Homogeneous Charge Compression Ignition (HCCI) Engines

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1 Homogeneous Charge Compression Ignition (HCCI) Engines Aravind. I. Garagad. Shri Dharmasthala Manjunatheshwara College of Engineering and Technology, Dharwad, Karnataka, India. ABSTRACT Large reductions in NOx emissions can be obtained by replacing Diesel or spark ignited combustion by HCCI combustion in reciprocating engines. Currently, HCCI combustion is limited to operating conditions with lean air/fuel ratios or large amounts of EGR. However, a numerical model shows that, even if high equivalence ratio HCCI operation were satisfactorily attained, the NOx reduction potential vs. DI-Diesel combustion would be much smaller. Thus, highload HCCI operation may best be obtained through highly boosted fuel-lean operation. Alternatively, HCCI combustion may be suited well for dual mode engine applications, in which spark ignition or conventional Diesel combustion is used to obtain full load. Avoiding wall impingement with heavy fuels is critical for achieving good emissions and fuel consumption, and it appears that a large degree of mixture in homogeneity can be tolerated from a NOx benefit standpoint. Keywords Need of HCCI HCCI Engine Concept Advantage & Challenges related to HCCI Future Aspects. INTRODUCTION HCCI is a form of internal combustion in which the fuel and air are compressed to the point of auto ignition. That means no spark is required to ignite the fuel/air mixture Creates the same amount of power as a traditional engine, but uses less fuel. A given concentration of fuel and air will spontaneously ignite when it reaches its auto-ignition temperature. The concentration/temperature can be controlled several ways: High compression ratio Preheating of induction gases Forced induction Retaining or reintroducing exhaust gases. Unlike conventional engines, the combustion occurs simultaneously throughout the volume rather than in a flame front. This important attribute of HCCI allows combustion to occur at much lower temperatures, dramatically reducing engine-out emissions of NOx. WORKING OF HCCI ENGINE Figure 1: Concept of HCCI engine. 170 Aravind. I. Garagad

2 Suction Stroke: Fuel air mixture intake take place. Figure 2: Suction stroke. Compression Stroke: Piston moves from bottom dead centre to top dead centre. Figure 3: Compression stroke. Combustion Stroke: Combustion of charge takes place. Figure 4: Combustion Stroke. Exhaust Stroke: Removal of exhaust gases takes place. Figure 5: Exhaust Stroke. 171 Aravind. I. Garagad

3 STARTING HCCI ENGINES Charge does not readily auto ignite cold engines. Early proposal was to start in SI mode and run in HCCI mode. It involves the risk of knocking and cylinder failure at high compression ratios. Now intake air pre-heating with HE and burner system allows startup in HCCI mode with conventional starter. ADVANTAGES Lower peak temperature leads to cleaner combustion/lower emissions High efficiency, no knock limit on compression ratio. Low Particulate Matter emissions, no need for Particulate Matter filter. HCCI provides up to a 15-percent fuel savings, while meeting current emissions standards. HCCI engines can operate on gasoline, diesel fuel, and most alternative fuels. In regards to CI engines, the omission of throttle losses improves HCCI efficiency. DISADVANTAGES. Higher cylinder peak pressures may damage the engine Auto-ignition is difficult to control HCCI Engines have a smaller power range. CHALLENGES. The auto-ignition event is difficult to control, unlike the ignition event in spark -ignition(si) and diesel engines which are controlled by spark plugs and in-cylinder fuel injectors, respectively. HCCI engines have a small power range, constrained at low loads by lean flammability limits and high loads by in-cylinder pressure restrictions. High HC and CO emissions. Control methods of HCCI combustion. The spontaneous and simultaneous combustion of fuel-air mixture need to be controlled. No direct control methods possible as in SI or CI engines. Various control methods are: Variable compression ratio: The geometric compression ratio can be changed with a movable plunger at the top of the cylinder head. This concept used in diesel model aircraft engine. Variable induction temperature: The simplest method uses resistance heater to vary inlet temperature. But this method is slow Now FTM (Fast Thermal Management) is used. It is accomplished by rapidly varying the cycle to cycle intake charge temperature by rapid mixing. Figure 6: Rapid mixing of cool and hot intake air takes place achieving optimal temperature as demanded and hence better control. 172 Aravind. I. Garagad

4 Variable valve actuation: This method gives finer control within combustion chamber Involves controlling the effective pressure ratio. It controls the point at which the intake valve closes. If the closure is after BDC, the effective volume and hence compression ratio changes. RECENT DEVELOPMENTS IN HCCI. Turbo charging initially proposed to increase power. Challenges for turbo charging: Exhaust gas temperatures low (300 to 350 c) because of high compression ratio. Post turbine exhaust gas temperature must be high enough to preheat intake fuel-air mixture in HE. Low available compressor pressure ratio. The exhaust has dual effects on HCCI combustion. It dilutes the fresh charge, delaying ignition and reducing the chemical energy and engine work. Reduce the CO and HC emissions. THE FUTURE OF HCCI. The future of HCCI looks promising Major companies such as General Motors, Mercedes-Benz, Honda, and Volkswagen have invested in HCCI research. Preliminary prototype figures show that HCCI cars can achieve in the area of 6.6 liters per 100km. HCCI prototypes. General Motors has demonstrated Opel Vectra and Saturn Aura with modified HCCI engines. 173 Aravind. I. Garagad Figure 7: Prototype HCCI car Saturn by General Motors. Mercedes-Benz has developed a prototype engine called Dies Otto, with controlled auto ignition. It was displayed in its F 700 concept car at the 2007 Frankfurt Auto Show. Volkswagen is developing two types of engine for HCCI operation. The first, called Combined Combustion System or CCS, is based on the VW Group 2.0-litre diesel engine but uses homogenous intake charge rather than traditional diesel injection. It requires the use of synthetic fuel to achieve maximum benefit. The second is called

5 Gasoline Compression Ignition or GCI; it uses HCCI when cruising and spark ignition when accelerating. Both engines have been demonstrated in Touran Prototypes, and the company expects them to be ready for production in about Figure 8: Touran Prototype by VW. 174 Aravind. I. Garagad