DEVELOPMENTS IN FIELD OF INTERNAL COMBUSTION ENGINES

ENVIRONMENTALLY SAFER POWER SOURCES AND TECHNOLOGIES

1. A significant amount of research and development is currently being pursued by government, industry and academia around the world to improve Internal Combustion (IC) engine efficiency and reduce emissions. The most challenging aspects of this research are as follows:-

(a) Improve engine efficiency and reduce emissions.

(b) Achieve (a) above while neither compromising the power nor performance.

(c) Achieve (a) and (b) above without dramatically increasing the cost and/ or complexity.

(d) Arrive at a reliable technology that can be implemented over a wide-ranging power spectrum, while achieving (a), (b) and (c) above.2. This paper covers the following two advanced areas of research that are expected to yield significant gains in this field:-

(a) Homogenous Charge Compression Ignition (HCCI).

(b) Split Cycle Engine Technology.

HOMOGENEOUS CHARGE COMPRESSION IGNITION (HCCI)

3. Homogeneous Charge Compression Ignition (HCCI) technology has been around for a long time, but has recently received rekindled attention. Early attempts at fruition of this technology saw multiple failures, owing to non-availability of sophisticated computer controlled digital electronics.

Homogeneous Charge Compression Ignition (HCCI) Engine[endnoteRef:2] [2: Professor Bengt Johansson, Homogeneous Charge Compression Ignition the future of IC engines?, Lund Institute of Technology, Lund University, Sweden. Sourced from the Internet at http://www.osd.org.tr/6.pdf on 21 Aug 13.]

4. The Homogeneous Charge Compression Ignition (HCCI) engine combines the best of the Spark Ignition (SI) and Compression Ignition (CI) engines. In HCCI, the fuel and air are mixed before combustion starts and the mixture is auto-ignited due to the increase in temperature from the compression stroke. Thus, HCCI is similar to SI as both engines use a premixed charge, and HCCI is similar to CI as both rely on auto-ignition for combustion initiation. However, the three have notably different combustion processes.

5. HCCI and SI Engines. In SI engine there are three combustion zones - a burnt zone, an unburned zone and a narrow reaction zone in between, where the flame chemistry takes place. This reaction zone propagates through the combustion chamber as the flame propagation. Even though the reactions are fast in the reaction zone, the combustion process takes some time as the zone must propagate from spark plug to the far liner wall. In HCCI, the entire mass of charge in the cylinder reacts all at once. The auto-ignition in HCCI engines depends on a kinetically driven oxidation process that does not rely on flame propagation. The reactivity of the charge in an HCCI engine depends on the heat absorbed and the combustion is not followed by violent energy release typically characterized as knock[endnoteRef:3]. Further, SI engines suffer from poor part load efficiency due to large losses during gas exchange and low combustion/ thermodynamic efficiency. These shortcomings are overcome in the HCCI engine. References to comparison of fuel efficiency in the paper from Lund University[endnoteRef:4] state an improvement of fuel efficiency from 15% to 30% at 1.5 bar BMEP. This is an improvement of 100%, equivalent to a reduction of fuel consumption of 50%. Additionally, comparison between HCCI, normal SI and Direct Injected SI (DISI) concepts in the same paper show a much higher fuel consumption benefit for HCCI than for DISI concepts. [3: Chia-Jui Chiang and Anna G. Stefanopoulou, Stability Analysis in Homogeneous Charge Compression Ignition (HCCI) Engines with High Dilution, Senior Member, IEEE. IEEE Transactions on Control Systems Technology, Vol. 15, No. 2, March 2007. Sourced from the Internet at http//www.ieeexplore.ieee.org/iel5/87/4105928/04105934.pdf on 29 Aug 13.] [4: Professor Bengt Johansson, Homogeneous Charge Compression Ignition the future of IC engines?, Lund Institute of Technology, Lund University, Sweden. Sourced from the Internet at http://www.osd.org.tr/6.pdf on 21 Aug 13.]

6. HCCI and CI Engines. The CI engine is much more fuel efficient than the SI engine, and hence the natural choice in applications where fuel cost is more important than first cost. The problem with the CI engine is the emission of Nitrogen Oxides (NOx) and Particulate Matter (PM) in exhaust. After-treatment to reduce NOx and Particulates is expensive and still not common. The average CI engine normally has a trade-off between Particulates and NOx. If the engine operates at conditions with higher in-cylinder peak temperature, the oxidisation of soot will be good but the thermal production of NO will increase. If, on the other hand, the engine is operated with lower temperature, NOx can be suppressed but PM will be high due to bad oxidation. Clearly, the CI engine must use exhaust after-treatment of NOx and/ or PM. In the CI engine, NOx is formed in the very hot zones with close to stoichiometric conditions and the soot is formed in the fuel rich spray core. The in-cylinder average air/ fuel ratio is always lean but the combustion process is not. This means that there exists a potential to reduce emissions of NOx and PM by simply mixing fuel and air before combustion. Such a process is applied in HCCI, which results in exhaust NOx emission less than 1/500 of the CI level and no PM generation. The HCCI engine operates with high compression ratio and lean mixtures giving CI engine equivalent fuel consumption or better. Due to premixed charge without rich or stoichiometric zones, the production of soot and NOx is minimal.

