Something new to the internal combustion engine world is HCCI — Homogeneous Charge Compression Ignition, yet to be seen in any production vehicle by any manufacturer. A few car manufacturers have been experimenting with this, among them Honda who wants to introduce HCCI with it’s IMA Hybrid system.
What exactly is HCCI? Basically. Homogeneous Charge Compression Ignition ignites an air fuel mixture without any flame, plus a large area can be ignited simultaneously, much different from a conventional spark flame burn where the mixture burns first from the area around the initial spark and proceeds to the rest of the combustion chamber. This is somewhat similiar to a diesel engine’s self-ignition. Diesel self-ignition is called SCCI, or Stratified Charge Compression Ignition.
Engineers have predicted a potential 20 to 30% boost in engine efficiency over conventional gasoline combustion engines, and an improved thermal efficiency of about 40% to 50%. Honda calls it’s HCCI technology Activated Radical Combustion, and have been experimenting with two-stroke motorcycle engines. A 4-cylinder HCCI engine for cars is also in development, though currently progress has enabled HCCI on the lower RPM ranges of about 0 to 4,000rpm only. However, during normal non-spirited driving the rev rarely goes above 4,000rpm anyway.
A key problem is how to measure ignition timing accurately. This is not a problem in diesel self-ignition, as hot air is compressed and ignition starts when fuel is injected into the combustion chamber filled with the hot air. In a HCCI gasoline engine, the air-fuel mix is already in the combustion chamber before the piston starts compressing it. According to Lawrence Livermore Laboratory, some ways to ovecome this would be to recirculate exhaust gases into the fuel/air mix to quickly raise its temperature, or to add dimethyl ether to the fuel/air mixture to improve combustion.
Another idea would be to compress the air-fuel mixture to the point where it is just about to ignite, then injecting a second dosage of low octane fuel that combusts easily. That fuel would detonate, increasing temperature and detonating the rest of the air fuel mixture. The ignition timing could start from whenever the engine injects that second dosage of fuel.
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At last a petrol engine without spark plugs.. Wow.. but i can predict this engine will have more torque but less hp.. since it can only have 4,000rpm only… anyways this new engine can help improve the fuel consumption on city driving and cruising.. Cheers!
I'm probably not understanding it correctly, but referring to the last paragraph, if we introduce a second fuel of lower octane, that would then disqualify this process from being completely HCCI, as the lower octane fuel will have to burn first, then propagate the flame front to the rest of the air-fuel mixture. If that's the case, why bother putting a second fuel type, when the same can be done with a spark plug?
still……we need alternatives to fossil fuel! H2O for exact.
Yea, agree with cacing there, so no spark plugs are used, but the ignition is still localised at first, not spontaneously for the whole area.
cold start combustion is easily resolve with some kind initial ignition, or glow plug, etc.
my doubt is the capabilty for higher speed engine, theres a posibility a preignition happened before TDC due to heat built up, which destruct the engine,
due to non regulated ignition, they might want to have somekind of variable copression ratio in relation with engine speed or load, perhaps a small clyinder/piston built in the chamber to alter volume of the chamber hence the reduce the comp ratio for extreme condition,
Yet another stunt from Honda.
cacing said,
April 17, 2007 @ 9:33 am
I’m probably not understanding it correctly, but referring to the last paragraph, if we introduce a second fuel of lower octane, that would then disqualify this process from being completely HCCI, as the lower octane fuel will have to burn first, then propagate the flame front to the rest of the air-fuel mixture. If that’s the case, why bother putting a second fuel type, when the same can be done with a spark plug?
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Agree with you here. Plus, introducing another low octane fuel also means that the car has to cary different octane ratings of fuel at any one time. Complicates stuff too much.
To answer your question, why no spark plug? Easy, the incoming fuel is too dilute to be ignited by a spark. Remember, we need a ratio of 14:1 for spark ignition. Now, the engineers are trying to save fuel, reduce emissions, clean up environment, etcs…
The fuel coming in is probably 30 or 40:1 ratio. then add a slight touch of injected fuel into a hot chamber and boom! we have ignition.