Homogeneous Charge Compression Ignition

What is HCCI?

As stated above, the acronym means Homogeneous Charge Compression Ignition. Yes, yes, but what does that mean? What does it do? An HCCI engine is a mix of both conventional spark-ignition and diesel compression ignition technology. The blending of these two designs offers diesel-like high efficiency without the difficult--and expensive--to deal with NOx and particulate matter emissions. In its most basic form, it simply means that fuel (gasoline or E85) is homogeneously (thoroughly and completely) mixed with air in the combustion chamber (very similar to a regular spark ignited gasoline engine), but with a very high proportion of air to fuel (lean mixture). As the engine's piston reaches its highest point (top dead center) on the compression stroke, the air/fuel mixture auto-ignites (spontaneously and completely combusts with no spark plug assist) from compression heat, much like a diesel engine. The result is the best of both worlds: low fuel usage and low emissions.

How Does HCCI Work?

In an HCCI engine (which is based on the four-stroke Otto cycle), fuel delivery control is of paramount importance in controlling the combustion process. On the intake stroke, fuel is injected into each cylinder's combustion chamber via fuel injectors mounted directly in the cylinder head. This is achieved independently from air induction which takes place through the intake plenum. By the end of the intake stroke, fuel and air have been fully introduced and mixed in the cylinder's combustion chamber.

As the piston begins to move back up during the compression stroke, heat begins to build in the combustion chamber. When the piston reaches the end of this stroke, sufficient heat has accumulated to cause the fuel/air mixture to spontaneously combust (no spark is necessary) and force the piston down for the power stroke. Unlike conventional spark engines (and even diesels), the combustion process is a lean, low temperature and flameless release of energy across the entire combustion chamber. The entire fuel mixture is burned simultaneously producing equivalent power, but using much less fuel and releasing far fewer emissions in the process.

At the end of the power stroke, the piston reverses direction again and initiates the exhaust stroke, but before all of the exhaust gases can be evacuated, the exhaust valves close early, trapping some of the latent combustion heat. This heat is preserved, and a small quantity of fuel is injected into the combustion chamber for a pre-charge (to help control combustion temperatures and emissions) before the next intake stroke begins.

Challenges for HCCI

An ongoing developmental problem with HCCI engines is controlling the combustion process. In traditional spark engines, combustion timing is easily adjusted by the engine management control module changing the spark event and perhaps fuel delivery. It's not nearly 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 and RPMs and throttle position, ambient air temperature extremes and atmospheric pressure changes. Most of these conditions are compensated for with sensors and automatic adjustments to otherwise normally fixed actions. Included are: individual cylinder pressure sensors, variable hydraulic valve lift and electromechanical phasers for camshaft timing. The trick isn't so much as getting these systems to work as it is getting them to work together, very quickly, 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 affordable.

Advantages of HCCI

• Lean combustion returns 15 percent increase in fuel efficiency over a conventional spark ignition engine.
• Cleaner combustion and lower emissions (especially NOx) than a conventional spark ignition engine.
• Compatible with gasoline as well as E85 (ethanol) fuel.
• Fuel is burned quicker and at lower temperatures, reducing heat energy loss compared to a conventional spark engine.
• Throttleless induction system eliminates frictional pumping losses incurred in traditional (throttle body) spark engines.

Disadvantages of HCCI

• High cylinder pressures require stronger (and more expensive) engine construction.
• More limited power range than a conventional spark engine.
• The many phases of combustion characteristics are difficult (and more expensive) to control.

It is clear that HCCI technology offers superior fuel efficiency and emissions control compared to the conventional tried-and-true spark ignition gasoline engine. What's not-so-certain yet is the ability of these engines to deliver these characteristics inexpensively, and, probably more importantly, reliably over the life of the vehicle. Continued advancements in electronic controls has brought HCCI to the precipice of workable reality, and further refinements will be necessary to push it over the edge into everyday production vehicles.