HCCI Overview

7. The HCCI engine integrates the advantages of both the CI and the SI engines, as follows:-

(a) High fuel efficiency through high compression ratio and rapid heat release.

(b) Low Nitrogen Oxides (NOx) and low Particulate Matter (PM) emissions due to low cylinder peak temperature (below 1700 K).

8. HCCI combustion has the potential to be highly efficient and to produce low emissions. HCCI engines can have efficiencies as high as Compression Ignition, Direct-Injection (CIDI) engines (an advanced version of the commonly known diesel engine), while producing ultra-low oxides of nitrogen (NOx) and Particulate Matter (PM) emissions. HCCI engines can operate on gasoline, diesel fuel, and most alternative fuels. While HCCI has been demonstrated and known for quite some time, only the recent advent of electronic sensors and controls has made HCCI engines a potential practical reality.

9. HCCI represents the next major step beyond high efficiency CIDI and Spark Ignition, Direct Injection (SIDI) engines. While a well-mixed charge minimises particulate emissions, compression ignition without throttling losses leads to high efficiency. A lower pressure fuel-injection system owing to pre-mixed charge and simpler emission control systems for HCCI engines also has the potential to reduce cost and complexity. HCCI technology could be scaled to virtually every size-class of transportation engines from small motorcycle to large ship engines. HCCI is also applicable to piston engines used outside the transportation sector such as those used for electrical power generation and pipeline pumping. With successful R&D, commercialised HCCI engines have the potential to save as much as a half-million barrels of primary oil per day by 2015[endnoteRef:5]. [5: Homogeneous Charge Compression Ignition (HCCI) Technology. A Report to the US Congress, US Department of Energy, Energy Efficiency and Renewable Energy, Office of Transportation Technologies. 2001.]

Requirements for HCCI

10. The HCCI combustion process puts two major requirements on the conditions in the cylinder:-

(a) The temperature after compression stroke should equal the auto-ignition temperature of the fuel/air mixture.

(b) The mixture should be lean enough to give reasonable burn rate, without causing uncontrollable combustion and corresponding rapid temperature rise.

11. Thus, HCCI is governed by three temperatures. We need to reach the auto-ignition temperature to get things started; the combustion should then increase the temperature so as to achieve have good combustion efficiency (typically around 1400 K) but it should not be increased to more than 1800 K to prevent NO formation.

Challenges for HCCI

12. An on-going developmental problem with HCCI engines is controlling the combustion process. In traditional SI, combustion timing is easily adjusted by the engine management control module changing the spark event and perhaps fuel delivery. However, it is not so easy with HCCI's flameless combustion. Combustion chamber temperature and mixture composition must be tightly controlled within quickly changing and very narrow thresholds that include parameters such as cylinder pressure, engine load/ RPM/ throttle position, ambient air temperature and atmospheric pressure changes. Most of these are achieved by advanced control mechanisms consisting of sensors performing automated adjustments to otherwise normally fixed actions. The most common sensors and actuating mechanisms used are individual cylinder pressure sensors, variable hydraulic valve lift and electromechanical phasers for camshaft timing. The challenge for the engine developer and manufacturer lies in getting these complex systems to work synchronously in critically time-constrained environments, sustain years of wear n tear, whilst keeping these advanced control systems affordable for mass usage.

13. Controlling the operation of an HCCI engine over a wide range of speeds and loads is probably the most difficult hurdle facing HCCI engines. HCCI ignition is determined by the charge mixture composition, its time-temperature history, and to a lesser extent pressure. Several potential control methods have been proposed to control HCCI combustion - varying the amount of Exhaust Gas Recirculation (EGR), using a Variable Compression Ratio (VCR), and using Variable Valve Timing (VVT) to change the effective compression ratio and/ or the amount of hot exhaust gases retained in the cylinder. VCR and VVT technologies are particularly attractive because their time response could be made sufficiently fast to handle rapid accelerations/ decelerations. Although these technologies have shown strong potential, performance is not yet fully proven, and cost and reliability issues have be addressed.

