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A Homogeneous Charge Compression Ignition (HCCI) engine is a combination of both diesel compression ignition and conventional spark-ignition technology. The mixing of these two designs provides diesel-like high efficiency without the expensive and difficult to deal with Nitrogen oxide (NOx) as well as particulate matter emissions. In its most basic form, it simply means that fuel (E85 or gasoline) is homogeneously combined with air in the combustion chamber, but with a very high amount of air than fuel. As the piston of the engine reaches its highest point, which is at the top dead center, on the compression stroke, the fuel or air mixture auto-ignites spontaneously and completely burns from compression heat, similar to a diesel engine. The result is the best of both worlds: low emissions as well as low fuel usage (Singh & Paswan, 2014).
How Does Homogeneous Charge Compression Ignition (HCCI) Work?
In a Homogeneous Charge Compression Ignition (HCCI) engine, which is based on the four-stroke Otto cycle, the fuel delivery control is very important in controlling the combustion process. On the intake stroke, fuel is added into the combustion chamber of each cylinder by the use of fuel injectors attached directly in the cylinder head. This is accomplished independently from air induction which takes place via the intake plenum. Towards the end of the intake stroke, air and fuel have been fully introduced and combined inside the cylinder’s combustion chamber.
Once the piston begins to move back up during the compression stroke, heat begins to build inside the combustion chamber. Once the piston reaches the end of this stroke, enough heat has collected to cause the air or fuel mixture to spontaneously combust and force the piston down for the power stroke. The combustion process is a lean, flameless, and low temperature release of energy across the whole combustion chamber unlike conventional spark engines. The whole fuel mixture is burned at the same time producing equivalent power, however it utilizes much less fuel and releases far fewer emissions in the process (Christine & Scott, 2017).
Towards the end of the power stroke, the piston reverses direction over again and starts the exhaust stroke, however before all of the exhaust gases can be evacuated, the exhaust valves close early, trapping some of the latent combustion heat. This heat is well-kept, and a small quantity of fuel is inserted into the combustion chamber for a pre-charge to assist control combustion temperatures and emissions before the next intake stroke starts.
Challenges for Homogeneous Charge Compression Ignition (HCCI)
A current developmental problem with Homogeneous Charge Compression Ignition (HCCI) engines is being able to control the combustion process (Collin, 2016). Combustion timing in traditional spark engines is adjusted without difficulty by the engine management control module altering the spark event and possibly fuel delivery, however, this is not nearly so easy when it comes to HCCI’s flameless combustion. The combustion chamber temperature along with the mixture composition needs to be tightly controlled within quickly fluctuating and very narrow thresholds that consist of parameters such as engine load and RPMs and throttle position, cylinder pressure, ambient air temperature extremes as well as atmospheric pressure changes (Tarun, 2011).
Many of these conditions are compensated for with sensors as well as automatic adjustments to otherwise normally fixed actions. Encompassed are: individual cylinder pressure sensors, variable hydraulic valve lift and electromechanical phasors for camshaft timing. The getting these systems to work isn’t the trick, the trick is getting them to work together, very fast, and over many thousands of miles and years of wear and tear. Perhaps just as challenging though will be the problem of keeping these advanced control systems reasonably priced.
Advantages of Homogeneous Charge Compression Ignition (HCCI)
- HCCI is compatible with gasoline and E85 (ethanol) fuel.
- Lean combustion returns fifteen percent increase in fuel efficiency more than a conventional spark ignition engine.
- It has cleaner combustion as well as lower emissions, especially Nitrogen Oxide, compared to a conventional spark ignition engine.
- In HCCI, the fuel is burned much quicker and at lower temperatures thus reducing heat energy loss compared to a conventional spark engine.
- Throttle less induction system in HCCI removes frictional pumping losses experienced in traditional spark engines which have a throttle body.
Disadvantages of Homogeneous Charge Compression Ignition (HCCI)
- More limited power range compared to a conventional spark engine.
- High cylinder pressures need stronger and more expensive engine construction.
- The many phases of combustion characteristics are more expensive and difficult to control.
- It is clear that HCCI technology provides superior fuel efficiency and emissions control compared to the conventional tried and true spark ignition gasoline engine nonetheless modifications have to be done to implement at present (Kumar, & Sooraj, 2013).
Recent developments in the Homogeneous Charge Compression Ignition (HCCI) technology have given very positive results to overcome the shortcomings of this technology. The technology has huge scope of use and it is used in a wide range of industries, which makes it a promising technology for the coming generations. Automobile giants such as GM, Ford as well as Cummins have been exploring the possibilities in the HCCI technology for more than fifteen years. General Motors has begun educational programs in different Universities to support the research work in this technology. HCCI has also enabled engineers to experiment with different blends of fuel mixture so that performance and efficiency of HCCI engines can be tested with different combinations of non-conventional fuels. As the demand of conventional fuels is increasing, scope of research and experimentation in HCCI technology will increase only with time. Of late, many companies have launched HCCI based automobiles in the market among which Mercedes F700, Volkswagen Touran, Opel Vectra, and Saturn Aura the prominent names.
Homogeneous Charge Compression Ignition engines have the potential to match or surpass the effectiveness of diesel fueled on Compression Ignition Direct Injection (CIDI) engines without the major challenge of controlling Nitrogen Oxide (NOx) and Promethium (PM) emission or a major impact on fuel refining capability. Furthermore, HCCI engines would most likely cost less than Compression Ignition Direct Injection (CIDI) engines for the reason that Homogeneous Charge Compression Ignition (HCCI) engines would probably utilize lower pressure fuel injection equipment, in addition, the combustion characteristics of Homogeneous Charge Compression Ignition (HCCI) would possibly allow the use of emission control devices that rely less on scarce and costly precious metals. Moreover, for heavy duty vehicles, successful development of the diesel fueled Homogeneous Charge Compression Ignition (HCCI) engine is an alternative strategy in the event that Compression Ignition Direct Injection (CIDI) engines cannot achieve future Nitrogen Oxide (NOx) and Promethium (PM)emissions standards.
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- Colin, W. (2016). This New Type of Engine Combines the Best of Both Gasoline and Diesel.
- Kumar, P., & Sooraj S. (2013). Homogeneous Charge Compression Engine.
- Singh, A., & Paswan, M. (2014). Homogeneous Charge Compression Ignition (HCCI) Engine.
- Tarun, G. (2011). Everything You Should Know About Homogeneous Charge Engines.