The Gasoline-Diesel Hybrid

September 11, 2007


General Motors recently invited journalists to test two prototype vehicles powered by HCCI, also known as “homogeneous charge compression ignition engines.” The HCCI engine is like a cross between a gasoline engine and a diesel engine—and can improve the fuel efficiency of a comparable gasoline engine by as much as 30 percent. For example, a Toyota Camry with a 4-cylinder HCCI engine could achieve highway gas mileage approaching 40 miles per gallon.

HCCI may not have a sexy name, but its potential to deliver substantial fuel savings is impressive. More importantly, the savings could come much sooner than other so-called “game-changing” technologies that receive a lot more press.

More Than Hot Air

In a gasoline engine, fuel and air are mixed together, compressed, and then ignited by a spark. In a diesel engine, only the air is compressed. Compressing the air causes it to heat up. When fuel is injected into the cylinder, the fuel combusts on contact with the hot air.

HCCI’s process begins like a conventional gasoline engine: fuel and air are mixed and compressed. But instead of lighting the mixture with a spark, the HCCI engine compresses the mixture to the point that it auto-ignites. In a conventional gasoline engine, auto-ignition is bad news since it can cause severe damage, but in an HCCI powerplant, the process is controlled and yields numerous benefits. HCCI deliver the efficiency of a diesel engine, but without generating the smog-inducing nitrogen oxides (NOx) that are inherent in diesel combustion. HCCI also can accommodate a range of fuels, including gasoline, diesel fuel, propane, E85, and natural gas.

Challenges Remain

HCCI faces a number of technical hurdles. One of the biggest challenges is the management of the combustion process over a diverse range of driving conditions. The HCCI process is fragile, and not yet tolerant of shocks such as cold starts or quick acceleration. To get around this limitation, companies like GM have built “hybrid” HCCI engines that use conventional spark-ignition combustion under some conditions, and HCCI under others. The result is a 15 percent increase in fuel economy in vehicles such as GM’s Saturn Aura HCCI concept vehicle. GM has yet to pair an HCCI engine with a hybrid-electric drivetrain, but the combination of the two technologies could increase gas mileage even further. In fact, some observers have suggested that an HCCI engine is perfect for a series-hybrid like the Chevrolet Volt, because a series-hybrid allows the engine to operate with little variation in engine speed and load.

Like many promising new technologies, HCCI has yet to make the leap from test track to production car. At least four major automakers are working on HCCI engines, and many are beginning to brand the technology—a sign that it is making its way toward mainstream vehicles. Unlike plug-in hybrids, electric cars, or fuel cell vehicles, HCCI represents a less radical shift in technology. But a 30 percent improvement in fuel economy is revolutionary, no matter how you get it.


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  • sean

    Good news. Too good.

  • quirky

    You could just reduce the weight of the car, no electric seats, no 10 speaker stereo, not opting for the dual zone aircon etc.

    But of course people are too lazy for that. Wait until you have to ride a push bike to work and then see hind sight set in.

    Read LATOC!!

  • Andreas

    Daimler(Chrysler) has a S-Class-Showcar (F700) with a Diesotto enginge on show at the IAA right now. Diesotto (Diesel and Otto-Engine, being Otto the name of the invetor of the ICE) seems to be the same technology than HCCI. This S-Class is announced to use 5.3l/100km. Pretty promising.

  • PW

    Like the beginning of the article says “General Motors recently invited journalists to test two prototype vehicles.” How many times have we heard similar statements, but it’s been obvious in the past that GM comes out with numerous prototype vehicles. They sit on the technology and keep producing the same old gas guzzling vehicles. They wonder why Toyota has overtaken them with extremely good sales of hybrid cars like the Prius.

  • Daniel

    I said in past post, I believe it from GM when and if it hits the show room floor.

  • Alex

    HCCI is very hard to accomplish in a piston engine. When you detonate fuel through the method of HCCI you create a powerful energy wave traveling at the speed of sound (around 700mph). Most gasoline engines can only handle explosions at 125mph. You can see why this technology has not been implemented earlier. The piston engine can not run at these speeds. The piston has to move at the speed of sound in a back and fourth motion several thousand times a minute in a small distance of only a few inches. No metal can handle these stresses. So in current HCCIs you now see they had to pollute the intake to dumb down the explosion, which reduces efficiency, severely. However, if you had an engine that has all of its moving parts going in one rotational direction even aluminum can survive. The quasiturbine engine is the only combustion engine that can generate the necessary compression and survive a true HCCI detonation, which is 50% more efficient than regular combustion and easily rivals the most advanced hybrid technology. You can find out more at the following website.

  • Hugh

    What happened to the GM electric car?

  • Max Reid

    I think GM is getting serious about alternatives and hope they will bring it to market.

    After all, 3 of their Minivans were dropped, SUV’s were lagging and now even pickups were going down.

    Today, the oil prices hit a record $79 / gallon. We dont know when this will impact gas prices.

  • Armand

    I don’t understand why automakers can’t make lighter cars. That would be the quickest and most efficient way to reduce fuel usage and emissions.

    I think the effort they are putting in engine development should also be put in chassis development, structural weight savings, and rigidity. Blaming safety requirements for weight gain is the biggest farce since…who knows what. Poor engineering is another name for it.

  • jaimi goodhand

    I think that hybrid cars are the way for the future and if enough people just stuck their hands in we would have a better future and live longer. Too many people don’t care about what will happen tomorrow but not me, i think people should really start helping science build us(people)a better future.

  • ashly osborne

    Who needs cars i mean come on, our ancestors didn’t need cars and they got around just fine in fact back in tho’s days people were never fat and they lived longer too. Cars are just destroying us, making us fat and killing us us too. Look at all the young teenagers dying coz of the crushes they have had because of cars.

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  • Joana

    I don’t understand why automakers can’t make lighter cars. That would be the quickest and most efficient way to reduce fuel usage and emissions.

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