2007 Saturn Aura / Opel Vectra HCCI Prototype

With all this talk about electric vehicles and hydrogen-powered cars, one

might assume that burning traditional fossil fuels like gasoline and diesel will come to an end sooner rather than later. This, of course, is pretty far from the truth; not only are we years and years away from filling up our cars with hydrogen at our local gas stations, but a concerted effort from most manufacturers is being made to improve the burning efficiency of fuels that are currently available. For instance, the progression of diesel technology over the last ten years is simply astounding; they've gone from crude, slow and smoky machines to being super-quiet and efficient.

But not every market has access to diesel fuels in the same capacity as mainland Europe, or have emissions standards that will allow for the proliferation of such technologies. North America is one such place where it's extremely difficult for diesel passenger cars to pass emissions (the Tier 2 Bin 5, specifically). It may be true that a number of carmakers have committed to clean diesel, such as BMW, Honda and Nissan, for our markets, but there's still a resounding focus on straight up gasoline.

The latest breakthrough regarding the gasoline-powered internal combustion engine is called the homogeneous charge compression engine (HCCI), which in short combines the benefits of a gasoline engine with those of a diesel. You might've heard a little bit about such a technology being that researchers from Mercedes-Benz (DiesOtto) and Volkswagen have been working on an engine of this type for quite some time now. The latest manufacturer to jump into the wonderful world of HCCI headfirst is General Motors. Not only have they just announced their version of this engine, but they've also fitted it to two working prototypes, which will make their official auto show debut this coming week in Frankfurt. Interestingly, the two cars they've fitted with the new HCCI engine are the Euro-only Opel Vectra, its platform-mate the North American-only Saturn Aura. Both vehicles are powered by 2.2-liter inline-four engines which are derived from the Ecotec lineup. HCCI does not affect the type of transmission used; the Vectra featured a 5-speed manual, while the Aura featured an automatic.

HCCI essentially combines the best of both worlds for gasoline and diesel engines. Diesel engines don't have spark plugs like gasoline engines; the combustion of fuel is done via extra high compression and temperatures where the fuel ignites by itself. The HCCI engine operates like this, but it uses gasoline or any blend of ethanol-enhanced gasoline (such as E85).

It's much more than just a diesel engine that runs on gasoline, mind you, as some pretty major changes have been made to the fuel system in order to make its claimed efficiency possible. For instance, the HCCI engine has a much lower compression ratio than a diesel, and it also burns at a much lower temperature. Technologies such as the latest direct fuel injection systems and variable valve timing allow the fuel to be atomized in such a precise yet accurate manner that facilitates successful burning in the HCCI's combustion cycle. Also, because the type of gasoline quality as well as the atmospheric conditions are critical to the combustion of fuel in an HCCI, special sensors are needed, which allow the valve's timing to be changed in real time.

By adapting what's essentially the diesel cycle, GM's engineers have been able to increase the efficiency of the engine by some 15-percent over the usual four-cycle gasoline engine of equivalent size. However, the really impressive part about HCCI is that unlike diesels, it doesn't have trouble meeting NOx emissions standards. Because combustion occurs at a much lower temperature than in a diesel engine, this type of pollutant isn't formed. This means that there's no expensive post-combustion treatment like AdBlue or any fancy particulate filter, a cost-related factor and the main stumbling block for manufacturers trying to introduce such technologies into mainstream cars.

Like a diesel engine, HCCI works best under constant loads that aren't full throttle (do we sense the use of HCCI in the Volt, or is that just us?), and will operate from a standstill to about 55 mph on the tester vehicles that GM made. As you know, not everyone is willing to creep up to speed, or will keep under 55 mph. For those times, the

car switches back into a regular, four-stroke gasoline engine power. We've yet to drive a car with this technology, but from what we understand it's more or less seamless with no differences in noise levels or vibrations.

There haven't been any established dates for the introduction of an HCCI-powered vehicle, but there's a great deal of interest in the idea. What's nice is that this technology wouldn't require that much of a leap forward because the elements that would make it work are already available today. Currently, development and reliability issues hold the introduction of such engines back. If for instance the sensors are off, the engine could stall, pre-ignite or worse, damage itself. GM needs to get the sensors right on the HCCI and calibrate such issues as switchover between regular gasoline and HCCI modes. That might take some time, but rest assured that it's a project in good progress. Expect to see engines like these on the streets by the end of the decade.