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GM's new engine: a leap forward for fuel efficiency, a step back for plain English

With all the focus on hybrids, electric cars and other alternative fuels, the one thing that's taken as a given is the death of petrol-fuelled engine.

With all the focus on hybrids, electric cars and other alternative fuels, the one thing that's taken as a given is the death of petrol-fuelled engine, the common perception being that the Otto cycle has reached the zenith of its development and is due to be put on the shelf as part of history. But have we really wrung every last bit of efficiency possible out of the petrol-fueled piston engine?

Recently, a two-page press release caught my attention. It was a press release like any other, not so different from the multitudes that cross my desk each and every day. "GM takes new combustion technology out of the lab and onto the road" is hardly a head-turning headline. But two paragraphs into the release were four words that awoke my inner pocket protector: Homogeneous Charge Compression Ignition.

I know, I know, your heart is doing little flutters too. But, before I lose you in a discussion of thermodynamics and intake charge distribution, let me state that GM's contention that this is the "most awaited advanced combustion technology of the past 30 years" may well be spot on. It achieves that exalted status, says GM, by combining the fuel economy of a diesel engine with the superior emissions cleanliness and drivability of a petrol engine. Fuel economy is improved by as much as 15 per cent over conventional petrol engines, yet an HCCI engine requires very little exhaust treatment (ie, catalysts) and certainly none of the "chemical laboratory" after-treatment that diesels need for emissions requirements.

It does this by combining the basic principles of both diesel and gas engines. Essentially, the much-modified 2.2L Ecotec engine I tested in a Saturn Aura recently operates in traditional petrol and a new 'diesel-like' modes. In "SI" (spark ignition) mode, it's a regular Ecotec with spark plugs firing a direct-injected air-fuel mixture. The prototype's SI light is always on at idle and whenever the engine is under heavy load.

However, like a typical diesel engine, when the one-off Aura's "HCCI" light is on, the 2.2L needs no spark plugs. This offers all manner of advantages, says Vijay Ramappan, HCCI Calibration Engineer for GM. Because the fuel is igniting itself, the combustion inside the cylinder happens all at once. That might not sound like a big deal, but when you are initiating combustion using a spark plug, the farthest reaches of the combustion chamber have to wait for the explosion to propagate outward.

This delay, measured in milliseconds, is one of the reasons for the petrol engine's relative inefficiency compared with a diesel. On the other hand, unlike a diesel (but as with a conventional petrol engine), even in HCCI mode, the Saturn's fuel and air are premixed. In a diesel engine, injecting the fuel directly into the cylinder serves the purpose of the spark plug - to cause the torque-producing explosion at exactly the opportune time.

While this helps with minimising the HCCI's emissions, without a spark plug or the diesel's injection of fuel to time the ignition precisely, getting the entire process to occur at the precise for optimum combustion becomes a far trickier process than in either gas or diesel engine. Unlike a petrol engine, a diesel (and since they are so similar, an HCCI engine as well) needs heat - lots of it - to generate an explosion. A diesel engine uses an incredibly high compression ratio (often as high as 17:1, compared with a petrol-powered engine at 10:1) to get things inside the cylinder percolating. GM's HHCI engine has no such option, since such a high compression ratio would create detonation when operate in its normal spark ignition mode.

What to do? Well, according to Ramappan, the trick is to run a second camshaft that shuts the exhaust valve down early. Doing so traps a bunch of the exhaust gases in the cylinder. Trapping those incredibly hot exhaust gases sends the temperature inside the combustion chamber sky high, more than enough for the incoming fuel-air mixture to self-ignite and operate in HCCI's 'diesel-like' mode. An HCCI-equipped engine offers a 10 to 15 per cent increase in fuel economy with very little change in driving abilities. At idle, it sounds completely normal because it operates exactly like the same spark plug-ignited four-banger that powers so many GM vehicles. Acceleration is similar and, above about 90kph, the sound is also identical as the engine is still operating in SI mode.

But when cruising at a steady speed below 90 clicks, the engine quickly flicks to its fuel-saving HCCI mode, with just a slight hint of the more robust engine noise that anyone who's ever drive a diesel-powered car is familiar with. However, since the vehicle is moving, it's barely noticeable. Ramappan says that HCCI's entry into the marketplace is still some time away. Getting the right amount of exhaust gas in the cylinder while simultaneously injecting the precise amount of fuel is beyond the current onboard computers' abilities.

But know this; General Motors is a leader in this technology. The company seems once again committed to taking on imports head-on and getting to the forefront of automotive technological development. motoring@thenational.ae

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