A comparison of a diesel engine (compression, left), petrol engine (spark, middle), and HCCI engine (compression, right). The HCCI engine is at least as efficient as diesel, and perhaps can be even more so.  (Source:

Another comparison, with traditional spark injection on the left, and the more efficient HCCI engine's compression burn on the right.  (Source: GM)

GM's new HCCI engine should deliver top-tier performance and fuel economy, and help lay to rest the diesel versus hybrid debate.  (Source: GM)

The 2010 Buick Lacrosse is among GM's many new models to feature gasoline direct injection. The HCCI engine technology, likewise, could soon be bringing even greater efficiency to the GM lineup.  (Source: AutoBlog)
GM hopes to finally lay to rest the diesel versus hybrid debate with its diesel-like new gas engine

There has been a reoccurring debate over the merits of diesel vehicles versus hybrid ones.  Both technologies help to increase fuel economy; however, people tend to be convinced that the one or the other is the only solution to these problems.

Now GM finally is preparing to lay these criticisms to rest with potentially the first compression-ignited gas engines to see rigorous real world testing, in preparation of market entry.

In order to understand why this is so significant, a discussion of engines is in order.  Diesel engines typically have two advantages over their gasoline burning counterparts.  The first is direct injection.  Traditional gas engines inject fuel into an intake, and the air-gas mix is then sucked into chamber, compressed, and finally ignited.  Diesel engines put it directly into the combustion chamber, a more efficient process.  This problem has already been tackled by Ford (EcoBoost), GM and many others, and direct-injection gas engines will soon be all over the market.

The second diesel advantage is compression ignition.  In a diesel engine, the compression of the gas/air mix triggers ignition, and the compression causes the combustion to be evenly distributed.  In a traditional gasoline engine, compression is limited by design so less-efficient spark ignition must be used.  This problem has yet to be solved in a production model.

GM's HCCI (homogenous charge compression ignition) four-cylinder engine unveiled at the company's research and development center this week may be the first engine to bring compression to the world of gas engines.  In early testing the engine deliver 15 percent better fuel economy and much lower NOx gas and CO2 emissions.

Uwe Grebe, GM's executive director of powertrain advanced engineering, says that the new engine greatly reduces these toxic gases.  He says that the technology can easily "be applied to many engines, including four- , six- and eight-cylinder engines."

The secret to the new engine is temperature control. Typically an engine runs at 3,800 degrees Fahrenheit, but the new engine runs from 0 to 60 mph and from idle to 3,000 rpm at 2,900 degrees Fahrenheit (ignition temperature).  This means that there are fewer pollutants, and compression can be used to ignite the fuel without risking damage to the engine.  Essentially this yields a gas-burning engine that runs as clean and efficiently as a diesel.

Vijay Ramappan, staff engineer for calibration, states, "From zero to 60, this engine can run completely in HCCI mode and basically work like a diesel."

GM is not revealing when the engine might see production deployments, or if it will utilize GM's direct injection gas technology to offer even greater benefits, a likely move.  Whenever the engine does see the market, though, it would make a dynamite partner to a hybrid powertrain, possibly delivering a gasoline-sipping engine with potential performance well north of 100 mpg in small cars.

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