The Homogeneous-Charge Compression Ignition (HCCI) engine has a long and spotty history, with the earliest designs being produced towards the end of the 19th century. But, due to its shortcomings at the time, HCCI never caught on like spark ignition or diesel injection engines and was mostly forgotten except for in the wet dreams of many mechanical engineers.

With regulations on internal combustion engines becomming more strict every year, HCCI’s high level of efficiency in both fuel economy and tailpipe emissions has attracted many manufacturers in recent years to begin investing in the research and possible development of modern production versions of these machines. Jason Fenske of Engineering Explained posted the great video above, which breaks down the basic theory behind Homogeneous-Charge Compression Ignition and its possible future on a production scale.

While several manufacturers have patents on hybrid versions that combine HCCI and spark ignition technology, Fenske is using data from one of the manufacturers that has been heavily researching an independent HCCI platform, Nissan.

A simulation of Nissan’s HCCI design. The color purple represents low temperature oxidation, dark blue is autoignition, and red is high temperature oxidation.

What Is Homogeneous-Charge Compression Ignition?

Before getting into the more technical aspects of HCCI, it’s important to understand the basic operation of both diesel and spark ignition engines. Check out our article taking a closer look at four of the major differences between a spark ignition engine and diesel powerplant.

Engineers often describe HCCI as having the highest heat efficiency of any internal combustion engine design. To start the combustion event, HCCI relies on a higher compression ratio that is similar to those found in diesel engines, but instead uses lean homogeneous injection of air and gasoline fed into the combustion chamber on the intake stroke.

The major advantage that HCCI has over diesel injection or spark ignition is its lack of an ignition point within the combustion chamber, such as an injector or spark plug. A spark plug or fuel injector creates a source of extremely high heat within the combustion chamber that can reach temperatures in excess of 4,000 degrees fahrenheit.

Graph showing the reduction in thermal emissions production when using HCCI, in comparison to both spark ignition and low temperature combustion (LTC) diesel engines. An equivalence ratio higher than one means that the air/fuel mixture is lean, below means the mix is rich. (Temperature is in kelvin)

HCCI instead ignites the entire volume of the air/fuel mixture simultaneously using only compression to reach its autoignition point. Removing any localized ignition source from within the combustion chamber greatly reduces temperatures within the cylinder, which in turn eliminates thermal NO_x production almost entirely.

 The Problem

The major challenge standing in the way of a production HCCI engine for may years has been its extremely narrow combustion window. By using homogeneous injection without a fuel injector or spark plug controlling the timing of the combustion events, we have to rely on accurately controlling cylinder temperatures to keep it within this window; too hot and you will run into severe detonation, too cool and the engine will misfire heavily.

However, with valvetrain technology such as variable valve timing (VVT) and variable valve lift (VVL) becomming more advanced and more affordable in recent years, we are now able to control how long the intake valve stays opened as a way of dynamically controlling the compression ratio to advance or delay the combustion event. Engineers found that when combining these more advanced forms of VVT and VVL with exhaust gas recirculation (EGR), they could control cylinder pressure and temperature fairly easily.

While manufacturers working on an independent HCCI platform are well into their research phase, we will still have to wait at least another five to ten years before we begin to see what kind of engine platforms are produced using this technology, unless the price of electric cars is reduced by more than half in that time. Since combustion is achieved using only compression, we could theoretically even start seeing large displacement four cylinder and six cylinder engines replacing V8’s in performance cars as a way of reducing parts and manufacturing costs.

Homogeneous-Charge Compression Ignition is one of the leading contenders to be the future of the internal combustion engine for both heavy equipment and production cars. With government regulators breathing down the necks of auto manufacturers, finding a way to benefit from HCCI’s vastly superior efficiency over both spark ignition and diesel injection engines would be the saving grace of the engine as we know it.