HCCI Variant - Reactivity Controlled Compression Ignition (RCCI) [endnoteRef:6] [6: Reactivity Controlled Compression Ignition (RCCI), Wisconsin Engine Research Consultants. Sourced from the Internet at http://www.w-erc/services/rcci on 27 Aug 13. ]

14. The Reactivity Controlled Compression Ignition (RCCI) engine, developed at the University of Wisconsin-MadisonEngine Research Centre laboratories, is a HCCI variant that uses dual fuel engine combustion technology. It uses in-cylinder fuel blending with at least two fuels of different reactivity and multiple injections to control in-cylinder fuel reactivity to optimise combustion phasing, duration and magnitude. The process involves introduction of a low reactivity fuel into the cylinder to create a well-mixed charge of low reactivity fuel, air and re-circulated exhaust gases. The high reactivity fuel is injected before ignition of the premixed fuel, using single or multiple injections, directly into the combustion chamber. Examples of fuel pairings for RCCI are gasoline and diesel mixtures, ethanol and diesel, and gasoline and gasoline with small additions of a Cetane-number booster (Di-Tert-Butyl Peroxide (DTBP)).

15. Appropriately selection of the reactivities of the constituent fuel charges may lead to the desired level of control over the process of combustion, which can be tailored to achieve optimal power output (fuel efficiency), at controlled temperatures (controlling NOx) with controlled equivalence ratios (controlling soot).

SPLIT CYCLE ENGINE TECHNOLOGY [endnoteRef:7] [7: A case for split-cycle engine technology, Scuderi Engine Technology and Scuderi Group homepage, http://www.scuderigroup.com/technology. Sourced from the Internet on 31 Aug 13.]

16. Efficient IC Engines. The three elements that drive IC engine efficiency are as follows:-

(a) High charge compression ratio.

(b) Engine operating regimes at wide open throttle (no throttling).

(c) Miller Cycle over expansion. 17. IC Efficiency and SI Engines. SI (Petrol/ Gasoline) engines have two features that prevent them from operating with no throttle and high compression ratios. Firstly, a SI engine must maintain a specific fuel - air ratio, known as the stoichiometric ratio. The engines load and speed is controlled by the amount of air the engine pulls in during its intake stroke. Since the air flow into the engine must be controlled to maintain the stoichiometric fuel - air ratio, the engine must be throttled when operating at part load conditions. Secondly, high compression ratio of the fuel air mixture (charge) results in premature detonation, characterised by knock. Although modern SI engines have found ways to improve performance by utilising variable valves to reduce the throttling losses and direct fuel injection (SIDI, as mentioned earlier in the paper) to obtain higher compression ratios, diesel engines still maintain a significant advantage on efficiency, especially when running at part load. Application of Miller over expansion in SI engines is also limited to such variable valve prototypes, and even then only for part load operation operations.

18. IC Efficiency and CI Engines. Conventional CI (Diesel) engines are able to achieve no throttling and high compression ratios. Diesel engines are able to run at wide open throttle because their combustion process does not depend on a specific fuel - air ratio. In addition, diesel engines compress only air, introducing the fuel only when the piston is close to Top Dead Centre (TDC). They do not have a premature detonation issue the way SI engines do. As a result, they can reach much higher compression ratios. The combination of no throttling and high compression ratio enables diesel engines to obtain higher efficiencies than SI engines, especially at part load operating conditions. However, the Miller Cycle has only been applied to diesel engines on a limited basis, since the reduced temperatures resulting from the over expansion tends to cause increased smoke emissions.

Whats a Split-Cycle Engine?

19. Split-cycle engines separate the four strokes of intake, compression, power, and exhaust into two separate but paired cylinders. The first cylinder is used for intake and compression. The compressed air is then transferred through a crossover passage from the compression cylinder into the second cylinder, where combustion and exhaust occur. Thus, a split-cycle engine is really an air compressor on one side with a combustion chamber on the other.

20. Split-cycle engines IC engine designs have thermodynamic advantages over conventional 4-stroke engines[endnoteRef:8], as they allow for dissimilar compression and expansion strokes, leading to significant reduction in exhaust and cooling system losses. [8: Split the Cycle to Crack the ICE, Automotive World. Sourced from the Internet at http://www.automotive world.com/analysis/powertrain/87437-split-the-cycle-to-crack-the-ICE on 28 Aug 13.]

Fig 1 - Split Cycle Engine

Earlier Split-Cycle Engines

21. Split-cycle engines appeared as early as 1914. Many different split-cycle configurations have since been developed. However, none has matched the efficiency or performance of conventional IC engines. Earlier split-cycle engines have had two major problems poor breathing (volumetric efficiency) and low thermal efficiency.

22. Breathing (Volumetric Efficiency). The breathing problem was caused by high-pressure gas trapped in the compression cylinder. This trapped high pressure gas needed to re-expand before another charge of air could be drawn into the compression cylinder, effectively reducing the engines capacity to pump air and resulting in poor volumetric efficiency.

