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Just a thought on HCCI and a rather lack of information(compare to other tech). What are the problems they are having? I have read from it needing cold compression, becoming volatile when things get hot yet it does need to a certain degree some compressing to cause homogenous ignition. The concept is very interesting with perfect burning of fuel. Does it also give higher bmep comparatively to a normal spark engine?
HCCI engines operate with a homogeneous charge; air and fuel is mixed to a homogeneous burnable mixture in the cylinder like in a spark ignition engine. However, unlike a spark ignition engine the mixture is ignited by the heat of compression. This is one of the challenges with HCCI; controlling the point of ignition. In spark ignition engines the point of ignition is easily controlled by the timing of the spark and in a diesel engine by the start of fuel injection. With a HCCI engine the ignition timing must be controlled by indirect means, for instance by controlling the charge temperature.
HCCI engines are typically limited to low bmeps. Like diesel engines they operate with air excess, often with exhaust gas recirculation. The purpose here is limit the heat release rate and the pressure rise during combustion. The high heat release rate of HCCI engines increase heat losses during combustion and can cause very high peak cylinder pressures. The high pressure rise can also give a very noisy combustion; the knocking engine sound you can hear from a diesel is due to higher pressure rise in a diesel. As lean mixtures burn slower than stoichiometric or slightly rich mixtures running in HCCI mode is typically limited to lean mixtures. At high loads HCCI engines can also suffer from a knocking type of phenomenon.
A HCCI engine does not have perfect burning of the fuel, infact they typically have high amounts of unburned hydrocarbons in the raw untreated exhaust. The combustion process does however produce very little NOx, so the exhaust can be treated using only an oxidation catalyst. Since NOx can't be reduced by a catalyst when there is excess air, this can be a significant advantage.
