Szybist). 0360-5442/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2009.12.012

series of tests, the instrumentation was limited to the fuel and exhaust systems, and thus the friction, coolant, and other losses are indistinguishable. Only a portion of the energy in the exhaust is available for recovery due to process irreversibilities, ambient conditions, etc. The exergy in the exhaust was determined by a 2nd law thermodynamic analysis as described by Rodriguez [1]. The difference in brake work fractions between the 1st and 2nd law analyses, also shown in Fig. 1, is because the 1st law analysis of energy distribution is normalized with the lower heating value of the fuel while the 2nd law analysis is normalized with the...
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