Fig 2 Poor Volumetric Efficiency

23. Low Thermal Efficiency. The thermal efficiency of earlier split-cycle engines has always been significantly worse because they all tried to fire like a conventional engine - Before Top Dead Centre (BTDC). In order to fire BTDC in a split-cycle engine, the compressed air, trapped in the crossover passage, is allowed to expand into the power cylinder as the power piston is in its upward stroke. By releasing the pressure of the compressed air, the work done on the air in the compression cylinder is lost. The power piston then recompresses the air in order to fire BTDC. By allowing the compressed gas in the transfer passage to expand into the power cylinder, the engine needs to perform the work of compression twice, resulting in poor thermal efficiency.

Recent Developments - The Scuderi Split-Cycle Engine [endnoteRef:9] [9: A case for split-cycle engine technology, Scuderi Engine Technology and Scuderi Group homepage, http://www.scuderigroup.com/technology. Sourced from the Internet on 31 Aug 13.]

24. A recently developed Scuderi Split-Cycle engine solves both the breathing and thermal efficiency problems with two unique and patented concepts:-

(a) Unique Valve Design. On the compression side of the Scuderi Engine, the breathing problem is solved by reducing the clearance between the piston and the cylinder head to less than 1 mm. This design requires the use of outwardly opening valves that enable the piston to move very close to the cylinder head without the interference of the valves. This effectively pushes almost 100% of the compressed air from the compression cylinder into the crossover passage, eliminating the breathing problems associated with previous split-cycle engines.

(b) Firing After Top Dead Centre (ATDC). The Scuderi split-cycle engine fires ATDC, thus eliminates the losses created by recompressing the gas. Firing ATDC is accomplished by using a combination of high pressure air in the transfer passage and high turbulence in the power cylinder. Because the cylinders in a Scuderi Split-Cycle Engine are independent from each other, the compression ratio in the compression cylinder is not limited by the combustion process. A compression ratio in the order of 75:1 is obtained, with pressure in the compression cylinder equal to that of a conventional engine during combustion. The pressure in the compression cylinder and the crossover passage reach more than 50 bar (725 psi) on the naturally aspirated (NA) engine and more than 130 bar (1885 psi) on the Turbo Charged (TC) engine. This high-pressure air entering the power cylinder creates massive turbulence. The turbulence is further enhanced by keeping the valves open as long as possible during combustion. The result is very rapid atomisation of the air/ fuel mixture, creating a fast flame speed or combustion rate faster than any previously obtained. The combination of high starting pressure and fast flame speed enables combustion to start between 11 and 15 degrees ATDC and end 23 degrees after ignition. The result is a split-cycle engine with better efficiency and greater performance than a conventional engine.

25. Claimed Advantages. According to Scuderi Group, tests indicate that the Scuderi engine shows gains in efficiency and reduced emissions over conventional four-stroke Otto cycle designs. The company also says that the Scuderi engine could be used as part of an air hybrid system, allowing recovered braking energy to be stored as compressed air. Laboratory tests of the prototype are said to match earlier predictions generated by computer models.

26. Patents. As of August 2011, Scuderi Group has claimed that its patent portfolio included more than 476 patent applications worldwide, more than 154 of which have issued as patents in more than 50 countries.

27. Partners. The Scuderi Group has partnered with several automotive engineering companies to assist with engineering the Scuderi engine's complementary components. German automotive supply company Mahle GmbH is working on the pistons, Swedish engine developer Cargine Engineering AB is assisting with the air-activated valves, Denver-based Gates Corporation is engineering the belts, and Germany-based Schaeffler Group is contributing to the valve train assembly. The engineering division of Germany's Robert Bosch GmbH is working on the timing mechanism of the engine. Various manufacturers, including Honda, Daimler AG, Fiat, and PSA Peugeot Citron, have signed non-disclosure agreements with Scuderi Group for business partnership.

28. Promise of the Scuderi Split-Cycle Engine. With the revolution of the Scuderi Split-Cycle Engine firing ATDC and its evolution into the various configurations of naturally aspirated, turbocharged, air-hybrid and diesel designs, the Scuderi Split-Cycle Engine Technology provides a simple but elegant solution to meet futuristic engine demands for increased efficiency, improved performance and lower emissions.

Conclusion

29. The HCCI and the Split-Cycle Engine technology together may usher a new era of IC engine technology - an era marked by unprecedented improvements to fuel efficiency and engine performance whilst offering reduced emissions. Indeed, proliferation of such futuristic engine technologies may soon lead to their induction into ocean going vessels - both the merchant marine as well as the navies of the world.