General Motors first showed off its prototype Homogeneous Charge Compression Ignition (HCCI) engines last summer in an Opel Vectra and a Saturn Aura. We had the opportunity to drive these vehicles at GM's Milford Proving Ground at a very early stage of development. Because HCCI only works at part throttle conditions (while engines in cars have to work under all conditions), these new power plants have the ability to switch modes on the fly between HCCI and spark ignition. When we drove the cars, the HCCI only operated when driving at up to 55mph. At higher speeds or loads or when idling, the engine was in normal spark ignition mode.

GM's researchers continued developing the HCCI engine over the intervening 10 months and it is now able to operate in HCCI mode at idle as well. They demonstrated the HCCI Aura to journalists in California earlier this week and, according to Mike Levine of PickupTrucks.com, it's making good progress. The basic hardware to make HCCI work exists with direct injection, variable valve timing and pressure and temperature sensors in the combustion chamber. The key now is to develop the control algorithms to manage the sparkless combustion. The beauty of HCCI is the potential to improve fuel consumption to almost diesel levels without the expensive aftertreatment systems. HCCI engines can also operate on conventional gasoline or even ethanol.

[Source: General Motors]

Last summer we had the chance to meet up with GM powertrain researcher Paul Najt and drive GM's prototype HCCI engines. Homegeneous charge compression ignition engines basically take some of the technical concepts behind diesel engines and combine it with a gasoline engine. The result is a new power system that achieves most of the benefit of diesel efficiency without the problems of soot and NOx emissions. As features like variable valve timing, direct fuel injection, cylinder pressure sensors and more powerful control systems have become available in engines, HCCI has become more practical. HCCI doesn't work well under all operating conditions, but engineers are learning how to blend HCCI and normal spark ignition operation seamlessly. The guys at Popular Mechanics had a chance to visit the GM research lab where the HCCI engines are being developed. There is a good interview with Najt and some explanation of how the engines work in a video on their site.


[Source: Popular Mechanics]

Click on the image to view high-res shots of the Mercedes F700 concept

Homogeneous Charge Compression Ignition, or HCCI, holds out a great deal of hope for green automotive enthusiasts. Mercedes-Benz has shown concept vehicles using the technology, which they refer to as DiesOtto, in their F700 concept vehicle pictured above. Like diesels, HCCI engines eschew spark-plugs in favor of compression ignition. Unlike diesels, HCCI burns gasoline, which could make them a popular choice in countries like the U.S. where gasoline is more widely available than diesel (and, currently in most areas, cheaper). HCCI engines could potentially allow for the emissions levels of gasoline with the efficiency of diesel and could be cheaper to produce than a comparably-powered diesel engine.

Over the hump to Number 5.

Click on the HCCI equipped Opel Vectra for a high-res gallery

At the General Motors Proving Ground in Milford, MI on Friday, the company provided their first public presentation of HCCI engines. Most of the major automakers, in addition to researchers at universities around the world have been experimenting with HCCI for much of the last three decades and now it's finally driveable. As part of their 2007 Powertrain technology show, GM brought a Saturn Aura and an Opel Vectra, each equipped with a 2.2L HCCI engine.

Dr. Uwe Grebe gave the assembled media an introduction to HCCI before we got to drive. Ever since Karl Benz's first MotorWagen in 1886, automotive engines have been dominated by the four-stroke cycle which consists of intake, compression, power and exhaust. All four-strokes operate on the same basic principals of opening a valve and drawing in the intake mix as the piston descends and compressing that mixture as it goes back up. The air fuel mixture is then ignited which pushes the piston down as the mixture burns and expands and then the exhaust valve opens as piston goes up and pushes out the exhaust. The ignition part is one of places where things vary.

Read on after the jump to learn how HCCI fits into the process.